Polymer Composite Processes 152

Polymer Composite Processes discusses the various common manufacturing methods for processing polymer-matrix composites (PMCs). Polymer-matrix composites are processed most frequently using open or closed molding methods. Open molding processes, which use a single-sided mold to shape a composite part, include lay-up molding, spray-up molding, and filament winding. Closed molding processes, which use a two-sided mold to shape composite material, include compression, injection, reaction injection, and resin transfer molding as well as pultrusion. The type and form of matrix materials affect which processes are appropriate to use.

Polymer-matrix composites are the most widely utilized composites in manufacturing. An understanding of the various processes used to create PMC parts is essential for any composite technician. After taking this class, a user will be able to distinguish between the various polymer composite processes used in manufacturing today.

  • Difficulty Beginner

  • Format Online

  • Number of Lessons 15

  • Language English


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Course Outline
  • Composite Processing
  • Polymer-Matrix Composites
  • Preforms, Prepregs, and Molding Compounds
  • Review: Composite Processing Basics
  • PMC Molding Processes
  • Lay-Up and Spray-Up Molding
  • Filament Winding
  • Debulking for Open Molding Processes
  • Review: Open Molding Processes
  • Compression Molding
  • Injection Molding
  • Reaction Injection Molding
  • Resin Transfer Molding
  • Pultrusion
  • Review: Closed Molding Processes
  • Describe the general composite production process.
  • Describe basic processing considerations for polymer-matrix composites.
  • Distinguish between preforms, prepregs, and molding compounds.
  • Distinguish between open and closed molding processes for polymer-matrix composites.
  • Describe lay-up and spray-up molding.
  • Describe filament winding.
  • Describe debulking for PMC parts made with open molding processes.
  • Describe compression molding.
  • Describe injection molding.
  • Describe reaction injection molding.
  • Describe resin transfer molding.
  • Describe pultrusion.
Vocabulary Term

advanced composite

A high-performance material consisting of long reinforcement fibers and polymer, metal, ceramic, or carbon matrices. Advanced composites are more expensive than engineering composites.

air valves

A mechanical device that regulates the flow of air with moveable parts that open, close, or obstruct one or more openings or passageways. Air valves direct air movement and regulate the amount of pressure.


A manufacturing process in which two or more components are joined together to create a finished part. Assembly may include the use of fasteners, adhesives, or welding to attach parts to one another.


A pressure vessel that promotes faster curing through a combination of vacuum and heat. Advanced composites often require autoclave curing.


A system or process that does not require human intervention. Automated composite manufacturing processes, such as filament winding and composite molding, typically use computer numerically controlled (CNC) machines.


To hold two or more materials together. Matrix and reinforcement materials bind together to create a composite part.


To burn an object's surface until it becomes black or discolored. Thermoset materials will char if exposed to heat after their initial curing process.


A pre-weighed amount of material used to fill a mold during one molding cycle. Charges for compression molding processes are often types of molding compounds.

chemical reaction

A process in which one or more substances are changed into another substance or substances. Chemical reactions alter the physical structure of a substance in some way.

chopped fibers

A discontinuous piece of fiber that has been cut to a length of 1 to 3 inches (2.5 to 7.6 cm). Chopped fibers do not provide as much reinforcement as continuous fibers do.

chopper gun

A pneumatic device that chops continuous strands of fiber into small pieces and sprays them along with matrix material onto the surface of a mold. Chopper guns are used during spray-up molding processes.


A device that holds an object by applying downward or inward pressure. Clamps are often used as workholding devices in various manufacturing processes.

closed mold

A type of mold with two halves that mate together, completely enclosing material, and apply heat and/or pressure to shape the material. Closed molds are used in various composite manufacturing processes. including compression molding, resin-transfer molding, and injection molding.

closed molding

A molding process that uses a two-sided mold to shape composite materials. Open molding processes, such as compression and transfer molding, are automated processes that produce parts with smooth, finished surfaces on all sides.


