40207_23

PULTRUSION 23

Brian A. Wilson

23.1 INTRODUCTION

The word pultrusion is used to describe a commercial fabrication process for the production of fiber reinforced composite elements. First mention of the process is recorded in a patent in 1951 with much of the early work in the 1950s attributed to W. Brandt Goldsworthyl. He performed much of the process develop-

ment and manufactured the equipment to produce structural elements by the method. A typical pultrusion machine is shown in Fig. 23.1. The process has a relationship to extrusion, which is used primarily with metals and describes the process of forming a shape using a closed die and pushing normally hot metallic materials through the die. Pultrusion differs in

Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7

Fig. 23-1 Typical Pulkusion machine. (Courtesy of W. Brant Goldsworthy and Associates Inc.)

The basic process 489

that it takes advantage of the strength of the longitudinal fibers in the section to pull the extruded shape through the forming die and the heated curing die to create a constant cross section structural element from a composite materials system. Hence the name pultrusion.

curing energy in the heated die to cure the composite into a hard structural product and using the resulting shape as a unidirectional strength element for commercial structures.

The primary advantages of pultrusion are as follows:

Use of this-process has now expanded both in the USA and around the world with many manufacturers equipped to produce the simple structural elements which are the main pultrusion products. The process was labeled for the first 30 years or so of its existence as a commercial production method. This was owing to

~roductioniscontinuous~ 0 labor requirements are low; 0 material scrap rate is low; 0 the requirement for support materials is

eliminated, i.e. breathers, bleeder cloth, sep- arator film, bagging film, edge tape.

the nature of the method, using longitudinal fibers and a simple thermoset resin system to produce a structural shape which had its primary strength in the longitudinal direction and properties in the transverse and third axis relating only to those of the resin system. The ability to incorporate three-dimensional strength first occurred in the mid 1960s when it became possible to include layers of mat/fabric and circumferentially wound layers of fiber both w i t h the body and on the surface of the structural shapes. Multi-directional reinforcement was used extensively starting in the 1970s2, 3. These developments, plus the introduction of thermoplastic resin matrices, have brought the pultrusion process into the manufacturing arena of the aerospace, defense and aircraft businesses as a relatively inexpensive and repeatable method of producing a constant cross-section structural element48 *.

In this chapter, the basic process, variations on the process, pultrusion equipment, materials including fiber, fabric, resin matrices, additives, tooling, curing, structural connection methods, equipment manufacturers, pultrusion fabricators and potential markets and applications will be reviewed"".

23.2 THE BASIC PROCESS

The basic pultrusion process as originally con- ceived consisted of creating a cured composite shape by pulling a bundle of resin impregnated fibers through a heated die, providing sufficient

23.2.1 VARIATIONS IN PROCESSING

The original process of pulling a bundle of unidirectional fibers through a curing die remained without variation until the middle of the 1960s. At this point, pultruders, both in Europe and the USA, developed the process of adding fabric in strip or sheet form and fiber mat to the pultrusion system in order to provide transverse strength and shear strength in the corner sections of complex pultruded shapes. In addition, the concept was developed for a filament winding head to be added to the pultrusion machine to provide a hoop wrap around a pultruded form. This process is known as 'pull winding' and is primarily used with tubular or rod shapes. It is not feasible to add the filament winding process to a structural shape that has a concave curvature, such as angles, channels and I-beams. A typical process of pull winding is shown schemati- cally in Fig. 23.212.

During the 1970s, the concept of pull forming was developed by Brandt Goldsworthy. Three versions of this forming process were developed.

The first was actually named 'curved pultrusion' by Goldsworthy Engineering. The method was developed under a NASA contract and resulted in a pure, constant radius section with a constant cross section. A curved die was used, with a reduced radius, smaller than the required part radius. This allows for a degree of

490 Pultvusion

COUNTER-ROTATING

MANDREL AND

\

LAYER 1. LAYER 2. LAYER 3. LAYER 4. LAYE&S. FIBERS FIBERS WOUND FIBERS FIBERS WOUND FIBERS

Fig. 23.2 5-Layer tube on double head pull winder. (Courtesy of Pultrex Ltd.)

spring back after the product is removed from the diel3. The die was split along its length, in the vertical plane. One half of the curved unit was fixed in place. The internal radius portion of the die was further split in half vertically, along the central radius plane, forming two quadrants. Each of the quadrants had a slightly longer circumferential length than the fixed half. The two moving quadrants moved past the fixed part of the die at the processing speed of the pultrusion. When each moving quadrant had exited the plane of the fixed die, it moved rapidly in a circular fashion to contact the end of the following quadrant as it too passed across the face of the fixed die.

23.3 PULL FORMING

The pull forming process is a highly sophisticated variation of the pultrusion process in which both curved and straight product sections are fabricated with the ability to change cross-sectional dimensions of the product. It can be thought of as a combination of pultru-

response to a specific production require- rnentl4,l5. The pull forming process still uses the basic principles of pultrusion since it has a continuous strand of fiberglass roving within the product and this roving is used to pull the product through the sequence of the forming process. Goldsworthy Associates have designed two versions of this process, a curved pull forming and a straight pull forming. These two versions of the process are briefly described in the following paragraphs but it should be noted that a major difference between pull forming and pultrusion is that pultrusion is a generic process which can be used to make many different cross-sectional shapes and products by changing the die in the machine (which creates the cross-sectional shape) and the materials of fabrication. Pull forming however, is essentially a custom process which must be tailored to a particular product design.

23.3.1 CURVED PULL FORMING

sion and compression molding. The pull In the curved version of the pull forming forming process was developed by process, a curved structure is manufactured Goldsworthy Associates and was created in using a selected combination of roving and

Pull forming 491

mat/fabric to satisfy the performance requirements of the produ~t'~J~. The die is a multi- cavity die. Several of these dies are mounted on the face of a wheel and are curved with the radius of curvature of the die matching the radius of its position on the wheel. The die is open faced and is also open on the two ends for entry of the fiber preform. The preform is impregnated with resin in the same manner as for pultrusion and the fiber and mat/fabric combination is pulled into the curved mold or die by the rotation of the wheel. As the preform is pulled into the die, a stainless steel band moves against the open face of the die and com- presses the impregnated preform into the contour which is in essence a compression molding cavity. After the die is closed, it is heated to cure the resin and following cure, the stainless steel band peels away from the face of the die/mold. A fixed pin which is mounted adjacent to the wheel strips the product out of the mold and into a storage bin. When the stainless steel band is moved against the mold it is held in place by an automated clamping system until the product is fully cured at which point the clamping system releases the band which then retracts into a 'parking' position. The

machine is shown in Fig. 23.3. It should be noted that the process does dif-

fer from pultrusion in that the fiber preform is placed into the mold rather than being pulled through it. Also, while the cross section of the mold can change, the resulting part is a constant volume design, equivalent to the volume of the preform which is initially compressed into the mold.

23.3.2 STRAIGHT PULL FORMING

Straight pull forming was also invented by Goldsworthy Associates in response to a specific customer requirement for the automated manufacture of hammer handles. In this process, the fiber is pulled as a preform through the automated machine. Many single cavity dies are mounted on a belt. As the fiberglass roving moves through the system, a section of bulk molding compound (BMC) is cut from a rope of this material, is opened up longitudinally and placed around the roving and clamped onto it. The BMC is then compressed in a small briquetting press which travels with the fiber preform as it moves through the system. Following the compression

Fig. 23.3 Curved pull forming process. (Courtesy of W. Brandt Goldsworthy and Associates Inc.)

492 Pultrusion

molding of the BMC, the press releases and the balance of the fiberglass roving is fed in and encapsulates the BMC material. Following the encapsulation, a shrink film is placed around the wet package and the process goes through a heated tunnel which shrinks the film around the product. Finally, the product is run through the frame of a C-shaped die in a press which provides the final compression molding and curing of the hammer handle. The product is pulled from the die and cut off and the dies con- tinuously exit the belt and are recycled back to the front end of the process.

This type of pull forming is a highly automated, sophisticated process which requires considerable capital investment for the design, manufacture and set up. Each potential product, which would be a candidate for the pull forming process, must have its design analyzed and the process modified to respond to the custom requirements of the particular product.

With the increasing popularity and explo- ration of the resin transfer molding process in recent years, the pultrusion process has been reviewed for the potential of developing a repeatable, precise cross-section of a preform using a loose fiber bundle and an adhesive binder in place of the resin system. This

process variation has not been successful to date and further development will be required.

23.4 PROCESS EQUIPMENT

The pultrusion fabrication machine consists of six different parts (Fig. 23.4): the creel; the resin bath; the forming die; the heated curing die; the pullers; the cut-off saw.

23.4.1 CREEL

The creel is the beginning of the pultrusion process and is the material storage system from which the fibers and mat, veil or fabric are drawn in the correct sequence to match the design requirements of the structural shape. Since pultrusion is a long run continuous process, fiber rovings are provided in the maximum size configuration possible. Continuous glass rovings are normally provided in 'center pull' packages between 14 and 23 kg (30 and 50 lb) in size. These center pull packages are typically stored on a bookshelf style creel. These creels have from three to six shelves and are capable of storing anywhere from 45 to 120 packages of this type of fiberglass. These creels are normally mounted on casters to create a

Mat Roving racks racks A Material

Pulling mechanism Cutoff saw

V Resin tank Preforming

guides

Finished product

Fig. 23.4 Schematic of pultrusion process. (Courtesy of W. Brandt Goldsworthy and Associates Inc.)

Process equipment 493

mobile system. The casters are provided with a foot locking device to enable them to be locked in place when this is required. The glass roving is pulled vertically from the package through ceramic-lined holes in the shelves above. The roving is collected above the creel and turned 90" by means of a ceramic textile type thread guide and then moved forward to the material accumulating section just prior to the resin bath and forming die. A series of ceramic guides or rollers can be provided in the fiber path in order to tailor the tension in the fibers to the required level. The pulling of fiber from the center of the package will automatically insert a twist in the fiber as it is led into the pultrusion machine. To eliminate this, some fiberglass rovings are provided in a center pull twistless condition where the natural twist has been off-set by a 'built-in' reverse twist.

Continuous fibers of fiberglass, carbon, aramid and polyethylene are normally sup-

plied on 'way wound' packages on standard diameter cardboard cores. A typical carbon fiber package mounted in a payoff creel is shown in Fig. 23.5. These fiber packages are designed to provide fiber payoff from the exterior of the fiber package and hence avoid twist. While fiberglass is normally supplied in the heavy center pull spools, it can also be obtained in the outside payoff type package. All of the cardboard cores are a standard three inch diameter with the exception of the aramid which is 90 mm (3.5 in) inside diameter.

This style of package requires the use of a multiple spindle creel design in which the packages are oriented normally horizontal but usually with a slight upward camber. Creels are available with package positions up to 1500 or 2000 on a single creel or combination of creel frames. A loaded multiple spindle creel is shown in Fig. 23.6. Ceramic guides are used to move the fibers to the front of the creel

Fig. 23.5 Carbon fibre spool on package holder with quick braking action. (Courtesy of Texkimp Ltd.)

494 Pulfrusion

Fig. 23.6 Loaded multiple spindle creel. (Courtesy of Texkimp Ltd.)

and into the pultrusion system. Tension requirements in this type of system are usually provided by spring attached tension straps which rub on a pulley attached to the package holder. This provides a braking action. The straps can be either independently loaded with weight or can be connected to a central mechanical system, such that tension for all of the spools can be varied by a single adjustment handle. Fiber packages for this type of system are normally 5 kg (11 lb). With this small size of package, it is normal for a large number of packages to be located on this type of creel assembly. In addition, package simula- tion bars are normally provided with these types of creels to give a uniform tension and to eliminate the possibility of a fiber from a small or 'almost used' package touching and abrad- ing a near-by forward mounted, full package.

