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Simulation of a rapid nip pressure strike and its effect on press felt samples

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AUTEX Research Journal, Vol. 8, No3, September 2008 © AUTEX

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SIMULATION OF A RAPID NIP PRESSURE STRIKE AND ITS EFFECT ON PRESSFELT SAMPLES

Tomi Hakala1 and Ali Harlin2

1Fibre Materials Science, Tampere University of TechnologyP.O. Box 589, FIN-33101 Tampere, Finland

Tel. +358 40 8490 963, Fax +358 3 3115 2955E-mail: tomi.hakala@tut.fi

2Professor, VTT Technical Research Centre of FinlandFIN-02044 VTT, Finland

Abstract:

Special technical textiles are used in papermaking to support, transfer, and dewater the paper web. Thesetextiles, paper machine cloths, have many essential functions connected to their position on the paper or boardmachine. Mechanical wet pressing uses press felts, whose porosity and resiliency are important for effectivedewatering. Water is squeezed out by two overlapping paper machine rolls, which form a nip. After squeezing,the porous felt should be void and return to its original thickness to ensure efficient dewatering. Friction forcesare also present at the nip, since abrasive interfaces occur between the cloths and the rolls by pressure, heat,and movement. Thus in time, the harsh papermaking process wears out the press felt, deteriorating its quality. Athigh machine speed, defects in press felts, rolls, or other parts of the nip environment can cause runnabilityproblems such as noisy run, that is, vibration in the pressing section, decreasing paper quality and outputcapacity.This study sought for a new way to simulate the ambiguous nip phenomenon on laboratory scale and to find outa way to predict this noisy run problem. A dynamic test method, the Hopkinson Split Bar, was used to define theease with which strike energy passed through from the upper roll to the lower roll and the damping of strikes bynew and worn felt samples.In our study, the elasticity of the press felt was strongly linked with the ageing time. Decreased elasticity lets a nipimpulse more easily through the press felt.

Keywords:

Paper machine cloths, nip, resiliency, vibration, runnability, Hopkinson Split Bar

Introduction

Special technical textiles are used in the forming, pressing,and drying sections of paper machines. These paper machinecloths, forming fabrics, press felts, shoe press belts, transferbelts, and dryer fabrics, transfer, support, and dewater thepaper web. Therefore, they have a very important role in theprocess.

Dewatering can be done in different ways, but in this paperwe focus on the press section. Mechanical wet pressing usespermeable press felts and high-load dense rolls to removewater maximally. Squeezing, also called the nip event, is done

by two overlapping rolls. The paper machine may have 1-4press nips, which usually need 2-4 press felts (Figure 1). Forexample, in a four-nip press section a linear load can varyfrom 70 to 140 kN/m (1,8-3,5 MPa). A press impulse variesaccordingly between 2,8-5,6 kPas at a machine speed of 1,500m/min and a nip length of 0,04 m, the numbers depending onthe design of the wet press section and the paper grade [6].

In optimal dewatering, the press felt should be void (porosity)and recover its original thickness after the nip (resiliency).Moreover, the paper web should not reabsorb water in theexpanding part of the nip. To inhibit rewetting, the paper weband felt are rapidly separated after the nip [6, 12]. Thus for

Figure 1. Left: dewatering event in press nip; right: three-nip press section [11].

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optimal drainage, the papermaking parameters and theproperties of the press felt are of prime importance.

The materials and structure of the press felt together with itsmanufacturing method define the properties of the press feltsuch as even pressure distribution in the nip, smoothness,resiliency, tendency to rewet, and durability. The papermakingparameters, such as paper machine velocity, type of rolls,and nip load, define together with the press felt nip width,dwelling time, void volume in the nip, and water flow velocity.These factors reflect directly on an important indicator, the drycontent of the paper sheet, which is continuously monitoredduring the process. To maintain the dry content at a certainlevel, changes in the properties and structure of the press feltshould be taken in account and reacted to. This is best doneby cleaning and conditioning the press felts during operation(4-6 weeks) and by using extra equipment such as suctionboxes, and high- and low-pressure showers [1, 6, 7].

