What is Compression Molding?

Not sure when to choose injection molding vs. compression molding? Xometry has the answers!

By Greg Paulsen · May 14, 2020

Compression molding is a process very similar to injection molding in that the process requires a machined steel or aluminum tool with cavities, which is then filled with the part material to form the final shape of the part. Compression molding uses rubber and fluorosilicone materials as well as some thermosetting materials.

But unlike injection molding, the material is not liquified and injected into the tool through a gate. Instead, a measured amount of pre-heated material called a charge is placed into the tool cavity and compressed and formed into the shape of the part as the tool closes. Because the charge is placed in the cavity, compression molding platforms are typically vertical presses moving up and down. Tooling for compression molding is typically cheaper than injection molding due to reduced requirements around parting, ejection, and gating.

Compression molding diagram

Pictured: Compression molding clamps a pre-form charge of rubber, thermoset, or silicone into a cavity to form the shape of the part

Compression molding makes cost-effective rubber and silicone parts. It is very useful for geometries that have undercuts, like bellows, phone cases, gaskets, and even footwear. Compression molding typically results in very low residual stress. In injection molding, a higher level of residual stress often causes warping, shrinkage, or sink deformation. Because of the tool simplicity, compression molding can be the most economical option for large parts.

A compression-molded rubber case for Feevr device

Pictured: A compression-molded rubber case secures a thermal camera and smartphone to the Feevr device

A recent example is the rubber housing for the Feevr contactless thermal imaging device. This required several undercuts and an uneven wall thickness, which made it an ideal candidate for compression molding. An added advantage of compression molding this part was that the final product did not retain any gating marks and knit lines typical of injection molding, which can sometimes change the part’s cosmetics.

When determining whether you should compression mold or Injection mold your parts, compression molding is still ideal for thin-walled and complex geometries as well as for rigid materials. Compression molding typically has slower production cycles than injection molding, which can become challenging in mass production. Compression molding may also have more flash, or excess attached material, than injection molded parts, which must be manually removed.

A Xometry molding engineer can help you identify the best molding process for your project depending on the design, requirements, total quantities needed, and your budget. Check out Xometry’s injection molding services page to find out more about molding options and design tips.

Posted in Manufacturability Tips

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