Stationed immediately after the fiber creel is the specialized type of creel which is designed to accommodate rolls of mat, veil or fabric. These materials are usually supplied in roll form with diameters between 305-610 mm

(12-24 in) on cores of 75-100 mm (3-4 in) in inside diameter. This special mat/fabric creel must be able to accommodate rolls of these dimensions and resulting weight and permit the rolls of material to be installed in a varying sequence of locations to match the design of the final structure. This type of material creel will normally provide horizontal feed. However, if vertical mat or fabric is required, then an independent custom unit must be provided. These units are usually of a carousel-type configuration. Figure 23.7 shows a typical mounting for feeding veil rolls into the system.

I Fig. 23.7 Feeding external veil material into pultrusion system. (Courtesy of Creative Pultrusions Inc.)

Clearly, the overall creel system for materials supply must be able to provide any combination or arrangement. It should be noted that in every fiber and fabric system used for pultrusion, there should be sufficient continuous roving in the system to sustain the required pulling force. As the various materials travel forward towards the resin application area it is important to control the alignment of the various fibers and fabric/mat strips which are going into the configuration. This will prevent knotting and twisting of the fiber reinforcement and also will ensure that the various fiber


materials remain in the correct relationship to each other and are placed in the correct zone of the pultruded product according to the product design requirements. This can be best accomplished by the use of orifice plates, creel cards combs and rollers (grooved and flat) to precisely and accurately place all the materials. The materials commonly used for these guides and rollers are primarily titanium oxide ceramic, stainless steel, tool steel alloys and chrome plated steel. These materials are also used for the various rollers associated not only with guiding the sheet materials but also with controlling them during their passage through the resin impregnation zone.

23.4.2 RESIN IMPREGNATION

Virtually all pultrusion processes utilize a resin impregnation bath to facilitate the impregnation of the resin into the fiber structure. The position of the resin impregnation bath in the production line can be varied and the manner in which the resin is applied to the fiber can have many different versions. A resin dip bath is most commonly used. During this process the fibers are passed over and under a series of rollers or bars which both spread the fiber to more easily accept the resin and also provide a massaging effect which 'kneads' the resin into the fiber bundles and structure.

The wetting speed of the fibers depends upon their pretreatment and on the resin formulation. Wetting is also affected by the type of sizing agent on the fiber, the possible presence of remaining lubricant on woven fabric and finally the type of binder which is used in mat and veil products. The resin bath is uniformly used for products that utilize all roving in their construction or for products that are easily formed from the flat fiber ply which emerges from the resin bath. However, in many of the more sophisticated products which are now made from pultrusion, it is impractical to dip all of the materials in the resin bath. When vertical mats are required or hollow profiles are produced, a tailored resin

bath is frequently used which matches the preform shape or orientation of materials which pass through it. These types of tailored channels or baths can also be used to orient the flat materials properly. This method permits the resin impregnation to take place without moving the reinforcements away from their optimum path or shape.

23.4.3 VERTICAL PULTRUSION

The vertical pultrusion process should be mentioned at this point since the primary difficulty in creating a vertical pultrusion is the placement and use of a resin bath. In the arrangement for vertical pultrusion the equipment used is essentially similar to that for the horizontal methodI8. Most equipment operates in the vertical position with the exception of the resin bath and roving creels which are generally positioned horizontally and the roving is fed in the regular manner, horizontally through the resin impregnation bath. This bath is located above the entrance to the forming die and the fibers exiting from the bath are turned 90" across a roller and then proceed vertically into the forming die. The advantage of using this vertical procedure is that a uniform arrangement of impregnated fibers can be achieved across the section being formed. The effects of gravity are removed from the fiber arrangement. During the horizontal process, gravity plus any nonuniform tensioning across the fiber group will result in some fibers sagging under their weight with resin and hence not be properly located as they enter the forming and curing dies. An additional advantage of the vertical process is that it is possible accurately to locate the internal mandrels whch are necessary for hollow shapes and tubes. This is particularly important in the fabrication of heavy, thick wall tubes. In the horizontal process, these internal mandrels will frequently deflect under their own weight and cause a nonuniformity of wall thickness around the hollow profile or tube. In addition, the vertical process allows the use of several different fiber

496 Pul trusion

entry points to the forming process with multiple resin baths. In the horizontal procedure, the multiple entry points have to come from the side of the main fiber path and it is not as easy to coa- lesce the fibers into a single merged preform.

23.4.4 USE OF THERMOPLASTIC MATRIX RESINS

Thermoplastics cannot be applied to roving easily using the standard resin bath, even with heating. Thermoplastic resins impregnated on roving are generally available as preimpreg- nated (prepreg) materials and are supplied by specialty companies who are experts in the prepreg process. Thermoplastic matrices improve the toughness of the composite and this is their major end use advantage. In general they have high softening temperatures and high physical properties coupled with a low fluid viscosity in their melted form. The prepregs are normally prepared using solvents and this provides some difficulty against environmental regulations in order to remove and dissipate the majority of the solvent. Some solvent has to be retained in the process in order to have the material be sufficiently pliable for machine and manual handling. In addition to the improved toughness of the thermoplastic composite, an important advantage of thermoplastic pultrusions is the ability to heat and reshape the product after f ~ r m i n g ' ~ - ~ ~ .

23.4.5 RESIN BATH LIFE

In a continuous pultrusion process, the pot life of the resin should be several days. However, if the volume of the resin bath can be kept small in relation to the resin being withdrawn, shorter pot-life resin, i.e. 3 4 h can be used. Shorter pot-life resins result in smaller batches and mixing becomes time consuming.

23.4.6 THE PULL WINDING TECHNIQUE

The pull winding process was developed in Europe and was used frequently by European

fabricators to provide pultruded tubular structures both round and square23, 24. The process combines the standard continuous unidirectional fibers of the pultrusion process with hoop wound continuous fibers. The longitudinal fibers are used for axial and bending resistance while the hoop fibers are used for hoop tension and compression resistance. The combination of the two processes of pultrusion and winding provides virtually unlimited possibilities. However, the increasing complexity of the process limits these combinations. Normally, longitudinal fibers are positioned at the inside and outside surfaces of the tube with one or two hoop wound layers positioned internally in the tube wall. These hoop wound layers are not truly 90" hoop wound layers because of the slightly helical nature of the winding and the lateral movement of a wound roving, one band width of advance with each circumferential pass. Depending on the diameter of the tube, the winding angle is typically anywhere from 80-87". The winding can be performed in both clockwise and counterclockwise direction and in addition to the hoop tension and compression resistance can also provide a degree of torque strength to the tubing. Single and double head pull winders are used with these techniques. The pull winding process is shown in Fig. 23.2 and the procedure for the use of these systems is self explanatory. The important control feature of the pull winding process is the control of the winding speed and position with respect to the linear motion of the pultruded form through the machine. This is achieved by sensing the speed of the longitudinal motion accurately and using a microprocessor control to coordinate the speed of the rotating head motor. This relationship defines the pitch of the winding which is defined as the linear distance moved during one revolution of the head. With the microprocessor control the pitch can be varied, thus providing variations in fiber content and angular position, as required. A paper by D.E. Shaw Stuartz5


defines the primary advantages of pull winding over other methods of tube manufacture as:

0 it is a fully automatic continuous process; it is dimensionally accurate and repeatable;

0 it produces tubes with a good external appearance and finish;

0 it can have built-in color; it can be made with thinner wall sections than conventional pultrusion or filament winding.

23.4.7 PULTRUSION DIES

Two types of dies are used in the pultrusion process: the forming die and the heating or curing die. Forming is normally accomplished immediately after the impregnation process although some shaping with the roving and mat/fabric inserts in a dry condition can take place prior to entering the resin impregnation processing step. Forming dies or guides are normally attached to the heating or curing die in order to provide the correct relationship between the forming and the heated curing step. For tubular or hollow profile pultruded products, a central mandrel support is necessary internal to the fiber form and it is necessary to extend this mandrel as a cantilever through the pultrusion die. It is also important to resist the forward drag on this central mandrel which will occur from the surface tension and adhesive forces of the resin on the roving or mat. Materials must form in sequence around this mandrel and must alternate from one material to another to prevent any weak areas at overlapping joints. The sizing of the slots, holes and clearances in the forming die must be carefully designed so that excess tension on the dry or impregnated fiber is avoided. The fiber is weaker in this condition than in the cured condition and filaments may be independently broken and distortion of the mat and fabric can take place. The forming die can also be designed to permit excess resin removal. This prevents an abnormally high hydrostatic force at the curing die entrance. Materials commonly

used for forming dies include Teflon@, high molecular weight polyethylene, chrome plated steel and a variety of tool steel alloys. The fabrication of the forming die is a custom process which is best performed at the pultrusion fabri- cator by a skilled tool maker/machinist.

23.4.8 INJECTION PULTRUSION

In this modification of the resin impregnation process, the resin is directly injected into either the forming die or into the initial segment of the pultrusion die. The resin is injected into the die under pressure and is forced into the inter- stices of the fiber system. The principal advantage of this system is that it limits the release of volatile resin components and reaction products.

An additional advantage of this process for laboratory or research and development pultruders is that it enables a rapid resin change without removal and cleaning of all of the resin bath components. In addition, dry fibers are not impregnated with resin before entering the die and therefore they can be positioned accurately, even with complex profile cross sections and multiple mat and fabric entries. The schematic of the injection pultrusion system is shown in Fig. 23.8. A typical resin injection pultrusion die is shown in Fig. 23.9. In addition to the advantages listed above, there are several disadvantages which must be weighed in a process trade off, before resorting to the injection pultrusion method. These disadvantages are:

0 a more complicated die design; 0 more parameters to control; 0 analytical support is required to predict wet

out behavior and design of the resulting dies;

0 fibers in the die are very tightly compressed and resin penetration is difficult, particularly with large wall thicknesses;

0 the tightly packed fibers can act as a filter and partially remove resin additives, particularly those in suspension.

498 Pultrusion

Fiber rack

doth J racks

I S - moving pulling mechanisms cutoff saw

t disengaged \ - Lc

I hydraulic rams I

finished pressurized product

resin tank

\ / preforming guides

Fig. 23.8 Injection pultrusion process. (Courtesy of American Composite Technology Inc.)

injection port thermostats

‘ 1 6% thermostats matrix

injection i fiber + 1 cure

I zone zone COmprBsEmn

zcne

Fig. 23.9 Resin injection pultrusion die. (Courtesy of American Composite Technology Inc.)

23.4.9 HEATING AND CURING DIES

There are three considerations in the design and use of a heated die for the curing of a pultruded composite section. The first of these is the positioning of the die relative to the machine access, the second is the actual method of heating to induce the necessary energy into the composite material to fully cure the resin system. The third includes the design features and construction materials for the die.

23.4.10 POSITIONING OF THE HEATED DIE

The heating/curing die must be firmly mounted to the frame of the pultrusion machine in order to react the pulling forces, which are created in pulling the product through the process. These forces are typically in the range of 5440-7250kg (6-8tons). However, some of the larger machines which are capable of producing parts up to 1.52 m (60 in) wide and heights of 0.60 m (24 in) (with narrower parts) can require pulling forces up to 16 320-18 000 kg (8-20 tons). The mounting method must not only permit these types of loading but must also be capable of mounting height adjustment in order to accurately align the axis of the die with the pullers which move the product out of the die. Following alignment and clamping, the mounting system must also not allow any deflection which might provide an angular or dimensional mis- match of the machine.