Many factors effect changes in press felts in the nip, the mostpowerful being mechanical forces such as pressure, friction,tension, and shear, which deform felt structure and make itlose its caliper and become more compact during its lifetime.Concurrently, the felt’s dewatering efficiency declines. In ourresearch, press felt deformations reached even singlematerials, yarns and staple fibres, the base materials in pressfelt production. The general polymer for yarns and fibres issynthetic polyamide 6, which has good properties such ashydrophilia and abrasion resistance. Forces in the nip deformthe cross section of the fiber from circular to flat and causeeven fibre breaks. The reshaping of fibres increases thedensity of the felt surface and decreases the size of capillarypores for dewatering. Furthermore, paper chemicals andseveral contaminants such as fillers, pitch, and sizing agents,increase interfiber locking and fibre wear, gluing fibers togetherand leaving hard particles to damage the fibre surface.Moreover, contaminants stick inside the press felt and thusfill the felt. Moving and rotating rolls, such as stretch and guiderolls, generate tensile stress, and in- and out-of-plane shearstress in the press felt. With compression, tensile and shearforces increase locking and the filling-in of felt structure.Forces acting in the nip and the phenomena inside the feltbring about a decline in resiliency and compaction of the pressfelt. All these phenomena help underline the importance ofmonitoring and maintaining the paper machine textiles duringpaper making [1, 6].

Compaction and resiliency loss affect the dewatering behaviorand runnability of the paper machine. Also other factors affectrunnability, but we focused on felt ageing and vibration (noisyrun).

Vibration can be generated in the nip by bad bearings, a speeddifference between rolls, unbalanced rolls, dirt on rolls, a flatarea on the rolls, roll and drive misalignment, unevendistribution in the nip, insufficient void volume, and/or defectsin the press felt. Such vibration affects negatively the quality ofpaper, whose caliper profile becomes uneven and strengthdeclines, quality problems likely to cause unscheduledshutdowns [6].

In our research, we observed only factors deriving from thepress felt and its ageing. We assumed that the vibrationfrequency in the nip depends on the resiliency loss,compaction, and wearing of the press felt.

Our paper is divided into the following sections: the theorypart concerns the principle of the Hopkinson Split Bar (HSB)

technique and the design of samples. The preliminary workbefore HSB testing and the parameters of the HSB device arediscussed in the experimental part, and HSB results are givenin Results and Discussion.

Theoretical part

For many decades, wet pressing has been investigated toimprove paper quality and to increase production capacity inpapermaking. Scientists have developed some presssimulators, for example, the Wahren-Zotterman and thehydraulic press simulator, which have been used to researchdewatering and z-directional distribution of filler materials inthe paper web [9, 10].

In our research, we used a rapid strike test method, theHopkinson Split Bar, with a porous textile piece to simulateone nip impulse.

Principle of the Hopkinson Split Bar technique

The Hopkinson Split Bar technique is well-known in materialcharacterization. By means of stress-strain curves, it ispossible to define a material’s resistance to deformation andenergy consumption. HSB test samples are usually rigidmaterials such as metals and ceramics, but nowadays alsoother testing materials are being researched such as polymercomposites, honeycombs, and concrete. In some cases,polymer composites have been reinforced with a woven fabric[2, 8].

The HSB technique, capable of testing compression, shear,and tension, is based on the progressive movement of elasticstress waves in solids and can be performed at low or elevatedtemperatures [2, 8]. In our tests, we used the compressiontechnique at room temperature.

The HSB device consists of three bars, the first of which is astriker bar, the second an incident bar, and the third atransmitted bar. The striker bar is accelerated by an air gun,and the sample is located between the incident andtransmitted bar. In a rapid strike test, the striker bar hit theincident bar, sending an incident pulse at a certain velocityinside the bar and to meet the sample surface. The samplethen collapses and usually hardens. Based on the materialproperties and the strike force, the incident pulse is dividedinto two pulses, reflected and transmitted pulse, the formermoving back and forth inside the incident bar until it dies out.The latter passes through the sample to continue inside thetransmitted bar (Figure 2) [2, 5].

Upon a strike impact, the caliper of the sample decreasesand its diameter increases. The sample can deformplastically, viscoelastically, and/or elastically, depending onsample material and structure. Our assumption was that feltsdeform mainly elastically, because their structure has beendesigned to sustain rapid nip impulses.

