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Manufacturing Methods of Polymer Matrix Composites

Dr. P. Alagusundaramoorthy Professor, Structural Engineering Division, IIT Madras, Chennai – 600 036

aspara0@iitm.ac.in 1. INTRODUCTION

The design benefit of polymer matrix composites (PMC) is the ability to select the type of

reinforcements, quantity and orientation required to meet the specific mechanical and

physical requirements of the composites at end-use. However, if there is no practical or cost-

effective way to place the optimum reinforcement during fabrication due to limitations of the

process, full design benefits in the application of polymer matrix composites may be

impractical or unattainable. To meet the wide range of needs that may be required in

fabricating composite products, the composite industry has evolved over a dozen separate

manufacturing processes as well as a number of hybrid processes. Each of these processes

offers advantages and specific benefits in the fabrication of PMCs are briefly explained in

this chapter.

2. HAND LAY-UP PROCESS

In hand lay-up process, catalyzed liquid resin is applied on the reinforcements that may be

kept on the finished surface of an open mold (Fig.1). Accelerators, catalysts and other

ingredients may be added to the resin and the composite laminate cures at room temperature

without external heating. Chemical reactions in the resin harden the material to a strong,

lightweight product. The resin serves as the matrix for the reinforcing fibers, like concrete

acts as the matrix for steel reinforcing rods. In production, a pigmented gel-coat is first

applied on the mold surface using a spray gun. When the gel-coat is sufficiently cured, layers

of reinforcement are placed in the mold and resin is applied by hand. Any air that may be

entrapped is removed using serrated rollers. The thickness and types of reinforcements used

are determined by the design. Generally, only one finished surface, the gel-coated side of the

product results from the hand lay-up process. It is possible to improve the “nonappearance”

surface of hand lay-up parts by sanding or by using a smooth release sheet such as

cellophane.

Most hand lay-up production involves the use of general purpose (orthophthalic) polyester

resins. Isophthalic polyesters, vinylester and epoxies are also used. Glass fiber chopped

strand mats and woven roving mats are used as reinforcements in hand lay-up process. The

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hand lay-up process is the simplest and most basic of composite manufacturing processes that

offers number of benefits such as, low tooling cost, no restrictions on the size of parts, lowest

capital costs, variety of colors and decorative finishes, potential for on-site fabrication,

suitable for prototyping and scale-up, maximum design flexibility, easily accommodates in

areas requiring more strength and special attachments and incorporates sub-assemblies etc.

The hand lay-up process has several inherent limitations which include, one finished or

“appearance” surface, most labor intensive, product quality is very dependent on operator

skill; shapes may be limited by the ability of reinforcing materials to conform to the mold,

and emissions of volatile chemicals from the resin system. Large products such as boat hulls,

auto and truck body parts, swimming pools, tanks, corrosion resistant equipment, furniture

and accessories, electrical and equipment housings and enclosures, duct and air handling

equipment are produced by hand lay-up process.

3. SPRAY-UP PROCESS

Spray-up is a method used for fabricating composite products in low-to-medium size

products. In spray-up process, glass fiber reinforcement and catalyzed resin are sprayed on a

mold from a specialized spray-up gun. This gun simultaneously chops continuous fiber

reinforcement into suitable lengths and mixes catalyst into the sprayed resin (Fig.2). Once the

material has been sprayed on the mold, brushes or rollers are used to remove any entrapped

air and to assure that the reinforcement is thoroughly wet out. Several layers of chopped

material or specialized reinforcements such as woven roving, fabric, etc. can be added to the

laminate depending upon the requirements for the product. Fillers such as calcium carbonate

and alumina trihydrate can be incorporated in the spray-up resin to reduce the cost and

improve fire/smoke performance.

General purpose (orthophthalic) polyester resin is the normal resin used in spray-up process.

