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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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