23.4.11 DIE HEATING

Die heating is probably the most critical control parameter of the whole pultrusion process. The die heating profile will control


the rate of polymerization of the resin system and the position of the resin gel point front w i t h the die. It also influences the degree of resin exotherm profile throughout the various wall thicknesses of the pultruded structure. The curing dies are typically heated with elec- tric strip heaters or hot oil jackets. The thermal curing using these two methods of heating is slow, owing to the fact that the tool steel of the die is a poor conductor and limits the rate of heat injection to provide a uniform cure. The thermal energy which is required to cure the composite material must all be applied through the outer surface of the composite shape. The heat input is required to produce gelation of the resin throughout the composite part. When the composite structure being pultruded has thick sections, this necessarily requires a longer heat input which slows down the pultrusion rate. Increasing the rate of heating to permit a faster pulling speed will not necessarily solve this problem and may result in premature curing of the outside skin of the profile with early onset of the exotherm in the resin system and potential overheating and cracking of the exterior surface of the part. Temperature conditions of the die are controlled by the internal placement of thermocouples and this can result in individual control of heating rates in various segments of the die.

Instead of using strip heaters or hot oil jackets, it is possible to heat the curing die by means of heated platens and a press. These heated platens will usually have several zones of heating control with imbedded thermocouples to sense the platen temperature. T h s method provides a uniform heating condition to the die. However, since the thermocouples are sensing the platen heating temperature rather than the die or product temperatures, the temperature differential between these zones must be well documented. This permits the platen heating temperature to be set to provide an adequate heating level in the composite product and absorb all of the con- duction and radiation losses from the system.

Use of the platen system will allow change out of dies with very little system down time.

During start up and also during shut down periods it is necessary to provide a cooling method at the front of the die to prevent early gelation of the resin system. During these phases, the entire die will heat up rapidly without the composite being pulled through it and acting as a heat sink. The cooling can be done dynamically through cold air impingement or by using a water jacket or tubes through which cold water will be pumped. Instead of these dynamic methods, a simple static method would be to provide an unheated zone at the beginning of the die to act as a heat sink and conduct away the excess heat during start up or shut down actions. These same unheated or cooled sections may also be useful at the exit of the die to remove contained heat from the product prior to its exit.

It has been mentioned that the heating profile within the die is the primary factor which controls the speed of throughput of material in the process. This was recognized early in the development of the pultrusion process by W. Brandt Goldsworthy and he added a radio frequency preheated system to his pultrusion machine designZ6rz7. The use of radio frequency heating in conjunction with the traditional die heating methods can significantly increase running speeds. This technique is basically limited to unidirectional reinforced rods, bars and shapes and cannot be used if carbon fibers comprise the reinforcement. Data from Goldsworthy Engineering Inc. shows speed increases of up to 400% based on a 2.54mm (0.1 in) wall thickness pultrusion. Speed increases of 100% (i.e. doubling the speed) are accomplished at wall thicknesses of 2.54 mm

During recent years it has become possible to evaluate the internal die profile with regard to both temperature and pressurez8. The sensors which are used to provide that data are essentially similar to strain gages with output leads attached to them. The pressure sensor is

(0.1 in).

500 Pultrusion

a unique development resulting from the music industry. These sensors were originally used as striking pads in electronic keyboards and the manufacturer has now developed them to measure pressure forces transverse to the plane of the sensor. The output from the sensor is transmitted via a pair of thin strain gage type wires. It is thus possible to insert both of these sensors (temperature and pressure) into the fiber resin system prior to entering the curing die. The sensors will then travel through the die internal to the product and will record both the temperature and the internal pressure in the die from the entrance to the exit. Once the sensors have traveled beyond the exit from the die, the connecting wires can be cut. The section of product which contains the two sensors and the lead wires is then cut out and discarded. The resulting data provides a complete process description from the entrance to the exit of the die.

23.5 DESIGN FEATURES AND MATERIALS

In considering the design of the heated curing die for a pultrusion system, it is possible to use a single cavity tool, a multi-cavity tool or several single cavity tools mounted in parallel. The choice between these three will depend upon the size, complexity, dimensional tolerance and surface quality of the pultruded product. Individual or single cavity dies are a frequent choice of pultruders, particularly for medium to large or sophisticated shaped products. Use of the multi-cavity tool or several single cavity tools mounted in parallel is generally restricted to very competitive and low cost shapes such as round and square cross section rod. The choice between an individual die and a multi-cavity die is frequently dependent upon the experience and prefer- ence of the manufacturing engineer in the pultrusion company. The multi-cavity tool can be an efficient arrangement for production conditions. In this case the die is two to three times the length of a single cavity tool. The longer die makes it possible to use different

heating zones in order to maintain control over the resin gel, curing and cooling of the pultruded section within the die. In addition, if one of the cavities in the multi-cavity tool is damaged, the complete tool is not out of production since the remaining useable heating zones can be modified to continue to provide an acceptable product.

The dies are usually made from steel, which can be hardened tool steel, or steel alloy which is treated, hardened and plated before use. The die must be relatively thick walled so that it can be heated uniformly and retain the heat input. Thick wall design also reduces distortion under temperature and pressure. It has been suggested that the cross-sectional area of the steel should be at least ten times that of the pultruded part. The steel used should be hard since it has to withstand the abrasive action of the composite being drawn through it. A typical hardness is a Rockwell C rating of 30, which can be obtained with a prehardened tool steel. Dies are manufactured from multiple pieces that are machined and joined together to create a design profile cavity. As the various sections of the die are connected together they must be properly aligned. This can be done using dowels for permanent alignment or by machining an alignment groove on the outside part of the die which can then be used for a hard metal key which can be driven into the groove to provide a pos- itive alignment for the entire tool. The die assembly is then match drilled and tapped for the assembly bolts which hold the parts of the die in position against the high internal pressure developed during the process. Following alignment and assembly, the exterior surface of the assembled die is ground flat. The internal cavity surfaces are polished using polishing wheels and buffing compounds to a high surface finish 0.254-0.762 wm (10-20 kin). At this point a bell mouth is machined around the entrance of the die to provide a smooth entrance for the resin impregnated fiber form. The radius will vary in size from a small radius for small simple structural

Design features and materials 501

profiles to a relatively large radius for a large and complex composite profile particularly where the fiber content is high. The bell mouth is polished and the finished die is hard chrome plated to a thickness of 0.025-0.050 mm (0.0014.002 in) with a Rockwell C hardness of approximately 70. The dimensions and surface quality of the pultruded product are a direct reflection of the condition of the die. Dies will thus not be removed from production to be inspected unless the shape or quality of the product begins to change. Chrome plated dies will normally have a life of 61 000-150 000 m (200 000-500 000 ft) of product run with their initial chrome plate application. Up to 305 000 m (1 x lo6 ft) of product have been produced from some hard chrome plated dies. Chrome plated dies require frequent inspection to insure that their internal shape and dimension is maintained. They should also be inspected for wear of the chrome plated surface since the wear process will proceed much more rapidly if the tool steel surface is exposed from under the chrome. Once the die has worn and produces product beyond allowable dimensional toler- ances, the die may be replated and repolished back to original dimensions. This process may be repeated several times.

23.5.1 CLAMPING AND PULLING

Three different types of clamping and pulling systems have been used in the pultrusion industry. Of these three, only two are now encountered. The original system used on the earliest machines in the 1950s and early 1960s employed a single clamp. This clamp was hydraulically controlled and contoured pads were used for gripping the part. The unit containing this single clamp was pulled by a continuous chain for a distance of 3.2-3.7 m (10-12 ft). Areversible motor was used to drive the chain and following the pull stroke, the puller/clamp released the product and returned to the beginning of the pulling stroke, clamped the product and pulled againz9. The obvious disadvantage of this system is that the

pultruded profile remains stationery until the puller returned to the beginning of its stroke. Because of the alternating pull and pause mechanism this system was known as the 'intermittent puller'. This system is still used on some very early machines, however, it is certainly not in widespread use.

A modification of the clamping/pulling system has become popular which provides a continuous pull. The clamping, pulling and unlocking cycles of this system are coordinated by the control system of the machine. The drive system used can be either a hydraulic cylinder, a threaded ball screw such as is used on lathes, or a chain drive mechanism. The clamping pads are still formed to match the contour of the profile being pulled. The two puller heads must operate in the space originally designed for the single puller. Consequently, limited lateral movements of approximately two feet each are sustained by the two pullers. The two puller system is shown in Fig. 23.10.

Fig. 23.10 Conventional two puller system. (Courtesy of Pultrex Ltd.)

Continuous belt pullers are used on basic commercial machines. These pullers can be used with single or multiple cavity molds. The cleated chain or caterpillar version of the continuous belt machine has many individually contoured puller pads attached along the chain length. The number of these contoured

502 Pultrusion

puller pads depends on the complexity of the part but generally varies between 12 and 60 pads. This large number of pads permits a lower unit pressure between the clamping pad and the pultrusion. The caterpillar type machine was designed and built in 1958 by Brandt Goldsworthy, Dennis Franks and Tom Bailey. Caterpillar type machines are preferred and still widely used in the industry.

23.5.2 CUT-OFF STATION

Every pultrusion machine utilizes a cut-off saw to cut the pultruded profiles to the required length for shipment and use. This saw is frequently of a radial arm type as shown in Fig. 23.11, but can be also a chop saw, orbital or band saw. It is mounted on a platform which moves down the pultrusion exit table at the same speed as the pultruded product. Carbide and diamond tipped saw blades are used for the cutting of glass and carbon pultrusions. However, these saw blades are not effective for cutting aramid fiber pultrusions. This fiber is known for its properties of toughness and resistance to abuse. These properties lead to difficulty in machining, grinding and cutting.

The most successful method of cutting

aramid fiber to date has been the high pressure water jet and presumably this could be adapted for use on a pultrusion machine. The inherent disadvantage of the high pressure water jet is its cost which is from $50 000-100 000. A com- promise solution to this problem would be to cut off the sections as smoothly as possible, using the diamond saw, to a slightly over length condition with very rough ends. The product length can then be subcontracted to a waterjet cutting source for final trimming. This will result in some wastage.

23.6 MATERIALS

Fiber properties to aid the designer are shown elsewhere in this text. Following the selection of the fiber type to suit the required design factors, the fiber must be oriented in the correct direction. It is understood that all of the fiber types must be available in continuous form in order to be useable in the pultrusion process. The most commonly used form of continuous reinforcement is roving. This is available in single and multiple strand configurationsm. Glass rovings are designated by their yield which is the number of yards per pound of material. The two most commonly used versions are at 112 yd/lb or 224 in/kg or 112 or 124 m/kg (56

1 or 62 yd/lb). The glass rovings are typically supplied in 18.1 kg (40 lb) hollow cylindrical packages with a center pull payout. A similar center pull spool is also available for both aramid and polyethylene fibers. Carbon fiber is typically available in either a 3K, a 6K or a 12K

!!- filament. It should be noted that the tow sizes of 1 the carbon are much smaller than the glass rov- - ing and package weights are 1-2 kg (2-5 lb) \ with an outside payoff designed for a package

holder style creel system. New versions of the - carbon fiber roving are available now in 40K, 160K and 320K tows. Use of these tows allows the fiber to be laid down very rapidly and consequently these versions of carbon fiber are attractive to pultruders. Typical properties of fibers used in PultruSiOn are shown in Table Fig. 23.11 Conventional cut-off saw. (Courtesy of

Creative Pultrusions Inc.) 23.13', 32.

