Plastic Product Material and Process Selection Handbook Part 15 pdf

This is a method of making chopped fiber mats of complex shapes that are to be used as reinforcements in different RP molding fabricating processes rather than conventional flat mats tha

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There are also flexible RP These RTP elastomeric materials provide special engineered products such as conveyor belts, mechanical belts, high temperature or chemical resistant suits, wire and cable insulation, and architectural designed shapes

Each process provides capabilities such as meeting production quantity (small to large quantities a n d / o r shapes), performance requirements, proper ratio of reinforcement to matrix, fiber orientation, reliability/ quality control, surface finish, materials used, quantity, tolerance, time schedule, and so forth versus cost (equipment, labor, utilities, etc.) There are products when only one process can be used but there can be applications where different processes can be used

Preform Process

The preform process has been used since the 1940s As time passed significant improvements occurred processing-wise, equipmcnt-wise, plastic-wise, and cost-wise This is a method of making chopped fiber mats of complex shapes that are to be used as reinforcements in different RP molding fabricating processes rather than conventional flat mats that may tear, wrinkle, or give uneven glass distribution when producing complex shapes in a mold Most of the reinforcement used is glass fiber rovings They arc desirable where the product to be molded

is deep or very complex shapewise Oriented patterns can be incorporated in the prcforms Different methods arc used with each having many different modifications They include a plenum chamber, directed fiber, and water slurry

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The rovings in a plenum chamber are fed into a cutter and after being cut to the desired lengths, fall either into a plenum chamber or perforated screen where the air is exhausted from under the screen A plastic binder of usually up to 5wt% is applied and is later cured As the glass falls into the plenum chamber, the air flow pattern and baffles inside the screen control its distribution Preform screen rotates and sometimes is tilted to ensure maximizing uniform deposits of the roving With the directed fiber system strands are blown onto a rotating preform screen from a flexible hose Roving is directed into a chopper where air flow moves it to a preform screen Use can be made of a vertical or horizontal rotating turntable This process requires a rather high degree of skill on the part of the operator; however, automated robots are used to provide a controlled system producing quality preforms

With water slurry chopped strands are in water (similar to that used by the paper pulp industry for centuries) It produces intricate shaped preforms that are tough and self-supporting Bonding together the preform can use cellulose fibers a n d / o r bonding resins Where maximum strength is not required, the cellulose content can be sufficiently high The fibers can be dyed during the slurry process

The correct manufacture of the screen is important for success Different shapes can be used to meet different product designs Recognize that cylindrical preforms are easier and less costly to produce than box-like sections Also it is important to recognize that during the rotation of a cylindrical part, the fibrous glass will flow uniformly onto the screen because most sections move at a uniform linear rate With a rectangular section it is difficult because the comers rotate in a wider circle than do the center sections and because the air flow is lowest at the corners Contouring the box shape can improve reinforcement distribution

Preform screens are usually made from 16-gauge perforated material with 1/8 in holes on 3/16 in centers This produces about 40% open area For some operations, a more open area is required Perforation patterns are also used to develop specifically designed reinforcement directional properties The screen is usually designed so that the outside contour is identical with the contour of the mating half of the mold A screen which is not of the correct size will cause a great deal of difficulty in the molding operation If the screen is too small, the preform will tear during the molding If too large, wrinkling and overlapping of the preform will result

The preform is usually heavy on the fiat top and light on the edges and corners Internal baffles may be added in the preform screen to control

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the airflow, thus giving a more uniform deposition of glass The exact area of the baffle usually has to be worked out on a trial-and-error basis until experience is developed Close cooperation with the preform- machine manufacturer is helpful

When molding a product with a variable wall thiclmess, it is possible to vary the thickness of the preform This is usually accomplished by baffling Another approach that can be used is to completely block off areas where no fiber is desired This action saves material that would otherwise be trimmed off and probably discarded It has also proven practical to combine two or more preforms into one molded part This technique is very useful where the thiclmess of the molded part prohibits the collection of the preform in one piece

