N E W Edition 2009

Steel belts for the production of wood based panels

2

The steel belt is one of the most versatile tools available to industry and is found in applications as diverse as baking, chemical processing, materials handling, wood based panel production and, in recent years, paper manufacturing too.

As far as the wood based panels industry is concerned, the steel belt is the fundamental machinery element in all continuous presses, be they rotation or double belt presses. In short, steel belts are essential to the ongoing success of this technology.

The publication of this handbook has only been possible through the much appreciated co-operation of companies such as Siempelkamp, Dieffenbacher, Hymmen, Pagnoni, Held and ,previously, Küsters and Metso Panelboard (Bison), who have contributed pictures, illustrations and valuable comments.

As a ‘neutral’ supplier to all these companies, and others, our position is clear. Whoever we are supplying, we do so with one sole objective – to deliver the best quality steel belt, together with all associated services, and to be a competent and reliable partner for OEMs and end-users wherever they are in the world.

We hope you find this book both interesting and informative.

Copyright © 2009 by AB Sandvik Process Systems, Sandviken/Sweden All rights reserved. Printed in Germany - PS-SB-440 ENG 2.09

Foreword

The production of this booklet has involved the input and support of a number of technical experts. Particular credit is due to the following people:

Editors: J.O. Jonsson, Senior Technical Manager WBP Industry AB Sandvik Process Systems S-81181 Sandviken Sascha Porst, Regional Sales ManagerSandvik Surface Solutions (former Hindrichs-Auffermann) Mühlenfeld 101D-58256 Ennepetal/Germany

Ralf Griesche, Marketing Manager Design & Engineering, Wood Division G. Siempelkamp GmbH & Co., D-47803 Krefeld/Germany

Detlef Kroll, Engineering Manager, Dieffenbacher GmbH & Co., KG Heilbronner Str. 20 D-75031 Eppingen/Germany

Andreas Lentner, Vice President Sales and MarketingHymmen GmbH - Maschinen - und AnlagenbauD-33613 Bielefeld

Ulrich Koletzki, Sales ManagerHeld Technologie GmbHWeigheimer Str. 11D-78647 Trossingen-Schura

Ennio Codogno, Technical ManagerPagnoni Impianti s.r.lI-20040 Aicurzio-(Mi)/Italy

Gottfried Bluthardt (former Sales Manager Presses)Metso PanelboardPress & Energy DivisionD-30559 Hannover

Consultant and German copy writing: Hansgert Soiné, Senior Consultant for Press History D-38173 Evessen/Germany

33

1. Historical review 41.1 Pioneering the way to particleboard 4

2. Cycle pressing technology 4

3. The Bison-Mende process 6

4. The first continuous flat bed presses: 10 Bartrev - Sandvik 5. The change of opinion in favor of continuous

pressing technology 10

6. Continuous pressing technology for the production of wood based panels with steel press belts 13

6.1 Continuous roller bed presses 136.1.1 Küsters/Contipress™ – History 146.1.2 The ContiRoll® system from Siempelkamp 166.1.3 Dieffenbacher ‘CPS®’ 22

6.1.4 Hymmen Presses 26

6.1.5 Held Technology 27

6.1.6 Pagnoni Easylam® 31

7. Steel belts for wood based panels 33 8. Sandvik Surface Solutions

press plates and endless press belts 36 9. Innovation & investment 42 10. Sandvik Site Service 45 11. Questions and answers 47

Steel press belts, a technology that has revolutionized production processes in the wood based panel industry

I N D E x

Sandvik is a high-technology

engineering group with advanced

products and a world-leading

position within selected areas –

tools for metal cutting, machinery

and tools for rock excavation,

products in stainless steel, special

alloys, metallic and ceramic

resistance materials as well as

process systems. Worldwide

business activities are conducted

through 300 companies and

representation in 130 countries.

Sandvik Process Systems is a world

force in the design and

manufacture of steel belts, press

plates and steel belt-based

industrial processing systems.

Markets served include food,

chemicals and pressing equipment

for wood and other materials.

4

1. Historical review

Dr. Fred Fahrni of Zürich, Switzerland, reported (HRW 1/57, p. 24) on a patent of the year 1889:

”Krammer suggests gluing wood chips on canvas with an adhesive, arranging them in parallel individual layers, with the layers alternating lengthwise and crosswise. The number of layers depends on the thickness of the product.“ In 1905, Watson tried to press thin wood platelets to boards of varying thickness.

1.1. Pioneering the way to particleboard

The world’s first particleboard factory was Torfit Werke AG in Bremen-Hemelingen, Germany, where a 10 tons/day-plant was in operation from 1941. In this plant, boards with a raw density between 800 and 1100 kg/m3 (‘Pek-Pressholz’) were produced from a mixture of wood chips with 8-10% phenolic resins on a single-opening press made by Becker & van Hüllen, with pressures applied up to 1000 N/cm2. In 1943, the company supplied ‘compact wood’ in raw densities around 600 kg/m3. Shortly thereafter, the company was bombed and was never reconstructed.

In 1942, Westdeutsche Sperrholzwerke, Wieden-brück, a plywood mill in Germany, purchased Kaurit -brand UF resins from BASF for producing boards made from beech veneer scrap with raw densities of 700-800 kg/m3. A similar production was then started by the plywood factory Schütte-Lanz in Mannheim, Germany.

About that time, Dr. Fahrni launched his idea of the three-layer boards with a middle layer of coarse chips and outside layers of thin, laminar chip material. This idea was first put into practice by the Keller plywood factory in Klingnau, Switzerland. Production of high-quality, lightweight particleboard named ‘Novopan’ was started in 1946.

In the late 1940s and into 1950s, a number of small units with capacities of about 10 tons/day were installed. Many were operated by furniture factories utilizing their own wood waste material. This was a boon for the furniture manufacturers as they could add their own board-making facilities to their existing saw mills.

2. Cycle pressing technology

Multi-daylight presses with accompanying loading and unloading systems were known from the much-older fiberboard industry, which preceded particleboard by many years. They are closely connected with the names of two machine works located in Krefeld, Germany: Siempelkamp and Becker & van Hüllen. (The latter withdrew from the market in the 1980s.)

Apart from the complicated handling of the thick mats and the panels in and out of the press, the time required for heat transfer from press to the mats was another obstacle in reaching the goal of higher capacities. Moreover, the steel platens had a ten dency to alter in shape under the effect of heat, which disrupted the layers of the small wood fragments in the mat. By using aluminum platens or, even better, brass, these cited disadvantages could be reduced, but the pressing time still remained unsatisfactorily long.

Faced with these disadvantages, multi- daylight pressing gave way to the single-opening press. The German Bison-Werke in Springe (SES Siempelkamp since 2007) developed a semi-automatic production system for particle boards known as the Bison system. It is a combination of a very long single-opening press with a rotating steel belt to carry the mat and moving stepwise through the press.

The first plant of this type was delivered in 1957 and soon became popular. Over the next few years, 100 such presses were built.

Since then, Bison, Becker & van Hüllen, Dieffen-bacher, Motala, Siempelkamp, Sunds and other machine works have delivered over 600 single- opening presses. Their length and width increased gradually over the years to meet growing capacity demands, finally reaching an optimum length of approx. 52 m.

The 1300C grade steel belt used for this process is supplied almost exclusively by Sandvik. It is a harden ed and tempered carbon steel with an average carbon content of 0.65% (see page 34). The single-opening pressing procedure is uneconomical for the production of thin panels, as the ratio between dead time and reaction time change unfavorably in relation to the output. Dead time is a constant, while pressing times are variable depending on final panel thickness.

Steel press belts, a technology that has revolutionized production processes in the wood based panel industry

5

The situation is similar to that of the sanding allowance required to obtain the correct panel thickness. The allowance is a constant; thus, the percentage of (expensive) resin-containing wood material which has to be removed in the sanding process increases with diminishing panel thickness. To overcome these deficiencies of cycle presses, the concept of continuous, nonstop presses was developed.

Single opening presses (SOP)

Graphite bar for maintenance-free lubrication of steel belts for

single opening presses

wear plate idlersgraphite pad

forming machine

▲

brush main press

Ø 1,400 mmØ 2,500 mm

drive steel belt support rollers tracking tensioning

Diagram of SOP (Single Opening Press)

6

3. The Bison-Mende process

The first commercially successful procedure for the continuous production of thin particleboard and fiberboard was the Bison-Mende process. The press is a derivative of a plant which had been designed for continuous bonding of rubber sheets and laminating of thermoplastic sheets (Auma press).

The process was further developed and adapted to the requirements of wood based panel production through close cooperation between the Mende panel-manufacturing concern and Bison.

The process first reached the market in 1971, and since then over 90 presses of this type have been put into service. Many are still operating throughout the world. An essential feature of the plant is a continuous steel belt which runs through the forming machine and around a heated press drum with a diameter of 3, 4 or 5 m. The forming station can also be separate from the steel belt. In these plants, panels with a thickness of 2-8 mm, max. 12 mm are produced, (lines are usually devoted to the production of thinner panels) and widths range from 1.2 to 2.5 m. The system was built by Bison and Metso Panelboard up to 2007, who purchase the basic press from Berstorff of Hanover, Germany, and offer it as complete turnkey unit ready for operation (see pages 7-9). Nowadays, the company BINOS in Springe/Germany are also producing Mende presses.

The total stress on the steel belt is very high in the Mende process, as the belt is exposed to high reversed bending and thermal stress. During each rotation, the belt is bent six times as well as being heated up and cooled down. The belt must already be maintained under high tension in order to be able to exert high pressure on the panel being pressed. Additional pressure is applied from up to three calibrating drums arranged around the large heated main press drum.

Apart from the stresses mentioned above, there are several other influences which can shorten belt life: hard particles falling between drum and belt, uneven shape of the mat, belt tracking problems, scratches on the belt, corrosion, etc.

During the early stages when Sandvik 1300C belts were used, the inherent operating conditions were hard to overcome. In particular, the high number of reversed bendings per rotation reduced the lifetime of the belts to a level which was barely economical.

Moreover, due to the required heat treatment of the weld, repair welding was very time-consuming.

For all these reasons, users showed great interest in a steel belt quality with higher strength and easier repairability.

This situation led Sandvik to develop Sandvik 1450SM – a high-strength steel belt grade – in the mid-1970s. In 1980, it was further improved and named 1650SM – and this steel belt grade is now used in continuous roller bed presses worldwide.

It is a precipitation-hardened, corrosion- resistant steel which obtains its excellent mechanical properties by a simple heat treatment in air.

Steel belt grade Sandvik 1650SM (PH) offers the following advantages, compared with Sandvik 1300C (see page 34/35):

• Very high strength• Very high fatigue strength• Optimal strength of weld (welding factor almost 1)• Ease of repairability• Easy to weld• Deformation-resistant As a result of the development of the high-strength steel grade, the service life of belts increased considerably, and the cost situation improved. By the 1990s, it became normal for a belt in a traditional Mende plant for particleboard to be used for 30-40 months, provided that the plant is well-maintained. The service life of belts in the new Mende plants for MDF, however, is only 9-15 months. The reason is that belts are much shorter and speeds much higher, which means an increased number of load reversals per time unit. Moreover, the pre-tension is higher in order to achieve the higher pressure on the mat for increased panel raw density. Finally, as the requirements for panel integrity are very stringent, markings from the repair welding must not be visible.

For many years the Mende system was the unrivalled No. 1 method for continuous production of thin particleboards. For thicker panels, the process was unsuitable due to the bending of the boards around the main press drum.

7

1 Cooling and belt return drum2 Infeed and first pressing drum3 Press belt return drum, heated,

line pressure 3,500 N/cm4 Press belt drive drum5 Press belt tensioning drums6 Main press drum, heated, available

in 3, 4 or 5 m diameter7 + 8 Pressing and calibrating drums

9 Infrared heating elements (600°C) to ensure uniform temperature of press belt

10 Steel press belt. Belt tension applies a pres-sure of approx. 20 N/cm2 to the product

11 Finished product outlet12 Hydraulic tensioning unit, which also ensures

accurate belt tracking13 Hydraulic control of infeed drum14 Press belt cleaning brushes15 Induction motor for cleaning brush

Press plant of Bison-Mende system

8

Bison-Mende system using a single belt for forming and pressing

Bison-Mende system using high-frequency preheating for increased capacity. Forming belt and steel press belt are separate.

