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Everything about compression molding

Join Date: 2018-08-19
compression molding

What is Compression molding?

Compression molding is a molding process in which a preheated material is put into a preheated compression mold. The mold is then closed and the temperature and pressure are used to force the material to fill all areas of the mold and harden the material before ejecting the product. The product may contain flash, which can be cut off. Compared with several other molding methods, there is no runner and gate system in the compression mold. Compression molding has the ability to make complex parts without having to spend too much time and steps processing the parts, while putting part cost and turnaround time first, so for manufacturers in many industries, compression molding is a an advantageous process.

The compression molding process can produce parts of varying length, thickness and complexity. The strength of the objects it produces is also high, making it an attractive process for many different industries.


Thermoplastic and thermoset composites are the most common types of materials used in compression molding. MDC has experience with the following compression molding materials:

  • Bulk Molding Compound (BMC)
  • Sheet molding compound (SMC)
  • Glass Mat Reinforced Thermoplasitcs (GMT)
  • Long Fiber Thermoplastics (LFT)
  • Carbon fiber

compression molding process includes four main steps:

  • Created a high-quality compression mold that exactly matches the size of the part and requires full consideration of the environment and performance requirements of the part. This mold is then installed in a press and heated.
  • Preform the raw materials of the parts. Preforming is a crucial step to help improve the performance of the finished part.
  • Insert the preformed part into a heated mold. The mold is then compressed at a very high pressure, typically in the range of 800 psi to 2000 psi (depending on the thickness of the part and the type of material used).
  • After releasing the pressure, remove the part from the mold. At this point, all resin burrs will also be removed.

When using compression molding methods, the following factors must be considered:

  • material
    Thermosets and thermoplastics are unique to molding methods. Thermosets do not change once they are heated and shaped, while thermoplastics can be reheated and cooled as needed. But it is important to ensure that no matter which material is used, the material should cover all areas and gaps in the mold and be evenly distributed.
  • pressure
    In the heated state, for composite parts that are compression molded, the higher the pressure, the stronger the final part.
  • temperature
    Molding temperature will vary. Some plastics require temperatures in excess of 700 degrees, while others require temperatures below 200 degrees.
  • cycle
    The molding cycle is also a factor. Material type, pressure, and part thickness are all factors that determine how much time the part takes in the mold. For thermoplastics, parts and molds will need to be cooled to a certain degree to make the parts rigid.

Compression molding application
Automotive part molds are the most common compression mold, Composite moulded automotive parts are lighter and stronger. In the construction industry, molded parts are more accurate and more durable...

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