Conventional Process

The more conventional processes used for unreinforced plastics also use RPs They include those reviewed in this b o o k - namely injection molding (IM), extrusion (EX), thermoforming (TF), foaming, calendering, coating, casting, reaction injection molding (RIM), rotational molding (RM), compression molding (CM), reaction injection molding, rotational molding, and others (Chapters 4 to 14 and 16) These processes arc usually limited to using short reinforcing fibers however there are processes that can use long fibers 21~ Since glass fibers are extensively used, specifically in IM, the glass fibers will cause wear of metals during processing such as plasticating barrels and molds

or dies Using appropriate metals that can provide a degree of extending their operating time can reduce this wear (Chapter 17) Information on processes used to fabricate RP products follows 37, 292,293

Compression Molding

TS plastics in reinforced sheets and compounds are usually used Also used arc reinforced thermoplastic sheets and compounds With TSs compression molding (CM) can use preheated material (dielectric heater, etc.) that is placed in a heated mold cavity The mold is closed under pressure causing the material to flow and completely fill the cavity Chemical crosslinking occurs solidifying the TS molding material

The closed mold shapes the material usually by heat and pressure With special additives the TS material can cure at room temperature It would have a time limit (pot life) prior to curing and hardening Based

on the compound's preparation, sufficient time is allowed to store and handle the compound prior to its chemical reaction curing action occurring (Chapter 14)

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Depending on what plastic is being molded, the clamping force may be from contact to over thousands of tons TS polyesters usually have just contact pressure There are plastics requiring pressure A force is also required to open the mold that is usually much less than the clamping force So one has to ensure that available opening clamping pressure is obtainaable Usually this requirement is not a problem Clamping pre- dominantly use hydraulic systems Also becoming popular are all electric drive systems a n d / o r with hydraulic/electrical hybrid systems The actual mechanical mechanisms range from toggle to straight ram systems Each of these different systems has their individual advantages (Chapter 4)

The mold is fastened on the platens These platens usually include a mold-mounting pattern of bolt holes or "T" slots; standard pattern is recommended by SPI Platens range from the usual parallel design to other configurations meeting different requirements The parallel type can include one or more floating platens located between the stationary and normal moveable platens resulting in two or more daylight openings where two or more molds or fiat laminates can be used simultaneously during one machine operating cycle

There are presses that include shuttle (molds in which usually two, or more, are moved so that one mold is positioned to receive material and then moves to the press permitting another mold to receive material with this cycle repeating; result is to permit insert molding, reduce molding cycle, etc.), rotary or carousal system, and "book" opening or tilting press.< 279

Applying vacuum in a mold cavity can be very beneficial in molding plastics at low pressures Press can include a vacuum chamber around or within the mold providing removal o f air and other gases from the cavity(s)

Flexible Hunger

This process is a take-off from compression molding that uses solid material male and female matching mold halves This unique process uses a precision-made, solid shaped heated cavity and a flexible plunger that is usually made of hard rubber or TS polyurethane This two-part system can be m o u n t e d in a press, either hydraulic or air-actuated Rather excellent product qualities are possible at fairly low production rates The reinforcement can be positioned in the cavity and the liquid

TS resin is poured on it Also used are prepregs, BMC, and SMC

The plug is forced into the cavity and the product is cured The plunger

is somewhat deeper and narrower than the cavity It is tapered in such a manner that contact occurs first in the lowest part of the mold

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Ultimate pressure usually used is up to 400 to 700 kPa (58 to 100 psi)

in the plunger, this causes the contact area to expand radially toward the rim of the cavity, thereby forcing the resin and air ahead of it through the reinforcement with the target of developing a void free product The pressure conforms to irregularities in the lay-up, permits wall thickness to be varied within reasonable limits, and makes a good surface possible against a metal mold surface The fact that the heat can

be applied only from the cavity side leads to long cure cycles, but the same factor tends to produce resin richness, and consequently greater smoothness on the outside of the molding

Flexible Bag Molding

An air inflated-pressurized flexible-type envelope can replace the plunger This process provides higher glass content and decreases chance of voids Limitations include extensive trimming and only one good surface

