Transfer Molding

Transfer molding forms thermoset parts by forcing preheated material from a pot into a closed mold, supporting inserts, low flash, and consistent encapsulation.

Overview

Transfer molding (thermoset transfer molding) loads a measured charge of thermoset compound into a heated pot, then pushes it through runners into a closed cavity where it cures. It’s common for epoxy, phenolic, and silicone compounds, especially when you need controlled fill around features or inserts.

Choose it for encapsulation, electrical insulation, and parts with inserts where you want better material control than open compression molding and less fiber/insert disturbance than some high-shear injection approaches. It handles thicker sections well and can produce clean cosmetic surfaces with stable dimensions after cure.

Tradeoffs: cycle times are driven by cure, not cooling; material waste in runners/culls is higher than true thermoset injection; and thin walls and long flow lengths are limited by viscosity and gel time. Tooling is still a hardened, heated mold with gating/venting that must be designed for consistent flow and air evacuation.

Common Materials

  • Epoxy molding compound
  • Phenolic
  • Melamine
  • Silicone rubber
  • BMC
  • DMC

Tolerances

±0.003" to ±0.010"

Applications

  • IC and semiconductor encapsulation
  • Electrical connector bodies (thermoset)
  • Coil and transformer potting/encapsulation
  • Automotive sensor encapsulation
  • Appliance switch housings
  • Bushings and wear-resistant phenolic components

When to Choose Transfer Molding

Transfer molding fits medium to high volumes of thermoset parts that need consistent fill, good surface finish, and controlled encapsulation. It’s a strong choice for electrically insulating compounds, thicker sections, and parts with metal inserts or embedded components where cure stability matters.

vs Standard Injection Molding

Choose transfer molding when the part must be a thermoset (heat/chemical resistance, electrical insulation) and you need curing in a heated mold. It’s also useful when you want gentler, more controlled fill around delicate inserts than some high-shear injection setups.

vs Overmolding

Choose transfer molding when the ‘overmold’ is a thermoset encapsulation step and you need a measured charge and controlled flow into a closed cavity. It works well for fully encapsulating electronics or sensors where void control and cure performance drive requirements.

vs Insert Molding

Choose transfer molding for insert molding when you’re embedding metal terminals, bushings, or components into a thermoset body and want strong adhesion and high-temperature performance. The process can reduce insert movement versus methods that rely on higher injection shear or open compression loading.

vs Compression Molding

Choose transfer molding when you need better control of flow into thin features, around inserts, or into multiple cavities, with less risk of trapped air and surface defects. Transfer molding typically yields more consistent cosmetics and fill, at the cost of runner/cull waste.

vs Liquid Silicone Rubber (LSR) Molding

Choose transfer molding when the silicone is supplied as a solid/compound (not liquid) or when you need a pot-and-plunger style process for encapsulation with specific thermoset compounds. LSR molding is better for very high-volume, automated liquid feed and very thin, long-flow elastomer parts.

Design Considerations

  • Keep wall thickness as uniform as possible to reduce cure gradients, sink-like read-through, and post-cure distortion
  • Place inserts with positive mechanical location features and specify allowable insert shift; uncontrolled floating drives scrap
  • Add vents at end-of-fill and around insert pockets to prevent burn marks, voids, and incomplete encapsulation
  • Minimize thin walls and long flow lengths; confirm maximum flow distance vs gel time with your molder early
  • Specify critical dimensions after post-cure condition (if required) and call out any secondary deflashing requirements
  • Provide a clear cosmetic spec (A/B/C surfaces) and gate/parting-line restrictions; cosmetics drive tool complexity and cost