Incidents of deformation are hard to verify, because theincident bar hits the sample many times before the sampledrops between the bars. The bar is set into motion by areflected pulse, which oscillates back and forth inside theincident bar. In our studies, data were collected only after thefirst strike. A load pulse lasted an extremely short time ofbetween 50-200 s [5].

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The following equations were used to calculate strain (ε) andstress (σ) [2]:

Average strain in the specimen:

dtL

Cd ri

t

t

t

s

)()( (t)00

0 εεεττεε +−== ∫∫•

. [1]

Average stress in the specimen:

s

t

A

tAEt

)()(

εσ = . [2]

A cross sectional area of pressure bar [mm2]A

scross sectional area of specimen [mm2]

E Young’s modulus of bar material [N/mm2]ε

ttransmitted strain [mm/mm]

εi

incident strain [mm/mm]ε

rreflected strain [mm/mm]

Ls

length of specimen [m]C

0velocity of sound in bars

t timeε strain rateτ shear stress

During compression, true stress and true strain calculationstook continuously into account any changes in specimencaliper and area, and their values were used for stress-straincurves.

Figure 2. Structure and operational principle of an HSB device, and plotting of recorded pulses on strain-timecoordinates [5].

εε

Figure 3. Resilient press felt damps part of nip strike (shock pulse), some part of strike goes through; same phenomenonsimulated in HSB device.

In our research, theHSB device struck thepress felt sample,simulating a singlenip impulse in thepaper machine. Thesamples were thusloaded through-the-thickness. The HSBtest was based alsoon the assumptionthat press felts actsdifferently in a rapidstrike test than in astatic compressiontest. Figure 3 showshow a shock pulse isdivided in the nip

when the press felt and paper web form a resilient layer. Thesame is simulated in the HSB device.

Principle of Press Felt Manufacture

Generally, the press felt is manufactured from woven fabric(s)and pre-needled batt layers. The base fabric(s) and severalbatt layers are connected to each other by needling. The paperside of the felt is generally needled with finer staple fibres.The middle and wear side of the felt is needled with coarserstaple fibres. The finer staple fibre on the batt layer makeswider contact with the paper web and has a more even uniformpressure. According to Vomhoff and Gullbrand, the diameterof the staple fibre greatly affects dewatering efficiency [12].After needling, the raw press felt is finished by heat-setting,calendaring, and edge trimming.

Press Felt Samples for HSB Testing

HSB testing was divided into two cases:Case 1: Several press felts with batt layers prepared

from different staple fibres. The staple fibresdiffered in molecular weight (M

w), which varied

from 81,000 to 155,000.Case 2: Two different structures of press felt: a normal

press felt and a special press felt for noisyrun positions.

Detailed information on the felt samples is given below.

.

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Case 1: Press Felt Samples

All press felt samples shared the same woven base fabricstructure (single layer) and the same number of batt layerson both sides (paper and wear side). Thus the mass perarea of each batt layer was the same. The polymer waspolyamide 6 (PA6) for yarns and staple fibres, but the latterdiffered in their molecular weight. All press felts were preparedat the Laboratory of Fibre Materials Science at TampereUniversity of Technology. Half the felt samples were aged for300,000 nip strikes in a PMC device, described in detail in [4].During ageing tests, the felt samples compacted substantially,as seen in Figures 4 and 5.

The PMC supplier had tested the felts for the extent and typeof ageing. For the HSB test, felt samples were labeledaccording to their ageing as a new sample (unaged), an agedsample, and a used sample. The aged felt samples were runup to 100,000 nip cycles with the PMC supplier’s special testdevice in their laboratory. The used felt samples wereremoved from the paper machine after 20 days of operation.

Experimental part

Because porous and anisotropic, the press felts werechallenging to test. Their anisotropy was related to the use ofstaple fibres and yarns and different design. Moreover,

Figure 4. Cross sections of new and aged press felt, aged 300,000 cycles; compaction ofstructure and wear of staple fibres clearly visible [4].

Changes during the ageing test

0102030405060708090

100110

0 100 000 200 000 300 000

Number of cycles

Pe

rce

nt

[%]

Air permeability Thickness Mass per area

Figure 5. Relative values of air permeability, thickness, and mass per area of press feltsamples during ageing test. Hundred percent represents a new press felt (unaged) [4].