Laminate

This refers to many different fabricated RP products such as high or

c o n t a c t / l o w pressure laminates It usually identifies flat or curved panels using high pressure rather than contact or low pressure It is a product made by bonding together two or more layers of laminate materials The usual resins are thermoset such as epoxies, phenolics, melamines, and TS polyesters A modification of this process uses TPs The type of materials can be endless depending on market require- ments Included are one or more combinations of different woven

a n d / o r nonwoven fabrics, aluminum, steel, paper, plastic film, etc High pressure laminates generally use pre-loaded (prepreg) RP sheets

in a hot mold at pressures in excess of 7 MPa (1015 psi) Compression multi platen presses are used; up to at least 30 platens producing the flat (also curved) sheets at high production rates Laminates are molded between each platen simultaneously Automatic systems can be used to feed material simultaneously between each platen opening and in turn after curing and the multiple platens open cured products arc auto- matically removed The contact or low pressure laminates use prepregs that cure at low pressures such as TS polyester resins Depending on the resin formulation just contact pressure is only required such as using hand operated rollers The usual highest pressure that identifies low pressure laminates is at 350 kPa (50 psi)

In the industry, for almost a century these laminates are used for their electrical properties, impact strength, wearing qualities, chemical resistance, decorative panels, or other characteristics depending on fiber-resin used with or without a surfacing material They arc used for

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printed circuit boards, electrical insulation, decorative panels, mechanical paneling, etc The major change in the process about a half- century ago was making the operation completely automatic, this significantly reduced labor cost

Hand Lay-Up

This low cost process has different names such as open, contact, or bag molding (due to different market uses at times different processing names are used that overlap a process) It is a very simple and most versatile process for producing RP products However, it is slow and is usually very labor intensive It consists of hand tailoring and placing of layers of (usually glass fiber) fibrous reinforcements either random oriented mat, woven roving, or fabric on a one-piece mold and simultaneously saturating the layers with a liquid plastic (usually TS polyester) (Figure 15.7) Usually it is required to coat the mold cavity with a parting agent Gel coatings with or without very thin woven or mat glass fiber scrim rcinforcement arc also applied to provide smooth and attractive surfaces Molds can be made of inexpensive metal, plaster, RP, wood, etc (Chapter 17)

Figure t 5,7 Layout of reinforcement is designed to meet structural requirements

Depending on the resin preparation, the material in or around a mold can be cured with or without heat, and commonly without pressure Curing needs include room tcmpcrature conditions, heat sources, vacuum bags, pressure bags, autoclaves, etc An alternative is to use preimpregnated, B-stage TS polyester or sheet molding compound (SMC), but in this case heat is applied with low pressure via a impermeable sheet over the material This process can produce compact

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structures that meet tight thickness tolerance simulating injection molded products

Generally, the process only requires low-cost equipment that is not automated However, automated systems are used Automation includes cutting and providing the layout of the cut prepreg in a mold

In turn, the designed RP assembly is delivered to a curing station such

as an oven or autoclave

This process can be recommended for prototype products, products with small to large production runs, molding very large and complex products, and products that require high strength and reliability The size of the product that can be made is limited by the size of the curing oven However, outdoor UV via outdoor sunlight curing or room temperature curing plastic systems permits practically unlimited product size Alternate curing methods are used that include induction, infusion (vacuum-pressure), dielectric microwave, xenon, UV, electron beam, or gamma radiation

The general process of hand molding can be subdivided into specific molding methods such as those that follow The terms of some of these methods as well as others reviewed here overlap the same technology; the different terms are derived from different sections of the RP and other industries

Vacuum Bag Molding

This process also called just bag molding It is the conventional hand lay-up or spray-up that is allowed to cure without the use of external pressure For many applications this is sufficient, but maximum consolidation may not be reached There can be some porosity; fibers may not fit closely into internal corners with sharp radii but tend to spring back Resin-rich a n d / o r resin-starved areas may occur because of draining, even with thixotropic agents With moderate pressure (hand rollers, etc.) these defects or limitations can be overcome with significant improvement in mechanical properties

One way to apply such moderate pressure is to enclose the wet-liquid resin material and mold in a flexible membrane or bag, and draw a vacuum inside the enclosure Atmospheric pressure on the outside then presses the bag or membrane uniformly against the wet lay-up An effective pressure

of 69-283 kPa (10 to 14 psi) is applied to the product Air is mechanically worked out of the lay-up by hand usually using serrated rollers The vacuum directly helps to remove air in the wet lay-up via techniques such

as using bleeder channels within the bag (using material such as jute, glass wool, etc.) to aid in the removal of air and also to permit drainage of any excess resin This layup is than exposed to heat using an oven or heat lamp

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Vacuum Bag Molding and Pressure