Directly before the press infeed, a discharge device can be opened above a pit for faulty batches.

Pressing station High-frequency preheater

Spreading station Cutting station

Pressing station Spreading station Cutting or sanding station

Typical data for a 3.2 mm panel

Rawboard density 800 kg/m3 ± 5%

Bending strength 35-50 kg/mm2

Wood portion of the panel 750 kg/m3

Glue portion of the panel 90 kg/m3

Electricity 300 kWH/m3

Heat required for dryer 2 GJ/m3

Heat required for press 0.4 GJ/m3

Steam required for refiner 500 kg/m3

9

With the Bison-Mende system, the pressed panel continuous ribbon returns above the plant and is sanded inline

(not shown in the picture) and cross-cut. The crosscut saw is designed so that it can operate with short cutting cycles.

View of the outfeed of the press in

the Bison-Mende system, showing the

press belt on the tension drum

10

4. The first continuous flat bed presses

BartrevBartrev presses were equipped with platens arranged as plate chains which rotated in the machine – supported on two rollers on each side – similar to caterpillar tracking. The individual platens were lined up horizontally without a gap between them: however, they were also covered by steel belts, which could be cleaned more easily than the plates. The lower steel belt also transported the mat through the pre-press, the HF heating electrode and the press. Only the chains had a drive; the steel belts were dragged along with them. However, the wear of the pressplates was too much so the experiment ended in the 1960s.

SandvikSandvik Process Systems was the first company to design and manufacture a continuous Double-Belt Press System and its design principle is still used for continuous belt presses today: Multiple rolling bars guide the steel belt on one side and establish the heat transfer to the product via the heat plate on the other side. The process is continuous and the product is moved by the upper and lower belt. A similar design is employed in double-belt cooler systems.

The idea of using "rolling bars in a row" for heating and cooling was progressive and established a long time before it was used for wood based panel production.

The Sandvik Double-Belt Press can be used at temperatures of up to 400ºC and can achieve tolerances as fine as 0,05 mm. Such a system was tested for pressing wood based panels and for laminating finished chipboards.

However, as an ́ outsider´ in the wood based panel industry, Sandvik decided against further development for this industry due to the cost and complexity of the system. In the early 80s Sandvik backed out of the business of designing presses for wood based panels entirely, leaving the market mainly to Siempelkamp and Dieffenbacher.

Today, all suppliers of press systems for the wood based panel industry are customers of Sandvik, using both their steel belts and their service.

Sandvik didn't leave the double-belt press market entirely though, using its experience to concentrate on the plastics, composite materials and laminate industries.

To this day Sandvik remains a supplier of such

press systems and has sold more than 40 systems for these applications. A further 200 Sandvik double-belt press systems have been used for other applications, e.g. artificial marble, polyester, acrylics, phenol, epoxy and other resins, silicon, rubber, antioxidants, wax, nylon, polyurethane foam and expanded polystyrene.

5. The change of opinion in favor of continuous pressing technology

In complete contrast to the situation in the early days of the panel industry, characterized by fierce and drawn out patent disputes, the changeover to continuous pressing took place peacefully. It was accompanied by the evolution of suitable steel press belts needed in the production process, as the newer belts were able to handle considerable tensile forces. In fact, it was the steel belt which sparked this development.

There were many years between the completion of the last Bartrev press and the appearance of the first küsters press®. After a period of industrial trials at a German particleboard mill, the first küsters press® was installed on a full-time commercial production basis at the Spano particleboard mill in Belgium. The change of opinion was then so surprising that it’s worth looking closer at the reasons behind this.

Küsters, a highly reputable supplier to the textile and paper industries, decided to enter a third field of activities and opened the Küsters press division in the wood based panel industry. Since 1999, the press

Sandvik press at Symalit

11

division has been an integrated part of the Metso Panelboard GmbH, Hannover/Germany, and the name of the press changed to Contipress™. Spano’s küsters press®, a pioneer in a new field, was a success right from the start.

In 2007 Siempelkamp took over the Hannover premises and, with that, production of the Küsters Contipress was ceased (although Siempelkamp has guaranteed to support existing presses in terms of both service and elongations).

The first unit, of course, raised a number of new challenges. Not only were there the start-up problems to be overcome, but also the up- and down-stream equipment had to be designed separately and contracted out to other suppliers, since the Küsters shops only made the press.This was the main reason for the slow acceptance of the küsters press® and also the reason for the later success of the Siempelkamp ContiRoll® press, which was part of a complete production line supplied from a single source.

Moreover, a lack of incentives to change to a new

technology hindered widespread acceptance at that time. Loading and unloading techniques of cycle presses worked perfectly and did not hamper the continuity of materials flow. There was no actual need to make pressing a continuous process.

In none of the publications on Bartrev presses was there a hint as to why the risk of such an innovation had been taken. Variable product lengths were not a high-priority reason, and ‘continuous = economical’ was not proven.

Spano, the first successful user of a küsters press®, kept their operating experience confidential for a certain period; however, important findings could not be concealed from the public for long. Interested companies had product samples made for their own laboratories and the findings of even the simplest test were clear : Particleboards with a raw density of only 640 kg/m3 produced in the küsters press® were as good as boards with a density of 680 kg/m3 which had been produced in cycle presses. The somewhat-lower bending strength was outweighed by improved internal bond transverse tensile strength.

The reasons favoring the continuous units’ acceptance are the substantial differences in pressing technology. In contrast to traditional cycle presses, continuous presses can be operated with a defined pressure and temperature profile over the whole effective length. The high number of hydraulic cylinders employed across the width of the production line, together with the use of a thinner heating platen, means that a continuous press is able to deliver more efficient steam release across the full production width. Oval steam nests, as used in cycle presses (where steam can only evaporate via the edges by opening the press), no longer exist.

However, material savings of 5% did not trigger any reactions by the competitors, as long as only one user turned this advantage into profit. This situation changed as soon as Siempelkamp launched its ContiRoll® and Bison simultaneously presented its Hydro-Dyn-press, the latter having a completely different design concept.

Apart from the material savings due to reduced density, further savings were achieved through minimal sanding allowance. This is possible because the continuous press imparts to the pressed panel very compact surface layers and is already quite accurately calibrated for thickness. The sanding removes 0.2-0.3 mm each side of the panel

Characteristic curve of the specific pressure in continuous

particleboard production: pressure build-up – high-pressure zone

– pressure drop – calibration

press length

spec

. pre

ssu

re

12

(continuous press), compared with 0.4-0.6 mm (single- opening press) and 0.6-1.0 mm (multi-daylight units). Continuous presses are economical consumers of electrical and thermal energy. They need minimal hydraulic forces, the only purpose of which is to maintain the pressure applied, or to decrease or increase it slightly, whereas cycle presses shuttle back and forth between zero and maximum.

Also with regard to driving forces, continuous presses show a favorable balance. Although press belt drives must be specified sufficiently strong for starting the press, their power consumption is considerably reduced during continuous operation. With cycle presses, especially multi-daylight presses for feeding, closing and opening, each load of panels requires high and sudden amounts of energy.

Savings of thermal energy result from the deaeration of the web in the continuous press’s conical infeed and absence of any dead time with radiation losses involved.

Panel-size variability is as easily achieved with modern multi-daylight presses as with continuous presses; however, with the former, capacity losses have to be accepted not only in case of reduction of width, but also of length.

As dead times are completely eliminated in continuous presses, their capacity, depending on product-specific heating times, remains constant. With cycle presses, however, the capacity decreases with decreasing product thickness, as the negative impact of their constant dead times gains importance; for panels below 8 mm thickness, the process becomes uneconomical and a technological risk due to the deaeration problems. On cycle presses, no thin panels

are produced (with one exception: hard fiberboard, which can be deaerated through the screens used in pressing).

Technological press data show a further plus for continuous presses. The pressing time factor approaches 4.0 s/mm for the optimal panel thickness of 16-19 mm. It strongly depends on press temperature, which can reach almost 240°C. Other factors are of less, but still measurable influence: an extension of the press length of 10 m means an advantage of approx. 0.5 s/mm, i.e., a 40 m long press can achieve a time saving of approx. 1.0 s/mm compared with a 20 m press. The pressing width obviously plays a more-important role with cycle presses than with continuous presses. This is due to the deaeration problems mentioned above. For cycle presses, an increase of press time of 0.5 s/mm per 300 mm effective width is mentioned. If it amounted to about 5.0 s/mm for one of the 1.2 m (4-ft) wide pioneer units, it would amount to 7.0 s/mm for an 8-ft unit. Spano, however, mentioned only a time difference of 0.5 s/mm, between the narrowest (1850 mm) and the widest (2630 mm) küsters press®.

Another possibility of press time reduction – with system-dependent efficiency – is cooling under pressure without loss of heat energy. Since a cooling section reduces internal steam pressure in the panel effectively, press times can be shortened and partly converted into curing time. Furthermore, the press can be entered with higher mat moistures, thus resulting in higher production capacity as a result of improved heat transfer. Other advantages are a decrease in board swelling and an increase in board moisture which is closer to the equilibrium moisture content.

Unfortunately, this cooling is in direct contradiction to the heating-up of the press. An economic cooling of single-opening or multi-daylight presses is not possible due to the mass of the heating platen, which has to be cooled down and heated up again in each pressing cycle. Even in continuous presses the amount of heat energy stored in the material of the ‘rolling system’ has been too high to allow economic cooling.

It was Küsters once again who, in 1997, solved this problem. The latest generation of küsters press® is equipped with an integrated cooling zone at the outlet end of the press. The ‘rolling system’ of the küsters press®, a chain system, is driven by pressure between heating platens, product and the turning Küsters/Contipress: heating and cooling

13

steel belts, whereas no separate drives are needed. The chain system can be easily divided into two separate systems within an extremely short distance, splitting the press into a heating zone and a cooling zone.

This means that the chain in the heating zone remains hot and the chain in the cooling zone remains cool all the time.

Therefore, in the cooling zone of the küsters press® only the steel belts have to be cooled in addition to the board, not the rolling system. The effective cooling zone length covers 25% - 30% of the total press length.

The first four küsters press®-based MDF lines featuring integrated cooling zones went into succesful operation in 1998 and by 2001 the first such paricleboard line – together with further MDF lines – had also been supplied.

The wear of roller elements, however, is an almost negligible cost factor. Today’s steel press belts are no longer the high-cost factor they were in earlier times either, as the alloys are constantly being improved, the belts made thicker and stronger and repair easier.

After more than two decades of the küsters press® operation, the estimates are becoming more reliable.

Cycle presses, based on industry experience, must be replaced after 20 years at the latest. Their frames and hydraulic elements are subject to wear and fatigue due to the pressing cycles. This does not apply to continuous presses. In fact, the oldest küsters press®, installed in 1977 at Spano, remains fully operable. Therefore, a lifetime of 30 years could be realistic, provided that the press is properly serviced.

There is one obvious argument against continuous presses: they need more-sensitive controls. Cycle presses are comparable to a blacksmith’s hammers, whereas continuous presses are more like punching tools. Continuous presses must be in thermal balance in order to function properly.

Sensitivity to the processed particle material, however, disappears with the increasing thickness of steel press belts. Such high-quality steels, together with the increased thickness, reduce wear and the belt-tracking problems which have sometimes caused significant problems.

It can be concluded, after comparing the various disadvantages and advantages, that the continuous press technology absolutely is the future as we have seen now in the new century.