Materials 503

Table 23.1 Typical properties of the major fibers used in the pultrusion process

Property E-glass S-glass Keular Spectra Carbon Carbon

Density, 2600 2491 1470 968 1720 1770

Tensile strength, 3447.5 4585.2 2964.9 1170.0 1896.1 2560.9 MPa

Tensile modulus, 72.4 86.9 131.0 26.0 379.2 473.6 GPa

Elongation at 4.8 5.4 2.3 3.7 0.5 1.81 break, %

(Aramid) (Type T300) Inter. modulus ~~

kg/m3

All the rovings discussed will yield the highest possible longitudinal properties. Fibers as rovings result in the maximum fiber content to be achieved in pultrusion. If the longitudinal rovings are used under near perfect conditions, a 65% fiber volume percent level should be achieved. In a product which utilizes 100% roving this material is normally in the longitudinal direction or axis of the pultruded product. Properties in the other two directions are dependent upon the resin system and the mechanical properties of the matrix resins are much lower than fiber properties. Transverse strength problems are overcome by inserting transverse fiber materials into the pultrusion. This is done either by using fabric or continuous strand mat. The latter is most commonly used. While the fabric is a standard woven textile form, the continuous strand mat has fibers oriented in a random mode, bonded with a thermoset resin binder which holds the mat together adequately for processing in the pultruded section. While mat is available in any of the fibers which have been previously discussed, the most common available mat is an E-glass version that has coarse fibers in an open or porous construction. This mat can be used either as a center ply in a pultruded structure or on the outer surface of the structure. Use of the mat greatly improves the transverse physical and mechanical properties. It should be noted, however, that the mat

is porous and its use on the exterior surface of a pultrusion might well leave porosities or voids in the surface. To counter this problem, a very fine filament, E-glass mat, commonly known as veil, can be used as the surface ply. Its presence during the pultrusion process will tend to bring more resin to the surface of the pultrusion and this will achieve a smooth, uniform surface, devoid of porosity or voids. The veil mats can also be placed internally in the composite and recent improvements in their structural properties have made this possible.

The random fiber mats in E-glass are used in weights of 0.15-0.6 kg/m2 (0.5-2 oz/ft2). The inclusion of these mats in the pultruded structure means that some of the longitudinal fibers will have to be removed to allow for the volume of the mat, veil or fabric. With the use of fabric or mats in the structure, the resin content by volume will increase in order to fill the openings in the mat or fabric. Thus while the transverse strengths increase, the longitudinal strengths usually decrease. Mats are also available in carbon fiber.

The random fiber structure of strand mat provides fibers in all directions. However, this random orientation does provide some problems in that the fibers may not provide a symmetrical balance within the structure. The initial solution to this problem was the use of woven fabric. However, the lack of tension in the fabric results in a lower strength capability

504 Pultrusion

of the pultrusion since under load, the fibers in the fabric will have to straighten and become tensioned prior to being able to accept load. One way of solving this difficulty is to use non-woven biaxial fabrics which are stitched or knitted together at the crossover points. Because of the nature of fabrication of these nonwoven materials, any ratio of fibers in the two directions can be provided. It is also feasible to utilize k45" fibers in conjunction with the 0 and 90" fibers. The biaxial fabrics are normally used as internal plies and not on the external surface. This is due to the nature of the nonwovens in that their transverse fibers will tend to be dis- placed by friction with the walls of the die during pulling. Fabrics using a +45" orientation without any longitudinal fibers are usually impractical for the pultrusion process. It should be noted that hybrid composites with tailored properties are possible using combinations of the fiberglass, carbon and aramid materials. The designer will read- ily determine the mechanical properties which are required from the nonwoven or woven fabrics. The rule of mixtures will apply for combination proper tie^^^.

When pultruded composites are used in outdoor weather conditions, the surface of the composite may be degraded with time by sun, wind, rain and ultraviolet exposure. In order to solve this problem, additional resin needs to be provided at the exterior surface of the pultrusion. This is done by incorporating the very fine filament veil mats which are typically fabricated from polyester or nylon34. These veil fabrics are available in a variety of weights and weaving patterns. They help the pultruder by providing a tough surface material which will protect the die wall from the abrasive nature of the fiberglass or aramid. In addition, the resin rich surface is created without any obvious fiber weaving patterns, plus the veil materials can be screen printed with company identification or decorative effects35.

23.6.1 MATRIX RESINS

Of all of the technology considerations in the pultrusion process, the most critical material is the resin system and its f~rmulation~"~~. Resin selection controls mechanical characteristics, electrical insulation, corrosion resistance, operating temperature and flame and smoke properties. It also has a significant effect on the process speed because of the required cure cycle for any particular resin. The selection of a resin system will also affect the production cost of the process. The two most commonly used resin types in pultrusion are the isoph- thalic polyester and the vinylester. These two comprise over 90% of all resins used in pultrusion. Epoxy resins and phenolic resins are also being increasingly used. Phenolic resins were traditionally avoided by pultruders because of their condensation reaction during cure. Condensation reactions produce large volumes of water vapor and this typically causes voids, channels, delaminations and porosity when there is no provision to remove it. While the pultrusion die does have an entrance and an exit, nonetheless the system is essentially a closed, pressurized volume. Table 23.2 provides typical mechanical properties for resin systems most generally used in pultrusion and several other chapters discuss the broad range of matrix materials39.

23.6.2 PHENOLICS

In consideration of the use of phenolic resin systems, the disadvantage of the condensation type reaction was certainly sufficient to cause delays in the potential use of phenolic resin in the pultrusion process. The amount of water vapor which is generated in the condensation process is very large and it has always been assumed that a phenolic pultruded structure would look somewhat like a sponge. However, phenolic systems have been pultruded in recent years and a phenomenon has occurred which is not well understood. During pulling of a phenolic/fiberglass structure through the

Materials 505

Table 23.2 Typical mechanical properties for resins used in the pultrusion process

Property Polyester Vinylester Epoxy

Density, 1100 1100 1300 kg/m3

Tensile strength, 77.2 81.2 75.4

Tensile modulus, 3.3 3.4 3.3 GPa

Elongation at 4.2 4.5 6.3 break, YO

Flexural strength, 122 134 115 MPa

Flexural modulus, 3.2 3.1 3.3 GPa

Heat distortion, 77 99 166 "C

MPa

pultrusion die, a high pressure jet of steam is noted at the exit from the die. How and why the steam is caused to come off in this manner is not known. However, in the experiments which have been run, the resulting pultrusion has not had any porosity problems and the processing tests are noted as being successful. Specific pultrusion grades of phenolic resin systems are now available from plywood manufacturing corporations such as Weyerhauser and Georgia Pacific. The avail- ability of these resins would certainly indicate that the anticipated processing problems have not occurred40, 41. The importance of phenolic resins is in their resistance to fire and their low smoke and toxicity production under fire conditions. All forms of composite materials, including pultrusions, are being used increasingly in mass transit, aircraft and civil engineering applications. In all of these areas of application, increasing contact of the composite material systems with the general public is occurring. Because of this, the fire smoke and toxicity requirements of specifica- tion control groups such as the FAA, the

Federal Department of Transport and the various state departments of transportation have imposed fire controls on composite materials. It is required that they will not bum or stimulate combustion, have minimum required smoke levels and also will not produce toxic fumes under flame impingement and high environmental temperature conditions. Considering all of the resins in the composite industry, phenolic resins will come closest to matching these flammability specifications. Because of these factors, it is anticipated that the use of phenolic resin systems in pultruded products in the future will increase dramatically.

23.6.3 COMPARISON OF RESIN SYSTEMS

In recent years, there has been use of epoxy and phenolic resins in pultrusion. G.A. Hunter of Shell Development Company compared the properties of resin systems42. He provided a three zone model of the pultrusion process within the curing die (Fig. 23.12). The sketch provides an excellent background for comparison of resin proper tie^^^.

Of the four primary resin systems used in the pultrusion process, the polyester and vinylester resins account for more than 90% of the marketplace. Phenolic and epoxy resins make up the balance of the market. In com- paring resin systems, one should review the internal contours and the heating profile of the heated resin die and examine the change in resin morphology as it proceeds through the die. The model of the pultrusion process given in Fig. 23.12 which shows the three zones of the heated die and the transition of the resin phase from liquid through the gel zone into the solid phase. The first zone shown in Fig. 23.12 is where the material enters the die at room temperature and expands as it absorbs heat which causes the hydraulic pressure in this zone of the die to rise. As the material pro- gresses into zone 2, or the gel zone, it has absorbed more heat, is beginning to cross link and changes from a viscous liquid into a non- flowing jelly type of material, then into a

506 Pultrusion

~~

" T H E G E L Z O N E " S T R I P H E A T E R

L I Q U I D P H A S E SOLID P H A S E &&( . . . .

ZONE 1 i ZONE 3 . . i V l S C O U S S H E A R i i S L I D I N G F R I C T I O N F O R C E S i

I.. ..... i F O R C E S *........ . * . . . . . . - . I

* * a . ........ ZONE 2

: C O H E S I V E F O R C E S :

Fig. 23.12 Three zone model of the pultrusion process. (Courtesy of Shell Development Company.)

rubber-like texture. As the material cures to a hard solid, shrinkage occurs which releases the hydraulic pressure forces and the product shape retracts from the internal surface of the die. This is zone 3. In this zone, because of the release of the product from the surface of the die, the sliding frictional forces are very slight. Depending upon the thickness of the part and the process speed, the bullet-nose shape of the

gel zone will expand or contract. Joseph Sumerak in 1985 quantitatively

described the internal dynamics of the pultrusion process. Taking test results from Sumerak's earlier work, Hunter showed the relationship of pull loads to processing speed for catalyzed and uncatalyzed resin systems (Fig. 23.13)w7. For the uncatalyzed resin case, the rising pulling load associated with

looot 71 W I T H 20 P H R C L A Y P t D - PULL LOADS ATTRIBUTED TO

VISCOUS SHEAR AND FRICTION O F

CATALVZED RESIN 8 0 0 -

0 - PULL LOADS ATTRIBUTED TO * 7 0 0 - PURE V ISCOUS SHEAR OF THE

UNC ATALYZ E D RESIN A I 6 0 0 - A CALCULATED PULL LOADS

ATTRIBUTED TO VISCOUS

SHEAR O F THE CATALVZED u)

0

J J 3 3 0 0 -

4 0 0 -

n 200 -

0 12 24 36

L I N E SPEED, I N I M I N .

Fig. 23.13 Pull loads compared with line speed for different types of resin systems. (Courtesy of Shell Development Company.)

Materials 507

B

4 0 >

- 1 -

- 2 -

- 3 -

increased processing rate, or line speed, is the result of increasing shear forces over the length of the die. In the case of the catalyzed resin and referring back to Fig. 23.12, viscous shear forces are generated only in the front portion of the die, i.e. zone 1. Within the gel zone, cohesive forces come into play for a small length of the die which is followed by the transition to the rubbery cured material which provides substantial friction forces. As the resin hardens and shrinks away from the surface of the die, the frictional forces are reduced significantly. It is obvious that the pull load is significantly higher for the catalyzed resin system, particularly as the line speed increases. This proves that the major portion of the pultrusion loads are generated in the gel zone and are cohesive forces and frictional forces resulting from the interface of the resin and the die.