To maximize properties in the product, higher pressure is needed in the conventional vacuum bag system A second envelope can be placed around the whole assemblage Air under pressure is admitted between the inner bag and the outer envelope after the initial vacuum cycle is completed Still higher uniform pressures can be obtained by placing the vacuum assemblage in an autoclave By this technique, an initial vacuum may or may not be employed Using an autoclave assures good results

Pressure Bag Molding

This process is used when more pressure is required than those processes just reviewed A second envelope (or structure) is placed around the whole assemblage and air pressure admitted between the inner bag and outer envelope, or between the inner bag and structure Application of pressure (air, steam, or water) forces the bag against the product to apply pressure while the product cures Using this combination of vacuum and pressure bags results in ease of air or gas removal and higher pressures resulting in more densification

Autoclave Molding

Very high pressures can be obtained for processing RPs by placing a pressure or vacuum bag molding assemblage in an autoclave This curing process may or may not employ an initial vacuum Some of the different RP processes are used in conjunction with the use of an autoclave oven H o t air or steam pressures of 0.36 to 1380 MPa (50 to

200 psi) is used The higher pressure will yield denser products If still higher pressures are required (avoid this approach unless you have considered the danger of extremely high pressures), a hydroclave may

be used, employing water pressures as high as 70 MPa (10,150 psi) The bag must be well sealed to prevent infiltration of high pressure air, steam,

a n d / o r water into the molded product In all these approaches, the fluid pressure adjusts to irregularities in the lay-up and remains effective during all phases of the resin cure, even though the resin may shrink Use of this process includes seamless containers, tanks, pipes, etc

Autoclave Press Clave

This process simulates autoclave by using the platens of a press to seal the ends of open chamber It provides both the force required to prevent loss of the pressurized medium and the heat required to cure the RP inside

Wet Lay-Up

This procedure is usually just called bag molding It is a method that is sometimes combined with bag molding to enhance the properties Because it is difficult to wet out dry fibers with too little resin, initial

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volumetric fraction ratios of resin to fiber are seldom less than 2:1 On a weight basis the ratio is about 1:1 Liquid catalyzed resin is hand- worked or automatically worked into the fibers to ensure wet-out of fibers and reduce or eliminate entrapped air

Bag Molding Hinterspritzen

This patented process allows virgin or recycled thermoplastics such as

PP, PC/ABS, etc to thermally bond with the bacldng of multilayer PP based fabrics providing good elasticity This one step molding technique provides a low cost approach for in-mold fabric lamination that range from simple to complex shapes

Contact Molding

Also called open molding or contact pressure molding It is a process for molding RPs in which the reinforcement and plastic are placed in a mold cavity Depending on the plastic used, cure is either at room temperature using a catalyst-promoter system or by heating in an oven without pressure or using very little (contact) pressure Contact molding gave rise to bag molding, hand lay-up or open-mold, and low- pressure molding It plays a significant role in molding RPs It is difficult to surpass if a few products are to be made at the lowest cost The process is basically what was reviewed for Bag Molding

Filament Winding

Filament winding (FW) is a fabrication technique for forming reinforced plastic parts of high strength/modulus and lightweight It is made possible by exploiting the remarkable strength properties of their continuous fibers or filaments encased in a matrix of a resinous material For this process, the reinforcement consists of filamentous non-metallic

or metallic materials processed either in fibrous or tape forms 488, 489 Frequently used is some form of glass: continuous filaments roving, yarn, or tape The glass filaments, in whatever forms are encased in a plastic matrix, either wetted out immediately before winding (wet process) or impregnated ahead of time (preimpregnated process) The plastic fundamentally contains the reinforcement, holding it in place, sealing it from mechanical damage, and protecting it from environ- mental deterioration The reinforcement-matrix combination is wound continuously on a form or mandrel whose shape corresponds to the inner structure of the part being fabricated After curing of the matrix, the form may be discarded or it may be used as an integral part of the structural item

Reinforcements have set pattern lay-ups to meet performance requirements (Figure 15.8) Target is to have them uniformly stressed

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Figure 1 5~8 Views of fiber filament wound isotensoid pattern of the reinforcing fibers without

plastic (left)and with resin cured

In winding cylindrical pressure vessels, tanks, or rocket motors, two winding angles are generally used One angle is determined by the problem of winding the dome integrally with the cylinder Its mag- nitude is a function of the geometry of the dome These windings also pick up the longitudinal stresses The other windings are circumferential

or 90 ~ to the axes of the case and provide hoop strength for the cylindrical section

