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A Guide to Everything You Need to Know About Compression Molding Tools

by Nice Rapid | Jun 27,2022 | Compression Molding

Did you ever play with Play-Doh when you were a kid? Remember those molding the clay to create something out of a lump of clay? Well, compression molding is almost the same, only more advanced.

This post will discuss the basics of compression molding and what goes on behind the process including the backbone of its operations - tooling.

What is Compression Molding?

Compression molding is an excellent, cost-effective manufacturing technique used in creating a variety of rubber products. Aside from rubber, it can also create different products from thermoplastics, thermosets, and elastomers.

These parts are produced in high-volume, dimensionally precise, temperature-resistant, and high-strength properties with great surface quality. Compression molding produces parts in a variety of shapes and sizes. It can make thin-walled containers and thick solid shapes that we use as kitchenware, helmets, electrical housings, airplanes, and automotive components.

Parts from compression molding can range from one ounce in weight to over a hundred pounds and can copy the strength and complex geometries of metal. It can also have its anti-corrosive and insulative qualities and take many forms.

The most vital component of this technique is the mold. It develops the final shape, which contains the male and female counterparts. Once the mold is together, the gap (or cavity) determines the shape of the part. The excess material will escape from the cavity and into the overflow grooves, which can be trimmed during finishing.

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Primary Components of the Compression Mold

A compression mold has different parts that each play a role in successfully molding a part. It includes the following:

  • Top and bottom plate
  • Alignment pins and bushings
  • Mold cavities
  • Flow or overflow groove

How Molds are Made

The steps in creating the mold in compression molding are simple. To easily understand the process we have outlined it here.

  • Designing: It requires experience in understanding what is needed by the project to design the mold. The design should ensure that the parts will be made correctly and reduce any signs of imperfections.
  • Ordering of Materials: The mold is made from either aluminum or steel.
  • Machining of the Metal Blocks: Mostly CNC machining is used in creating the molds. These are made using computer-controlled cutting tools, carefully removing material from the blocks to shape the elements of the molds including overflow grooves and cavities.
  • Finishing: The tool will go through a fine-tuning process to ensure that the mold will not cause any flaws in the final product.
  • Mold Hardening: to endure the durability of the mold, it goes through a hardening process. Here that mold is exposed to a high temperature and followed by a cooling stage.

Choosing the Mold Material

Compression molds are commonly made with either aluminum or steel. There are advantages and disadvantages to each of these materials. Here are some of the basics about each material to have a better understanding of when they are appropriate to use.


Many of the compression molds you see are from machined out hot rolled steel (HRS). This material is thermally stable. It can be in molten form when heated and becomes solid when cooled after forming. It is not recommended for compression molding rubber as it tends to warp when exposed to higher molding temperatures.


Aluminum is a soft metal commonly used as a mold because of its durability. However, since it is a soft metal, it is more suitable for prototyping parts and low-volume runs. It is relatively easy to machine, faster, and easier to achieve a finer surface finish. One of the disadvantages of aluminum can easily get damaged by clamping pressure.

What is a Flash?

In compression molding, flash is formed when the material leaks between the mold’s surfaces. It is mainly a cosmetic defect but could also impact the sealing capabilities of the part. During the process, the flash is removed by hand or by using a knife, scissors, and other cutters after the parts are molded.

To avoid flash, the mold should be clean as the dirt can cause the parting lines to mismatch. Another way is to check that you have proper clamping pressure and if the molding tool is worn down.


Compression molding is an excellent manufacturing technique for producing parts without the significantly high cost of production like in injection molding. To meet the requirements, you need to put a lot of effort into the mold-making process. The compression mold will dictate how your part will turn out.

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