Sumerak showed that a significant part of the internal pressure does develop inside the

- I N I T I A L C U R E C Y C L E --- S E C O N D H E A T C Y C L E

S T A R T I N G V O L U M E - 9 . 0 9 8 M L

N E T V O L U M E L O S S - 0 . < 5 M L

V O L U M E T R I C S H R I N K A G E - 6 . 0 4 %

1 . r l r . l : 1 1 1 . 1 I ,

die and is proportional to the speed of processing. Hunter provided evidence that the pressure loss in zone 3 of the die occurs well before the material cools. Thus it is not thermal contraction but volumetric shrinkage due to the cure of the resin. The coefficient of thermal expansion of the steel material of the die also enters into this equation. For a differential temperature of 121°C (250"F), the hottest temperature section of the die for a 12.7 mm (0.5 in) diameter pultrusion will be 0.3% larger than the entrance. Thus pressure and volumetric shrinkage together play a major role in pultrusion dynamics. Insufficient pressure causes sloughing problems and insufficient shrinkage can cause excessive pull loads. The resin rate of shrinkage affects the rate of pressure decay and is controlled linearly by the cure rate of the resin. Thus a delicate balance between pressure, cure rate and shrinkage must be obtained for a clean pultrusion process to take place.

Fig. 23.14 Volume change of polyester resin during cure. (Courtesy of Shell Development Company.)

508 Pultrusion

- 2 -

-3-

23.6.4 SHRINKAGE

Hunter ran shrinkage tests on a typical polyester resin and a standard Shell epoxy resin system42. The volume change of the polyester and the epoxy resin are during cure are shown in Figs. 23.14 and 23.15. The data shows that the polyester shrinks almost twice the degree of epoxy. However, Hunter reports that the net shrinkage is not nearly as important as the profile of that shrinkage. The polyester continues to expand after the gel point which is followed by a high shrink rate that gradually tapers off. In comparison, the epoxy resin shrinks before it gels and continues to shrink at a steady rate until it is fully cured. This information sheds new light on the understanding of the pultrusion characteristics of epoxy resins compared to polyesters. In most composite manufacturing processes, the application of pressure during the curing phase of the resin is always beneficial to the resulting product. Pultrusion is no exception to this rule and it is beneficial to generate pressure in the pultrusion die up to the point where the resin

N E T V O L U M E L O S S - . 3 2 M L S .

V O L U M E T R I C S H R I N K A G E - 3 .57 %

3t G E L P O I N T .A

is fully cured. Similarly, the pressure during the gelation phase ensures that the product is tightly held against the surface of the die and consequently a smooth surface will be generated with the pressure preventing sloughing. Thus, from a comparison of the test results, it is obvious that the polyester shrinkage profile is superior to the epoxy in terms of providing gelation and cure under pressure. In addition, the sudden high initial rate of shrinkage following gelation for the polyester resin is also beneficial in that it results in a fast pressure drop and hence frictional force reduction.

In comparison, the epoxy resin begins to shrink well in advance of gelation and gels under a condition of declining hydraulic pressure. Thus much of the hydraulic pressure is lost by the time that gelation occurs due to the effect of volumetric shrinkage. Following gelation, the rate of shrinkage is very slow such that it causes the friction forces to only reduce gradually. Thus in the gel zone for epoxy resins there is insufficient hydraulic pressure to prevent sloughing. This explains why a

i 2 2 . 8 " C Q U E N C H

Fig. 23.15 Volume change of epoxy resin during cure. (Courtesy of Shell Development Company.)

Materials 509

problem is frequently encountered when epoxy resin is substituted directly for a polyester without consideration of the curing and shrinkage properties. Hunter postulates that a simple solution has been found to compensate for these shrinkage characteristics in epoxy resin systems. The presence of fillers, whether fiber or powdered, in the resin system, reduces the amount of total volumetric shrinkage. Also the pressure from thermal expansion is directly proportional to the amount of filler or fiber reinforcement volume. Thus the increase in reinforcement to resin volume ratio, either by fiber or powder fillers, will reduce the shrinkage tendency and the hydraulic pressure will be increased at the same time. Even though the epoxy resin shrinks prior to gelation, sufficient pressure will remain to prevent sloughing. This explains why it is always beneficial to have a higher fiber to resin ratio for epoxies in the pultrusion process than for polyester resins.

23.6.5 CURE RATE

As previously mentioned, shrinkage rate is a direct result of the known cure rate of the resin. It is beneficial in the pultrusion process to have a high shrinkage rate to initiate a quick pressure drop to reduce frictional pull loads. From this point of view, it is important for the epoxy to have a fast cure rate. This will also provide a shorter gel zone which will result in a faster processing rate. Cure rates of polyesters may be varied chemically by changing the amounts and types of peroxide catalysts which are used to initiate them. It is not simple to change the cure rate of an epoxy resin chemically.

Curing agents for epoxy resins are selected based on the desired performance parameters for the epoxy in the final product or structure. Some considerations of pot life and manufacturing process also influence this selection. Thermal accelerators can be used. However, the effect of increasing cure rate accelerator

0 0

5 0

0 0

5 0

I I I I

7 5 1 0 0 1 2 5 1 5 0 1 7 5 2 0 0

G E L T I M E T E M P E R A T U R E , D E G . C

Fig. 23.16 Gel time compared with temperature of epoxy and polyester resin. (Courtesy of Shell Development Company.)

510 Pultrusion

4 0 0 0 ~

3 5 0 0 W u) 0 3 0 0 0 - n

2 5 0 0 -

0 2 0 0 0 -

1 5 0 0 -

2 w

>.

u)

2 1 0 0 0 - u) - 5 5 0 0 -

content may be to reduce the pot life. Hunter ran experiments to provide data on gel times of epoxy resin at different accelerator levels. In addition, he checked on the gel times of polyester resin using two different curing agents. Figure 23.16 shows the data which resulted from these two evaluations. The data on the epoxy resin system shows that significantly more heat is required to generate the same gel times even though the accelerator content is doubled. Figure 23.17 shows the viscosity versus time of the epoxy resin at two accelerator levels and two temperatures. This graph clearly indicates that pot life is sacri- ficed by increasing the accelerator level. Pot life is also affected by temperature and Fig. 23.17 illustrates that a small increase in temperature will reduce the time to double the initial viscosity by almost half. Heat can be generated during the mixing process of the epoxy formulations and because of this it is important to minimize the mixing times when using high shear mixers that generate heat within the body of the resin system. The implication of the experimental data presented in these figures is that the most efficient method of increasing cure rate in the epoxy resin is to increase the die temperature.

-

23.6.6 REINFORCEMENT VOLUME

The general relationship of glass fiber content to pull loads in epoxy resin system is shown in Fig. 23.18. These data were derived from an experiment in the Shell laboratories where the reinforcement volume was decreased and the pull loads were recorded until sloughing

The fiber volume was then increased until the sloughing was eliminated and was increased further until pull loads became too large. The data shows that there is a plateau in the pull load curve spanning approximately 2% of the glass fiber content range. This is the optimum level for pultruding the 12.7 mm (0.5 in) diameter epoxy rod used in the test program. Below the optimum range, sloughing occurs owing to the insufficient hydraulic pressure at the gel point. Above the optimum level, the pull loads rise owing to the high pressure both during and after the gel zone (referring back to Fig. 23.12). Both polyester and epoxy systems respond similarly to the different types of reinforcement materials which are contained within the pultruded structure. For both of these resin systems, the minimum reinforcement level to prevent sloughing when using a continuous mat and roving is somewhat less than that for an all rov-

0- P T D . A C C E L E R A T O R L E V E L A T 2 l . C

0 1 I I I I I I 1 1 1 1 1 1 I J 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3

T I M E , H

Fig. 23.17 Epoxy resin viscosity compared with time and temperature. (Courtesy of Shell Development Company.)

Materials 511

1 0 0 0

0 0 0

4

0 2 6 0 0 4

A 2 3 n

4 0 0

1 1 2 " D I A M E T E R R O U N D

P R O C E S S E D A T 1 2 " I M I N

1 1 2 V L O . S I N G L E E N D R O V I N G

4 7 - 6 4 E N D S - 1 6 x C L A Y

T O O H I G H ,

L O A D S A R E

E X C E S S I V E

- T O O L O W ,

O C C U R S

-

7 8 7 9 0 0 0 1 0 2 0 3 0 4 0 5 8 6 0 7

G L A S S C O N T E N T ( N O N - C O M B U S T I B L E S ) , % B Y W T . ' 'OF W H I C H A P P R O X I M A T E L Y 2 X IS R E S I D U A L C L A Y

Fig. 23.18 Epoxy resin pull loads compared with glass content. (Courtesy of Shell Development Company.)

ing part. Table 23.3 lists the target reinforcement volumes for the epoxy system for a variety of reinforcement systems. These values were generated by following the same procedure as for the data in Fig. 23.18. The data shown in Table 23.3 are qualitative rather than quantitative values. They may be used to estimate the required reinforcement volume.

Table 23.3 Target fiber volume ranges for epoxy pultrusion

wt.%

All glass roving reinforced composites Multi-end type rovings Single end type rovings

Glass roving and continuous mat reinforced composites

78 77-81

3.175 nun (1/8 in) thick cross sections 6.35 nun (1/4 in) thick cross sections

6447 71-74

Carbon fiber reinforced composites All unidirectional tows 67-74

(57-65 VOlYO)

23.6.7 DIE TEMPERATURE CONTROL

In polyester pultrusion, there is a wealth of prior experience7 which can be used to provide a temperature set point to produce the desired surface and internal quality of the part with the controller being a thermocouple located some short distance from the entrance to the die. This creates a situation which is independent of the actual exotherm temperature in the curing process.

For epoxy pultrusion it is vital that the peak exotherm be understood and controlled. It should not exceed 225°C (437°F) in the hottest region of the part and die. At this temperature homopolymerization will take place within the epoxy resin system and the resin does not need the curing agent to stimulate the cure. The mechanical and physical properties of the structure are degraded under these conditions by the presence of the unused curing agent. For most thin profiles (up to 12.7 mm (0.5 in) thick), a single heating zone is sufficient. The thermocouple should be located in the center

512 Pultrusion

C I T

1 2 ' /MINUTE

1 8 ' S T R I P HEATERS

1'- - TEYPERATURE O N THE SURFACE OF THE R O D ---- TEYPERATURE I N THE CENTER OF THE R O D

I 1 I I 1 I I I 0 6 1 2 1 8 2 4 30 3 6 4 2 4 8

D I E L E N G T H , I N

Fig. 23.19 Die temperature profile of single zone heating. (Courtesy of Shell Development Company.)

of the strip heater to minimize the overshoot and lag time for the temperature controller. For a 12.7 mm (0.5 in) thick cross section the temperature set point of 200°C (392°F) on the surface of the die will yield an internal peak exotherm temperature of 225°C (437°F). Figure 23.19 is a graphic example of this single strip heater profile. Figure 23.20 depicts the graph of the temperature set point versus pull loads

in the epoxy resin system. If the temperature set point is too low, the resin cure rate will decrease which increases the size of the gel zone. At the same time, the rate of shrinkage and the rate of hydraulic pressure decay is reduced and this results in more pressure within the larger gel zone which increases pull loads. As the die temperature increases, the conditions begin to favor reduced pull loads.

7 0 0 - m A

6 0 0 - 0 A

A 4 3 n 5 0 0 -

4 0 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0

D I E C O N T R O L T E M P E R A T U R E , " C

Fig. 23.20 Pull load compared with die control temperature. (Courtesy of Shell Development Company.)