6. Continuous pressing technology for the production of wood based panels with steel press belts

Apart from the extrusion press, there is no continuously working press system that does not use steel press belts. They all function according to the principle of ‘single-opening presses.’ The steel belts known from the Bartrev press were only carriers of the mat as they are still used in single-opening cycle presses. The lower belt is extended to provide a forming zone, if mat- forming is discontinuous. 6.1 Continuous roller bed presses

The steel belts used in today’s continuous presses are driven components subject to high tensile stresses, depending on the strength of the frictional forces, and are the principal element of such presses. Without them, nothing would happen.

To overcome the main problem – to avoid or minimize gliding friction acting on the belt – the concept of the ball bearing has been transferred to the belt support within the press: the press platen corresponds to the (fixed) internal ring of the ball bearing, the roller elements of the press systems to the balls and the steel belt to the movable outer ring of the bearing.

Movable elements and a number of procedures make the difference between discontinuous pressing techniques:• Carpets formed by roller chain strands in close

succession (see page 15) or roller bars extending over the whole plate width (see page 20) and rolling off between the press plates and the steel belts, with their return strands running above/ below the press.

• End drums with diameters increasing with the press belt thickness, their surface mostly ribbed with friction linings for transmission of the driving forces and for safe belt control.

• Infeeds which can be adapted to the different configurations of chips, strands, flakes and fibers, to the degree of compaction – i.e. thickness and density of the web, and to different kinds of raw materials.

• More or less ‘flexible,’ sometimes ‘ articulated’ upper and lower press plates, which serve in the short high-pressure zone on the infeed side, together with the movable press infeed, to control the mat thickness and to influence the density.

14

• Steel press belts are active, driven elements, which draw the product through the pressure zones, tensioned and guided hydraulically. The precise synchronization of upper and lower belts eliminates any shearing forces on the product.

The press characteristics of these systems are called isochoric, which means that they work with varying pressures on the inevitable local differences of raw density. Isochoric systems calibrate products to a uniform thickness independent of such density differences. All traditional single-opening and multi- daylight presses have the same press characteristics.Different thickness of the roller elements results in different heat transfer between heating plate and steel belt. This difference can be compensated by a higher infeed temperature of the heat carrier. All systems work at a speed of abt. 4.5 s/mm for particleboard and abt. 9.0 s/mm for MDF. The speed is also closely related to press length (reducing pressing time) and press width (increasing pressing time). 6.1.1 Küsters/Contipress™ – HistoryAfter the last Bartrev press had gone out of service (refer to section 4), some time went by before the Küsters company in Krefeld launched a new generation of continuous flat bed presses. In 1977, the first küsters press® was put into service at the Spano company’s particleboard mill in Belgium.

Since then, the names of both the press (Contipress™ by Siempelkamp) and the customer

(METSO Panelboard) have changed but the outstanding feature remains a rotating carpet of individual rollers without a separate drive. The numerous small rolls are of varying lengths; therefore, a wave- like arrangement of the roller links is necessary. This arrangement ensures a uniform pressure distribution on the pressed surface, as every point is constantly rolled-over (see page 15).

The design allows very narrow arcs of the roller carpet, which simplifies a subdivision of the complete press, in order to obtain a highly efficient cooling zone on the outfeed side. The narrow pressureless area of the separating line remains covered by the steel press belt (see page 12).

The carpet of small rollers of only 12.5 mm in diameter with its narrow center distance reduces the bending stress of the steel press belts by the counter-pressure of the compressed product – a stress which inevitably increases with the space between bigger roller elements.

The roller carpet elements are recirculated in a closed carpet above and below the press, on rollers in a heat-insulated channel, where most of the residual heat is conserved.

The fact that the rollers in the roller carpet are only 12.5 mm in diameter leads to a large surface contact between the steel belts and the heating platens. This in turn gives a very efficient heat transfer, meaning that the temperature difference between the heating platens and the belt surfaces is very low. For a standard particleboard press, it can be as low as 10-20°C. This efficient heat transfer allows lower

The latest generation Contipress™ with self-stabilized press body structure

15

heating oil temperatures, giving energy savings and reducing the risk of fire. Conversely, if the panelboard mill’s line is limited in productivity, a higher oil temperature similar to other presses on the market may be used, thereby increasing the effective capacity of the line.

After several years of research work, a new roller carpet design is now available to press users, thus allowing the mill operators to change the working width of their board production lines by more than 2 feet. With the redesign of the roller carpet to incorporate somewhat wider roller segments, the presses can make a swing of two feet with no loss in product quality or operating efficiency. Thus the new roller carpet design gives the panelboard producers additional flexibility to meet the changing demands from the marketplace (see above).

The press frame construction in combination with the press profile control system is another important feature of the Contipress™. The ‘window-frame’ is manufactured out of a solid plate of steel where the ‘window’ is milled out. The upper heating platen, designed as a rigid counter-bearing, is connected to the upper part of the frame.

The control of the press profile was uncompromising right from the beginning: the lower heating platen, 70 mm thick, softly yielding, rests on numerous hydraulic cylinders with a center distance of only 300 mm over the whole pressing

width, resulting in one cylinder per foot of width. The hydraulic cylinders in each press frame have their own HNC controlled, proportional hydraulic valve which is integrated, together with its thickness (press platen distance) measuring system, into a computer-controlled system. Groups of individual frames can be controlled by either distance or pressure, whichever is preferred. The use of differential cylinders (with 2 pressure chambers of different size resulting in 4 pressure steps) over the width of the press frames allows so-called "cross correction", delivering varying pressure over the press width. This keeps the spreading forces of the consolidating product in balance with the pressure applied and enables all variations resulting from wood species, spreading density, wood moisture and steam pressure to be compensated for. The accurately controlled calibration at the outlet end makes it possible to guarantee thickness tolerance of less than ± 0.10 mm.

Up to 2007, Contipress™ has been manufactured to lengths of more than 50 m, with a product width up to 3.30 m and operating press speeds of up to 90 m/min. Worldwide service for these presses is delivered by Siempelkamp Maschinen- und Anlagenbau, Krefeld.

The roller carpet is composed of a great number of rollers (12.5 mm diameter) connected by the links. The roller carpet elements used at the

outside of the roller carpet are 550 mm wide, allowing a width variation of the product of more than 2 feet.

As a result of the different roller lengths, the fabric shows a special pattern with the characteristic wave-like lines formed by the links; this

arrangement ensures good pressure distribution and leaves no line marks on the panel.

16

Siempelkamp ContiRoll System –3 sizes

6.1.2 The ContiRoll®system from Siempelkamp

In 1984, Siempelkamp entered the market with a system based on a continuous rod carpet. The technique was not new. Early patents existed in Sweden and are still used by Sandvik for special plants. It was considered an important breakthrough for this German company at that time to have turned to the production of wood based panels via the continuous method. The firm had established itself as a market leader in their particular sector of press technology. It is considered to be the principal company among the various competitors who supply turnkey plants ready for operation from a single source; certain components not made by Siempelkamp are out-sourced and integrated into the overall production process, e.g. such equipment that is required for fiber preparation and panel sanding.

The first ContiRoll® was delivered to Louisiana- Pacific Corp., USA, for MDF and was hailed as a major news event in the panel field. Its effective length was 16 m. Suddenly, continuous press technology had become accepted for both particleboard and MDF.

The ContiRoll® was constantly improved and adapted to the new findings of continuous press technology. One of the basic features, however, remained unchanged: the frame construction, which mirrored that used in cycle- pressing. Heavy longitudinal girders take up the high counteracting forces resulting from the high belt tension which is necessary for continuous pressing; the foundations thus have to support only the dead weight of the press, eliminating the need for anchoring rods with additional foundations to take up horizontal forces (see page 21).

Both the upper and the lower press platens in

17

the feed area are 60 mm thick. The lower plate is the fixed counter support for the flexibly mounted upper platen, which can be accurately adapted to the conditions and requirements in the press gap thanks to its double-acting pressing hydraulics. Product thickness and density profile are very flexibly controlled over the whole length. This is also to the advantage of lower-density MDF in the area of 450 kg/m3, which is very much in demand today. The upper end of the spectrum is covered by thin boards of densities of abt. 960 kg/m3. These boards came under the designation of ‘HDF,’ High Density Fiberboard. In view of the fact that ‘light’ (i.e. low density), ‘standard’ and ‘thick’ (both in the middle range of around 750 kg/m3) are now made on generally similar ContiRolls, one can see the numerous possibilities of a wide range of applications of MDF boards made by the continuous press.

Across the effective press width, the product thickness is also accurately controlled by a close sequence of press plungers. Thickness control on the longitudinal and transversal profiles is part of a very complex press control system and enables thickness tolerances as small as +/- 0.1 mm.

The radii of the press infeed can be optimized for any mat thickness by means of the hydraulics. Following the infeed drums, the steel belt is fed into the curved press infeed section. Here the required thermal energy is immediately provided for the high-density outer layers of HDF, which, with the close thickness tolerance, means that the sanding step can be eliminated but the panel can still attain a surface which can be laminated or wet-finished.

All parameters determining the raw density and the compaction geometry can be stored and the programs requested within the complex plant control system.

The roller carpet for transmitting the pressure forces, which is typical for this system, is formed by rotating rods going across the full width of the press plate – below the upper platen and above the lower one (see page 20).

By using these full-width roller elements, edge impressions on the steel belts – as they might be caused by narrower rollers – are avoided. (The marks resulting from such impressions are sometimes visible on the pressed panel.) Rods used as full-width roller elements cause no gliding friction at all but merely rolling friction; therefore, the lubricant consumption is

low. The rods are flexibly supported by chains at both ends, which ensure orderly return and safe re- entrance. The chains do not serve for guiding, as the rods run freely at a secured distance of 2 mm through the pressing zone, and back in the flat, heat-insulated ducts above/below the press frame, in which the steel belts are also returned.

Continuous, full-width roller elements offer another advantage: the uninterrupted, homogeneous heat transfer from the heating plate to the steel belt. The combination of components chosen by Siempelkamp for the ContiRoll® press, i.e. heating plate – full-width rods – steel belt, is matchless from the heat-technological point of view, as it minimizes the number of heat transfer functions. The integrity of heat transfer from the heating plate to the rods is further improved by the microfinish of the heating plate surface, which ensures full contact with the rods. The high share of supporting area between plate and rods has another positive effect: it reduces belt displacement.

The first generation ContiRoll® worked to speeds of 300 mm/s. With today’s improved technology, the presses are running at speeds of up to 2000 mm/s. The presses are longer, and very thin panels can now be produced. A 33.8 m long thin panel ContiRoll® can produce 2-mm-thick MDF at a speed of 1650 mm/s, with a daily capacity of 660 m3.

Over time, the many presses built and learning processes that took place were of special importance for the steel belt. It was found that the belt lifetime depends mainly on the design of the components of the press which are in direct contact with the belts. Right from the beginning, drums with a big diameter had been used for the drive and return terminals in order to keep the bending stress of the belts low. This, of course, only makes sense if bending radii at other points are equally large. Therefore, the design of the ContiRoll® press avoids joints and break points along the path of the steel belts. Transitions between different radii are gradual. Guiding elements for the steel belts are designed as roller ‘baskets’ with the belt-supporting rollers arranged on a circular arc section with big radius. This applies both to the feed terminal of the press and to the controls of the return strands of the belts.

Siempelkamp attached great importance to the compilation of binding specifications for the steel belts. They have been available for quite a long time and

18

include all relevant data such as dimensions, material specifications, technical-physical values, tolerance data, finish, grinding pattern in the weld area, etc. By now, comprehensive information on endurance and fatigue strength has been added to these data.

The quality of the steel belts has a great influence on the quality of the pressed board – the higher the pressing speed, the stronger the influence. This applies especially to the belt geometry in the area of the longitudinal welds. As a result of the grinding of the seams, there is a slight ‘depression’ in this area compared with the surrounding surfaces of the two belt halves. Although the thickness differences are minimal, any improvement – even if only 0.01 mm – is welcome. Due to the inherent stiffness of the steel belts, with lower pressures they do not come into close contact with the rods in the area of the ‘depression,’ with the consequence that the heat transfer is different from the neighboring zones. This results in a different technological effect on the pressed product. The end product, therefore, can show differences in the technical-physical values and

the surface color. The development of the belts was forced by Siempelkamp with special regard to this aspect, which is of particular importance in the production of thin MDF, which can be prepared for market without going through the sanding step. The slightest thickness variations within the boards can cause problems when lacquer is applied.

