What is Compression Molding? (Its Process, Examples & More)

What is Compression Molding

What is Compression Molding?
Compression Molding is a plastic molding process in which the heated polymer is placed into an open heated mold, and then it is compressed to get the desired shape.

Compression molding process is generally used for molding of thermosetting plastics.

The raw material used for compression molding can be either in powder form, liquid form or in the form of pallets.

In plastic compression molding, the plastic is preheated with the help of infrared heaters or ovens which softens the plastic and helps in proper deformation.

Well, this was just an overview, but further I have discussed a lot more important things like, compression molding process, examples of compression molding, advantages of compression molding, disadvantages of compression molding as well as applications of compression molding.

So let’s dive right into it.

Compression Molding Process (3 steps)

compression molding process

The compression molding process is done in 3 steps.

  • Step 1: Loading of the raw material into the lower half of the mold
  • Step 2: Compressing the raw material between the two molds
  • Step 3: Opening the mold and removing the molded plastic

Let me explain each of these steps in detail.

Step 1: Loading of the raw material into the lower half of the mold

The molds used in compression molding process are made into two halves as it involves loading and removing the plastic from the mold.

In order to get the proper consumption of plastic during the molding, the amount of plastic to be loaded in the mold is precisely calculated.

If the amount of plastic kept in the mold is less, then it will not be able to produce the desired shape.

So the calculation is required for the precise amount of raw material to be kept in the mold.

During this step, the lower half of the mold is first heated to maintain the temperature of the plastic during the molding process.

Step 2: Compressing the raw material between the two molds

The raw material kept between the two molds is compressed by pressing the raw material between the upper mold and lower mold.

Because of the high pressure, the raw material (or plastic) takes the shape of the mold cavity.

The dimensional accuracy of the final molded plastic depends on the quality of mold design as well as amount of raw material placed in the mold cavity.

Step 3: Opening the mold and removing the molded plastic

The last step in compression molding of polymer is the opening of molding halves and then removing the molded plastic. 

The molded plastic is removed by pressing the knockout pin towards the upper side (see above image).

Well, this step is performed only when the plastic is cooled and perfectly solidified.

Compression molding machine

What is compression molding machine? Compression molding machine is the type of press machine that is oriented vertically and contains two platens molding halves for compressing the polymers.

In a compression molding machine, the pressure can be applied in two ways.

  1. By upstroke of bottom platen or
  2. By downstroke of top platen

The pressing operation is usually carried out by hydraulic power.

The molds used for compression molding are much simpler than the molds used for injection molding process.

In compression molding process, there is no need for any gating system as the raw material is directly placed in the mold cavity.

Compression moulding materials

The materials used for compression molding are listed below.

  • Epoxies
  • Urea formaldehyde
  • Phenolics
  • Melamine formaldehyde

By using these thermosetting plastics in compression molding, the components like plugs, switches, sockets, dinner plates, cookware handles, etc can be manufactured.

Having said that, let’s now discuss the advantages and disadvantages of compression molding process.

Advantages of compression molding

Advantages of compression molding process are mentioned below.

  • Compression molding process is less expensive and it has low maintenance cost.
  • Scrap material is negligible in this process.
  • There are no residual stresses in the molded plastic.
  • Molds can be prepared with very less cost of manufacturing.
  • Labor cost is also very less as most of the work is done by compression molding machine.
  • Good dimensional stability is achieved by this process.
  • Very less shrinkages are seen on the material.

Disadvantages of compression molding

Disadvantages (or limitations) of compression molding process are mentioned below.

  • The cycle time required for compression molding is more as compared to that of other similar processes.
  • This process leaves a parting line on the molded plastic.
  • The depth of the molded plastic is limited to 2-3 times their diameter.

Applications/Examples of compression molding

Applications of compression molding process are mentioned below.

  • Compression molding is generally used for molding of large, flat or slightly curved parts.
  • Examples of compression molding products are;
    • Brush handles
    • Trays
    • Circuit breaker
    • Cookware knobs
    • Blower fan blades
    • Automotive parts
    • TV cabinets
    • Dinnerware plates
    • Radio cases
    • Electrical plugs and sockets, etc.

External links:
Compression molding: Image by ariel cornejo, CC BY-SA 4.0, via Wikimedia Commons

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