What is Compression Molding?

“Molding” is an umbrella term encompassing all of the processes that utilize molds to shape material (typically rubber or plastic) into the desired parts and products. Manufacturers are able to employ a variety of molding methods to suit different product and production requirements. One of the most commonly used methods is compression molding. 

What Is Compression Molding? 

Compression molding of rubber compounds is a technique that involves the compression of pre-formed and/or pre-heated raw material in a heated mold cavity (sort of like making a waffle in a waffle iron).  The material experiences high pressures within the closed mold, forcing it to conform to and, once cooled, retain the shape of the die cavity. Below, we provide a more comprehensive overview of the process as well as highlight some of the advantages it has over other manufacturing methods. 

An Overview of the Compression Molding Process

While all molding methods operate under the same basic principles, they differ in the exact steps taken to create a molded part or product. Compression molding operations typically follow these steps: 

  1. Designing and creating/sourcing the mold. Before the compression molding process can begin, the manufacturer must first obtain an appropriate two-part compression mold to create the desired component. 
  2. Creating the pre-forms. At the start of the compression molding process, the manufacturer must form the molding material into a pre-form. The carefully weighed pre-form might have the general shape of the desired component but be larger to ensure the material reaches and fills all areas of the mold cavity during the molding process. 
  3. Heating the mold. Before the pre-forms are placed in the mold cavity, the mold must be heated to facilitate the curing of the material that has a heat sensitive catalyst milled into it
  4. Placing the preforms. The pre-forms are placed into the cavity of the heated mold, and then the mold is closed.  
  5. Applying heat and pressure. Heat and pressure is applied to the mold and, consequently, the pre-form. This causes the material to fill and conform to the shape of the mold cavity. Any excess material is free to leak out of the mold cavity. 
  6. Ejecting the molded component. Once the component has been formed, it is released from the mold. 
  7. Removing excess material. If the molded component has excess material (i.e., flash), it is removed. 

Advantages of Compression Molding

Compared to other molding and manufacturing methods, compression molding offers a number of advantages, including the following: 

    • Broad design flexibility. The compression molding process is highly versatile. It can be used to produce a wide range of components in various shapes, sizes, and complexities. 
  • Strong parts. Compression-molded parts generally have little to no residual stress. 
  • Less waste generation. Compared to injection molding and transfer molding, compression can generate less material waste. This quality makes it ideal for molding operations involving expensive materials.  
  • Low tooling costs.  The compression molding process requires tooling that is much simpler and, consequently, less expensive than the tooling used in the injection molding or transfer molding processes. 
  • Suitability for large parts. Since material is loaded directly to the mold cavity, the part has less weight limitations except for those imposed by the size and pressure capacities of the press and the way the mold is designed.

Partner with the Experts at ETI for Your Compression Molding Needs

If you’re looking for the right partner for your next compression molding project, turn to the experts at Elastomer Technologies, Inc. (ETI). We are an industry-leading contract molding provider. One of our core service offerings is compression molding. We can compression mold a variety of materials, including butyl, EPDM, nitrile, polyisoprene, silicone, and Viton®, into parts of a wide range of sizes and complexities. 

To learn more about our compression molding capabilities, check out our compression molding services page. 

To discuss your specific molding needs with one of our experts, contact us today. 



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