Compression Molding

Compression molding forms thermoset or rubber parts by pressing preheated material in a heated mold, suited to thick, durable, electrically or thermally resistant components.

Overview

Compression molding shapes preheated thermoset or rubber charges in a heated, matched metal tool under high pressure. The material flows to fill the cavity, cures, and permanently sets, making this process ideal for rugged parts that must hold properties at elevated temperatures, resist chemicals, or provide electrical insulation. It handles thick sections, high glass-fill, and fiber-reinforced compounds better than most melt-flow processes.

Use compression molding for medium to high volumes where tooling cost must stay reasonable and part geometry is not extremely intricate. Expect modest feature detail, visible parting lines, and some flash that may need deflashing. Tolerances are looser than precision machining or high-end thermoplastic injection, but secondary machining can tighten critical interfaces. Tradeoffs: slower cycle times than thermoplastic injection, limited undercuts, and more variability from charge placement—but strong, dimensionally stable, high-heat parts at competitive piece cost.

Common Materials

  • Phenolic thermoset
  • Epoxy molding compound
  • Bulk molding compound (BMC)
  • Sheet molding compound (SMC)
  • Silicone rubber
  • Natural rubber

Tolerances

±0.005" to ±0.010" on molded features; tighter on critical surfaces with post-machining

Applications

  • Electrical switchgear housings and covers
  • High-temperature electrical insulators and terminal blocks
  • Automotive brake and clutch components
  • Cookware and appliance handles and knobs
  • Pump and valve housings in corrosive environments
  • Rubber seals, gaskets, and vibration isolators

When to Choose Compression Molding

Choose compression molding for thermoset or rubber parts that need high heat resistance, electrical insulation, or structural performance in thicker cross-sections. It fits medium to high production volumes where you can accept moderate feature detail and tolerances, often with secondary machining on critical interfaces. It works best for relatively simple parting-line-based geometries without complex undercuts.

vs Standard Injection Molding

Pick compression molding when you need thermoset materials, high glass-fill, or thick, heavily reinforced sections that are difficult to fill with thermoplastic injection. It is better suited for high-heat, dimensionally stable, electrically insulating parts where permanent cross-linking is a benefit, and ultra-fast cycle times or very fine cosmetic details are not the priority.

vs Overmolding

Choose compression molding when the entire part can be a single thermoset or rubber material and you do not need to bond onto a rigid substrate in one molding step. It simplifies tooling and processing for rugged, one-material components, especially where heat resistance and structural performance matter more than integrated soft-touch or multi-material features.

vs Insert Molding

Select compression molding when you can press-fit or assemble metal inserts after molding, or when the inserts would not tolerate the pressures and temperatures of thermoplastic injection. Compression molding works well when the main driver is a robust thermoset matrix with moderate integration of hardware, and you can accept secondary assembly instead of fully encapsulating inserts in-cycle.

vs Transfer Molding

Use compression molding instead of transfer molding when part geometry is simple, flash control is not ultra-critical, and you want lower tooling cost and simpler molds. It is a good fit for larger, thicker parts where slightly higher material waste or less precise flow control from transfer pots is not justified.

vs Liquid Silicone Rubber (LSR) Molding

Choose compression molding for high-consistency (gum) silicone or rubber parts where cycle time can be slower and geometry is straightforward. It is often more economical for large, thick, or lower-volume rubber components where the high tooling and metering equipment costs of LSR injection are not warranted.

Design Considerations

  • Keep wall thickness relatively uniform and avoid extreme thick sections to reduce cure time, sink, and internal stresses
  • Provide generous draft on cavity and core surfaces, especially on deep features, to aid ejection and reduce part damage
  • Plan parting lines along simple, accessible surfaces and accept that some flash and post-trim operations may be required
  • Avoid fine ribs, sharp corners, and very thin features; compression-molded materials do not fill tiny details as well as thermoplastics
  • Specify which dimensions are critical and allow secondary machining or grinding on those features rather than tight tolerances across the entire part
  • Design charge locations and overflow/flash lands so material flow paths are short and balanced, improving fill and reducing porosity or knit issues