It is possible to wind domes with a single polar port integrally with a cylinder comparatively easily without the necessity of cutting filaments Cutting is obviously not desirable, since it interrupts the continuity of the basically orthotropic material The usual procedure in winding multiported domes is to add interlaminate reinforcements during the winding operation where the ports arc to be located

It is possible to wind integrally most of the bodies of revolution, such as spheres, oblate spheres, and torroids Each application, however, requires a study to insure that the winding geometry satisfies the membrane forces induced by the configuration being wound

FW can be carried out on specially designed automatic machines Precise control of the winding pattern and direction of the filaments are required for maximum strength, which can be achieved only with controlled machine operation The equipment in use permits the fabrication of parts in accordance with properly designed parameters so that the reinforced filamentous wetting system is in complete balance and optimal strength is obtained The maximum strength is achieved when filaments in tension carry all major stresses Under proper design and controlled fabrication, hoop tensile strengths of filament wound items can bc achieved of over 3,500 MPa (508,000 psi), although strength of 1,500 MPa (218,000 psi) is more frequently achieved

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Since this fabrication technique allows production of strong, light- weight parts, it has proved particularly useful for components of structures of commercial and industrial usefulness and for aerospace, hydrospace, and military applications Both the reinforcement and the matrix can be tailor-made to satisfy almost any property demand This aid in widening the applicability of FW to the production of almost any item wherein the strength to weight ratio is important FW is used in different shapes such as the usual circular and elliptical shape to produce rectangular shapes

FW structures present certain problems because of the lack of ductility

in the glass reinforcement These can be partially solved by proper design and fabrication procedures Reinforcements other than glass can

be used to obtain good ductility, but some of these have lower temperature strength and characteristics Proper construction constitutes

a well-proved means of utilizing an intrinsically nonductile reinforce- ment to obtain a high degree of confidence in the structural integrity of the end product Since glass has high strength and is a relatively low- cost product, glass filaments are still the major reinforcing material Other filaments for applications requiring properties such as higher temperatures or greater stiffness include quartz, carbon, graphite, ceramics, and metals alone or in combinations that include glass fibers

A further difficulty with the basic materials is that they do not lend themselves readily to simple concepts and to simple comparisons The matrix components are essentially the same plastics as those used for conventional reinforced plastic laminates Epoxy plastics are more widely used than others, although phenolics and silicones give structures with higher temperature properties Thermoset polyesters are used for many commercial structures in which cost is a problem and high temperatures do not prevail

For certain FW vessels the low modulus of elasticity of the glass-plastic material is a serious disadvantage Only moderate improvements in modulus of elasticity by modifications in glass composition or in processing tend to be feasible Any significant improvement in modulus

of elasticity requires changes in the glass composition There are effective additives to the glass to increase its modulus without pro- portional increase in density such as beryllium oxide

Interlaminar shear constitutes possible limitations on FW parts Although the absence of interweaving (such as fabrics) boosts tensile strength by eliminating cross fraying, shear strength is limited by the bonding of the reinforcement to the plastic In conventional woven cloth laminates, the high points of one layer tend to interlock with the

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low points of adjacent layers This results in strengthening of the composite against shear failure Compared to other plastics or matrices epoxy gives better interlaminar shear because of its inherently better bonding By proper design, the low values of interlaminar shear can be minimized

FW structures have lower ultimate bearing strengths than conventional laminates, for they are more rigid and less ductile Accordingly, they have less ability to absorb stress concentrations around holes and cut- outs The original higher tensile strength permits allowable design stresses under these conditions Since cutting, drilling, or grooving for attachments or access openings reduce the high mechanical strength of filament wound structures, proper design is necessary Damaging machining operations are to be avoided after final curing of the part Destructive "cut-outs" or attachment holes are to be eliminated by incorporating the use of premolded plastic or metal inserts into the designs

Techniques cannot be used for every structural element The shape of the part must permit removal of the winding mandrel after final curing Reversed curvatures should be eliminated whenever possible, since it is difficult to wind them and hold the filaments under tension In order to meet this problem, fusible, expandable, and multiparty mandrels are often required

