Transfer Molding

Transfer molding drives preheated thermoset material into closed cavities for precise, flash-controlled parts around inserts and electronics at medium to high volumes.

Overview

Transfer molding is a thermoset molding process where preheated compound is forced from a transfer pot into closed cavities using a plunger. It excels at molding brittle or highly filled thermoset materials, tightly encapsulating inserts and electronics with good dimensional control and relatively low internal stresses. Typical parts are small to medium, with moderate to high production volumes.

Use transfer molding when you need robust, high-temperature thermoset parts, especially with fragile inserts, lead frames, coils, or electronic components that cannot tolerate the higher shear of standard injection molding. It offers better surface finish and detail than compression molding but has higher tooling costs and more complex runners and flash control. Tradeoffs include slower cycle times than thermoplastics, irreversible curing (no re-melt), and the need to manage flash, venting, and post-cure. When engineered well, it delivers durable, stable parts ideal for electrical, electronic, and under-hood environments.

Common Materials

  • Epoxy thermoset
  • Epoxy molding compound (EMC)
  • Phenolic thermoset
  • BMC thermoset (bulk molding compound)
  • Polyester thermoset
  • Silicone rubber

Tolerances

±0.002" to ±0.005" on critical features, looser on large overall dimensions

Applications

  • Semiconductor and IC encapsulation packages
  • Electrical and automotive connectors
  • Sensor and coil housings with encapsulated windings
  • Relay and switch bodies
  • High-temperature appliance and breaker components
  • Encapsulated electronic modules with metal inserts

When to Choose Transfer Molding

Choose transfer molding for thermoset parts that need good dimensional stability, high-temperature resistance, and reliable encapsulation of inserts or electronics at medium to high volumes. It is especially useful for brittle, heavily filled, or abrasive compounds that do not flow well in standard injection molding. Best fit is small to medium parts where flash control and package reliability matter more than ultra-low part cost.

vs Standard Injection Molding

Pick transfer molding when you need thermosets, lower shear, or reliable encapsulation of delicate inserts and electronics that might be damaged by high-pressure thermoplastic injection. It also handles more abrasive and highly filled compounds with less wear risk to tooling than standard injection molding of thermoplastics.

vs Overmolding

Choose transfer molding instead of overmolding when you are encapsulating thermoset compounds around sensitive electronics or complex insert stacks in a single cure step. It is better suited when the base structure is a metal or lead frame and the goal is a sealed package, not a soft-touch second shot on a thermoplastic base.

vs Insert Molding

Use transfer molding over conventional insert molding when the material must be a thermoset and the inserts are fragile, tightly spaced, or part of a lead frame. The lower-flow, pot-fed process reduces movement or damage to inserts and improves wet-out around fine features and terminals.

vs Compression Molding

Choose transfer molding rather than compression molding when you need better dimensional control, finer details, and reduced variation in part-to-part weight and thickness. The controlled transfer of material into a closed cavity reduces knit lines, improves fill of thin sections, and typically shortens cycle time once tooled.

vs Liquid Silicone Rubber (LSR) Molding

Select transfer molding over LSR molding when you need rigid or glass-filled thermoset parts or hermetic-style encapsulation of electronics instead of flexible elastomer parts. Transfer molding is better for epoxy and phenolic compounds where high temperature, creep resistance, and long-term dimensional stability are critical.

Design Considerations

  • Avoid extreme thin walls; target uniform wall thickness and clear minimum section limits for reliable cavity fill and reduced short shots
  • Add generous draft and radii to reduce demolding stress on brittle thermoset parts and to extend tool life
  • Clearly define gate and transfer pad locations, keeping flow across critical surfaces and away from sensitive inserts and sealing interfaces
  • Specify insert locations, positional tolerances, and allowable movement; design insert features (holes, undercuts) to mechanically lock into the thermoset
  • Plan for flash control: define acceptable flash thickness and parting-line locations on non-critical surfaces to simplify secondary deflashing
  • Call out only truly critical dimensions tightly and allow more open tolerances elsewhere to keep tooling, processing, and inspection costs in check