Isophthalic polyesters and vinylesters are also used if required. Spray-up parts are typically

cured at room temperature; however, mild external heat is sometimes applied to accelerate

the rate of cure. Typically, spray-up reinforcements are glass fibers chopped into lengths of ½

inch to 1½ inch. Spray-up process lends itself to the use of additional reinforcements as well.

Extra reinforcements in the form of mats, tape or others can be placed by hand as required.

Additional resin can be applied using the spray-up guns. In addition, similar to hand lay-up

process, various types of core materials can be embedded in the laminate including

honeycomb, PVC or polyurethane foam, plywood, balsa, corrugated sheet etc. Two types of

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spray-up guns such as catalyst injection (catalyst is mixed with the resin) guns and two pot

spray-up applicator guns are used.

Like hand lay-up, spray-up process uses low cost tooling and simple manufacturing

procedures. In addition, spray-up is well suited to produce complex shapes also and virtually

no restriction on the size of parts that can be manufactured. Spray-up also lends itself more

readily to mechanization of the process than hand lay-up process. This can help to offset the

labor intensive nature of open-mold fabrication. However the products made using this

manufacturing method has only one appearance surface and the labor per unit produced is

relatively high in comparison to other processes. It requires good operator skills. The

emission of volatile chemicals from the resin system takes place during manufacturing. Large

unit products such as boat hulls, auto and truck body parts, swimming pools, tanks, corrosion

resistant equipment, furniture and accessories, electrical and equipment housings and

enclosures, duct and air handling equipments etc are produced by spray-up process.

4. COMPRESSION MOLDING

Compression molding process dominates the PMCs production. There are four main

compression molding processes such as, (i) sheet molding compound (SMC), (ii) bulk

molding compound (BMC) including transfer molding, (iii) wet system compression molding

and (iv) reinforced thermoplastic sheet compression molding. Each of these four processes is

based on compressing the composite materials under hydraulic pressure, ranging from 250 to

3,000 psi, in matched metal dies and holding the configured, densified material in the desired

shape until the resin system has been cured completely.

4.1 Sheet Molding Compound (SMC)

Sheet molding compound (SMC) is a totally integrated compound in sheet form that

incorporates all reinforcements, resin, fillers, chemical thickeners, catalysts, mold release

agents and other ingredients. SMC may also include pigments and shrink control agents.

Conceptually, SMC is the composite equivalent of sheet steel that can be stored for a period

of time prior to molding into a complex shape. All of the liquid and dry SMC ingredients,

with the exception of the glass fiber reinforcement, are mixed in either a batch mixer or a

continuous mixer (Fig.3). The resulting compound is a liquid “paste” with a viscosity

generally between 40,000 and 100,000 centipoises. The paste material is delivered to an SMC

machine where it is metered through doctor blades onto separate upper and lower plastic

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“carrier” films, usually polyethylene. Glass fibers in a predetermined amount are delivered

through a cutter, where they are chopped to a specific length and uniformly distributed over

the lower paste bed. The upper and lower paste beds are brought together forming an SMC

“sandwich”. The sandwich progresses through a compaction belt and is consolidated. The

compacted SMC sheet is rolled into convenient lengths, packaged in a nylon sleeve to reduce

evaporation of resin volatiles, and stored in a temperature controlled area. The SMC material

is then allowed to thicken or “mature” over a period of several hours to several days until the

proper molding viscosity has been attained. Chemical ingredients in the formula, such as

magnesium oxide or magnesium hydroxide, thicken the resin. This cause the viscosity of the

resin paste to increase over time from its initial low viscosity to a matured viscosity of

6,000,000 to 75,000,000 centipoises. Once the SMC reaches the desired molding viscosity, it

is delivered to a molding press where the sheet is cut into appropriate sizes and shapes.