Siempelkamp pioneered the use of thicker steel belts firstly with 2.7 mm and, more recently, 3.5 mm! Such belts offer a number of significant advantages to the user which more than compensate for the higher price of the belts, thus offering a higher heat potential. This has a technologically favorable effect in the infeed area of the press, i.e. reduction of the heating factor. The belts have a higher thickness rigidity and are, therefore, less susceptible to damage. The considerably higher transversal rigidity results in a stable and smooth belt run.

Another aspect of steel belt development was the reduction of belt width. Originally the belt edges covered the side plates of the chains accompanying the rod carpet to protect them against contamination.

Siempelkamp’s ContiRoll® Press, 7’ x 55,3 m for MDF, Location: Yildiz Kimya, Turkey

19

Therefore, the belts had to be much wider than the heated press plates. The temperature of the belt edge zones beyond the heated area was much lower than that of the belt areas in contact with the mat, which caused comparatively high tensions resulting in edge damage (cracks). Today’s belts are narrower than the rods of the roller carpet, and their width in turn exceeds the heating plate width only slightly.

Contamination of the side chains is avoided by suction devices which clean off the edge zones of the belts at the infeed. This method is more efficient than the former protection method.

Along with the reduction of the belt width, Siempelkamp attached great importance to the machining of the belt edges. They are now specially machined according to the latest standards of mechanical engineering in order to eliminate edge and peak tensions, thus gaining longer belt life.

The aforementioned reduction of the belt width can, of course, only be seen in relation to the heating plate width.

Another development was the use of very wide

steel belts for new, wider presses. These belts could not be made just by joining two belt strips by a longitudinal weld and Siempelkamp cooperated closely with the steel belt manufacturers. The width of the strips had to be planned carefully as did the position of the welds in the upper and lower belt in the press. Under no circumstances may the welds be positioned exactly opposite each other (this, of course, also applies to belts composed of three strips; however, in that case it is much easier with a view to the economical use of belt material).

The care of steel belts, which are the most- expensive components of a press, is not only important in the press itself. Siempelkamp has incorporated comprehensive precautions for protection of the steel belts well ahead of the press, and ample control and cleaning equipment at proper points in the press. The first safety device in an MDF line is a ‘sifter’ for eliminating undesirable foreign matter from the fiber stream ahead of the production lines. The sifter is followed by equalizing spreading rollers (at the fiber spreaders) which break up fiber

Siempelkamp ContiRoll® Press

20

bundles and glue lumps before the mat is formed. As with all types of pressed products, a metal

detector incorporated into the line will trigger the removal from the line of any mat portion containing metal particles. The sensitivity of the coil is automatically adjusted by the setting of the thickness of the boards to be pressed. As a further precaution in particleboard lines, a magnet is installed directly after the spreading machines so any magnetized metal parts are removed from the mat before they pass the metal detector. (Magnets are not used in MDF lines, as they would be unreliable due to the felting of the fibers.)

At the prepress, sensors scan any thickness increase in the mat which could cause asymmetrical pressure application and belt displacement in the ContiRoll® press and trigger automatic rejection. Ahead of the ContiRoll®, a thickness control instrument is positioned to prevent accidental accumulations of particles or foreign matter on the mat from getting into the press, where they could cause extreme local stresses and consequent belt damage. Defective mat sections are automatically rejected. Lines for the production of thin panels, when threatened, are automatically stopped by a secondary control unit.

With the new thin panel presses a compactor is used to press the mat to nominal thickness and thereby removing of risk of steel belt damage.

At the press outfeed on thin panel lines, cleaning and safety devices are mounted for both belts to ensure that product residues do not adhere to the belts and are not carried along on the belt.

The efficient cleaning systems for the steel belts feature rotating metal brushes for the product sides of the belts and plate-type scrapers for the inside of the belts to prevent accumulation of lubricant.

The protection of the steel belts begins even before production commences, namely, after the erection of the press, when the belts must be drawn into the press enclosure. To make sure that this job is done properly, the winding of the belts in the delivery coil, for instance, is prescribed in the above-mentioned specifications, considering the local erection situation.

Siempelkamp’s scope of delivery always includes the unwinding and drawing-in facilities, which are temporarily fixed to the press during erection and which ensure safe and careful handling of the belts during the installation. These facilities are, of course, designed in accordance with all relevant regulations for the safety of erection personnel. Furthermore, in the Erection Manual for ContiRoll® presses, 25 of 41 pages are dedicated to the installation of the steel belts, clear evidence of the importance attached to the proper handling of these valuable components.

Rod carpet typical of Siempelkamp ContiRoll® press

21

View of ContiRoll® infeed end. On the left, the belt infeed drums are directly followed by the return and entrance of the rod carpet. In

the high- pressure zone, the frames are arranged in pairs. On the right can be seen how closely the press cylinders are spaced to enable

precision adjustment of the flexibly supported upper press plate.

View of ContiRoll® outlet end

with rod carpet return, belt tension

terminal and drive

22

6.1.3 Dieffenbacher ‘CPS®’Dieffenbacher Conti-Panel System – an innovative, user-oriented technological concept

With the successful introduction of the continu ous Conti-Panel System, Dieffenbacher has underlined its position as a world-leading manufacturer of presses and press lines for wood-based panels. This innovative concept is the result of many years of experience in hydraulic press manufacture, in conjunction with the use of the most-modern high-tech control elements. The ‘CPS’ is a synergistic product from the domain of metal forming and plastics molding presses, achieving dynamic accuracies in the range of some thousandths of millimeters, e.g. on parallel motion control systems. The ‘CPS’ embodies a close cooperation with the user : It is a highly flexible press with a low rate of wear, fully meeting users‘ demands:

• High specific performance based on high availability

• Operator-friendly controls• Transparency in modular setup, remarkable

robustness• Good access and simple maintenance facilities

At Dieffenbacher, the principle for further developments and long-term sales strategy is to include the user and his concrete ideas of innovation and technology in the realization of such complex lines. Thus it is ensured that only reliable, mature and optimized systems will go into operation.

The Dieffenbacher Conti-Panel System features an ‘all-round’ press; it is suited for the production of particleboard, MDF, HDF, OSB, LVL and other wood-based panels. High capacities, consistent finished product quality and maximum flexibility are guaranteed, among other factors, by:

• an online adjustable infeed geometry (wedge compactor)

• high advance speeds of up to 2000 mm/s• fast compression phase• optimized heat supply at the right time• adjustable position or pressure control (online)• minimum thickness tolerances• highly flexible forming profiles, length- and

crosswise• modular tie rod design with pressure cylinders

arranged on the outside• optimum range of width adjustability

Widest Dieffenbacher press at Footner/Canada: heating plate widthapprox. 4000 mm board width for approx. 3900 mm product width

23

Operational principle of the Dieffenbacher CPS: Heat supply and distribution

Depending on the length of the press, the Dieffenbacher Conti-Panel System is provided with a suitable number of heating circuits.Pressure and heat are transferred by platens that are heated through thermal oil channels and protected by thin protection plates. The heat is transmitted via rolling rods onto rotating steel belts and hence onto the material to be pressed.By the use of suitable material pairs – unhardened heating plates with high thermal conductivity and well-hardened thin protection plates combined with wear-resistant function elements – the CPS meets the criteria imposed by the user with regard to an effective heat transfer (see page 24).To increase the heat supply, the rolling rod and steel belt systems are guided back in an insulating channel. A preheating device for the rolling rods supplies additional heat energy that is available right at the inlet. With this feature, it is also possible to achieve a high degree of process stability.Heat transfer from the heating platens onto the steel belts is a dynamic process using the turning rolling rods that ensure a uniform, uninterrupted heat supply over the entire width.

Operational principle of the Dieffenbacher CPS: Pressure build-up and distribution

For the Conti-Panel System, Dieffenbacher uses the proven modular frame design with the pressure cylinders arranged along both sides of the press, on the outside.This leads to advantages that have been proven in practical use, such as good access for maintenance purposes and gentle operating conditions for the hydraulic components to ensure a long service life. The open frame design comprises a stationary frame bottom part (table) and a moving upper part (ram). Forces are transmitted via removable pull rods that are fastened to the table.In the middle of the press, calibrating (multipot) cylinders are installed in the frames. By means of these cylinders, it is possible to select the cross profiles that are necessary according to technological requirements in keeping with the customer‘s request.

The cylinders are controlled by proportional control valves and form closed-loop control circuits, together with the digital position pickups. Thus panels with optimum thickness tolerances and high process stability can be produced.By a deliberate separation of functions and the highly versatile design of the heating platen system, the press profile can be set online in an optimum manner in longitudinal and transverse directions to suit the respective product demands.As a result of a new concept, the frame supports are able to compensate for thermal expansion. The advantage is that the platen temperature can be changed without any restriction and without interrupting the production. This is also a major benefit for the CPS user.The press infeed section (see page 24) is specially designed to meet the requirements of panel production using different raw materials, i.e. particleboard, MDF or OSB.The wedge compactor equipped with an intelligent double joint system can adjust automatically to the respective mat height and can be set to any angular position. Hence the compression speed curve can be set in an optimum manner in this zone, which is an important phase in the technological process. Maximum pressure and high temperatures are available at the earliest moment possible. Thus the crucial conditions for the shortest possible pressing times and panel products are optimized for minimum sanding allowance, and best possible density profiles are achieved.

24

The rolling rods of the CPS are connected coaxially with the links of the hollow chain by resilient rods

Infeed of the rolling rods by means of gear wheels and spring leafs at both ends.

Footner OSB production with belt width 4,080 mm, belt thickness 3.0 mm.

Heated press plate

Protection plate

Rolling rods, 21 mm thick

Hollow chain

Steel press belt

Connecting rod

25

Dieffenbacher press for

particle boards

Dieffenbacher press for

MDF boards

Smallest Dieffenbacher press for LVL production: heating plate

width approx. 1300 mm board width, approx. 1400 mm product width

26

6.1.4 Hymmen IsoPress® and IsoRoll®

Hymmen is the only supplier in the field of continuous double belt presses to offer both isobaric and isochoric press systems, and is leader in various product applications which make use of these technologies. Isobaric double belt presses:

The isobaric press is based on the environmentally friendly air cushion system and is characterized by an optimal distribution of pressure and an exact control of the temperature profile. Isobaric presses generate pressures up to 100 bar and temperatures up to 400°C. An additional specific advantage of this technology is the ability to achieve a heating- and cooling process without a pressure interruption. The working width of an isobaric press can be up to 2300 mm, while the press length depends on capacity requirements and the press technology.

The isobaric Double Belt Press is suited to the lamination of board material as well as the pressing of roll material & granulates, such as:

• Decorative laminates (CPL)• Technical laminates (Copper cladded laminates for printed circuit boards, ski-laminates)

• Melamine lamination of flooring & furniture boards

• PVC & rubber flooring• Lightweight conveyor belts• Plastic cards & further high-tech products made out of composites

With an experience based on having sold more than 170 double belt presses, Hymmen takes a worldwide dominating position on the market for the continuous production of decorative and technical laminates as well as laminate floorings.

Isochoric double belt presses:

The strengths of the isochoric double belt press are in the field of pressing soft materials out of granulate or roll material together to one product with a defined material thickness.Isochoric presses can be constructed for heating- as also particularly combined heating- and cooling processes.

