Printed from Automotive Composites Alliance website Jul 23, 2018.

Manufacturing Processes

What manufacturing process do I select?
Compression Molding
Liquid Composite Molding (LCM)
Injection Molding
Reinforced Reaction Injection Molding (RRIM)
Structural Reaction Injection Molding (SRIM)
Resin Transfer Molding (RTM)
Vacuum Infusion Processing (VIP)
Hand Layup


In this section, those manufacturing processes typically used to make products found in automotive composites market are covered. Unique to the composites industry is the ability to create a product from many different manufacturing processes. There are a wide variety of processes available to the composites manufacturer to produce cost efficient products. Each of the fabrication processes has characteristics that define the type of products to be produced. This is advantageous because this expertise allows the manufacturer to provide the best solution for the customer. In order to select the most efficient manufacturing process, the manufacturing team considers several factors such as:
  • User needs
  • Performance requirements
  • Size of the product
  • Surface complexity
  • Appearance
  • Production rate
  • Total production volume
  • Economic targets/limitations
  • Labor
  • Materials
  • Tooling/assembly
  • Equipment

What manufacturing process do I select?

Selecting a manufacturing process is often made depending on the volume of parts being made. The chart below is a guide to selection of a process.

Composites Manufacturing Process Selection Guide

Low = 0 - 5000 parts
Medium = 5000 - 15,000 parts
High = 15,000 - 100,000+ parts

Production Volume
Low Medium High
Compression Molding
    Liquid Composite Molding (LCM)
Injection Molding
Vacuum Infusion Processing (VIP)   X  
Resin Transfer Molding (RTM)   X  
Hand Layup X    

Compression Molding

Compression molding is the most common method of molding thermosetting materials such as SMC (sheet molding compound) BMC (bulk molding compound), and LCM (liquid composite molding). This molding technique involves compressing materials containing a temperature-activated catalyst in a heated matched metal die using a vertical press.

The molding process begins with the delivery of high viscosity uncured composite material such as SMC, BMC, or a mat or perform covered with a medium viscosity resin paste (LCM) to the mold. Mold temperatures typically are in the range of 300o - 320o F. As the mold closes, composite viscosity is reduced under the heat and pressure approximating 1000 psi. The resin and the isotropically distributed reinforcements flow to fill the mold cavity in the case of SMC and BMC. In LCM the reinforcements do not move only the resin paste flows throughout the mold.

While the mold remains closed, the thermosetting material undergoes a chemical change (cure) that permanently hardens it into the shape of the mold cavity. Mold closure times vary from 30 seconds up to several minutes depending on part design and material formulation.

When the mold opens, parts are ready for finishing operations such as deflashing, painting, bonding, and installation of inserts for fasteners. By varying the formulation of the thermoset material and the reinforcements, parts can be molded to meet applications ranging from automotive class 'A' exterior body panels to structural members such as automobile bumper beams.

SMC Molding – SMC arrives at the press in roll form or folded in a large box. The SMC is encased in nylon film. The film is stripped and the sheet is cut into predetermined shapes to be loaded into the open heated matched metal dies. The charge is of a predetermined weight. The compression press is closed for a set time and the part is cured. The part is deflashed and ready to assemble or ship.

BMC Molding – BMC (bulk molding compound) consists of a highly filled resin paste combined with short (1/8 to ½ inch) glass fibers. It can be a mass of mix take right from the mixer, but it is usually extrude into "logs" and cut to the weight required for charging the compression or injection mold. In compression molding the weighed log is inserted in the open mold and the heated mold is closed. Once the material is cured the mold is opened and the part is removed and deflashed. In injection molding BMC logs are placed in the hopper of an injection molding machine and the material is forced into a closed mold and cured.

Liquid Composite Molding (LCM)

In liquid composite molding a fiberglass preform matching the shape of the finished part or fiberglass mat (for simple planer shapes) is loaded with the correct weight of a medium viscosity resin pasted is placed in the open dies and the compression press is closed. The glass does not move during molding leading to uniform orientation of the glass and thus uniform strength.

Preforming – Preforming uses a metal screen made to the shape of the inside of the molded part. A vacuum is drawn through this screen and strands of fiberglass rovings are chopped onto the screen with enough binder material to hold the preform together. The preform is then cured and removed from the screen.

Injection Molding

In this process short glass SMC or BMC are injected into a heated closed matched metal die by screw or ram injection and allowed to cure. Usually these injected parts will be lower than compression molded parts, but require little or no deflashing.

Reinforced Reaction Injection Molding (RRIM)

In this process, two components (polyol and isocyanate) which are reacted together to produce a polyurethane are brought together into a mixing head where other constituents such as fillers are combined. The ingredients are mixed very rapidly and then injected under pressure into a closed mold where a preform has been placed. A combination of chopped roving and preform may be used.

Structural Reaction Injection Molding (SRIM)

In this process the reinforcement is placed in the mold, the mold is closed and under pressure. The resin is then injected into the closed mold. The usual molds used in this process are telescoping molds allowing the resin to be injected into a closed cavity. The mold is then fully closed, which spreads the resin throughout the reinforcement. Fiberglass mat or performs are the usual reinforcement used in this process. The resins used are unfilled with polyurethane being the main resin.