The carrier films are removed and a pre-cut, pre-weighed charge is molded in a heated,

matched metal mold mounted in a hydraulic press. Mold temperature range between 250°F

and 350°F. Molding pressures may vary from 500 to 2,500 psi. Molding cycles are normally

from 1 to 4 minutes depending on the thickness of the part, mold temperature and the

quantity and form of the catalyst used. For applications requiring a high surface finish, such

as exterior automotive parts, coating is often applied during the mold cycle. This may affect

the overall cycle time. After curing, parts are removed from the mold. Molded parts may see

a variety of secondary operations such as deflashing, piercing, stud insertion, bonding and

other finishing. The choice of secondary operations will depend on the part’s complexity and

requirements of the intended end use.

SMC materials have been used primarily in high volume applications in the automotive,

appliance, electrical, and construction markets. Since low shrink, low profile resin systems

were introduced in the late 1960s, SMC has been closely identified with applications for

automotive grille opening panels (GOP’s) that pioneered the first major commercial use of

low profile SMC in high volume Class A automotive body panels. Successful applications

like the GOP’s along with Corvette body panels, lead to the selection of SMC for body panels

on the Pontiac Fiero car. General motors (GM) designers selected low shrink, low profile

SMC for all of the horizontal body surfaces including the hood, the roof and the rear deck on

this breakthrough vehicle. Success with the application of SMC in Pontiac Fiero car is now

leading to the introduction of Class A SMC parts in several other major body panels.

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4.2 Bulk Molding Compound (BMC)

The process consists of preparing easily handled putty like molding compound that contains

all resins, catalysts, fillers and reinforcements in a “ready-to-mold” form (Fig.4). BMC

typically comprises approximately 30% resin, 50% fillers and additives and 20%

reinforcement. The material is available in colors and can have excellent fire retardant and

mechanical properties. BMC is used to produce complex shapes with close tolerances. The

physical characteristics of the BMC are determined primarily by the choice of resin. There

are several specialized BMC compounds for specific end uses, including, electrical grade,

low shrink/general purpose, appliance/structural, automotive grade and corrosion resistant

etc.

Glass fiber chopped strands are the primary reinforcements used in BMC compounds. Other

types of fibers are used for special properties. The primary limitation with BMC is the loss of

strength caused by degradation of glass fiber reinforcements during energy-intensive mixing.

This, in turn, reduces mechanical strengths because of short fiber reinforcement length. The

process tends to be limited to the production of small, complex parts. Major applications of

BMC include air conditioner components, pump housings circuit breakers, computer and

business equipment components, garbage disposal housings, motor parts, power tools, gear

cases, electrical insulators and circuit board covers etc.

4.3 Wet System Compression Molding

In wet system compression molding process, liquid resin is poured or pumped onto dry

reinforcement in press-mounted and heated matched metal molds. Hydraulic pressures of 250

to 1000 psi forces the liquid resin to flow through the reinforcement and holds the materials

in place until cure is complete. Cure temperatures of 250 to 350°F are typical. Wet system

resins are normally thermosetting polyesters; however, vinylester, epoxy and other resins can

also be used. Typically, these resins are filled with inert materials such as clay, calcium

carbonate, and alumina, as well as release agents, pigments and catalysts to form a complete

liquid system, which requires only the addition of heat for curing.

Reinforcements commonly used in wet system molding include preforms, chopped strand

mats and continuous strand mats. Equipment for wet system molding, including the hydraulic

presses and matched die tooling, is essentially the same as for SMC and BMC. But because

of the lower molding pressures of the wet system, equipment and tooling savings may be

possible. Wet system process applications continue to be developed for a number of major

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markets including, automotive, appliance and equipment, construction, electrical, agriculture,

consumer/recreation, and corrosion resistant service. Wet system molding will continue to be

an important high-volume process for producing composites. With continued demand for low

cost, high performing products in markets such as automotive and electrical components are

produced. Wet system compression molding provides many of the benefits of composites on

a cost-effective basis.