Typical production fields are…

• Fibreboards• Cork or rubber flooring • Industrial laminates• Rubber boards• Heavy transport belts

Hymmen double belt press Iso Press® Type HPL

Hymmen double belt press Iso Roll® Type ISR

27

6.1.5 Held Technologie GmbH

The beginning of isobaric press technology In 1975, the Held company brought the first isobaric Double Belt Press (DBP) to the world. Further pioneering achievements followed, bringing technological leadership to the company in its field of isobaric presses:• World´s first isobaric high-pressure DBP providing

a surface pressure of 100 bars (1,450 psi),• World´s first isobaric high-temperature DBP to

provide a heating temperature of 400°C (750°F),• World´s first isobaric high-speed DBP for

production speed as high as 48m/min (160 fpm).

At the beginning of the 1970s a DBP was nothing new; its principle had been known for quite a while. Even the idea of exerting completely uniform pressure upon a surface by means of an isobaric cushion had already been conceived. However, until then it seemed impossible to press a fluid medium against a moving steel belt and hold it there without leakage. Only the seal system developed by Held made an old dream come true: continuous production under isobaric pressure.

A business partner in the furniture industry had triggered the development by requesting a machine that could make thermo-set edge-banding strips on a roll. He had enough of wasting material only because the available strips of fixed length never quite fit his application.

This challenge eventually led to the building of the first isobaric DBP, on which resin-impregnated paper could be laminated into endless edge-banding material under the influence of pressure and temperature. With a useful width of 500 mm (20 in.) the machine had only relatively modest dimensions. Its reaction zone, i.e. the working area between the steel belts, could not yet be heated; therefore, the necessary process heat had to be loaded on to the belts by heating the inlet drums. For reasons of simplicity air was chosen as the pressure medium for the pressure cushion. At 7 bars (100 psi) the generated pressure was far below the level reached by modern oil-supported DBPs. But at least it was possible for the first time to produce from roll to roll, i.e. continuously, even though production speed was rather modest compared with today’s numbers: 1.8 m/min with a 2.1 m long reaction zone. Today, ten times that output is achieved.

The machine proved so useful that a licensee was soon found and the concept developed by Held went on to form the basis of all isobaric DBPs used around the world.

Evolution of the high-performance press – high pressure and high temperature

This success encouraged research into how to improve efficiency and productivity. Higher cushion pressure would permit the use of more cost-effective paper with lower resin content, but would result in increased leakage of the air from the pressure cushion, leading to considerable waste of energy which would render the economic advantage useless. A pressure medium more viscous than air promised to be kept more easily on the moving belts without leakage. The solution was the oil-supported DBP. The oil-cushion brought a very helpful side-effect: an extremely efficient lubrication of the seal system, an advantage, whose importance became apparent only much later when air-cushion machines had problems handling high process pressure. Oil-cushion machines could be run with 80 bars continuously without compromising the life expectancy of the seals or generating blisters on the product. They were also found fundamentally more advantageous with regard to safety. Like all liquids, oil is incompressible and therefore cannot

World´s first Isobaric press (1975), 200°C, 7 bar

Basic design of an isobaric DBP

28

store any mechanical energy by being reduced in volume. In contrast, all gases are compressible, so under pressure they contain a considerable amount of energy. In the event of an accident, air cushions can release their stored energy in an explosion.

After a way had been found to feed process heat directly into the reaction zone where it was really needed, further benefits were reaped: higher output (the additional supply of energy permits higher production speed) and energy savings (lower thermal losses by reducing heat radiated by the inlet drums).

First, indirectly heated heat bridges, and later on directly heated heat bars provided solutions that went far beyond the original goal and permitted the processing of new materials demanding much higher process temperatures (up to 400°C or 750°F). Excellent heat conduction through direct contact of the metallic heat bars with the steel belts allows the transfer of large amounts of thermal energy with high efficiency. The directly heated reaction zone is an outstanding feature which proves extremely useful in particular with thicker materials that can be processed at significantly higher speeds.

Better quality through lower inlet temperature Heating the reaction zone provides further important advantages: melting and tearing of thin plastic films or detrimental pre-curing of laminate surfaces by heat radiated by the hot belts in the nip region of the press, which can happen when heating the belts only via the inlet drums, is now easily avoided when running with a low inlet drum temperature.

If a resin impregnated web does not enter the press sufficiently flat, stripe patterns from variations in gloss level are unavoidable. One might imagine that

choosing a higher production speed could reduce this effect by reducing the web’s residence time in the inlet region. Unfortunately, this will not help, since the higher speed will also require a faster curing resin, which in turn will also pre-cure in less time. A number of protective devices have been devised to avoid the effect, but have mostly proven ineffective, since they can only influence the very outer part of the nip.

A sufficiently low inlet drum temperature is needed to eliminate the problem and ensure that surface curing only starts when the material in under pressure in the reaction zone. Of course, this temperature pattern can only be achieved if enough heat can be applied to the belts inside the pressure zone, enabling the high process temperature to be reached here and here alone. This also makes possible the processing of thin plastic films, which would otherwise tear and melt at first contact with the hot belts in the nip of the press.

Decorative laminate – HPL vs. CPLThe quality of a laminate is highly dependent upon the pressure level it was subjected to during the manufacturing process. The old DIN 16 926 standard (since superseded by EN 438) stipulated a working pressure of 70 bar for the production of HPL (High Pressure Laminate), which air-cushion machines could not provide. As a result, continuously-produced laminate was, for a long time, considered a low quality product.

Today, while the quality of Low Pressure Laminate is still not comparable with that of genuine HPL, the market share now being won by CPL shows that there are applications for which low pressure CPL is perfectly well suited. Furthermore, as oil-supported DBPs are now available with working pressure of up

Service friendly: high-performance heating platen can be drawn out User friendly: setting the gap-width is just a push button operation

29

to 80 bars, this differentiation no longer makes sense. Indeed, the only differentiation that remains relevant is that of press type (cycle or continuous press). And it is Held's isobaric, hydro-static DBP >system contilam< that holds the advantage, delivering continuous HPL (CHPL), a material that offers the lowest production costs and optimum laminate uniformity.

DBP or cycle press?The principles of isobaric DBP operation offer a number of other important advantages.

A DBP can produce uniform quality more easily than a cycle press as every piece of laminate is subjected to exactly the same pressure, temperature, and residence time. Differences in temperature history between various pieces of laminate cannot occur in a continuous press, but are unavoidable in a cycle press due to each laminate's relative position in a multi-daylight press. And for very long pieces of laminate, there is no alternative to a continuous DBP.

With regard to the basic question: "Cycle press or continuous press?", a comparison with the printing industry may be of interest. When Gutenberg invented book printing, he used a cycle press. However, today's printed matter is, with few exceptions, manufactured on high-speed rotary printing presses. Why so? Because they are more economical. This is why the future belongs to continuous processes, and why examples can be found in every industry.

ProfitabilityFurther aspects contribute to the superior profitability of DBP-based laminate manufacture. Almost complete automation means the entire process can be run with just two operators (one to supply material and one to unload finished product) and also results in low handling losses.

In times of rising energy costs, a directly-heated, hydro-static DBP >system contilam< is particularly economical. Heat is supplied directly to the pressure/heating zone, i.e. where it is needed. The pressure medium only needs to convey pressure - it does not need to carry process heat. This means there is no need to circulate a large amount of pressure medium and heat/pressure losses are reduced to a minimum. Since a DBP never needs to open its gap for loading and unloading of material, it uses energy even more efficiently. And last but not least, trim waste only occurs on two sides while cycle press laminate needs to have all four sides trimmed.

High-gloss laminate – heating and cooling under pressureCost-effective manufacturing of high-gloss laminate is another application for Held's isobaric DBPs due to their efficient cooling without affecting process pressure. The cooling process is particularly economical as heating/cooling zones are thermally isolated from each other, and therefore remain at their respective temperature levels. Heat only needs to be extracted from the material and the thin steel belts (1 mm thick) passing the press whereas, in a multi-daylight-press, the thick press platens (ca. 100 mm thick) must also be cooled, rendering a process with heating and cooling rather uneconomical. Isochoric or isobaric? One question remains regarding DBP operating principles: "Which is best: isochoric (mechanical, supported by rollers) or isobaric (fluid, supported by air or oil)?"

Isochoric presses are well established in the processing of compressible materials such as particle board made from wood chips. Their inherent calibrating effect is an advantage here; the unavoidably

High-performance DBP for decorative laminate, two formats, 1.30 m and 1.50 m, 80 bars, 30 m/min (CLEAF, Italy)

30

non-uniform compaction and hence non-uniform density is acceptable for this type of product.

Other materials need processing with a completely uniform surface pressure. Those with demanding visible surfaces which soften during the manufacturing process, for instance, can only be produced continuously on an isobaric DBP where there no risk of pressure differences impacting on the material and distorting the pattern. For similar reasons, copper-clad laminate for demanding electronic applications require a process as free from tensions as possible in order to eliminate the risk of tensions trapped in the laminate being released during soldering and warping the product. As well as delivering the necessary uniformity of pressure, an oil-supported, hydrostatic DBP offers the additional advantages of high profitability and the ability to

be used in cleanrooms - a prerequisite for this application - making it the ideal choice.

Continuous high-gloss laminate, two formats, 1.35 m and 1.55 m, 80 bars, 30 m/min (Westag & Getalit)

High-temperature press suitable for clean-room operation, pressure

zone 0.7 m x 3.1 m, 400°C, 80 bars, 20 m/min

World’s largest isobaric DBP for decorative laminate. Press platens 5.20 m x 2.30 m, 80 bars, 48 m/min (SIT – Gruppo Mauro Saviola, Italy)

31

6.1.6 Pagnoni Easylam®

In the late ‘90s, Pagnoni Impianti, the Italian manufacturer of single/multi-opening presses for panels production (particleboard, MDF, plywood, HPL) and short cycle lamination (with melamine or phenol papers), entered the continuous press market with its Pagnoni Easylam®, a double steel belt press specifically designed to apply glued overlays and to flatten veneer on boards.

In these applications the press is the bottleneck of the whole process: conventional multi-daylight presses are very slow and high frequency presses have often failed to live up to users’ expectations.

The innovative technology behind the Pagnoni Easylam® not only removes this bottleneck through its inherent speed but also offers the additional advantage that the lack of press loading/unloading dead time allows the use of fast reaction glues, further increasing the production capacity of the line.

The pressThe Pagnoni Easylam® press is modular in design and its main elements are the drums, the hot platens, the rollers, the bearings and the steel belts.

The press structure consists of 2 m long modules. The upper part moves vertically in relation to the lower one, while the lower section lays on base beams which are fixed to the foundation. The structure is open on one side to allow easy replacement of the steel belt.

Two motorized drums and two idle ones, 1200 mm diameter, provide the drive and guiding/tensioning of the steel belts. The in-feed drums are heated with thermal oil, through axial rotary joints, and the temperature in each drum can be independently adjusted.

The sliding platens are placed between the upper/lower structures and the relevant steel belt. They are made up of modular elements, positioned side by side. Each element consists of a canalized platen (with heating circulation to ensure correct working

Continuous press Pagnoni Easylam® for engineered parquet, pressing hot platens 1000 x 4200 mm

32

temperature), and a frame which is fixed to the base platen and supports the bearings and the rollers. Each frame can be easily disassembled for maintenance purposes. Two heating circuits enable different temperatures to be set in the platens.

This new press belongs to the family of isochoric presses, but is significantly different from other such presses in that there is no rod carpet moving inside the press; instead, independent rollers rotate on their own axis on a bed of ball bearings. With this patented system, the squareness of the rollers and axial feed is guaranteed by the accuracy of mechanical construction, thereby avoiding any difficulty in the steel belts guidance. This ensures outstanding precision (max. 1 mm deviation) around the theoretical movement axis, even with asymmetrical loads. Furthermore, the rollers always remain warm, enabling considerable savings in energy costs.

Pagnoni Impianti uses Sandvik 1,4 mm thick stainless steel belts. The out-feed drums drive the belts; their tension is provided by 2+2 cylinders placed on the in-feed drums. The guide is controlled by lateral sensors which, based on the position of the belt with respect to the longitudinal press axis, adjust the pressure in the tensioning cylinders. The out-feed drums are equipped with cleaning scrapers and rotating brushes to remove any glue waste. The inner part of the belts is also continuously cleaned by brushes, moving transversally.