A material composed of two or more unlike materials that are bonded together without losing their individual properties or characteristics. Composites consist of a matrix material and a reinforcement material.

compression molding

A closed molding process that forces polymer-matrix composite material into a heated mold cavity using high amounts of pressure. Compression molding either holds thermoset PMCs between the mold halves until they cure, or it holds thermoplastic PMCs until they cool and solidify.

computer numerical control

CNC. A self-contained system of computers and precision motors that executes program instructions to guide machine tool components and manufacture parts. Computer numerical control machines are used to automate many manufacturing processes.


Curving inward, like the interior surface of a bowl. Concave composite parts made with open molding processes have smooth surface finish on the outside, such as boat hulls.


To combine multiple layers of composite materials into a solid, compact mass. Composite parts consolidate during debulking methods.

continuous fibers

A fiber that covers an entire dimension of a part without a break or interruption. Continuous fibers provide greater reinforcement than chopped fibers.


Curving outward, like the exterior surface of a circle. Convex composite parts made with open molding processes have a smooth finish on the inside, such as a storage tanks.


To cause a material to bond and solidify by permanently cross-linking its molecules through heat, light, time, or chemical means. When liquid thermoset composite matrices cure, they become hard, strong, and stiff.

cycle time

The time it takes to perform one step of a manufacturing process and forward it to the next step. Cycle times for closed molding composite processes are shorter than those for open molding composite processes.


A three-dimensional (3D) shape that has two end faces that are parallel circles connected by a perpendicular plane. Cylindrical objects can be solid or hollow.


The process of removing air pockets and consolidating the layers of a composite material. Debulking increases the density of the composite part.


A tool containing a recess that shapes the width and height of a composite material. The die using in pultrusion is heated to cure the composite as it passes through.

feed barrel

The holding chamber of an extrusion or injection molding machine in which material is melted and mixed. The feed barrel is typically heated and contains a reciprocating screw to moves material through the barrel.


Gradually moving an object through or into another object. During injection molding, a hopper feeds pellets of material into the feed barrel.


A thin strand of composite reinforcing material that is longer than 0.005 inches (0.13 mm). Fibers provide greater reinforcement than whiskers or particles.

filament winding

An open molding process during which continuous fiber reinforcements are soaked in matrix material and then wrapped around the outside of a rotating mandrel. Filament winding is an automated process that uses computer numerical controlled (CNC) machines.


An inert substance added to a composite to take up space and reduce material shrinkage. Filler can also reduce material weight and/or cost.


An inert substance added to a composite to take up space and reduce material shrinkage. Fillers can also reduce material weight and/or cost.


A final process performed to complete a part. Finishing operations surface, condition, edge, and clean parts and are usually the last steps in the machining process, occurring before inspection and testing.

gel coating

A layer of material that is usually applied to the mold's surface before forming the composite part. Gel coatings protect the surface of the composite and give it a smooth finish.


A mechanical device on a filament winding machine that places reinforcement fibers at precise orientations. The guide moves back and forth along the length of the mandrel.

hand roller

A cylindrical device that rotates as an operator moves it over top the surface of a layer of composite material. Hand rollers help remove air bubbles and excess matrix materials during lay-up and spray-up molding processes.


A substance that starts and controls the curing process. Hardeners are mixed with thermoset-resin matrices during reaction injection molding (RIM) and resin transfer molding (RTM).


A substance that starts and controls the curing process. Hardeners are mixed with thermoset-resin matrices to make molding compounds as well as during reaction injection molding (RIM) and resin transfer molding (RTM).


A large, funnel-shaped device located on top of the feed barrel on extrusion and injection molding machines. The hopper serves as the entryway for pellets of composite material into the feed barrel.


The body or outer shell of a boat. Hulls are often made of lightweight and strong materials, like fiberglass-reinforced plastics.

inert gas

A gas that does not chemically react with the substance it contacts. Inert gases include nitrogen, helium, and argon.

injection molding

A closed molding process that melts polymer material, mixes it with reinforcements, and then injects into a mold. Injection molding can produce polymer-matrix composite (PMC) parts using either thermoplastic or thermoset matrices.

lay-up molding

An open molding process during which reinforcement material is positioned into a mold and saturated with a matrix material. Lay-up molding is a manual process that shapes composite materials using a single-sided mold.


In a straight line along a single axis. The guide of a filament winding machine moves linearly along the length of the mandrel to precisely place reinforcement fiber.