Materials 513

With section thicknesses up to 12.7 mm (0.5 23.6.8 RESIN MIXING in) a two zone heating profile can be used and this will probably eliminate the need for radio frequency (RF preheating) to prevent internal cracking. The pull loads are much lower and the temperature decay rate is less than the single zone heating profile. Thus the two zone version will result in a higher degree of cure than the single zone.

For thick pultrusions with sections beyond 12.7 mm (0.5 in), the RF preheating method results in faster processing rates without cracking. Figure 23.21 illustrates the die temperature conditions for the same 12.7 mm (0.5 in) diameter rod pultruded with RF preheating. Preheating serves to reduce the temperature differential between the die entrance and the gel zone which results in less volumetric expansion due to temperature and hence less pressure. The temperature lag between the surface and the center of the part is also reduced by RF heating, thus the gel zone is smaller which reduces pulling loads. Increase of the processing rate through the machine will bring the pressure, the gel zone size and the pull loads back to normal conditions. This is how RF heating permits faster processing rates without increasing the pull loads beyond the standard levels.

The best approach to mixing resin system components is to precisely follow the stated recipe for the amount of material to be added and the degree of mixing following the addition. As the ingredients such as catalyst accelerator, pigments, viscosity and extenders, internal mold release and fillers are added, the resin is mixed for a very short time of up to one minute (Table 23.4). The curing agent is left out of the mix until the mix is ready to be added to the pultrusion resin bath. The addition of filler materials requires a high shear mixing and this should be minimized since it generates significant quantities of heat. Just prior to start up, the curing agent should be added and blended at a reduced mixer speed (high shear mixing is only needed for filler addition.) Following start up of the system, make-up replenishment resin batches should be scaled to the depletion rate for the run. If the resin consumption is 7.5 1 (two gallons) per hour, add 7.5 1 (two gallons) per hour. This addition will assist the pot life of the epoxy resin and will dilute it with fresh resin on a frequent basis. If a small bath size can be used, this will increase the dilution effect of the replenishment material. Depending on the size of the bath, this technique can enhance the pot

C I T 2 0 0 c Q

1 8 ' / M I N U T E r

1 8 ' S T R I P H E A T E R S

2 5 0 k

Y

- T E M P E R A T U R E O N THE SURFACE O F THE ROD ---- TEMPERATURE I N THE C E N T E R O F THE R O D W I- 5 0 -

01 I 1 I 1 I I I I I 0 6 1 2 1 8 2 4 30 36 4 2 4 8

D IE LENGTH, I N

Fig. 23.21 Die temperature profile with FF heating. (Courtesy of Shell Development Company.)

514 Pultrusion

life of the resin by 800% or more. A large master batch of the resin can be mixed and set aside in advance of the production run. This batch without the curing agent will be stable for up to three days. The curing agent can then be added to a small make-up batch and mixed in just prior to the addition to the system resin bath. A typical formulation used in batch mixing is shown in Table 23.4.

Even with the best of conditions in terms of a small resin bath and frequent addition of new batches, the resin mix will ultimately become too viscous for good fiber wetting. A good tip is to provide a large hole in the resin bath with an appropriate plug. This allows a quick drain and a clean and refill with only a momentary pause in the process. With this step, the pot life of the resin bath will be reset to zero. It should also be noted that the plug in the drain hole should not be threaded since a very small amount of resin can cure and lock up the threads. A preferred plug would be hard rubber with a rim, similar to the knock out drain plugs in the floor of an automobile.

23.7 START-UP PROCEDURE

A key factor in trouble free start up is to use the minimum required amount of reinforcement. However, if too much reinforcement is eliminated in start up, sloughing will take place in the die because of insufficient total pressure in the gel zone. Once the sloughing is

Table 23.4 Two part batching for long production runs

initiated there may be potential for significant build up on the surface of the die and this may be difficult to remove. The most troublesome spots in the die are the low pressure, remote areas such as a comer or a small radius. There is a standard process used in pultrusion of purging the die with pure mold release just prior to entry of the resin. Experience and recent tests have shown that the pure mold release is not necessary and may lead to related problems. The normal types and quantities of mold release recommended for use with epoxy were determined by tests at resin suppliers. Any levels of mold release in excess of those recommended will not provide additional benefit to the process. A high concentration of mold release may result in a 'squeeze off' at the die entrance which could work its way back into the resin bath. If a pre- lubrication step is used, this squeeze off resin quantity must not be allowed to get back into the resin bath.

Prior to the resin entering the bath, the die temperature must be stabilized at the set point. For most parts an initial throughput speed of 25.4-30.4 cm (10-12 in) per min is recommended until the cured stock is in the pullers, to minimize loads imposed on the dry fiber. Process rate increase can then be made gradually and for epoxy resins the processing speeds normally will not exceed 45.7 cm (18 in) per min. It would appear that 30.4-35.5 cm (12-14 in) per min provides the best combina-

Part 'A' Part 'B'

(in order of addition) [PHR, (wt.%)] 1. Epon@ resin 9310 100.00 2. Epon Curing Agent@ 9360 0.67 3. Mold Accelerator 837 Wiz Int. 1846 0.70 4. Zylac 907 0.40 Blend the above for 30 s then add clay 5. ASP400P 20.00 Blend clay for no more than 5-10 min

Epon Curing AgenP 9360

33 PHR based on resin weight OR

Recompute based on the total weight of Part 'A' (33/121.77) 100 = 27.1

and use 27.1 PHR to Part 'A'

Courtesy: Shell Chemical Co.

Additives 515

tion of processing parameters, pull loads and surface gloss. If RF preheating is used, perform the start up without it at a reduced speed 15.2-20.3 cm (6-8 in) per min to minimize pull loads. When cured stock is through the die and in the pullers, the RF preheater can gradually be started up. The resin temperature entering the die should be monitored and as it reaches about 71°C (160°F) gradually increase the processing speed. Do not recycle the resin which is squeezed off at the die entrance.

23.7.1 TROUBLESHOOTING

The standard problem encountered with epoxy resin pultrusions is poor surface finish or sloughing. The reasons for this have been discussed earlier in this chapter. If the reinforcement level is low then the cure for this is to obviously add some reinforcement. However, if the redorcement level is in accor- dance with the specifications, then additional reinforcement will increase the pull loads beyond standard. In this case the die temperature profile is probably too low. There is an instrument on the market known as the Gelstar Thermal Analyzer. The thermal analyzer can be used to obtain a temperature profile within the die. From this, the size of the gel zone can be estimated. This is proportional to the lag time between the die and internal temperature profiles. If the temperatures appear to be within limits but the lag time is too large then the processing rate is too fast for the particular cross section within the die. At this point either reduce the process rate or use RF preheating to minimize the problem. These steps will reduce the size of the gel zone.

If sloughing is encountered, the part should automatically clean itself up. The typical purge techniques that are common in the industry can be used with epoxy resins. If a portion of the die refuses to clean up, a trick is to insert a copper kitchen ‘Chore Boy’ in the area of the part which has the problem. This will often push the offending plug out or catch onto it and pull it out. The copper gauze will

provide a mild scrubbing action that will clean the surface of the die.

23.7.2 SHUT DOWN

Standard shut down procedures have been developed for polyester resins. The key step is to remove the resin bath or bypass the reinforcement around it. The dry reinforcement should be completely pulled through the die. None of it should be cut out or removed. At this point the die will be free from build up and ready for a restart.

The resin drain from the bath should be placed in a container in an area with good ven- tilation and spill protection. A metal or plastic tray with a surrounding high lip will be satis- factory for spill protection. The resin containers should only be half full. Eventually when it does exotherm, it will become hot, expand and may overflow the container if it is too full.

23.8 ADDITIVES

Inorganic fillers are used to reduce shrinkage at polymerization. They also extend the volume of the resin phase to provide a low cost formulation. These are primary additives. Fillers can frequently constitute the largest proportion of a formulation, second only to the base resin. Fillers are classified according to their particle size, as either coarse fillers or fine. Coarse fillers have an average particle size in excess of 8 pm and are generally the nonfibrous type with low surface area and low oil absorptions. They can be highly loaded into the resin and are easily wetted out by the resin system. Their disadvantage is that they tend to provide poor compound cohesiveness and to introduce localized resin rich pockets and possibly to increase fiber agglomeration during secondary molding processes (as in pull forming). The large particle size filler can be filtered out by a high density roving preform which can lead to large voids in the interior of the pultruded structure.

The most common of these fillers are calcium carbonate, aluminum silicate and

516 Pultrusion

alumina trihydrate. Silica and coarse talcs are also examples of coarse fillers. Calcium carbonate is primarily used as a volume extender to provide the lowest cost resin formulation where performance is not critical4s.

Fine fillers have an average particle size of 5 pm or less and have high surface areas which can produce high viscosities in the formulations. These fillers provide a high order of cohesiveness and will tend to lubricate the pultrusion system. They also help to reduce localized shrinkage owing to their more complete distribution within the polymer. Kaolin clays, hydrous alumina silicate, fine talc, col- loidal silica and precipitated calcium carbonate are examples of fine fillers. Clay fillers are used to improve corrosion resistance and where electrical properties are required. They also impart a superior surface finish to the pultruded product. Alumina trihydrate improves flame and smoke generation properties and occurs in applications where consumer or governmental codes are imposed to decrease flammability.

Fillers used in the pultrusion process should contain less than 0-5% free water content and should be uniform and free from contamination. Foreign material in the filler may cause localized reaction with off gassing of volatile byproducts and voids or could affect the uniformity of coloring within the pultrusion. Fillers are mixed into the resins in quantities up to 50% of the total resin formulation by weight (100 parts filler per 100 parts resin). Limits of filler addition are based on the viscosity of the system which results from the particle size of the filler and the characteristics of the resin. Wetting agents are sometimes used to add a volume of filler material without increasing formula viscosity. Wetting agents can be added to the filler by the supplier or as an additive during the formulation process for the resin. Air release agents are added in the same manner and will result in more efficient packing by reducing entrapped air in the liquid resin. They also tend to reduce void content in the finished product.

23.8.1 PIGMENTS

Pultruded products are normally associated with bright colors and these colors are normally created by adding suitable colored pigments to the resin system which are then cured into the matrix material. Pigments are generally of three types: (a) dyes; (b) organic pigments; (c) inorganic pigments. These three pigments are characterized by their own individual proper tie^^^. Dyes have good transparency and acceptable brightness. However, they have poor heat resistance and tend to migrate in solution. Organic pigments also have acceptable brightness and brilliance but are not normally as good as the dyes. Weather resistance and W absorption tends to be a problem and the colors may deteriorate and fade after long periods of W exposure.

Inorganic pigments are generally the materials of choice. These are usually natural or synthetic metallic oxides, sulfides, or other salts which are heat treated and converted to a dry powder at 600-1100°C (1112-2012°F). Inorganic pigments have superior properties of brightness to those of organic pigments or dyes and are very resistant to weather and migra- tion and have a very high stability under light exposure. The major problem, however, in incorporating these pigment additives into resin formulations are the effects which they have on the cure cycle of the resins. It is possible for them to be involved in the polymerization reaction during the curing of the resins and to become chemically attached into some of the reactive sites. This has a strong effect on the properties of the resulting composite material and will require a change in the temperature/time curing cycle. A large per- centage of pultruded composite components are used outdoors and W exposure is a problem. Under these circumstances, titanium dioxide, an excellent UV absorber and whitener, is used as a part of the formulation. Its presence would also naturally create a paler color in the pultruded composite and hence additional quantities of inorganic pigment are

normally required to make the color bright. Zinc sulfide is also a UV absorber which is frequently used in pigment systems. Both titanium dioxide and zinc sulfide seem to have little effect on the ultimate mechanical and physical properties of the pultrusions such as the strength, moduli and impact resistance.