4.4 Reinforced Thermoplastic Sheet Compression Molding

Reinforced thermoplastic sheet molding has gained significance as a high productivity

process for fabricating automotive, appliance and consumer products. The process utilizes a

precombined sheet of thermoplastic resin and glass fiber reinforcement. These sheets are cut

into blanks, which are preheated to a specified temperature and loaded into matched metal

compression molds. Under pressure, heat-softened blanks flow and fill the mold. The mold is

maintained at a temperature, which causes the sheet to solidify, and allows demolding of the

part (Fig.5). Though it is possible to use modified steel stamping presses for this process,

most of the reinforced thermoplastic sheet today is processed on high-speed compression

presses with pressures ranging from 1500 to 3000 psi.

A variety of thermoplastic resins are now available in sheet form. Polypropylene is being

used most extensively in production. Similar sheet is also produced with thermoplastic

polyester, polycarbonate and nylon. Continuous glass fiber mat is the primary reinforcement

for this process. It provides good flow characteristics and high strength. Advantages of the

reinforced thermoplastic sheet compression molding process include, fast molding cycles,

unlimited shelf life of the input sheet and blanks, broad spectrum of properties, recyclable

scrap and potential for parts consolidation in comparison to metals. Current applications for

reinforced thermoplastic sheet compression molding include, automotive bumper beams,

automotive load floors, radiator supports, battery trays, package shelves, concrete pouring

forms, access flooring, chair shells, military containers, materials handling pallets and trays,

helmets, electrical enclosures etc.

5. REINFORCED REACTION INJECTION MOLDING (RRIM)

In the reinforced reaction injection molding (RRIM) process, two or more reactive resins are

metered and impingement mixed under high pressure, injected into a mold to form a

thermosetting polymer, and then cured in the mold. A schematic of the RRIM process is

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shown in (Fig. 6). The RRIM process requires special resins and reinforcements. A number

of resins, including epoxies, polyesters, nylons and polyurethanes have been successfully

developed for RRIM processing. Today, polyurethane is the predominate resin in RRIM. The

basic RRIM reinforcements are chopped or hammer milled glass fiber and glass flake.

Variations of the RRIM process include structural RIM or SRIM. In this process, chopped

fiber preforms or mats are positioned in the mold cavity. The mold is clamped and resin is

injected into the mold cavity. The reacting resin remains liquid long enough to completely fill

the mold, and penetrate the reinforcing fibers. Then the resin quickly cures. RRIM

composites have a number of processing advantages including very fast cycle time, low

labor, low mold clamping pressure and low scrap rate. Presently, transportation is the

principal market for RRIM products. Automotive and truck applications for RRIM parts

include Class A body panels, bumper beams, spare tire covers, floor pans and other similar

products. The advent of controllable reactivity resins such as polyurea/amide has introduced a

trend toward larger machines, larger clamps and larger parts. Very large RRIM molded parts

weighing over 100 pounds have already been produced. In 1988, a 120-pound SRIM pickup

truck bed was introduced. 6. RESIN TRANSFER MOLDING (RTM)

Resin transfer molding (RTM) process is a closed mold process in which matching male and

female molds, containing pre-cut or preformed fiber reinforcement, are clamped together.

Resin is pumped into the mold through injection ports, designed to contain the resin under

pressure. A perimeter gasket seals the edges of the mold (Fig.7). Air vents are located at the

gasket line to evacuate air. Pumping pressures from 40 to 50 psi (280-345 kPa) are typical.

Tooling must be designed to withstand excess pressures. The principal resins used today are

vinylesters, orthophthalic and isophthalic polyesters. Low-shrink, and low-profile polyesters

have also been developed especially for RTM processing to improve surface appearance.

Epoxies, urethanes, acrylic/polyester hybrids, and nylon resins are also used nowadays.

Reinforcements used in RTM process are typically glass fiber continuous strand mats and

chopped strand preforms. Special mats, which contain thermoplastic binders, have recently

been introduced for RTM process. These mats can be heated and thermoformed into very

accurate preforms. Preforms allow faster loading cycles and more complex part geometry. A

variety of woven and no woven glass fiber products have been developed for the RTM

process. Other forms of high performance reinforcement such as carbon and aramid fibers

can also be incorporated in the preparation of RTM laminates either alone or as part of a

hybrid system.