The cost of FW parts is low only when volume production is achievable Manufacturing processes should be mechanized and completely automated to obtain, by extensive and careful tooling, the close tolerances which are required in filament wound structures to meet high-strength but low-cost objectives Precision winders with carefully selected mandrels and speed controls, special curing ovens, and matched grinders are required It takes time to develop this equipment, and a high initial investment is necessary Once the original tooling cost has been amortized, the unit cost of individual filament wound parts becomes relatively low, since the basic materials have a low cost

as in the longitudinal direction 1

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Injection Molding

As reviewed over 50wt% of all RPs go through conventional injection molding machines (IMMs) Practically all thermoplastics are used Both short and long glass and other fibers arc injection molded The RP compounds that are thick and pasty (BMC, etc.) are principally processed through ram IMMs with some going through screw IMMs (Chapter 4)

Marco Process

During the 1940s to 1960s this process was extensively used to fabricate many different RP products It was the take-off for resin transfer molding (RTM) and bag molding (BagM) Reinforcements are laid up in any desired pattern as in R T M and BagM Low cost matched molds (wood, etc.) confine the reinforcement In this process the usual liquid catalyzed TS polyester surrounds the mold in its open trough (Figure 15.9) From a central opening (hole) in one of the mold halves

a pressure is applied so that the plastic flows through the reinforce- ments With proper wet-out of fibers voids are eliminated

Figure 1 5 9 Use is made of vacuum, pressure, or pressure-vacuum in the Marco process

This method when first used was the reverse of RTM By 1960 the Marco method used vacuum pressure at the parting line and also used a vacuum for a push-pull action where pressure was applied in the center hole similar to what is now used in RTM Pressure was applied through the center hole alone or in a combination with a vacuum from the trough area to aid the flow of the liquid plastic

15 Reinforced plastic 487 Pultrusion

This process can produce products that meet very high structural require- ments, high weight-to-strength performances, electrical requirements, etc

It is a continuous process for fabricating RPs that usually have a constant cross sectional shape (I-, U-, H-, and other shapes) The reinforcing fibers are pulled through a plastic (usually TS) liquid impregnation bath through rollers, etc and then through a shaping die followed with a curing action The material most commonly used is TS-polycster with glass fiber Other plastics, such as epoxy and polyurethane are used where their improved properties are needed When required, fiber material in mat or woven form is added for cross-ply properties

There are also systems eliminating the plastic bath so that the plastic is impregnated in the die This approach is a take-off in extruding wire and cable coating systems providing controlled impregnation (Chapter 5) Cleverly designed die have been used that include rotating sections providing complex pultruded products

In contrast to extrusion, in this process a combination of liquid plastic and continuous fibers (or combined with short fibers) is pulled continuously through a heated die of the shape required for continuous profiles Glass content typically ranges from 25 to 75wt% for sheet and shapes, and at least 75% for rods RP shapes include I-beams, L- channels, tubes, angles, rods, sheets, etc

Reactive Liquid Molding

Reactive liquid molding (RLM) proceeds in two steps: (1) preform formation by organizing loose fibers into a shaped preform, and (2) impregnation of the fibers with a low viscosity reacting liquid Heat transfer in the mold may thermally activate the reacting material or mixing activated by impingement of two reactive streams as in the polymerization of polymers (Chapter 1) Simulations of flow and reaction, a relatively recent innovation in RLM, allow determination of vent and weld line locations, fill times, and control of racetracking in terms of gate locations when injected molded, mat permeability, and processing conditions Commercial success requires (1) fast reaction and (2) efficient preform formation Using higher mold temperatures and preheating the preform can decrease cycle time for thermally active systems Low pressure and temperature processing by RLM allow the use of inexpensive lightweight tools, especially for prototyping RLCM allows customizing reinforcement to give desired local properties and part consolidation via complex 3D geometries

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Resin Transfer Molding

Resin transfer molding (RTM) includes the use of reinforcements (RRTM) It is a closed mold, low-pressure process in which a preplaced dry reinforcement fiber construction (such as woven and nonwoven fabric or a fiber preform) with or without decorative surface material is impregnated with a liquid plastic through an opening in the center area

of a mold (Figure 15.10) The resin at about 50 psi (0.3 MPa) pressure moves through the reinforcement located in the mold cavity The air inside the cavity is displaced by the advancing resin front, and escapes through vents located at the high points or the last areas of the mold to

be filled When the mold has filled, the vents and the resin inlet(s) are closed After curing via room temperature hardeners a n d / o r heat, the part is removed This process provides a rather simple approach to molding designed RP parts in relatively low-cost molds (using low pressure), and the molds are manufactured in a short time