23.8.2 STRUCTURAL CONNECTIONS

Pultruded FRP composites can be joined using various methods of assembly including mechanical fastening (with plastic or metallic bolts or screws or by doweling with dowels or rivets), mechanically interlocking connections (where molded or laminated inserts lock into the sections being fastened), adhesive bonding, or a combination of these methods50.

When structural components are assembled special attention must be given to the rigidity, geometry, fabrication and assembly requirements involved. It has been determined that mechanical fastening and/or mechanically interlocking connections are most suitable for structural connections. Some connections depending upon the geometry and the stresses developed in connection can be strengthened by also adhesive bonding in addition to mechanical fasteners. Each of these systems have their own advantages and disadvantages which are discussed in the following paragraphs.

23.8.3 MECHANICAL FASTENERS

If properly designed and fabricated, mechanically fastened connections are the most reliable method of joining pultruded structural sections. In recent years a significant amount of information through empirical test- ing and prototyping of connections has become available on the bolting and riveting of composites. There are many standard references in this field51-53.

The advantages of a mechanically fastened joint are:

Additives 517

0 surface preparation of the composites is not required;

0 inspection of the joint is relatively easy; 0 the joint can be assembled and connected

up to full strength rapidly; 0 ability to disassemble and reassemble.

The disadvantages of a mechanically fastened joint are:

in preparing the joints, holes must be drilled and sealed, structural members must be trimmed and any required gusset plates must be fabricated; drilling of holes cuts the longitudinal strength fibers in the area of the hole. This can cause high stress levels and stress concentrations; strength of the joint is dependent upon the bearing strength of the composite material; the strength of mechanically fastened joints is also dependent upon the strength of the fastener.

Fastener parameters include:

0

0

0

0

0

0

0

0

0

0

0

clamping force/installation torque limits; washer size for transmission of load; fastener size; hole size and tolerance; joint type; geometry of the fastener layout; composite thickness; rate of loading and the direction; static or dynamic loading; failure criteria; high stress concentrations around fastener.

23.8.4 ADHESIVE JOINTS

Adhesive joints have become popular for the connection of the composite materials since there is not degradation of the composite itself by the bonding process. Adhesives are usually available in solid, paste or liquid form and are classified as either inorganic or organic materials. The majority of the structural adhesives are derived from the organic group and can be

518 Pultrusion

thermosetting resins, thermoplastics, or elas- tomers. These adhesives are known for their properties of being strong, tough, insoluble and useable over a wide temperature rangeM. The primary advantages of an adhesive joint are:

0 the properties of the composite material remain intact and are not degraded;

0 adhesives are generally stronger than the composite material being bonded, consequently any failure mode is usually forced into the surrounding primary structural material; good distribution of joint stresses; adhesive bonding can be used to bond dis- similar materials; adhesive joints can be particularly forgiv- ing. Flaws in the joints do not generally degrade the strength of the joint. The main reason for this is that the critical stress location in a bonded joint is usually at the end of the overlap and flaws tend to occur in the center of the joint where the stress level is low; good fatigue and impact loading characteristics.

Disadvantages of the adhesive bonded joints are:

0 surface pretreatment such as cleaning and

0

0

0

0

0

0

0

etching must be carefully performed; the preparation and mixing of the adhesive in the correct proportions is critical. Manufacturers instructions must be followed precisely; there is a time limit or shelf life of the adhesive following its preparation; the component parts must be carefully located ;sing holding tools and fixtures and maintained in position with pressure across the joint during the curing of the adhesive;

0 while adhesive joints are normally designed and stressed in shear, there is an additional failure mode owing to peel stress. This is a tensile stress which devel- ops a maximum value at the free end of a

double or single lap joint. The stress can be particularly severe in thick, double lapped joints; adhesive bonded joints take time to cure while the resin is setting up and hardening. During the cure cycle, the component parts must be restrained in a fixed position.

0

23.8.5 MECHANICAL ADHESIVE COMBINATION

Some connections are stronger using a mechanically fastened/adhesive bonded joint. Advantages of mechanically fastened/adhesive bonded joints are:

0 higher overall capacities; 0

0

0

0 increased joint rigidity.

The disadvantage of mechanically fastened/adhesive bonded joints is that they are labor intensive.

greater resistance to environmental and thermal deteriorations; less subject to peel stress failure than ‘bonded only’ joint; improved fatigue and impact characteristics;

23.9 APPLICATIONS FOR PULTRUDED PRODUCTS

Applications for pultruded products are many and varied but generally are commercially oriented. The process shows up in the consumer and recreation market, electrical equipment products, corrosion resistance, civil engineering and construction and transportation (automotive, truck, bus and rail).

23.9.1 CONSUMER/RECREATION

The combination of strength, stiffness, fatigue resistance and aesthetic design and coloring makes pultruded products ideal for the consumer recreation market. Applications are: fishing rods, archery bows and arrows, hockey sticks, tent poles, ski poles, playground equipment, fence posts and baseball bats.

Applications for pultruded products 519

23.9.2 ELECTRICAL EQUIPMENT

In this marketplace, strength, electrical insulation and safety are primary attributes of pultruded The following are the most significant applications: fuse holders, ladders, tool handles, electrical conduits5(', cable traysy and power rail covers for subway trains.

Figure 23.22 shows a set of pultruded ladders and Fig. 23.23 shows the installation of a power rail cover for a subway train.

Fig. 23.22 Pultruded ladders. (Courtesy of Creative Pultrusions Inc.)

Fig. 23.23 Third rail cover for rapid transit rail system. (Courtesy of Creative Pultrusions Inc.)

23.9.3 CML ENGINEERING/CONSTRUCTION MARKET

In this market the properties which are required are strength, modulus, corrosion resistance and nonslip surfaces. Applications are: gratings, stairs, guard rails, bridges and platform^^^, crash barriers, ladder cages, structural supports614, sign posts and signs, light and pedestrian bridges.

Figure 23.24 shows an installation of gratings and hand rails in a chemical plant. Figure 23.25 shows a pedestrian bridge and Fig. 23.26 shows the Aberfeldy foot bridge in the UK. This latter application is one of the most interesting developments in the application of pultruded sections. This bridge was designed and erected by Maunsell Structural Plastics Division of Maunsell Engineering in London. The bridge spans the River Tay in Scotland and C O M & S

520 Pultrusion

, ,

Fig. 23.24 Grating and handrail installation. (Courtesy of North West Fibre Mechanics Ltd.)

Fig. 23.25 Pedestrian bridge in Pennsylvania. (Courtesy of Creative Pultrusions Inc.)

Fig. 23.26 Aberfeldy footbridge. (Courtesy of Maunsell Structural Plastics Ltd.)

two sections of a golf course. The bridge is a double cable stay design and all of the components are made of composite materials with the bridge decking and guard rails being pultruded products. Maunsell has also installed a composite vehicular bridge at Bonds Mill in England which was opened in 1994.

23.9.4 TRANSPORTATION MARKET

This market is potentially very large. It includes automotive, trucks, busses, light rail, subway trains and passenger trains. The products which are being pultruded for this large array of industries are as follows: drive shafts for trucks66, leaf springs67, 68, bumpers, frames and cross members, transportation container bodies, roll up doors, refrigerated truck components, frames for light rail cars, interior structure for passenger trains and subway

load carrying beds for small trucks and frangible airport approach masts.

References 521

Figure 23.27 shows a bus interior, fabricated from pultruded sections and Fig. 23.28 dis- plays a frangible airport approach mast.

23.9.5 MISCELLANEOUS

Another market which is using pultruded products is the oil and gas industry for off shore oil well platforms. Application here is for floor gratings, hand rails, stairs and storage buildings and living quarters on the platforms. Another emerging application is in the constant cross section blade for the Darius design of windmills for alternate power.

This is just a brief review Of the current aPP1i- cations. The future of the pultrusion process and its applications is only limited by the scope of human imagination. The market will continu- ously increase and it is predicted that by the year 2000 the total volume of pultruded products will have tripled over the 1995 levels.

Fig. 23.27 Bus interior showing pultruded composites. (Courtesy of Creative Pultrusions Inc.)

..” . - -----c?9.

REFERENCES

1.

2.

3.

4.

5.

6.

Goldsworthy, W. Brandt, US Patent, 2 871 911 Apparatus for Producing Elongated Articles from Fiber-reinforced Plastic Material; Issued 2/12/59. Birsa, R. and Taft, P., New Materials Approach for Providing Transverse Strength in Pultruded Shapes, RP/C Reinforced Plastics/Composites ‘84; Composites go to the Market; Papers presented at Technical Sessions of the 39th Ann. Conf., New York. Jan 16-19,1984, Session 1-A, p. 4, 627, SPI Reinforced Plastics/Composites Institute. Taft, P. and Birsa, R., Transverse Strength for Pultruded Parts, Plusf. Engng., 1984,40, (5), 634.

Barking: Elsevier Applied Science, 1986, p. 1-46, Fig. 23.28 Frangible airport approach mast. 012. (Courtesy of Creative Pultrusions Inc.)

522 Pultrusion

7. Martin, J., Pultrusion, Plastics Products Design Handbook. Part B. Processes and Design for Processes, (ed Miller, E.), New York: Marcel Dekker Inc., 1983, p. 37-74.

8. Martin, J. and Sumerak, J.E., Pultruded Composites - The Case Against Aluminum Extrusions, Pultrusion Technology, Inc., RP/C Reinforced Plastics/Composites ‘84; Composites go to the Market; Papers presented at Technical Sessions of the 39th Annual Conference, New York. Jan 16-19, 1984, Session 1-D, p. 5, Confer. 627, SPI Reinforced Plastics/ Composites Institute.

9. Laguan, O., Pultrusion: Economic Aspects, Applications and Design, Rev. Plast. Mod., 1985, 50(349), 61-6 (Spanish).

10. Spencer, R.A.P., Developments in Pultrusion, In Developments in GRP Technology - 1, (ed. Harris, B.), Barking: Applied Science, 1983, p. 1-36, 6272.

11. Martin, J.D., Pultrusion: The Other Process, Plast. Engng., 1979,35(3), 53-7.

12. Beck, D.E., New Processes and Prospects in Pultrusion, Goldsworthy Engineering Inc., Composite Solutions to Material Challenges: 38th Ann. Conf Preprint, Houston, Tex., February 7-11, 1983, Session 6-B, p. 4, Confer. 627, SPI, Reinforced Plastics/Composites Institute.

13. Roubinet, P., Curved Pultrusion, Composites Plast. Renf. Fibers. Verre Text, 24(4), July/Aug 1984,69-73 (French).

14. Goldsworthy, W. Brandt, New Technology for Continuous Reinforced Plastics Processing: Its Called ’Pulforming’ and It Permits Extrusion of Variable Cross Section Parts and Curves, Mod. Plast. Int., 1979,9(9), Sept, 100-1.

15. Goldsworthy, W. Brandt, Pulforming - The Changing Shape of Composites.

16. Ewald, G.W., Curved Pulforming - A New Manufacturing Process for Composite Automobile Springs, Working Together for Strength, 36th Ann. Conf., Washington, D.C., February 16-20, 1981, Session 16-C, p. 1-6, Confer. 012 SPI Reinforced Plastics/Composites Institute.