A manufacturing process that involves removing material to form an object. Machining includes methods such as milling, turning, and drilling that remove metal using cutting tools.


A rotating, cylindrical device around which reinforcement fibers saturated with matrix material are wrapped. Mandrels are used during the filament winding process to shape composite parts from the inside out.


A process that requires an operator to entirely perform the process and control the variables. Manual composite manufacturing processes include lay-up and spray-up molding.


A material that binds together the reinforcing fibers of a composite and hardens to give the part shape and protect the fibers from damage. The matrix is usually a viscous material that can be a variety of polymers, metals, or ceramics.

mixing chamber

An enclosed space where two or more materials combine together. Mixing chambers are used in reaction injection molding (RIM) and resin transfer molding (RTM).


A substance consisting of two or more separate substances that are combined together but not chemically bonded. Mixtures include different types of composites.


A hollow cavity that holds heated liquid material and imparts its shape on the material as it cools. Molds are often used during processing of polymer-matrix composites (PMCs).


A category of manufacturing process that uses a reusable cavity, or mold, to shape material. Molding processes can use open, single-sided molds or closed, two-sided molds.

molding compound

A type of composite in which the matrix and reinforcement have already been combined before they are sent to the manufacturer. Molding compound is available in bulk form and sheet form.

near-net shape

Having dimensions that are close to a final form after initial manufacturing steps. Parts that are near-net shape require little subsequent machining.

nip rollers

A mechanical device on a filament winding machine that draws continuous fibers through a resin bath. Nip rollers help keep the fiber under the appropriate amount of tension.


A colorless, odorless, tasteless gas that naturally exists in the atmosphere. Nitrogen is sometimes used in autoclaves to prevent oxidation during the curing cycle.

open mold

A type of mold with a single-side and no rigid second piece to enclose the part and apply pressure. Open molds are used in lay-up and spray-up molding as well as filament winding.

open molding

A molding process that uses a single-sided mold to shape composite materials. Open molding processes, such as lay-up molding, spray-up molding, and filament winding, are manual or automated processes that produce composite parts with one finished side and one unfinished side.


The direction and angle of reinforcement fiber in a composite part. Fiber orientation affects the strength of a composite part.


A small piece of material with a shape that is isotropic, or equal in all directions. Particles can be used in composites to provide small amounts of reinforcement or as fillers.


The distance around the outside of an object. The perimeter of an open mold is sealed with a plastic film during vacuum bagging.


Polymer-matrix composites. A composite made from a thermoset or thermoplastic matrix and reinforcement materials. PMCs are not as strong or heat resistant as metal-matrix and ceramic-matrix composites.

polymer-matrix composites

PMCs. A composite made from a thermoset or thermoplastic matrix and reinforcement materials. Polymer matrix composites are not as strong or heat resistant as metal-matrix and ceramic-matrix composites.


A tough but flexible reinforcement structure that is roughly the same shape as the final product. A preform must be combined with resin in order to create a composite.

preimpregnated material

A ready-to-mold reinforcement material that has been coated with ceramic or resin matrix materials. Preimpregnated materials, or prepregs, are available in several formats, including sheets, tapes, and tows of fiber.

preinitiated resin

A thermoset material that has already been combined with the hardener. Preinitiated resin allows for an even distribution of the hardener throughout the resin.


A ready-to-mold reinforcement material that has been coated with ceramic or resin matrix materials. Prepregs, or preimpregnated materials, are available in several formats, including sheets, tapes, and tows of fiber.

pressure vessel

A closed container whose contents are held at a pressure different from the pressure of the air outside the vessel.

primary processes

A main manufacturing operation that contributes to the completion of a part. Primary processes often form the initial shape of the part.

production costs

Any expenditure directly related to starting or running a manufacturing operation. Production costs include any expenses associated with purchasing necessary machines, materials, and labor.

production rates

The time it takes for a manufacturing process to create a set number of finished parts. Production rates are generally quicker for automated processes than manual processes.