Figure 5. Relative values of air permeability, thickness, andmass per area of press felt samples during ageing test.Hundred percent represents a new press felt (unaged) [4].

Case 2: Press Felt Samples

A PMC supplier manufactured two press felts for the HSBtests, a normal press felt and a special press felt for noisyrun positions. Both felts had a laminated structure with twoseparated woven base fabrics with several batt layers. Thefelts differed in their base textile structures and needlingdensity, and they had a total mass per area of over 1,800 g/m2.

materials with different titre,diameter, fibre directions, andentanglement also affected feltbehavior, all factors complicatingHSB testing and result analysis.

Rapid strike tests were run in dry andwet conditions. With the HSBtechnique, we also tested shoepress belts and a paper grade, butthe results are not included in thispaper.

Pre-work before the HSB tests

Small, circular pieces, of diameter12 mm and thus the same as that ofthe HSB bars, were punched fromthe middle of felt samples. Naturally,the punching slightly damaged thesamples, for example, by looseningfibres and pulling out yarn on theedges. The edges were not sealedafter punching, because that (gluingor melting) would have affected theresults. Open edges and cuttingdamage may have promoted sampledeformation.

Half the pieces were saturated withtap water and kept in glassware for24 hours to guarantee thoroughsaturation before HSB testing(Figure 6).

Before each rapid strike test, thepieces were measured for thicknesswith a thickness gauge, causing aslight compression of the piece. Thepressure gauge foot pressure waslow and the same for all samples

and thus negligible. In both cases, the caliper of the dry feltvaried between 3.1-4.9 mm for new samples. For aged andused samples, the caliper was 2.2-3.3 mm. The wet feltswere slightly thicker because of swelling from water, naturalof hydrophilic polyamide and porous structures.

Assembly of Felt Sample in the HSB Device

After preparation, the felt sample was assembled betweenthe incident and transmitted bars and located carefully at thebars’ mid-point (Figure 7). The strike direction was alwaystaken into account, so that the felt’s paper side was always

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facing the incident bar. An elastic band pulled both bars gentlyto the sample, generating a slight pre-load on the sample.Though the assembly steps were the same for both wet anddry conditions, we added a few drops of water on the wetsample with a dropper before shooting. A board box wasplaced under the sample to collect it after the strike.

HSB Device Parameters

Table 1 shows the HSB device parameters during tests.

Table 1. HSB device parameters for cases 1 and 2.

Results and Discussion

All measured data were plotted on stress-strain figures, whichshow the pressure needed for a certain yield. Evaluation andanalyses were made from those figures.

The HSB results were divided by case. For case 1, HSB resultsof new and aged samples with the same staple fibre molecularweight were evaluated first with their results in wet and dryconditions evaluated thereafter. Finally, HSB results wereevaluated based on molecular weight.

For case 2, HSB results of new, aged, and used sampleswere evaluated in wet and dry conditions. Then the HSB resultsof normal and special felt were compared.

Results of Case 1

In terms of stress on the same level of strain, new and agedfelt samples differed dramatically in dry or wet conditions.This applied to all felt samples with different molecular weights.A gap can be seen between the stress levels in Figure 8,which gives the curves of felt sample V5. The letters andnumbers after the sample symbol represent ageing cycles: 1and 2 are new felt samples, AW 1 and 2 are those after100,000 ageing cycles, BW 1 and 2 are those after 200,000ageing cycles, and CW 1 and 2 are samples after 300,000ageing cycles.

Usually in wet conditions, the stress level of each felt samplewas lower and the strain (yield) slightly higher than in dryconditions. Less power was thus needed to achieve the sameyield because of the water lubrication inside the felt sample,which decreased the friction between yarns and fibres. Alsothe structure reshaped and deformed more easily. Figure 11shows a good example of this phenomenon.

Another observation was that in wet conditions the stress-strain curve began unevenly. We interpreted this so that thewater squeezed from the felt sample at the beginning of astrike caused small stress peaks on the line.

A problem occurred at the beginning of the HSB test. Most ofthe new felt samples were hard to measure because of a

Figure 6. Small circular pieces were punched from felt samples and half of them saturated with tap water. Before each HSB test, theircaliper was measured with thickness gauge.