Pagnoni Easylam characteristicsSpecific pressure: max 80 N/cm2

Inlet drums temperature: max 150°CHot platens temperature: max 120°CMechanical speed: max 30 m/minHeating circuits: no. 4 independentPress opening/closing control: with electronic control

Pagnoni Easylam® is available in several standard sizes with widths varying from 350 mm to 2300 mm and lengths from 1900 mm to 8200 mm. The flexibility in size makes it easy to combine press length and width to meet customers’ technical and commercial needs.

Designed for versatilityThe Easylam continuous press has been designed by Pagnoni’s technical team to deliver maximum flexibility through the ability to:

- respond quickly to production changes- process panels that have been inaccurately or randomly positioned on the introduction belt- carry out different processes with the same press

Pagnoni Easylam® gets great results in terms of production capacity, final product quality and operating costs for a whole range of products/processes including panel veneering, two and three layers engineered flooring, five components doors (honeycomb frame + thin MDF + veneer), five components doors post (chipboard + MDF + veneer), thin plywood (up to 7 ply), tops for postforming made with MDF or chipboard overlaid with glossy HPL.

Pagnoni Easylam® panel veneering 2300x8200 mm (A view of the security roller installed before the upper inlet drum)

33

7. Steel belts for wood based panels

Steel belt manufacturer Sandvik has a number of significant ‘firsts’ to its credit, not least of which was the development of the world’s first steel belt in 1901. This was used for the transportation of sawdust, and the company has maintained its close connection with the wood industry to this day.

In 1957, Sandvik was responsible for introducing the steel belt to the wood based panel industry, when the first press to be fed by a steel belt, a single-opening intermittent press, was developed by Bähre & Greten in Springe, Germany.

Just a few years later, Sandvik itself designed the basis of the presses that have had such a major impact on this industry over the last two decades, building the first double belt press with a roller bed.

With this level of involvement over the years, it will come as no surprise to learn that the largest single market for Sandvik steels belts is the WBP industry.

While the company does still make its own double belt presses (only for fiberglass reinforced thermoplastics), as far as the WBP industry is concerned, it is as a specialist manufacturer of steel belts that its name is best known. The company’s steel belts are now at the heart of press systems worldwide.

Over the years, special grades have been developed to cope with the changing and ever-more-demanding requirements. Improvements continue to be made in the manufacturing process, but the essential qualities that make the steel belt ideal for this process remain virtually unchanged.

A properly manufactured steel belt combines hardness and strength with flexibility, a smooth surface and excellent heat transferability. It is durable, resistant to corrosion and easily maintained. Once the steel belt was introduced to the market in the 1950s, it didn’t take long for WBP producers or press manufacturers to recognize this unique combination of qualities, and Sandvik has continued to build on these fundamental benefits ever since.

Steel grades 1300C and the new 1320CThe steel grade used in single-opening presses has traditionally been Sandvik 1300C, a hardened and tempered carbon steel. Over the 50-plus years that these presses have been in use, more than a thousand 1300C belts have been supplied, and some have given their users well over two decades of reliable service.

The late ‘90s saw the introduction of a new steel grade suitable for this application, Sandvik 1320C, a low carbon, dual phase steel (with a microstructure of martensite and ferrite) which has excellent mechanical and fatigue properties but is particularly notable for its welding characteristics.

By eliminating the need to carry out post-weld annealing (even though the weld is harder than the parent material, it will run without cracking), installation costs are lower than those for the 1300C grade. In fact, with the correct tools, it is possible for a customer to carry out the welding in-house.

High strength steel belts 1500 SM + 1650 SMWhen the Bison-Mende process for the continuous production of thin board was introduced in 1971, it became clear that steel grade 1300C lacked the fatigue strength necessary to withstand the comparatively high reversed bending stresses that this type of press generates.

In this process, the tension of the steel belt contributes proportionally, along with two or three press rollers, to the total pressure, causing permanent reversed bending stresses and considerable pulling forces on the belt. A new steel grade, 1450SM (later modified to 1500SM), was introduced to cope with these demands and was complemented in 1980 by the development of the improved grade 1650SM. Both grades are used today and are characterized by their high strength, both inherent and in the weld area.

As continuous pressing technology developed further, the demands placed upon the steel belt also increased. Belts were called upon to handle production of MDF as thin as 1 mm but, at the other extreme, also relatively coarse-surfaced oriented strand board (OSB) and thick MDF and particleboard. Demand grew for heavier and stronger belts. In every case, however, uniform surface quality and close thickness tolerances remained as essential belt parameters.

Ever-higher belt speeds and pressures call for thicker steel belts, and, at the same time, the belts’ mechanical properties (strength and hardness) have to be optimized to ensure operation for many years.

34

January 1999 - Cancels edition March 1994

The Sandvik 1300C belt grade is made of hardened andtempered carbon steel and is characterized by:

■ Very good static strength■ Very good fatigue strength■ Very good thermal properties■ Excellent wear resistance■ Good repairability

Chemical composition, nominal values %

C Si Mn Cr

0.65 0.25 0.65 0.20

Static strengthStandard strength at room temperature, nominal values

Position Yield Tensile Elongationstrength strength A 5

MPa ksi MPa ksi (%)

Matrix 1200 174 1280 186 11Transverse weld(heat treated) 880 128 990 144 3

Hardened and Tempered Carbon Steel BeltSandvik 1300C

January 1999 - Cancels edition March 1994

The Sandvik 1320C belt grade is a dual phase carbonsteel, which matrix consists of 75% martensite and 25%ferrite. Sandvik 1320C is characterized by:

■ Very good static strength■ Very good fatigue strength■ Very good thermal properties■ Excellent wear resistance■ Good repairability

Chemical composition, nominal values %

C Si Mn Al Nb

0.15 0.50 1.80 0.04 0.03

Static strengthStandard strength at room temperature, nominal values

Position Yield Tensile Elongationstrength strength A 5

MPa ksi MPa ksi (%)

Matrix 1250 181 1340 194 5Transverse weld(not heat treated) 890 129 1000 145 4

Micro Alloyed Dual Phase Carbon Steel BeltSandvik 1320C

Sandvik’s range of steel belt grades enables the company to offer the appropriate product for all requirements. Most common in the WBP industry are grades 1300C, 1320C, 1500SM and 1650SM, for belts from 1.2-3.5 mm thickness, with the width of the belts between 1,200-4 ,620 mm.

] for more details please ask for our separate data sheet. ] for more details please ask for our separate data sheet.

35

The Sandvik 1500SM belt grade is made of a low carbon,martensitic precipitation-hardened, stainless steel of type15-5PH and is characterized by:

■ Excellent static strength■ Excellent fatigue strength■ Good corrosion resistance■ Very good wear resistance■ Very good repairability

Chemical composition, nominal values %

C Si Mn Cr Ni Cu Nb & Ta

0.07 1.0 1.0 15.0 5 3.5 0.3

Static strengthStandard strength at room temperature, nominal values

Position Yield Tensile Elongationstrength strength A 5

MPa ksi MPa ksi (%)

Matrix 1420 206 1500 217 7Transverse weld(not heat treated) 1100 160 1150 166 5

Precipitation Hardened Martensitic StainlessSteel Belt

Sandvik 1500SMPrecipitation Hardened Martensitic StainlessSteel Belt

The Sandvik 1650SM belt grade is made of a low carbon,martensitic precipitation-hardened, stainless steel of type15-7PH and is characterized by:

■ Excellent static strength■ Excellent fatigue strength■ Good corrosion resistance■ Very good wear resistance■ Very good repairability

Chemical composition, nominal values %

C Si Mn Cr Ni Cu Ti Mo

0.08 1.5 1.0 14.0 7 0.7 0.3 0.8

Static strengthStandard strength at room temperature, nominal values

Position Yield Tensile Elongationstrength strength A 5

MPa ksi MPa ksi (%)

Matrix 1580 229 1600 232 7Transverse weld(heat treated) 1250 181 1300 189 3 Transverse weld (not heat treated) 1100 159 1150 167 5

Sandvik 1650SM

] for more details please ask for our separate data sheet. ] for more details please ask for our separate data sheet.

36

8. Sandvik Surface Solutions Stainless steel press plates and endless press belts for the production of laminates and coated particle boards – fundamentals, maintenance and trends –

Today, it is quite common to use both discontinuous short-cycle presses and continuous double-belt presses for the production of: • Premium laminates for worktops in kitchens and similar postforming elements, • Standard laminates, • Thin laminates or abrasion-resistant laminates for the flooring industry • Direct coating of wooden particle boards,

Short-cycle presses are mainly used if there is a demand for a multitude of different decors and surface textures and this is imparted by the use of press plates.

For high-capacity, mass production of such products, continuous production lines are used (Fig. 1). The process itself, the joining of the resin impregnated décor paper (with or without cover foil) to the particle board, is performed on a double-belt press.

The necessary pressure is applied by a high pressure air cushion (alternatively by oil pressure).Examples of double-belt production systems are shown in Figs. 2 and 3.Two endless press belts are necessary for production: an upper and a lower endless press belt, both of which circulate around the infeed and outlet drums. The belts transfer the necessary pressure and temperature in the various zones of the reaction process by means of the pressure cushion, while the

surface texture of the endless press belt – which is in contact with the product surface – applies the final texture onto the laminate.

The press belts are made out of endless, and subsequently hard chrome-plated stainless steel belts. The upper belt usually features a deeper and more intensive texture, while the lower belts are sometimes only ground or smooth (also known as satin-finished belts).

The most important technical requirements for endless press belts are:

• Highlifetime-generallymeasuredbythetotal amount of product produced over the belt's lifetime; a longer belt life means fewer shutdowns and lower production costs.

• Highwearresistanceofthesurfacetextureunder thermal loads.

• Highmechanicalresistancetoloads-stressin longitudinal direction, thermally-caused stress and pressure load from the pressure cushion.

• Highchemicalresistancetotheresin,foilsand the mated coating itself.

• Lowadhesionofcoatingparticles,foils,resins.• Reproduciblesurfacetextureinveryfinedetail.

37

DimensionsThere are approximately 35 double-belt press systems in use around the world for the production of coated particle boards and laminates. The belts are usually 1100-2300 mm wide with an overall length of 9-15 m. The typical thickness varies from 1,2-2 mm.

MaterialDue to the stringent requirements previously mentioned, martensitic stainless steel belts are generally used for this application. The thermal conductivity of this material is comparable with austenitic stainless steel but the thermal expansion is significantly lower.

Therefore, this kind of steel is not as susceptible to buckling/bumping under thermal loads caused by the different temperature zones within the laminating process.

Fig. 1: Example of a production line based on a double-belt press system

1. Decoiling station

2. Double-belt press

3. Cooling device

4. Decor paper coiling device

5. Grinding machine

6. Edge cutting station

7. Coiling system

8. Cutting station

Fig. 2 and Fig. 3: Double-belt press for the continuous production of decorative and technical laminates

7 6 5

1

3

8 4

2

38

The benefits of this material include:

• Excellentmechanicalvalues(stress/strain).• Goodthermalconductivity.• Goodsurfaceshardness.• Ahighly-controllablebehaviourintheetching process.

• Goodjoint-weldingcapabilities.• Goodbehaviourwithinthepolishingprocess.

The purity of the raw material is very important for the quality of the final product. By avoiding material defects during the milling of the coil material, the possibility of surface defects on the final finish can also be minimized.

The typical production process begins with preliminary grinding of the raw, coiled martensitic material. Next, the coil edges are chamfered (Fig. 4).

The coil material is then cut to the correct length and the butting edges prepared for the welding. Today, two geometrical different welding approaches are generally used: the 'cross-wise' welding seam (usually at 85 degrees) and the 'spiral' weld seam.

