Compression Molding

Compression molding forms thermoset or rubber parts by pressing a pre-measured charge in a heated mold, producing durable parts with low scrap.

Overview

Compression molding presses a pre-weighed charge of thermoset compound (or rubber) into a heated mold, then cures under pressure. It’s common for medium-to-high volume parts that need heat resistance, electrical insulation, chemical resistance, or high strength from filled materials (glass/mineral). Tooling is typically simpler than many multi-gate injection tools, and material utilization can be good, especially for sheet/bulk molding compounds.

Choose it for thicker sections, large projected areas, and parts where minor flash is acceptable or can be trimmed. Expect longer cycle times than injection molding due to heat-up/cure, and more post-processing (deflashing, trimming, secondary ops). Tight cosmetic requirements, very thin walls, and complex undercuts are harder; material flow can limit fine details and very long flow lengths. Part-to-part variation is usually driven by charge placement, cure control, and flash management.

Common Materials

  • Phenolic
  • Melamine
  • Epoxy (molding compound)
  • SMC (glass-filled polyester)
  • BMC (glass-filled polyester)
  • Silicone rubber

Tolerances

±0.005" to ±0.010"

Applications

  • Electrical connector housings
  • Circuit breaker components
  • Automotive under-hood heat shields (SMC)
  • Appliance handles and knobs
  • Industrial gaskets and seals
  • Composite covers and enclosures

When to Choose Compression Molding

Pick compression molding for thermoset/rubber parts that benefit from high-temperature performance, electrical insulation, or filled compounds, especially with thicker walls and larger footprints. It fits medium to high volumes where tooling simplicity and material robustness matter more than ultra-fast cycles or ultra-thin features. Plan for trim/deflash operations and process controls around cure and charge placement.

vs Standard Injection Molding

Choose compression molding when the part is thermoset/rubber or heavily filled (SMC/BMC) and you want robust properties with simpler tooling and fewer flow-related defects. It also suits larger, thicker parts where injection pressures and long flow lengths become problematic, and where minor flash and trimming are acceptable.

vs Overmolding

Choose compression molding when you don’t need a multi-material bond and the functional requirements are met by a single thermoset/rubber compound. It’s often simpler and more economical than managing two-shot sequencing, adhesion validation, and tighter dimensional control needed for overmold interfaces.

vs Insert Molding

Choose compression molding when inserts can be placed without complex gating and you can tolerate some flash control around the insert features. It can reduce shear and flow forces on delicate inserts compared to injection, and it’s well suited to thermoset electrical parts with metal terminals that benefit from heat resistance.

vs Transfer Molding

Choose compression molding when the geometry is relatively open and you want the simplest mold architecture without pots/runners for transfer. Compression can also reduce material waste from runner systems and can be a better fit for large-area parts and SMC/BMC charges.

vs Liquid Silicone Rubber (LSR) Molding

Choose compression molding for silicone parts when volumes are lower, tooling budget is tighter, or the part is thick and doesn’t need the high throughput and automation typical of LSR injection systems. It also works well for sheet/solid silicone charges where metering and cold-runner LSR infrastructure isn’t justified.

Design Considerations

  • Keep wall thickness as uniform as practical; large thick-to-thin transitions drive knit/flow issues and cure distortion
  • Specify parting line, flash land, and acceptable flash limits; trimming/deflashing method should be agreed early
  • Avoid deep, narrow ribs and long thin flow paths; use generous radii and draft to support charge flow and release
  • Place critical dimensions away from the parting line and from areas likely to be trimmed or deflashed
  • Call out insert locations and retention features clearly; design for positive fixturing so inserts don’t shift during press closure
  • Define cure-related requirements (post-cure, Tg/heat resistance, dielectric targets) so the molder can set the right cycle and material system