Figure 7. Assembly of a circular piece of felt sample (inside redoval) between incident and transmitted bars; piece held by elasticband (not shown).

Parameters Units Case 1 and 2

Rated pressure bar 0,4 (1

Length of striker mm 600

Length of 2. and 3. bars mm 1800

Diameter of all bars mm 12 (2

Speed of striker m/s 11,1-11,5

Material of bars Aluminum

1) This was the minimum pressure capable of generating a big enough pulse to

be measured from the bars.2) Aluminum bars of diameter 22 mm were also used, but problems emerged

with signal quality.

All samples were tested at normal (room) temperature.

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Figure 8. Stress-strain figures of press felt sample in dry (left) and wet (right) conditionswith different ageing cycles.

Figure 9. Stress-strain curves of normal and special press felts for dry conditions; new felt samples at left, aged samplesin middle, and used samples from paper machine at right.

Figure 10. Stress-strain curves of normal and special press felts for wet conditions; new felt samples at left, aged samplesin middle, and used samples from paper machine at right.

very weak transmitted pulse, aproblem related to the elasticity ofthe press felt. The incoming pulsewas now mainly converted toreflected rather than transmittedpulse, a phenomenon welli l lustrating the big resil iencepotential of a new press felt. But theaged sample curves showedsomething different: the reflectedpulse decreased, and thetransmitted pulse increased. Thiswas explained by the compactionand wear of the felt sample; that is,the felt had lost almost all itsresiliency, and the incoming pulsewent easily through it. The resiliencyloss went hand in hand with loss ofcaliper, mass per area, and airpermeability. Figures 4 and 5 showthe drop of these properties.

Unfortunately, the differencebetween felt samples of varyingmolecular weight staple fibre wasinsignificant, and results did notfollow the molecular weight order.Consequently, HSB results are notdiscussed in detail in this paper. The

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main reason for the lack of difference was the HSB techniquewe used: the strike bar was so fast and its impact so powerfulas to cover any divergence.

The first HSB tests with technical textiles were promising,even if the results seemed not to give any new information ofthe press felt material properties (polyamide 6). However, wedecided to continue with the tests and studied next thesignificance of the felt structure.

Results of Case 2

Figures 9 and 10 show stress-strain curves of both press felttypes at different ageing stages (new, aged, and usedsamples). At each stage, both felt types had three referencesamples. Figure 9 shows dry condition and Figure 10 wetcondition results. The sample symbol P114 stands for normaland P106 for special felt type.

On the stress-strain curves, the highest point for the newsamples was much lower than that on the aged and used feltcurves. This was the same for dry and wet conditions.

In dry conditions, the strain (yield) of the new sample wasslightly bigger than that in the aged and used samples.Moreover, aged and used samples differed only slightly instress and were almost the same in strain. A notable differenceoccurred in the used sample curves, where the plots weregrouped by felt type.

In wet conditions, the strain values diverged widely due toimpact of water. Generally, wet reference samples variedmore and the stress-strain curves were more mixed than indry conditions.

After dry and wet testing, deformation (diameter increased)was obvious in the felt samples, though the diameter of allthe wet samples increased more markedly (Figure 11).

We should note that the sample did not deform immediatelyafter one strike, for the compressive stress wave continuedinside the incident bar after the first strike, moving the bar andcausing it to hit the sample again. This continued until thewave motion died away. Consequently, we could notdetermine how many strikes were responsible for thedeformation of a single sample, since during the HSB tests,and without a high-speed camera, we could not count theirnumber.

The results were similar for the first case. The increase instress was linked with the wear of the press felt, the two felt

Figure 11. Diameter of new press felt samples after testing in dry and wet conditions.

types differed only slightly, and the reference samples variedconsiderably.

Conclusions

We should note that the HSB results here are not absolutevalues, but unique to the tested felt samples, because insuch dynamic tests, measurement methods and techniquesaffect the results. Here the results and conclusions are validonly for the press felts, HSB test device, test parameters, andcircumstances presented in this paper.

In HSB testing, measuring problems emerged with new(unaged) felt samples. Only a portion of the loading pulsewas transmitted from the incident bar to the transmitted bar.The signal in the transmitted bar was so weak as to bedisturbed by noise. The problem was caused by the elasticityof the new sample, which damped the strike very efficiently.The problem with the aged samples was that their stress-strain curves did not differ markedly, but those for the referencesamples did. Several striker velocities were tested to solvethis problem, but there was a limit as to how low the velocitycould be decreased.