17. Goldsworthy, W. Brandt, Pulforming Makes Curved Pultrusions, Brit. Plast. Rubb., Nov 1985, p. 36.

18. Nepasicky, J and Kannebley, G., Advantages and Limitations of Vertical and Horizontal Pultrusion Processing. Examples of Typical Applications.

19. Beever, W.H. and O’Connor, J.E., Pultruded Thermoplastic Composite Structures, Int. SAMPE Symp. Proc., 32, 1309,1987.

20. Beever, W.H. and OConnor, J.E., Polyphenylene Sulphide Pultruded Type Composite Structure, 42nd Ann. Conf., Plastics/Composites Inst., 1987.

21. Wood, AS., Pultrusion is Poised for New Growth and It Won’t be All Thermosets, Mod. Plast. Int., 1976, 6(6), 47-9.

22. Goldsworthy, W. Brandt, Thermoplastic Composites: The New Structurals, Plast. World, 1984,42(9), 56-8.

23. Kidd, A.C., Winding and Profile Production - Tape, Filament, Pultrusion-Extrusion, Reinforced Plastics, In Proc. Electrical Symp., Bristol, Feb 1975, Paper 5, p. 18 Preprint 627-61

24. Smith, A., Pull Winding Techniques Improve Pultruded Products, Pop. Plast., 1988, 33(4),

25. Shaw Stewart, D.E., Pullwinding Conf. Proc., 2nd Int. Conf. on Automatic Composites, Paper 15, Noordwijkerhout, The Netherlands, 26-28 Sept. 1988.

26. Goldsworthy, W. Brandt, US Patent, 3 674 601 Augmented Curing of Reinforced Plastic Stock; Issued 7/4/72.

27. Goldsworthy, W. Brandt, US Patent, 3 793 108 Augmented Curing of Reinforced Plastic Stock; Issued 2/19/74.

28. Parry, T.V. and Wroksky, AS., Effect of Hydrostatic Pressure on the Tensile Properties of Pultruded CFRP, J. Mater. Sci., 1985, 20(6),

29. Bibbo, M.A. and Gutowski, T.G., Analysis of the Pulling Force in Pultrusion, Antec 86. Plastics - Value Through Technology. Proc. 44th Ann. Techn. Conf., Boston, April 28-May 1, 1986, p.

30. Anon, Pultruded Fibre-Reinforcements, Plast. News (Aust.), Nov. 1979, 20.

31. Hill, J.E., Goan, J.C. and Prescott, R., Properties of Pultruded Composite Containing High Modulus Graphite Fibers, S A M P E Qtly, 1973,4,

32. Spencer, R.A.P., Advances in Pultrusion of Carbon Fibre Composites, Carbon Fibres, 2nd Int. Conf., London, Feb 1974, Paper 21, 140-7, Confer. 51FlC.

33. Evans, D.J., Designing with Pultrusions: From the Idea to the Application, Composite Solutions to Material Challenges: 38th Ann. Conf Preprint, Houston, Tex., February 7-11,

42-3.

2141-7.

1430-2.012. SPE.

(2), 21-7.

References 523

1983, Session 6-A, p. 5, Confer. 627, SPI Reinforced Plastics/Composites Institute.

34. Browning, J., Synthetic Surface Veils for GRP Laminates, Int. Reinf. Plast. Ind., 1986, 5(5) 14/6.

35. Werner, R.I., Improvements in Means of Evaluating Weathering Characteristics of Pultrusions, Rising to the Challenge: 35th Ann. Conf. New Orleans, LA, Feb 1980, Section 4-E, p. 6, Confer. 627. SPI Reinforced Plastics/ Composites Institute.

36. Heritt, R.W., New, High Performance, Fast Curing Epoxy Resin System for Composites, Composite Solutions to Material Challenges: 38th Ann. Conf. Preprint, Houston, Tex., February 7-11,1983, Session 19-D, p. 7.627, SPI, Reinforced Plastics/Composites Institute.

37. Kershaw, J.A., New Epoxy Resin Systems for Pultrusion, Shell Development Co., Composite Solutions to Material Challenges: 38th Ann. Conf Preprint, Houston, Tex., February 7-11, 1983, Session 6-42, p. 4, 627, SPI Reinforced Plastics/Composites Institute.

38. McQuarrie, T.S., New Generation Resins for Pultrusion, 33rd Ann. Conf., Washington, D.C., Feb 1978, Section %E, p. 5, Confer. 627 SPI Reinforced Plastics/Composites Institute.

39. Howard, R.D. and Sayers, D.R., Development of New Methacrylate Resins for Use in Pultrusion, RP/C Reinforced Plastics/Composites '85. 40 Years of Innovative Technology; Proc. 40th Ann. Conf., Atlanta, GA, January 28-February 1, 1985, Paper 2-A, p. 5, 627, SPI Reinforced Plastics/Composites Institute.

40. Boinot, E and Daspet, Y., Phenolic Resins: A Boon to the Building and Transport Industries, Composites Plast. Renf. Fibres Verre Text, 26, 3, May/June 1986,97-100.

41. Lo Scalzo, E., Phenolic Resins in Advanced Composites, Mat. Plast. Elast., 1988, 3, p. 124-9 (Italian).

42. Hunger, G.A., Pultruding Epoxy Resin, Reprinted from 43rd Ann. Conf. and Focus '88. Proc. 43rd Ann. Conf. SPI Reinforced Plastics/Composites Institute

43. Kiernan, D., Tessier, N. and Schott, N., Modification of Epoxy Resins for Improved Pultrusion Processing, US Army Materials & Mechanics Research Center, RP/C Reinforced Plastics/Composites '85, 40 Years of Innovative Technology; Proc. 40th Ann. Conf., Atlanta, GA, January 28-February 1,1985, Paper 2 4 , p 6 627, SPI Reinforced Plastics/Composites Institute.

44. Sumerak, J.E., Understanding Pultrusion

Process Variables, Mod. Plast., March 1985. 45. Sumerak, J.E. and Martin, J.D., Pultrusion

Process Variables and their Effect Upon Manufacturing Capability, RP/C Reinforced Plastics/Composites '84; Composites go to the Market; Papers presented at Technical Sessions of the 39th Ann. Conf., New York. Jan 16-19 1984, Session 1-B, p. 7, Confer. 627, SPI Reinforced Plastics/Composites Institute.

46. Sumerak, J.E., Understanding Pultrusion Process Variables, Mod. Plast., 1985,62(3), 58-64.

47. Sumerak, J.E., Understanding Pultrusion Process Variables for the First Time, Conference, Atlanta, GA, January 28-February 1, 1985, Paper No 2-B, p. 8, Confer. 627, SPI Reinforced Plastics/Composites Institute.

48. Armstrong, R.F., Calcium Carbonate, Encyclopedia of Chemical Technology, Vol. 4, 2nd ed., Wiley Interscience, New York, 1964.

49. Anderson, R. and Riddel, R., Effects of Pigments on Pultrusion Physical Properties and Performance, Molded Fiberglass Co.; Morrison Molder Fiberglass Co., Composite Solutions to Material Challenges: 38th Ann. Conf Preprint, Houston, Tex., February 7-11, 1983, Session 6-H, p. 4, Confer. 627, SPI Reinforced Plastics/ Composites Institute.

50. Rufolo, A., Design Manual for Jointing of Glass Reinforced Plastics, US Naval Material Laboratory Report, Navship 250-6341, August 1951.

51. Hodgkinson, J.M., de Beer, D.L. and Matthews, EL., The Strength of Bolted Joints in Kevlar R.P., Proc. Composite Design for Space Application, The Netherlands, (esa SP-243) 15-18, October 1985.

52. Hollaway, L. and Baker, S., The Development of Nodal Joints Suitable for Double Layer Skeletal System made from Fibre/Matrix Composites, Part 7, Paper 21, Proc. 3rd Intern. Conf. on Space Structures (ed. Nooshin, H.), Barking: Elsevier Applied Science, 1984.

53. Green, A.K. and Phillips, L.N., Crimp-Bonded End-Fittings for Use on Pultruded Composite Sections, Composites, 1982,13(3), 219-24.

54. Hart-Smith, L.J., Adhesively Bonded Joints for Fibrous Composite Structures, Douglas paper 7740, Long Beach, California, 1986.

55. Anderson, R., Use of Pultruded Reinforced Plastics in Energy Generations and Energy Related Applications, Working Together for Strength, 36th Ann. C o d , Washington, D.C., February 16-20, 1981, Session 22-B, p. 1-3,

524 Pultrusion

Confer. 012. SPI Reinforced Plastics/Composites Institute.

56. Morara, F. and Eva, G., GRP Conduits and Poles, Agrosistemi, Macplas, 1985, 10(67),

57. Pultrusions for Cable Rack, Brockhouse Group, Eur. Plast. News, 1982, 9(4), p. 36.

58. Mallick, P.K., Qiauw, L.K. and Fesko, D.G., Design and Evaluation of a Pultruded Hybrid Beam, Working Together for Strength, 36th Ann. Conf., Washington, D.C., Febraury 16-20, 1981, Session 1 7 4 , p. 1-5, Confer. 012. SPI Reinforced Plastics/Composites Institute.

59. Head, P.R., GRP Walkway Membranes for Bridge Access and Protection, 13th Reinforced Plastics Congress, 1982, Brighton, November 8-11,1982, Paper 20,97-91, BFP Publn. 293, BPF, Reinforced Plastics Group.

60. Head, P.R., Pultruded Box Beams, Fibreforce Composites, Ltd.; Maunsell Structural Plastics; UK Dept. of Transport; Windfoil Ltd.

61. Anderson, R.A. and Thomas, C., Development of Large Hollow Rectangular Tubes for Structural and Electrical Markets - A Unique Application for Pultrusion, Rising to the Challenge: 25th Ann. Conf., New Orleans, LA, Feb 1980, Section &A, p. 5, Confer. 627. SPI Reinforced Plastics/Composites Institute.

136-9.

62. Starr, T.F., Structural Applications for Pultruded Profiles, TECHNOLEX, Composite Structures 2; Proc. 2nd Intern. Conf. Composite Structures, Paisley, September 1616,1983, p. 192-216,627.

63. Tickle, J.D., Halliday, G.A., Lazzarou, J. and Riseborough, B., Designing Structures With Pultruded Fibre Glass Reinforced Plastic Structural Profiles as Compared to Standard Steel Profiles, 33rd Ann. Conf., Washington, D.C., Feb 1978, Section SF, p. 8, Confer. 627.SPI Reinforced Plastics/Composites Inst.

64. Owens-Corning Fiberglas Europe SA, Fiberglas in Action: FRP Lighting Poles, Burssels, 1977, Publn, 13-Ch. 4-5, p / 4 12 ins. 16/2/77 6272-6R.

65. Mutch, W., Composite Utility Pole, Plast. World 1987,45, (8), 43.

66. Kliger, H.S., Yates, D.N. and Davis, G.C.R., Driveshafts: The Next Step for Composites?, Aufomot. Engng, 1980,88(3), 1OC-3.

67. Roubinet, P. and Delacroix, B., Industrial Development of Composite Leaf Springs, Composites Plast. RenJ Fibres. Verre Text., 26,(3), May/June 1986, p. 79-83 (French).

68. de Goncourt, L. and Sayers, K.H., Composite Spring Systems, Composites Plast., Rent Fibres Verre Text, 1988,28(3), 145-50 (French).

69. BTR Permali RP Ltd, Pultrusion Protects Passengers, Europ. Plast. News, 15(3), 1988,46.

40207_23

Documents