Resin Transfer Molding (RTM)

Resin Transfer Molding or RTM as it is commonly referred to is a "Closed Mold Process" in which reinforcement material is placed between two matching mold surfaces — one being male and one being female. The matching mold set is then closed and clamped and a low-viscosity thermoset resin is injected under moderate pressures (50 - 100 PSI typically) into the mold cavity through a port or series of ports within the mold. The resin is injected to fill all voids within the mold set and thus penetrates and wets out all surfaces of the reinforcing materials. The reinforcements may include a variety of fiber types, in various forms such as continuous fibers, mat, preforms, or woven type construction as well as a hybrid of more that one fiber type. Vacuum is sometimes used to enhance the resin flow and reduce void formation. The part is typically cured with heat. In some applications, the exothermic reaction of the resin may be sufficient for proper cure.

RTM as a process, is multi-compatible with a variety of resin systems including polyester, vinyl ester, epoxy, phenolic, modified acrylic and hybrid resins such as polyester and urethane. Typically, it requires a resin viscosity of 200-600 centipoise to penetrate all surfaces of the mold cavity. Advantages of the RTM process include:
  • As a closed mold process, emissions are lower than open mold processes such as spray up or hand lay up
  • The mold surface can produce a high quality finish (like those on an automobile)
  • This process can produce parts faster - as much as 5-20 times faster than open molding techniques.
  • Complex mold shapes can be achieved. Cabling and other fittings can be incorporated into the mold designs.
Disadvantages of the RTM process are:
  • High production volumes required to offset high tooling costs compared to open molding techniques.
  • Reinforcement materials are limited due to the flow and resin saturation of the fibers.
  • Size of the part is limited by the mold.

Vacuum Infusion Processing (VIP)

Vacuum infusion is a variation of vacuum bagging where the resin is introduced into the mold after the vacuum has pulled the bag down and compact the laminate. The method is defined as having lower than atmospheric pressure in the mold cavity. The reinforcement and core material are laid-up dry in the mold. This is done by hand and provides the opportunity to precisely position the reinforcement. When the resin is pulled into the mold the laminate is already compacted; therefore, there is no room for excess resin. Very high resin-to-glass ratios are possible with vacuum infusion and the mechanical properties of the laminate are superior. Vacuum infusion is suitable to mold very large structures and is considered a low volume molding process.

The mold may be gel coated in the traditional fashion. After the gel coat cures, the dry reinforcement is positioned in the mold. This includes all the plies of the laminate and core material if required. A perforated release film is placed over the dry reinforcement. Next a flow media consisting of a coarse mesh or a "crinkle" ply is positioned, and perforated tubing is positioned as a manifold to distribute resin across the laminate. The vacuum bag is then positioned and sealed at the mold perimeter. A tube is connected between the vacuum bag and the resin container. A vacuum is applied to consolidate the laminate and the resin is pulled into the mold.

Molds – Molds are similar to those used for conventional open-mold processes with additonal structure.

Major Advantages – Vacuum infusion can produce laminates with a uniform degree of consolidation, producing high strength, lightweight structures. This process uses the same low cost tooling as open molding and requires minimal equipment. Very large structures can be fabricated using this method. Vacuum infusion offers a substantial emissions reduction compared to either open molding or wet lay-up vacuum bagging.

Hand Layup

Hand lay up is the oldest and simplest method used for producing reinforced plastic laminates. Capital investment for hand lay up processes is relatively low. The most expensive piece of equipment typically is a spray gun for resin and gel coat application. Some fabricators pour or brush the resin into the molds so that a spray gun is not required for this step. There is virtually no limit to the size of the part that can be made. The molds can be made of wood, sheet metal, plaster, and FRP composites.

In a particular hand lay up process (otherwise known as wet lay up), high solubility resin is sprayed, poured, or brushed into a mold. The reinforcement is then wet out with resin. The reinforcement is placed in the mold. Depending upon the thickness or density of the reinforcement, it may receive additional resin to improve wet out and allow better drapeability into the mold surface. The reinforcement is then rolled, brushed, or squeegeed to distribute the resin uniformly, to remove entrapped air and to compact it against the mold surface.


Composites Institute, 1998, Introduction to Composites, 4th Edition, Society of the Plastics Industry, New York, NY.

Murphy, John, 1998, Reinforced Plastics Handbook, 2nd Edition, Elsevier Science Inc. NY, NY.

Rosato, Dominick V.,1997, Designing with Reinforced Plastics, Hanser/Gardner, Cincinnati, Ohio.

Shook, Gerald, 1986, Reinforced Plastics for Commercial Composites Source Book, ASM, Metals Park, OH.

Strong, Brent, 1989, Fundamentals of Composites Manufacturing: Materials, Methods, and Applications, Society of Manufacturing Engineers, Dearborn, MI.

U.S. Department of Agriculture Forest Service, 1987, Wood Handbook: Wood as an Engineering Material, Agricultural Handbook No. 72, Washington, DC.

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