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Everything You Need to Know About Compression Molding

Compression molding was developed in the 1850s, yet this traditional technique of creating rubber products is still popular today.

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It is an ideal method for creating rubber parts in low, medium, and bulky volume, which is one among several benefits it offers.

All that is required is to take a rubber compound and create a preform whose shape is similar to that of the end product, but larger than the final shape.

This molding process is also made possible by using heat and pressure, which helps to mold the plastic resin or thermoset rubber into the desired shape.

Let’s take a closer look at the compression molding process and what makes this method of molding parts peculiar.


What is Compression Molding?

Compression molding is a method of molding in which a pre-heated polymer is placed in a heated, open mold cavity.

Next, the mold is closed using a top plug, for the polymer to take the shape of the mold.

It is worth noting that thermoset composites are commonly used in compression molding.

This molding can produce objects of different shapes whose length and thickness vary.

As such, it has found several use cases in various industries.


How is Compression Molding Carried Out?

Different steps see to the full creation of a product that adopts the shape of the compression mold.

For instance, if thermoset composites are used during the molding process, these steps will include:

  1. Creation of a high strength tool with two parts whose dimension is the same as the product to be formed. The tool is installed in a press and then heated.
  2. The thermoset composite is preformed into a shape similar to that of the tool.
  3. The preform is transferred into a heated mold and compressed under very high pressures. The pressure may range between 800psi to 2000psi, and it is highly dependent on the thickness of the part and the type of material used.
  4. The part is finally taken out of the mold after the pressure has been released and it is left to cool. Resins left in the mold are also removed and may be recycled at a later time.

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5 Advantages of Compression Molding

There are several advantages of compression molding, which has made it a popular technique.

Some of these advantages are:

1. Design of High-Quality Products

Advanced composites used for molding are lighter, stronger, and stiffer. In the same vein, they are less corrosive than to metallic parts which have given them an advantage over the latter.

Manufacturers who still rely on metal parts have also discovered that it is easier to convert an object that was initially designed for metal into a compression molding part. As a result, there is the ability to match the metal part geometry by employing this technique.

2. Little Waste of Material

During this molding process, less material is lost.

The latter can be largely attributed to the fact that it does not rely on sprues, gates, or runners.

These are tooling features that materials that rely on other production methods have to undergo before they enter the mold cavity. What this does, in the end, is consume more material which adds an additional cost.

3. Creation of Complex Parts

Compression molding enables very complex parts to be created. These parts have complicated geometry in comparison to typically laminated composites.

This advantage is evident in longer fibers being generated in this type of molding than the plastic injection molding process, and as such, this yields stronger and stiffer materials.

What this also means is that complex parts with various areas of application can be produced while also prioritizing part cost and turnaround time. Nonetheless, it’s worth pointing out that their production speed is not comparable to plastic injection molding.

Hence, it is the common ground between laminated compound fabrication and injection molding.

4. Suitable for Large Parts

Large parts will require bulk materials for them to be manufactured. In this aspect, the form of molding that is most suitable is compression molding.

What’s more, the material is placed directly on the mold cavity and as such, there is no limitation to the weight of the part that can be created, except for the tonnage and size of press needed.

On the contrary, the volume of the injection barrel poses a limitation to the part’s weight that can be produced in injection molding presses.

5. Suitable for Small Production Runs

Compression molding is a cost-effective solution when it comes to designing small parts.

The reason cannot be far-fetched since the cost of manufacturing molding equipment is lower. The same goes for setting up a press to run the production parts.

Despite this, the higher capital cost of an injection molding tool may be better at certain times since it also offers a lower cost to the parts it designs.

As a result, consideration has to be given to the cost and benefit while determining the best molding process to use for a new product.


Compression Molding Vs. Injection Molding

Several comparisons can be made between compression molding and injection molding to ascertain their pros and cons.

First off, injection molding involves the heating of plastic before it is injected into the mold cavity.

It is a method of molding complex shapes as well as changing sizes and uptime efficiencies.

What’s more, injection molding is more suitable for a lower volume run since line changes can be made easily as well as choices for various materials.Contact Us for PET Compressor Parts and Repair

Benefits of Compression Molding Over Injection Molding

For starters, compression molding offers low tool maintenance. As such, it is easier to run higher volumes with less downtime.

On the same note, this molding process lacks a gate as is evident on injection molding, and for this reason, the vestige gate is avoided, and visual appearance is improved.

It offers the ability to have more efficiency during color changeovers.

On the other hand, this molding method has a limitation when it comes to sophisticated geometrics and larger diameter closures that are over 48mm.

Here, injection molding is suitable since it allows complex part geometrics which could help to achieve special closure features.



The benefits of compression molding have made it an important molding process in the design of rubber parts.

These advantages translate into cost benefits which have given some advantages over injection molding and extrusion molding processes.

Thus, as a manufacturer looking to design quality products at the lowest cost, you need to give this molding process considerable thought.

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