The 'cross-wise' weld seam is easier to produce - the risk of bumping or edge failures is low (Fig. 5). On the other hand, the shorter weld seam has to withstand the complete stress of the belt in longitudinal direction. If there are variations within the welding process, the weld seam might be visible on the final belt and product. This risk is avoided by means of grinding and heat treatment of the weld seam.

The 'spiral' weld seam distributes the longitudinal belt stress over a much longer weld length, which gives a better load condition for the weld seam. On the other hand, the weld seam is much longer and more difficult to produce.

However, the pros and cons of one method over the other are mostly theoretical. To date, no statistically-significant advantage has been proven for either method in terms of the final life time of endless press belts. One reason might be statistical failures such as belt damage caused by accidents in the production environment. In other words, belts are generally destroyed in other ways before the belt weld has any impact on life expectancy.

After the welding process, the belt is ground

Fig. 4: The typical and most important steps for the production of an endless press belt

coil

pad grinding

edge cutting gloss adjustment final quality check packaging

printing etching

cutting edge preparing welding

hard chrome plating

39

(Fig. 4). Grinding is performed by means of rotating, flat, continually-oscillating grinding pads and is undertaken in different directions to avoid the risk of grinding marks.

Grinding is carried out on both the inner and outer side of the belt to ensure the material remains completely flat at the end of the process.

The next stage is a multi-step, printing process, during which the texture is applied to the outer side of the belt. After cleaning of the outer surface, the first level of the texture is finished in the first etching process. Subsequent processes of printing and etching are repeated until the final three-dimensional texture is reached. There could be as many as eight printing and etching steps to produce a single final texture.

The belt is then hard-chrome plated to protect against chemical and physical damage, the final width is adjusted and the belt edges finished.

Sometimes, the surface gloss level of the belt is adjusted by mechanical or chemical methods before and/or after the chroming; this might affect the whole belt or just locally to achieve a desired surface texture.

Mechanical polishing does not affect the texture, whereas chemical polishing does remove some material and can therefore affect the final texture over multiple polishings. Each method is effective if controlled properly.

The final quality inspection is only a small part of the whole quality check chain. Prior to and after each single production step, all belt parameters are checked to avoid later failures. Nevertheless, the production risk is high: material defects in the raw material can be extremely difficult to recognize prior to the final etching process as they are often hidden under a perfect outer surface of the coil material.

Packing details are previously agreed with the customer. When packaging continuous belts for shipping, they are often tensioned in a special belt transport fixture before being crated and made ready for despatch.

The above is just a general description of the production process. Additional variables exist depending upon the texture of the belt, such as the special surface treatment methods necessary for preparing high-gloss endless press belts (Fig. 6) or belts with special matt-gloss surface effects.

Lifetime and maintenance of endlesspress belts The surface quality of the belt directly affects the quality of the final product. The lifetime of the press belt is a significant economic parameter for the production line. Therefore, the lifetime of the belt is of utmost importance.

Prior to the first use, the belt needs to be 'tempered' on the double-belt press system. During this procedure the belt temperature is increased in incremental steps. The tempering process takes approximately one to two shifts.

Independent from the initial state of the belt, the lifetime - defined by the maximum length of produced material - also depends upon further parameters, such as:

• Belt maintenance: - regular cleaning of the belt (Fig. 7) - regular checks of the surface for damage and effecting repair, if possible - regular checks of the belt edges and belt-edge maintenance

Fig. 5: Geometrical failures during the welding process of endless press belts - examples: A) bumping, B) edge failure

A B

40

•Unexpected and unfortunate damage - by dropping small parts within the production unit, these small parts go between the drum and the belt or between the two belts and can damage either or both. - mechanical damage caused during transport, installation or setup of the belt

• Failures in the process or in the material feed can result in adhesion of the material, in turn resulting in mechanical damage.

• Compliance with the machine parameters recommended and prescribed by the machine designer (pre-stressing of the belt and over- aggressive hydraulics in belt steering)

As noted, there are many factors that will affect the ultimate cycle life of a given press belt. Therefore, a general statement about the expected lifetime of endless press belts is difficult. Furthermore, the lifetime is dependant upon the type of surface texture and on the client's range of accepted texture finish on the final product. This latter restraint changes depending upon specific customers, markets and production standards.

Assuming careful handling and regular maintenance for the whole process chain and proactive care of the press belt today, the typical lifetime of a belt is in the range of three to five million running meters. In the event of major mechanical damage to a textured belt, repair will not be possible due to the detailed texture on the surface of the upper belt. However, minor surface damage might be repaired and corrected by de-chroming and re- chroming of the belt, if that damage has not penetrated through to the textured surface. Normally, the producers keep a stock of spare belts, the quantity of which is based upon the individual client's past experience of the belt lifetime due to incidences encountered.

Todays producers are offering training workshops for production and maintenance staff where correct handling and maintenance is taught and practiced.

OutlookThe production of textured, endless press belts today has reached a very high technological level but there are still improvements expected from on-going research and development. The actual lifetime of some belts has been doubled within the last few years. One of the reasons for this advance is a continuous improvement in raw material quality and major improvements in metallurgical and mechanical treatment of the weld seam.

In the past, endless press belts were mainly used for the production of so-called standard and mass surface textures. Nowadays, more belt customers are requesting much more complicated textures. One example is the high-gloss, 'mirror-finished' surfaces which are in production - other examples are deeper textures such as the "handscraped-textures" (Fig. 8) with the touch of a real 'weathered and worn' wood

Fig. 7: Endless press belt before and after refurbishment / cleaning

Fig. 6: Endless press belt with high-gloss surface

41

surface or the rough, broken stone surfaces. Therefore, a larger variety of textures and surface

textures on the press belts is expected in the future where the final product offers a higher quality rating and higher value impression. This leads to even more complex production steps within the endless press belt production process.

Fig. 8: Endless press belt on belt carrier unit with "handscraped" texture

42

9. Innovation and investment

Alongside the development of new grades with improved physical and mechanical properties, Sandvik has also responded to market demand for new dimensions in terms of increased width, thickness and length.

In 1997, this resulted in yet another technological breakthrough, with the supply of the first 3 mm thick steel belt. This was designed to meet the high stress demands placed upon the press by harsh-textured OSB (oriented strand board) material and was incorporated into a ContiRoll®.

Shortly thereafter 3 mm belts were also used for a continuous CPS press for production of LVL (Laminated Veneer Lumber) panels. Now 3 mm belts are standard for the production of OSB and LVL with improved belt life due to the high thickness and stability. For the special production of thin board (1.5-3 mm) Siempelkamp has introduced a new press de-sign with 3.5 mm thick steel belts, which are designed to provide greater resistance to damage caused by glue lumps and other defects. The first such line has been operational since the end of 2007 and has been showing good results.

With the growth enjoyed by the WBP industry, Sandvik has been proactive in terms of research and development, recognizing the need to continually im-prove the tolerances it can offer – thickness deviation, flatness and straightness – and its ability to meet ever-shorter delivery times.

The company´s most recently-installed production line is used primarily for the manufacture of high-strength steel belts for double-belt presses, and the system, so efficiently automated that it can be managed by a single operator, incorporates the very latest manufacturing technologies to ensure the highest quality at all times. For instance, lasers are used throughout production to monitor and control the surface of the belt.

The line enables Sandvik to produce belts up to 3,5 mm thick, 180 m long and up to 4620 mm wide – ground on both sides to perfect tolerances, and the increased flexibility resulting from the investment means faster delivery times for customers. Further investments in the range of US $40 million are planned at the belt-producing facility in Sandviken, Sweden, with almost 50 per cent of that sum going into a new press belt production line.

Steel belt production

43

Maximizing customer productivity

Whatever the length, width or grade, every Sandvik steel belt is designed to offer reliable service for many years. This is achieved through a series of stringently controlled manufacturing processes.

Belts wider than 1580 mm have to be longitudinally welded from 2 or 3 single strips. The first step for each individual strip consists of flattening, straightening, trimming and preparing the edges for welding. This is followed by automatic longitudinal welding and heat treatment, which hardens and strengthens the steel to the required values.

After cooling, the longitudinal weld is smoothed and ground on both sides, the belt edges cut parallel to the exact width, and the cut edges rounded. Finally the ends of the belt are prepared for endless welding, which is achieved after the belt is drawn onto the press. The cross weld is then ground to ensure that no visible trace is left on the panel.

Once in operational service, though, accidents in WBP mills may happen and, without the appropriate remedial treatment, such mishaps can threaten the working life of a steel belt.

Levelling and grinding unit at Sandvik workshop

44

The world’s widest grinding line – capable of grinding belts up to 4620 mm wide – installed in Sandviken/Sweden

At the beginning of 2000, to meet increasing worldwide demand for heavy steel belts ground on both sides, Sandvik installed a new grinding machine at its steel belt factory in Sweden. The machine, the largest of its kind in the world, gave Sandvik the ability to produce steel belts up to 15 tons in weight, up to 4620 mm wide and up to 3.5 mm thick. This project formed just part of a major ten year investment programme put into place by Sandvik to maintain its role as a leading supplier to the WBP industry.

45

10. Sandvik site service

Serving the world’s WBP business with efficient repair solutionsRound-the-clock service has been one of the key requirements of the WBP industry since its earliest days and Sandvik has satisfied this need with the development of a genuinely global service network.

While this remains a key aspect of Sandvik’s service offering, the company also recognises the need for basic repairs to be carried out by in-house personnel in order to keep downtime to the minimum. This has led to the development of the Sandvik QuickTools portfolio, an evolving range of tools designed make repair work as easy as possible for Sandvik engineers and trained in-house maintenance crews alike.

Sandvik QuickDiscTM

The QuickDisc™ is probably the most widely used repair tool in the range, and combines proven metal-cutting technology from Sandvik Coromant with the specific belt repair expertise gained by Sandvik Process Systems in the WBP field.

The tool is clamped to the belt surface by means of suction pads and a rotating tipped cutter cuts out a circular area of belt containing the damage. An identically-sized disc, cut from belt off-cut, is welded in place and the join ground until virtually invisible. The cutting system has no limitations in terms of steel grade or belt thickness. Precision discs from 70-300 mm diameter can easily be cut out and inserted with help of the QuickClamping tool.

Sandvik QuickClamperWhen inserting a circular blank into a damaged steel belt, thick copper or aluminum plates are used to achieve proper cooling to the weld area. These plates are kept at place with a device equipped with pneumatic system for pressure and holding. The body of the QuickDisc is used on the bottom side of the belt and the QuickClamper is used on the upper side to provide maximum space for welding and inspection of the repair.

Service in Brazil

Steel belt repair kit

Sandvik's QuickDisc™

46

Sandvik QuickGrinderAn air driven belt grinding unit, customized for safe and efficient removal of material remaining from repair or installation welding. A 50 mm wide grinding belt is used to smooth out variations in thickness over a wide area; a safety guiding system is also provided to avoid grinding mistakes by digging or slipping.

Sandvik QuickBlasterThe Sandvik QuickBlaster is an innovative remedial solution for deformed belts. It offers a quick, cost effective method of flattening out deformed steel belts without interrupting production. Sandvik developed the shot peening method of repairing heavily deformed and troughed belts in the 1980s. Using the same principle, today’s QuickBlaster tool is used by hundreds of press owners to repair belt deformation without having to turn the belt.

Sandvik QuickCutterThe use of ever-thicker belts has created a real challenge in terms of in-house cutting. Sandvik’s QuickCutter is an easy-to-use hand machine equipped with a cemented carbide cutting disc and guided by an accurate linear unit to achieve a straight direct weldable cut.

Sandvik QuickExpertThe reduction of costly downtime has been the driving force behind the development of the QuickExpert programme, with efficient and frequent inspections prolonging belt life and allowing scheduling of planned repair and maintenance. This initiative has also seen the development of an on-line tool enabling users to calculate the actual cost savings that could be achieved in their own businesses by reducing downtime.

