Intro to Compression Molding 170

This class will teach you about the compression molding process, as well as the materials and equipment associated with it.

  • Difficulty Beginner

  • Format Online

  • Number of Lessons 16

  • Language English


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Course Outline
  • Objectives
  • What Is Compression Molding?
  • Advantages of Compression Molding
  • Disadvantages of Compression Molding
  • Compression Molding Applications
  • The Compression Molding Press
  • The Mold
  • Types of Molds
  • How to Create a Part
  • Process Variables
  • Fiber Orientation
  • Warpage, Voids, and Blisters
  • Sink Marks and Excessive Surface Roughness
  • Troubleshooting Defects
  • Compression Molding Safety
  • Summary
  • Describe compression molding.
  • Describe the advantages of compression molding.
  • Describe the disadvantages of compression molding.
  • Identify common compression molding applications.
  • Identify elements of a common compression molding press.
  • Describe molds.
  • Identify types of molds.
  • Explain how a part is created.
  • Define common process variables.
  • Identify common fiber orientations.
  • Distinguish between warpage, voids, and blisters.
  • Describe sink marks and excessive surface roughness.
  • Describe ways to troubleshoot defects.
  • Describe common safety measures for compression molding.
Vocabulary Term

applied force

The energy or effort provided to a machine to perform work.


Tiny bubbles on the surface of a composite part that are caused when volatile gasses are trapped during compression molding.

charge amount

The volume of composite material used to create a part. The charge amount is precalculated by an engineer.

charge location

The orientation of the composite material within the mold. Charge location is used to influence fiber orientation.

closing rate

A measurement of the time it takes to completely close the mold.


A material made by combining a binding resin with small filaments of solid material. Composites have the strength of metal, the light weight of plastic, and the rigidity of ceramics.

compression force

The physical load exerted on the composite material.

compression molding

A method of manufacturing composites whereby a charge is placed into a mold and then a matching mold applies compressive force to the heated charge.

continuous fiber

A type of fiber that covers the entire dimension of a part without a break or interruption. Continuous fibers are used in mat and cloth reinforcements.

cure time

The amount of time it takes for the molecules in a composite to cross-link. Longer cure times increase the cost of making the composite and the possibility of error being introduced to the curing process.

daylight opening

The distance between the opened molds in a compression molding press.

ejector system

A system of pins that are mounted within the compression molding press that are used to eject parts from the mold.


A molding process in which material is pushed or drawn through a die to create objects of a fixed cross-sectional profile.

fiber orientation

The physical arrangement of fibers inside a composite part. Mold design and charge location influence fiber orientation.


The overflow of excess composite material that can leak out during the compression process. Depending on part requirements, secondary operations are usually necessary to remove flash.

fully positive mold

A type of mold that has flush edges and completely closes to keep material inside the mold.

gel time

The amount of time it takes for the composite material to solidify into a gel. The gel point is reached when crosslinking begins to dominate the nature of the resin.

hydraulic cylinder

The actuator of a compression molding press. The hydraulic cylinder transfers the pressure and flow of fluid in a hydraulic pump into mechanical force and motion in order to open and close the mold.

hydraulic press

The machine used to create compression molded parts. The hydraulic press uses fluid power to create the necessary compressive force.

injection molding

A molding process in which resin is heated in a barrel and then injected into a mold by a reciprocating screw. The resin then cools in the mold and is ejected as a solid part.

knit line

An area of a molded part where separate material flow from two or more cavity entry points meet. Compression molding tends to produce a good interpenetration of material across the knit line.


A method of protecting employees from accidental machine startup through proper locking and labeling of machines that are hazardous to nearby employees.

mechanical properties

The characteristics of a material that determine how it reacts when it is subjected to some type of force that attempts to stretch, dent, scratch, or break it.

mold temperature

The temperature to which the mold is heated by the platens. The mold temperature must be high enough to cure the part, but not so high as to burn it.

multi-cavity mold

A type of mold consisting of multiple identical side-by-side designs that can produce multiples of the same part in a single cycle.

open flash mold

A type of mold that has space around its edges to allow excess material to push out to create flash. On some parts where surface finish is not critical, allowing flash can help produce a more consistent fiber orientation.


A flat metal surface that travels along metal rods to open or close a mold. Platens are used to heat the mold halves.

process control

An approach to manufacturing that attempts to closely monitor and control all aspects of the process. Process control methods involve collecting and analyzing data to help reduce error.

quality management

A process improvement method that focuses on increasing customer satisfaction at a reasonable cost to the company.


A viscous substance made from either synthetic or natural polymers. In essence, a resin is a polymer that has not yet hardened into its final form.

resin transfer molding

RTM. A resin impregnation system in which a closed die is filled with resin while the reinforcement fibers are held inside. Resin injection prevents styrene from being released into the air.


A breathing device worn to prevent inhalation of hazardous substances. Respirators may purify air from the environment or supply air to the wearer.

semiautomatic mold

A type of mold that is automatically opened and closed by the machine, but that requires an operator to place the charge.

semipositive mold

A type of mold that has drafted angles that make the part easier to remove and space around its edges where material can push out to create flash.

sheet molding compound

SMC. A rolled-up sheet in which the ingredients are not mixed together all at once. Instead, a pre-mixed, pre-initiated paste of resin and filler is applied to a moving sheet of film onto which strands of fiberglass are applied.

single cavity mold

A type of mold consisting of a single design that produces one part in a single cycle.

sink marks

Depressions on the surface of a part caused when a part contracts as it cools. Sink marks may also be caused by a charge that is too small to fill the mold completely.

split mold

A type of mold consisting of an upper and lower half that are compressed together to form a single part.

surface roughness

The most important variable in surface finish. Surface roughness is a measure of the level of unevenness of the part's surface.

thick molding compound

TMC. A common composite base material in which the ingredients are mixed together all at once. Thick molding compound is less flexible but allows operators to fill a mold faster.


Tiny bubbles in the internal structure of a composite part that are caused when volatile gasses are trapped during compression molding.

volatile organic compound

VOC. A chemical that releases vapors that are harmful to a person's health and the environment.


The deformation of a part after molding. Warpage is commonly caused by poor heat exchange and uneven cooling.