Compression Moulding of Rubber vs Thermosets vs Composites and Carbon Fibre

Compression moulding is used for three very different families of material: rubber (elastomers that are vulcanised), thermosets (SMC, DMC/BMC, phenolic, melamine and urea that crosslink into rigid parts), and advanced composites such as carbon fibre prepreg and glass mat thermoplastics. The press and the principle are the same: place a charge, close a heated mould, apply pressure. But temperatures, pressures, cycle times and mould design differ a great deal.

At Prime Tools our core work is thermoset compression moulds. Because buyers often compare processes, this guide explains all three side by side so you know what kind of mould and toolmaker your part needs.

The three families at a glance

Rubber compression moulding

Rubber compression moulding is the oldest and simplest rubber process. A pre-weighed pre-form of uncured rubber compound is placed in each cavity. The heated mould closes and the rubber flows and vulcanises. After cure the flexible parts are stripped out, often by hand or with air.

Typical parts: O-rings, flat gaskets, oil seals, grommets, diaphragms, bushes, bellows, rubber-to-metal bonded parts and keypads.

Mould features: multi-cavity plates, overflow (flash) grooves around each cavity, tear-trim edges so flash breaks off cleanly, and good venting. Because rubber is flexible, some undercuts can be stripped without side actions.

Pros: low tooling cost, low waste, good for large or thick rubber parts and low volumes. Cons: more flash than injection moulding and longer cycles.

Thermoset compression moulding

This is our speciality. A charge of SMC, DMC/BMC, phenolic, melamine or urea is placed in a heated steel mould. Under pressure the material flows, fills the cavity and cures into a rigid part. A hardened shear edge between the two mould halves controls flash.

Typical parts: electrical enclosures and meter boxes (SMC), switchgear and lamp holders (DMC/BMC), handles and electrical parts (phenolic), tableware and wiring accessories (melamine and urea).

Mould features: through-hardened tool steel cavity and core, precise shear edges, cartridge heaters with thermocouples, ejector systems, guide pillars and support pillars, polish or chrome plating.

Read the full guide: compression moulding explained, or see our compression moulds.

Composite and carbon fibre compression moulding

Composite compression moulding covers several processes:

• Prepreg compression moulding: carbon or glass fibre fabric pre-impregnated with resin is cut, stacked in a specific layup, placed in a matched metal mould and cured under pressure. It gives very high strength-to-weight parts with fast cycles compared with autoclave curing.
• Carbon SMC and forged composites: chopped carbon fibre in resin, pressed like SMC, giving a distinctive marbled look and complex shapes.
• GMT and LFT (thermoplastic composites): glass mat thermoplastic sheets are heated and pressed in a cooler mould. Used for automotive underbody shields and seat structures.
• Headliner compression moulding: automotive headliners are formed by pressing heated fibre or foam composite layers in a mould.

Mould features: high accuracy and surface finish, very even temperature (sometimes with oil heating), shear edges or pinch-off edges, and robust guiding for precise thickness control.

How to choose the right process and toolmaker

1. Is the part flexible? If yes, it is rubber. Look for a rubber mould specialist.
2. Is it rigid, heat resistant, electrical or outdoor? It is a thermoset. That is our core work.
3. Is extreme strength-to-weight the priority (sports, aerospace, premium automotive)? Consider carbon or glass composites.
4. What volume? Compression suits low to medium volumes for all three; for very high volumes of small parts, injection variants may be better.
5. What press do you have? Tonnage, platen size and heating decide what you can mould. Read our compression moulding press guide.

Choosing the right material within each family

Rubber: NBR for oil resistance, EPDM for weather and water, silicone for high temperature and food contact, FKM (Viton) for fuels and chemicals, natural rubber for mechanical strength and resilience. Each compound has its own cure temperature and shrinkage, so the mould is sized for that compound.

Thermosets: SMC for large strong panels and enclosures, DMC/BMC for compact electrical parts, phenolic for heat and low smoke, melamine and urea for hard, light-coloured surfaces. Read SMC vs BMC and our DMC guide.

Composites: carbon fibre prepreg for maximum stiffness and lightness, glass prepreg for lower cost, carbon SMC for complex shapes, GMT/LFT for tough, recyclable automotive structures.

Mould steel and surface choices

Rubber moulds often use pre-hardened or through-hardened steels with chrome plating for release. Thermoset moulds need through-hardened tool steel because glass fibre and fillers are abrasive, plus polish and often hard chrome. Composite moulds need high-polish surfaces and very stable steel so the thickness stays exact. In all three, even heating and accurate guiding decide part quality.

Frequently asked questions