COMPRESSION MOLDING: Process Capabilities, Parameters & Types

Compression molding

Hello Readers, welcome to your own website to understand each and every topic related to the manufacturing process where we transform complex content into simpler ones. In this article, we are focused to cover compression molding, process capabilities, parameters, types of compression molding, materials used in this process, material properties affected by this process, advantages, disadvantages, and applications.


In compression molding, a preshaped charge of material, premeasured volume of powder, or viscous mixture of liquid-resin and filler material is set straight forwardly into a heated mold cavity that typically is around 200°C (400°F) but can be much higher.

Forming is finish under pressure from a plug or from the upper portion of the die because of this the process is some what looking same to a closed die forging of metals.

Pressures range from about 10 to 150 MPa (1400 to 22,000 psi).

There is a flash formed, which continuously is eliminated by trimming or by some different methods.

Regular parts made are dishes, handles, container caps, fittings, electrical and electronic segment, washing-machine agitators, and housings.

By the compression molding process the fiber-reinforced parts with chopped fibers also are formed mostly.

Compression molding is utilized fundamentally with thermosetting plastics, with the unique material being in a mostly  polymerized state.

In any case thermoplastics and elastomers are prepared by compression molding.

Restoring times range from in between 0.5 to 5 minutes, relate on the material and on part thickness and shape.

If there is thicker the material, the more it will take to cure.


Three types of compression molds are available:

•  Flash type: for shallow or flat part.

•  Positive type: for high-density parts.

•  Semipositive type: for quality production.

Undercuts in parts are not suggested in any case, dies can be designed to open sideways to permit expulsion of the molded part.

All in all, the complexity of parts created is small than that from injection molding, but the dimensional control is better.

Surface areas of compression molded parts may range up to about 2.5 meter square

Due to their relative simplicity, dies for compression molding typically are less expensive than those which are used in injection molding.

They normally are made of tool steels and may be chrome plated or cleaned for an improved surface finish of the molded product.

The material is placed between two molds, and pressure is applied.

The molds may be either at room temperature or heated to accelerate hardening of the part.

The material should be a bulk molding compound, which is a viscous, sticky mixture of polymers, fibers, and additives.

Generally, it is molded into the shape of a log, which subsequently is cut or sliced into the desired shape.

Fiber lengths generally range from 3 to 50 mm (0.125 to 2 in.), although longer fibers of
75 mm (3 in.) also may be used.

Sheet molding compounds also can be processed by compression molding.

These compounds are same to bulk molding compounds, only the difference is that the
resin fiber mixture is laid between plastic sheets to make a sandwich that can be
handled easily.

The sheets have to be removed prior to placing the SMC in the mold.

Compression molding cycle is followed by two step:
1. Preheating
2. Pressurizing.

Compression molding is generally used for thermosetting plastics.


1. The quantity of charge (molding material) put into the mold.

2. Pressure of the molding process: Range of pressure 2000-3000 psi (13.8-20.7 MPa)

3.Mold temperature: Temperature range 300ºF to 375ºF (149⁰C- 191⁰C)

4.Cure time variables: The duration needed to harden thermosetting material to partial complete polymerization is called cure time


Types of compression molding
Types of compression molding (a) positive (b) semi positive (c) flash and (d) Die design for making compression molded part.

1. Sheet molding compound (SMC):

Setting a reinforcement, like as a glass mat, in between sandwiching layers of a thermoplastic.

2. Bulk molding compound (BMC):

Billet is utilized with efficiently set and provide long reinforcement.


Thermosetting polymers.

Fiber reinforced composite.


Ultra High Molecular Weight Polyethylene (UHMWPE).

Long Fiber reinforced thermoplastics.




Anisotropic property (fiber orientation).

Shrinkage and wrapage.


It has a lowest cost.

There is more uniform density.

In this molding, uniform shrinkage due to uniform flow.

In compression molding improved impact strength due to no degradation of fibers during flow.

It has a  dimensional accuracy.

In this molding internal stress and warping are minimized.


For compression molding curing time is large.

In this molding uneven parting lines present.

The scrap cannot be reprocessed.


It is used for dinnerware.

It is also used for buttons.

Used in knobs.

In appliance housings.

It is used in radio cases.

This molding is used for automotive exterior panels especially for commercial vehicles.

Also used in ash trays & electrical parts.


We have covered all the important concepts related to compression molding. Hope you all are crystal clear with understanding all the concepts mentioned here. If you have any questions please use the comments section to get in touch with us. Till then have fun and always keep reading!

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