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RTM parts can be molded at the rate of several parts per hour per mold depending upon

design and process considerations. Closed RTM molds release fewer volatile materials than

open molds. Mold cycle times are as much as four times faster than hand lay-up and spray-up

processes. RTM tooling is potentially lower in cost than tooling for other closed mold

processes. Molded parts have two finished surfaces. Secondary reinforcements (cores, ribs,

bosses, etc.) can be easily incorporated. Shrink control systems can be employed to produce

improved surfaces. Thermoplastic skins can be incorporated during molding. Several styles

of doors and antenna dishes were first molded as RTM parts to evaluate the acceptance and

design features before an SMC capital investment was made.

7. PULTRUSION

The pultrusion process has been used for making profile shapes such as rods, beams,

channels, plates, etc. since 1948. In the pultrusion process (Fig.8), reinforcements are pulled

continuously through a thermosetting resin bath, shaped into a specific constant cross-section

and cured to hardness while being held in the desired shape. The term pultrusion was coined

to differentiate the process from “extrusion” where plastics or meals are pushed through a die

opening. Polyester is the principal resin. Some vinylesters, phenolics, and epoxies are also

used. Epoxies are used primarily for “hot sticks” and other high performance electrical

applications. Epoxy pultrusion resins are usually anhydride cured rather then amine cured

formulations. Various additives and fillers are used in pultrusion resins to improve specific

properties or lower the cost. Usual fillers are calcium carbonate, clay, and hydrated alumina.

Reinforcements commonly used for pultrusion include; continuous fiberglass rovings or

tows, continuous strand mats, chopped strand mats, graphite fibers, surfacing mats and woven

tapes. Pultruded parts can achieve a high degree of reinforcement orientation. For products

such as electrical strain rods and oil well sucker rods, this is a definite design benefit.

Pultrusion is a highly automated, low labor process. The tooling and capital equipment

required for pultrusion is generally less expensive than for other high-output production

processes.

8. FILAMENT WINDING

Filament winding is a process for producing products that are surfaces of revolution.

Reinforcing fibers and resins are applied to a rotating mold surface or mandrel following a

machine controlled geometric pattern (Fig.9). A variety of thermosetting resins can be used in

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a filament winding process. The most common resins include, polyesters, bisphenol A

vinylesters, epoxies and furans. All types of reinforcing fibers can be used in filament

winding. These reinforcing fibers can be in a variety of different forms including continuous

strand and tows, woven and unidirectional tapes, chopped strands and continuous or chopped

strand mats. Glass fiber rovings are most frequently used. The advantages of filament

winding processing include, highly controllable properties, variable strength capability, low

labor cost, broad range of reinforcement and resin options and process can be highly

automated etc. Filament wound products can be found in every major market segment.

Proven applications include, pipe and tank products for oil and gas, chemical processing

industry and water/waste water treatment, fittings for pipe and tanks, air and gas pressure

bottles, aircraft fuel wing tanks, rocket motor and shell casings, gun barrels, automotive and

truck drive shafts, aircraft bodies, automotive leaf springs, sailboat masts, tennis racket

frames and railroad tank cars.