Chen et all studied low-impedance samples and used ahollow aluminum transmitted bar and a circular piezoelectrictransducer in the middle of the transmitted bar to overcomethe above problems. According to them, the transducer wasmuch more sensitive to detect low-amplitude transmittedforces [3].

Though the HSB tests could not assess the significance ofthe felt structure or its raw material, they confirmed theassumption that worn and compacted press felts contributeto noisy run, because a stress pulse easily passes throughan aged or used felt. This was evident in the stress-straincurves of new and aged samples. Another observation wasthat new felt samples had good damping ability, a desiredproperty that caused some measuring problems in HSBtesting. A third observation was that the stress-strain curvesdiffered markedly when felt samples were tested in dry andwet conditions. In most HSB tests, the water saturated sampleshowed lower stress values at the same strain than the drysample, a difference issuing from water lubrication andswelling of the sample. That is, water lowered the frictionbetween the fibre materials and loosened the wet felt structure.These are the significant observations to be made of ourHSB testing.

Though further action is needed to improve the dynamicaltest method, the results testify to the resilient behavior andwear of the felt and therefore to the promise of HSB testing.

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Further Action

The conventional HSB technique used here needs someequipment modification to improve its suitability for resilientsamples, signal accuracy, and clarity of results. The mainmodifications would be longer bars to increase the waveamplitude, more sensitive transducers to detect forces, a barcatcher to limit the strike force of the incident bar to one, acontrollable spring to hold the sample between the incidentand transmitted bar, and a high-speed camera to record thestrike and the deformation of the felt sample. Suchmodifications require investment, construction time andpretests, and are thus no short-term projects.

Acknowledgements

We wish to thank research scientist Mikko Hokka, for operatingthe HSB device. We are also grateful to the PMC supplier fortheir press felt products and co-operation.

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1. Adanur, S. Papermachine Clothing. Asten 1997, CRC Press.395 p.

2. Apostol, M. Strain Rate and Temperature Dependence ofthe Compression Behavior of FCC and BCC Metals.Dissertation. Tampere 2007. Tampere University ofTechnology. Publication 649. 112 p.

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4. Hakala, T., Wilenius, T., Harlin, A. Laboratory ageing TestDevice for Press-Felt Clothes of Paper Machine. AutexResearch Journal. 7(2007) 1, pp. 71-79.

5. Hopkinson Split Bar (HSB). 2007. Institute of MaterialScience, TUT. [WWW]. [Cited 03.04.2007]. Available at:http://www.tut.fi/umits/mol/materiaalioppi/ehopkinson.html.

6. Paulapuro, H. (edit.). Papermaking Part 1. StockPreparation and Wet End. Book 8. TAPPI 2000, Fapet. 461p.

7. Paulapuro, H. Wet Pressing – Present Understanding andFuture Challenges. Presented at the 12th FundamentalResearch Symposium, Keble Collage, Oxford, UK,September 17th-21st, 2001. FRC, pp. 639-678.

8. Resnyansky, A. D. The Impact Response of CompositeMaterials Involved in Helicopter Vulnerability Assessment:Literature Review – Part 1. April 2006. Defence Scienceand Technology Organisation.DSTO-TR-1842 Part 1. 84 p.

9. Szikla, Z. On the Basic Mechanisms of Wet Pressing.Dissertation. Espoo 1992. KCL Paper Science Center. 219p.

10. Szikla, Z., Paulapuro, H. Z-directional Distribution of Finesand Filler Material in the Paper Web Under Wet PressingConditions. Helsinki 1986. The Finnish Pulp and PaperResearch Institute. Report 583, pp. 654-664.

11. Technology solutions for every need. Paper Machine PressSections. 2006. Brochure of Metso Paper, Inc. 16 p.

12. Vomhoff, H., Gullbrand, J. New Insights in the Mechanismsof Rewetting. Das Papier 2004-T194, ipw 11/2004, pp. 40-43.

∇∆∇∆∇∆∇∆∇∆

Dry, P106

Wet, P106

Wet, P114

Dry, P114

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