Sandvik QuickSlitter & QuickDresserThese are among the latest additions to this continuously developing tool series and are used on belt edges. The QuickSlitter is used to slit a slice from the complete belt length before the QuickDresser is used to shape the edge to its original form (using carbide inserts from Sandvik Coromant).

Sandvik QuickGrinder, designed for the efficient removal of excess

material or deformations resulting from the welding process, either

in new belt installations or following repair work such as disc inserts.

Shot peening with the QuickBlaster

Sandvik QuickCutter

Belt inspection

47

1. Lifetime of the belts

1.1 How long can a belt last?Steel belts are designed to have the same operating life as the press itself but, in practice, their working life will be limited by general wear and tear. The belts with the longest known operational life were installed by Sandvik in a Siempelkamp line in Meppen/Germany, now owned by Sonae group. These ran for 13 years – producing thick MDF – before finally needing repla-cing in 2001.

1.2 What can reduce the working life of a belt?Many factors can adversely affect the life of a belt but primary causes include:• Too many instances of damage sufficiently severe to

mark the board• An excessively high percentage of rejects or lower

grade board• Accidents during operation• Increased belt speed, e.g. up to 120 m/min for thin

board (glues and resins allow faster processing, hence more revolutions and bending cycles)

• Inexperienced staff

1.3 Why do I need spare belts?Accidents can and do happen and downtime costs can be very high. As a result, the loss of only a few days’ production can equate to cost of a new belt.If an accident happens, Sandvik’s support people will do their utmost to help, but manufacturing a new belt will still take several weeks. Your insurance company may well insist that you keep at least one spare belt to keep your premium down.

1.4 Are belts more at risk when a new plant is started up?

Experience shows that it is particularly important to hold a spare belt or belts when starting up a new plant as the risk of damage through accidents is higher than normal due to inexperienced maintenance and operational people. However, even if personnel are well-trained, the start-up of new systems/processes still carries an increased risk – such as from foreign particles, double-matting – especially when running thin board.

2. Problems and maintenance

2.1 Can variations in mat thickness cause problems?The risk is greatest when you start up production and/or change thickness, or when you have a combination of thin mat, high density and high speed: the resistance of the mat may be too low in relation to the pressure of air inside the mat, causing it to burst and resulting in a local variation of the mat thickness. This could in turn lead to high stresses in the belt of such magnitude to cause permanent deformation of the belt; such local deformation will eventually result in cracks of the steel belt.

During normal running, the risk is lower unless you have a very light and dust-prone wood fiber. Speed of steel belt and feeding belt must match exactly.

2.2 What other issues can result in damage to steel belts?

There are many, but here are a few:• Metal objects in press (but most modern lines now

have metal detectors)•Uneven mat forming•Glue lumps•Tracking problems•Chain breakage•Mistreatment of belt edge(s)•Poor cleaning – too much dust and chips•Fires caused by oil and dust• Unsuitable lubricating oil or excessively high

temperature that carbonizes oil• Troughing•Roller and chain condition can also affect belt life •Wrong pressure setting

2.3 What causes belt tracking problem?Belt displacement can be caused by a number of different factors, but 80-90% of such problems result from mat-forming. Other possible causes include:• Deviations, distortions and tolerances in respect to

the optimum straightness.• Varying pressure, temperatures and speeds.• Uneven tensioning of chain or rollers can have an

extremely negative impact on belt tracking.Sandvik belts are tested in our production plant without using a belt tracking control unit. Every steel belt is verified according to known specification.

11. Questions and answers for DBP

48

2.4 Why are white marks visible after inserting a round belt piece?

Uneven thickness and flatness in the weld leads to non-uniform hardening of the glue due to different heat transfer properties. Also, the heat transfer properties of the weld itself are about 5% lower than those of the parent material. It is very difficult to make a repair invisible, especially with thin board.

Best results will be obtained by using Sandvik’s Quick-Tool repair system to create and insert a replacement disc and then treating the repair using the Sandvik belt-grinding unit. Other important factors behind an effective invisible repair include the type of glue used and the kind of wood being processed.

2.5 Why shot peening?One of the most common problems in the press is belt deformation in the form of elongation of the back or roller side, caused by rollers or a chain making an impression on the surface. The result is a curled or troughed belt and the most efficient method of repair is shot-peening. Using compressed air, hardened steel balls of different size (depending on the belt thickness and grade) are blasted onto the belt surface in order to achieve an elongation, or to even out the stresses between product side and back side of the belt.

When the differences in stress and elongation are eliminated, the belt is once again returned to its original flat condition.

2.6 What routine inspections are recommended for the belt itself?

Visual inspections of:

•Belt edges• Belt surface (clean inside at regular intervals and

inspect as most fatigue cracks start from inside; oil on the board product indicates a crack as oil seeps through)

•Belt shape•Belt tracking•Brake parts, dirt and lubrication

2.7 What is the highest suitable belt temperature for repairs?

100°C. It is impractical for technicians to work at higher temperatures.

2.8 How is a belt cleaned in the machine?Most double belt presses are provided with metal brushes and, in normal use, these will provide all the cleaning necessary. However, if the belt surface is still not clean, a small belt grinder can be used or a Scotch Brite type brush.

Very sticky and coarse impurities must be removed with mechanical scrapers and then ground off. Sandvik can also supply a mobile belt grinder for removal of surface deposits. In some cases, dry ice can be used for cleaning, but it is not very efficient and raises costs considerably. Laser cleaning is not cost effective. Another possible way of cleaning is to add a few % of melamine adhesive and run production for a few days.

2.9 Can a Sandvik belt and a Berndorf belt be used in the same DBP?

Yes, both belts are controlled individually and therefore do not influence each other. The only consideration is the position of the longitudinal welds (to avoid overlapping).

2.10 Can the upper belt and lower belts be of different thicknesses?

Yes, but their respective speeds will have to be adjusted individually. Check the adjustment by marking both belts at the inlet and outlet to see if the marking coincides.

2.11 What is the best way to detect a crack?Most cracks can be detected by the naked eye – look out for visible oil marks on the board or visible light reflection on the belt surface (edge cracks). Other methods include:

•Magna flux test•Penetrant test•MilliQ test

2.12 How long can a crack be before you have to repair it?

Transversal cracks are the most common and can be up to 25 mm at the belt edge or 50 mm inside the belt before they must be repaired.

Longitudinal cracks can be longer than this as risk of belt rupture is much less, but remember that it’s easier to repair a short crack than a long one! Repair according to Sandvik instructions PS-SB-4451 and 4452.

49

2.13 What about scratches to the belt?Small scratches without burr do not affect belt life and no remedial action is needed as long as they do not influence the product. Larger scratches or those with burr should be treated to minimize risk.

2.14 Dents and deformationsSee Sandvik instructions PS-SB-4450.

3. Belt thickness and belt quality

3.1 Is it possible to change to thicker belts?The basic answer is yes, but the plant manufacturer must be consulted first and Sandvik cannot guarantee any improvements in results. Most presses are designed so that the thickness can be increased to the next nominal standard dimension, i.e.

• from 1.8 to 2.0 (2.3) mm• from 2.0 to 2.3 mm• from 2.3 to 2.7 mm• from 2.7 to 3.0 mm

3.2 Is it better to work with a thicker belt?Thicker belts are certainly more resistant to hard particles and dents and less susceptible to troughing. However, a thicker belt is also more sensitive to fatigue due to surface defects, and the bending stress is higher.

3.3 What’s the difference between using a 1500SM and a 1650SM steel belt?

Both are martensitic hardened steel: 1650SM has a higher strength and better fatigue values, but in practice 1500SM has worked without problems. Previously, the main reason for using 1500SM was that 1650SM was not available in a 2.7 or 3.0 mm thickness. This is no longer the case so we would now recommend SM1650SM. However, 1500SM is still available for users who want a spare belt in the same grade.

3.4 What type of ground belt surface finishes does Sandvik offer?

Sandvik grind both belt sides for all surface finishes, but the tolerances vary with lowest values for SK-O and best SK-2

•Mill finish (SK-0)•Ground one side (SK-1) •Ground both sides (SK-2)

3.5 Why is grinding necessary?Grinding improves the thickness and flatness of a steel belt, especially over the longitudinal weld. It also gives the defect-free surface needed for production of unsanded board.

4. Running-in of belts

4.1 What tension should the belt be run in at start-up?At 100% (approx. 50 N/mm2).

4.2 What determines this specific tension?Proper belt tracking.

4.3 When should the protective tape be removed from the belt?

The tape on the inside of the belt should be removed when drawing the belt into the press. Remove the tape on the product side before test run and test heating.

4.4 How should the press be heated-up during running-in?

Normally a modern press can be heated-up with approx. 1°C/min.

4.5 How should belt tracking be performed during set up?

Drums and belt tracking rollers should be adjusted manually during test run. The system should only be switched to automatic once the belts are running uniformly.

4.6 Is oil supplied during running in?Yes, by drop metering pump (at 30% of the usual ‘with product’ oil feed rate).

4.7 Should any checks be carried out before produc-tion is started?

Speed difference on drives of belts and rollers should be checked and, if necessary, adjusted.

50

5. Safety devices

5.1 What safety devices are normally used?

•Fire / heat / smoke detecting systems.•Chip screen with magnet trap.•Scrap trap.•Glue lump trap in dryer.•Metal detector in mat-forming section, etc.•Scrapers on belt insides.•Safety switches for belt tracking.•GreCon DIEFFENSOR.

6. Oil

6.1 What sort of oil is usually used and recommended?

Klüber or Optimol.

6.2 How is the oil fed during production?The oil is fed in drops via a metering pump. Visual inspection is used to assess the correct amount.

6.3 Does the oil need cleaning?No.

6.4 How is oil removed from roller side of the belts?Belt scraper to tray (a certain amount is allowed to pass the belt scraper).

7. General

7.1 Is there a difference between belts for MDF and particle board?

Yes, thin MDF is more demanding, requiring closer tolerances for thickness and surface finish of the belts.

7.2 Can different widths be produced in the press?Yes they can but in order to avoid deformation of belts the press should not run too long with narrower width belts (maximum 2-3 days at a time with up to 600 mm [2 ft] difference).

7.3 Does Sandvik recommend one particular press manufacturer?

No, not at all. All OEMs have their advantages and, as long as they use Sandvik steel belts, we’re more than happy to work with them all.

51

Sandvik Group

The Sandvik Group is a high technology engineering group, with advanced products and world-leading positions in selected areas. Operations comprise the Tooling, Mining and Construction, and the Materials Technology business areas. The group has 50,000 employees and representations in 130 countries.

Sandvik Materials Technology

Sandvik Materials Technology is a business area within the Sandvik Group and a world-leading manufacturer of high value-added products in advanced stainless materials, special alloys, metallic and ceramic resistance materials, as well as process plants. The product areas comprises Tube, Strip, Wire, Kanthal, Process Systems and Med Tech.Sandvik Process Systems itself made up of two product centres, Belts (incl. Press Plates) and Industrial Processing.

Quality Assurance

Sandvik Materials Technology has a quality management system approved by internationally recognised organisations. We hold for example: ASME Quality System Certificate as a Materials Organisation, approval to ISO 9001 and QS 9000, as well as approvals from LRQA, JIS and other organisations as a materials manufacturer.

Environment

Environmental awareness is an integral part of our business and is at the forefront of all activities within our operation. We hold approval to ISO 14001.

52

Sandvik’s position in the WBP and laminate industry is virtually unique, with the company operating in partnership with double-belt press manufacturers and the panel producers themselves. As well as being one of its major strengths, this also places the company in a position of responsibility.

The investment that has been put into R&D, the global network of service teams, the ever- increasing range of belt grades and new repair techniques that maximize the operational life of steel belts … the combination of all of these factors is a clear demonstration of Sandvik’s determination to live up to its leadership responsibility and to continue supporting this industry far into the third millennium.

Partners in press belt performance

www.smt.sandvik.com/spsPS

-SB

440

EN

G 2

.09

P

rint

ed in

Ger

man

y