Compression and Transfer Molding of Thermoset Materials

Compression Molded Thermosets

Thermoset materials have excellent physical property characteristics when compression molded. Structural parts molded from BMC or SMC can be an excellent alternative to cast or machined aluminum, and result in significant weight savings without sacrificing strength. Phenolics and Polyesters can also deliver exceptional cosmetic qualities. Superior heat resistance, compressive strength, flexural modulus (stiffness) and dimensional stability are all proven attributes of compression molded thermosets.

Reliance Engineering has extensive experience in compression molding of the following materials:

  • Phenolics
  • Bulk Molding Compound (BMC)
  • Sheet Molding Compound (SMC)
  • Thermoset Polyester, Vinyl Ester
  • Epoxy
  • Diallyl Phthalate (DAP)
  • Silicone

Compression Molded Thermosets – Parts


What is Compression Molding and Transfer Molding?

Compression molding of thermoset materials is a manual process that transforms granular, bulk or sheet materials into a molded shape by placing the material into an open mold cavity and then closing the mold. Thermoset transfer molding includes the additional process of pre-forming a “pill” of material before being injected into a closed mold via the mold’s transfer pot, runner and gate system.

Thermoset materials are cured by an irreversible chemical reaction under heat and pressure. The result of this reaction is a highly cross-linked molecular structure and why Compression and transfer delivers molded thermosets can retain their properties at elevated temperatures.

The compression process starts with granular bulk or sheet materials added into the mold cavity. Closing the mold delivers the force needed for the material to flow in the mold which is heated to 300 degrees or higher. This process starts the curing. Since there is no runner and gate system in a compression mold, the mechanical properties of compression molding are better than identical parts molded by transfer molding or thermoset injection molding.

Transfer molding generally requires applying a preheated preform of material into the mold pot before a plunger applies pressure to the material loading, forcing it through a runner and gate into the mold cavity. Thermoset transfer molding is considered for more precise precision geometrics. It creates less flash than compression molding. Transfer molding is excellent for insert molding because the molding machine clamp opens and closes vertically, and retention and seal off of the insert is more easily accomplished through a closed mold.

The extra material left behind in the mold must be disposed of, since cured thermoset material from sprues and runners cannot be reground and reprocessed, as opposed to thermoplastic injection molding. When the mold cavities are filled, the parts must cure to a solid form. The mold opens for part removal, and parts are ejected and removed by hand or automated equipment.


Thermoset Transfer and Compression Molding Advantages

  • High temperature thermoset materials
  • Increased dimensionally stable
  • Decreased wall thickness variations