Figure 15 t 0 Cut away example of a mold used for resin transfer molding

Rotational Molding

In rotational molding (RM), a solid (powder or pellet) or liquid with or without reinforcing fibers and principally TPs are used (Chapter 13) With reinforcement is is called RRM Reinforcement is placed in a mold that only has a cavity to form the outside of the part to be made The mold is rotated simultaneously about two axis at similar or different speeds depending on the part configuration The material is forced against the walls of the cavity It first goes through a heating period to

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melt the plastic followed by a cooling period to solidify the plastic Small to large parts are molded Because they are not subjected to pressure, relatively low cost molds can be used

Squeeze Molding

This method is a take off between RTM and hand lay-up The reinforcement and a room temperature curing TS polyester resin are put into a mold In turn, the mold is put into an air pressure bag where the resin is slowly forced through the reinforcement in the mold cavity

at low pressures of about 200 to 500 kPa (30 to 75 psi) The RP is cured at room temperature in unheated molds It is a slow process so one or a few products per day are usually molded

Infusion Molding

RTM can also incorporate vacuum to assist plastic melt flow With vacuum-assisted R T M the process is called infusion molding 3~176 This process could be identified as a take-off to the Marco process 3

SCRIMP Process

The Seeman Composites Resin Infusion Process (SCRIMP | is a gas- assist resin transfer molding process Glass fiber fabrics/thermoset vinyl ester polyester plastic and polyurethane foam panels (for insulation) are usually used They are placed in a segmented tool A vacuum is pulled with a bag so that a huge amount of plastic can be drawn into the mold

It is similar to various reinforced plastics molding processes It is adaptable to fabricating large RP products such as a transportation bus weigh about 10,000 kg (22,000 lb) that is 3200 kg (7000 lb) lighter than steel units

Soluble Core Molding

This technology is also called fusible core, soluble core technology (SCT), lost-wax, loss core, etc molding This technique is a take off and similar to the lost wax molding process used during the ancient Egyptian times fabricating jewelry In this process, a core is usually molded of a low-melting-point eutectic alloy (zinc, tin), water-soluble

TP, wax formation, etc During core installation, it can be supported by the mold core pins, spiders, etc The core is inserted in a mold (IM,

CM, casting, etc.) and plastic injected or located around the core When plastic has solidified and is removed from the mold, the core is removed by melting at a temperature below the plastic melting point through an existing opening or will require drilling a hole in the plastic

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Lost- Wax Process

When this soluble fusible core molding technique was first used it involved a bar of wax wrapped with RPs (such as glass fiber-TS polyester resin) After the RP is cured (bag molding, oven, autoclave, etc.) in a restricted mold to keep the required shape, the wax is removed at low heat by drilling a hole or removing the ends The result

is very high strength RP product Its shape can be rectangular, round, curved, etc This process was used during 1944 to fabricate the first all plastic airplane using the bag molding process fabricating principally RP sandwich monocoque construction 1, 424

Spray-Up

This process has been a popular system with RP production for over half century With time passing, significant new developments occur particularly in the spraying equipment An air spray gun includes a roller cutter that chops usually glass fiber rovings to a controlled short length before being blown in a random pattern onto a surface of the mold This action can be manual or automatic Suppliers of spray-up equipment continue to produce cleaner, reduced styrene emissions (as low as 2.2%), higher capacity, more uniform spray pattern, and more versatile.29s, 442 Types and performances of spray guns are many such as external or internal mixing gun, distributive/turbulent mixing gun, air atomized, airless, etc

As the fibers leave the spray gun simultaneously the gun sprays the usual catalyzed TS polyester plastic (with styrene monomer, Chapter 2) The chopped fibers are plastic coated as they exit the gun's nozzle The resulting, rather fluffy, RP mass is consolidated with serrated rollers to squeeze out air and reduce or eliminate voids A closed mold with appropriate temperature and pressure produce products

The reinforced plastic sheet material is prccut to the required size depending on the part size to be molded The precut sheet is preheated

in an oven, the heat required depends on the TP used [such as PP or