A specialized molding process for composites that creates a part with a continuous shape that is sawed into various lengths. Pultrusion makes polymer-matrix composite (PMC) parts most frequently using a thermoset resin.

raw material

An unprocessed or lightly processed component used to make the finished part. Raw materials in manufacturing include metal, polymer, and ceramic.

reaction injection molding

RIM. A closed molding process that mixes together thermoset resin, hardener, and reinforcements and injects into a heated mold cavity at the same time. Reaction injection molding can create polymer-matrix composite (PMC) parts.

reciprocating screw

A device in the feed barrel of an injection molding machine that rotates and moves back and forth to mix and inject molten material through to a mold. Reciprocating screws move back and forth while also rotating in the barrel.


Able to be reused or reprocessed multiple times. Thermoplastics are recyclable materials and can be melted and reshaped several times.


The part of the composite that provides strength, stiffness, and the ability to carry a load. Reinforcement materials can be in the form of whiskers, particles, or fibers.

release agent

A substance that is applied to a mold surface to make it easier to remove the molded part. Release agents are commonly used in lay-up and spray-up molding processes.


Able to deliver accurate and dependable results over a long period of time. Automated processes are often highly repeatable compared to manual processes.

resin bath

A container filled with liquid matrix material for wetting fibers. Resin baths are used in filament winding to saturate fibers with matrix material prior to wrapping them around a mandrel.

resin transfer molding

RTM. A closed molding process mixes together thermoset resin and hardener and then injects them into a heated mold cavity containing a preform. Resin transfer molding can create polymer-matrix composite (PMC) parts.


A raw polymer, usually in the form of liquid, beads, or pellets, that is not yet molded into its final shape. Liquid resins are applied to reinforcement fibers during thermoset PMC processes.


Resistant to bending. Rigid materials are stiff and inflexible and are useful in applications where a part must hold its shape.


A multi-point cutting tool with a series of teeth on its edge. Saws are is used in pultrusion to separate a cured composite into parts of various lengths.


Unusable material produced during a manufacturing process. Scrap is a waste product of manufacturing that can increase overall production costs.

secondary processes

An additional manufacturing operation required to bring a workpiece into tolerance and create a final part. Secondary processes can include finishing and assembly.

spray-up molding

An open molding process during which chopped strands of reinforcement material combined with a matrix material are sprayed onto the surface of a mold. Spray-up molding is a manual process that shapes composite materials using a single-sided mold.

strength-to-weight ratio

The relationship between a material's strength and its weight. Materials with a high strength-to-weight ratio are light but also very strong.

surface finish

The degree of roughness and variation on the surface of a manufactured part. Surface finish affects how a composite part will function.

tensile strength

A material's ability to resist forces that attempt to pull it apart or stretch it. Materials with high tensile strength tend to deform, bend, or stretch before breaking.


A pulling force that is directed away from the object and attempts to move, stretch, or elongate the object. Fiber tension affects the strength of a composite part.


A group of plastics that can be repeatedly heated, cooled, and shaped. Thermoplastics are used less frequently as a composite matrix material than thermosets.


A group of polymers that are permanently hardened by curing or heating. Thermosets often have high rigidity and thermal stability.


3D. Having a length, depth, and width. Preforms are three-dimensional reinforcement structures used during some composite processing methods.


An acceptable deviation from a desired dimension that meets specifications. Parts outside of a required tolerance must be discarded or reworked.


To be constant and unchanging. Uniform composite parts can be created using closed molding processes, such as compression molding.


The absence of matter, including air and gases. The pressure inside a vacuum is lower than the pressure outside the vacuum.

vacuum bag

A thick, stretchy plastic bag that covers the composite laminate and the rest of the vacuum bagging materials. Vacuum bags must be able to withstand vacuum forces and curing temperatures without breaking.


A debulking method in which a bag is placed over the mold and a vacuum pump draws the air out. Vacuum-bagging removes any voids or excess resin while consolidating the composite part.


Having a high resistance to flow. Viscous materials tend to be sticky or syrupy.


A tiny air pocket that has been trapped in the composite material during molding. Voids are undesirable because they compromise the integrity of the finished product.

wetting out

The process of saturating the reinforcement material of a composite with the matrix material. Wetting out must be done thoroughly to give the composite part its full strength.


A thin strand of composite reinforcement that is less than 0.005 in. (0.13 mm) long. Whiskers are shorter than fibers but longer than particles.