9. INJECTION MOLDING

The basic principle of injection molding involves the property of thermoplastic materials to

soften when subjected to heat and to harden when cooled. In most injection molding, granular

thermoplastic resin is fed from a supply hopper into one end of a heated metallic cylinder. A

screw or auger operating inside the cylinder rotates, conveying the resin from the hopper into

the hot cylinder. Upon heating, the resin softens to a semifluid state, thus conditioning it for

injection into the mold. The injection molding machine screw, in addition to rotating, can

reciprocate backward and forward along the axis of the cylinder. This action performs two

important functions: metering the resin for the melt volume required to fill the mold and

transferring the metered resin charge into the mold. In addition to plasticating the resin and

metering the melt, an additional function of the injection molding machine is to retain the

mold, clamping it closed under pressure during injection and ejection of the finished part

after cooling takes place. The mold cavity represents the exact shape of the part to be

produced. Molds may contain one or more cavities depending upon part size and molding

machine capacity. Molds are usually maintained at a temperature that is relatively cool in

comparison to the temperature of the molten plastic being processed. Once the semifluid resin

is in the cool mold, the material again becomes solid, taking the shape of the mold cavity.

The significant elements of the injection molding process are, melting or plastification of the

thermoplastic resin, metering and transfer of the melt into the mold, clamping system for

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retaining the mold, causing it to open and close at appropriate times in the molding cycle, and

for ejecting the molded part, and machine controls to execute the proper sequence of process

steps and timing and to control the temperature of the resin during processing. Some of the

reinforced thermoplastics that are commonly injection molded include nylon, PVC, acetal,

polyethylene, polypropylene, ABS, SAN and polycarbonate. Reinforcements are typically

chopped glass fibers in the range of 0.125-0.5” in length. Fiber loadings vary from 5% to

40% by weight.

10. OTHER PROCESS

10.1 Centrifugal Casting

In centrifugal casting, reinforcements and resin are deposited against the inside surface of a

rotating mold (Fig.10). Centrifugal force holds the materials in place until the part is cured.

With centrifugal casting, the outside surface of the part, which is cured against the inside

surface of the mold, represents the “finished” surface. The interior surface of centrifugally

cast parts can be given an additional coating of “neat” or pure resin to improve surface

appearance and provide additional chemical resistance in the part. Centrifugal casting

commercially produces large diameter composite pipe and tanks. Advantages of centrifugal

casting include a finished exterior surface and containment of volatiles during processing.

The primary limitations of centrifugal casting are the ability to spin molds of large size and

relatively low productivity per tool.

10.2 Extrusion

Extrusion is a popular process for producing reinforced thermoplastic products. In extrusion,

heated, relatively low viscosity material is forced under pressure through a shaping die and

cooled, (or cured in the case of thermosets), until hardened. Although changes in cross-

section are possible by using movable “tractor” dies, extruded products normally have

constant cross-sectional geometry. Any of the common thermoplastic polymers and some

thermoset compounds can be extruded.

10.3 Vacuum Bag/Pressure Bag/Autoclave Molding

The vacuum bag, pressure bag and autoclave molding processes are similar to the hand lay-

up process previously described. The significant difference between the processes is the use

of pressure during curing. Each of these processes uses both wet system and prepregs in the

materials application step. With the set system, reinforcements such as mat or woven roving

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are placed in the mold. Liquid resin is applied to the reinforcement and spread by brush over

the surface of the reinforcement. A flexible plastic film (or “bag” as it is commonly known)

is placed over the reinforcement and sealed to the mold around the perimeter of the part.

In vacuum bag molding, vacuum is used to expel the air remaining between the bag and the

mold surface. In the case of wet system molding, squeegees distribute resin to eliminate wet

or dry spots in the reinforcement. Once the bag has been evacuated, atmospheric pressure is

retained until cure is complete. External heat can be applied to accelerate the cure.

Pressure bag molding is a variation on vacuum bag processing when higher than atmospheric

pressure is required. A flexible bag is placed over the reinforcement and resin. Materials can

be in the form of prepregs or wet system. Pressures upto 50 psi are applied to the bag until

cure is complete.

With autoclave molding, the vacuum and pressure bag processes are essentially combined.

Either the prepreg or wet system is employed. The mold and composite part, with vacuum

applied, is moved into an autoclave for curing. Both heat and external pressure are applied by

means of high pressure steam. Autoclave molding today produces many high performance

composite aircraft parts.

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