Compression Moulding Explained: Process, Advantages and Limitations

Compression moulding is a process in which a measured charge of material is placed into an open, heated mould cavity, the mould is closed in a press, and heat and pressure make the material flow to fill the cavity and then cure (or vulcanise) into the final shape. It is the main process for thermoset parts made from SMC, DMC/BMC, phenolic (Bakelite), melamine and urea-formaldehyde, and it is also used for rubber and advanced composites.

We have built compression moulds at Prime Tools since 2015. This guide explains the process the way we explain it to new customers: what happens in the mould, what the process is good at, where it struggles, and how the mould design decides whether your parts are good or full of flash.

How compression moulding works, step by step

1. Prepare the charge. The material is weighed. SMC is cut from sheet and stacked; DMC is cut from a log; phenolic powder may be pre-formed into tablets and pre-heated. Getting the weight right shot after shot is the first rule of good moulding.
2. Load the mould. The operator (or a robot) places the charge into the lower half of the heated mould, in a position decided during design so the material flows evenly.
3. Close the press. The upper half comes down. The press first closes quickly, then slows as the mould touches the material, so air can escape and fibres are not damaged.
4. Flow and fill. Under pressure, the material flows outwards and fills every part of the cavity. The shear edge, a precise sliding fit between the two halves, traps the material and lets only a thin film of flash out.
5. Cure. The mould is held closed while heat cures the material. For thermosets, the resin crosslinks; for rubber, it vulcanises. Thicker walls need longer cure time.
6. Open and eject. The press opens and ejector pins push the part out. The part is deflashed, inspected and, if needed, post-cured or machined.

Materials used in compression moulding

• SMC (sheet moulding compound): large, strong parts such as enclosures, meter boxes, covers and panels. See SMC moulds.
• DMC / BMC: compact, detailed electrical and heat-resistant parts. See DMC and BMC moulds.
• Phenolic (Bakelite): handles, switch parts, commutators, brake components.
• Melamine and urea-formaldehyde: tableware, wiring accessories, closures.
• Rubber: seals, gaskets, grommets. See rubber vs thermoset compression moulding.
• Composites and carbon fibre prepregs: high-performance panels and parts.

Advantages of compression moulding

• Large parts at sensible cost. Big panels and enclosures can be moulded in one piece on a press.
• Strong parts. Long fibres in SMC are not broken up by a screw or narrow gate, so parts keep high strength.
• Lower tooling cost for large parts. No runners, sprues or hot runners. A compression mould is often simpler than an injection tool of the same size.
• Little material waste. The charge is close to the part weight. There are no runners to regrind.
• Low internal stress. Short flow paths mean less fibre orientation and less warpage.
• Thermoset benefits. Heat resistance, flame performance, electrical insulation and dimensional stability.
• Good for low and medium volumes. Economical where injection tooling would not pay back.

Disadvantages and limitations

This is the section people search for most (“which is not an advantage of compression moulding”). Here are the honest limitations:

• Longer cycle times. Cure takes time, often from under a minute to several minutes depending on wall thickness, compared with seconds for many injection parts.
• Flash. A thin film forms at the shear edge and must be removed. Flash grows as the mould wears.
• Less suited to very intricate parts. Very small, complex parts with fine detail, undercuts and side actions are often better suited to transfer or injection moulding.
• Manual handling. Charge loading is often manual, so consistency depends on operator discipline unless automated.
• Limited undercuts. Side cores are possible but add cost and complexity.
• Inserts need care. Moulded-in inserts must be located well or the flow will move them.

So, a common exam answer: “high production speed for intricate parts” is not an advantage of compression moulding. Its strengths are large, strong parts with low waste and lower tooling cost.

Compression vs transfer vs injection moulding

What makes a good compression mould

Most moulding defects (flash, short fill, blisters, sticking, warpage) are mould problems or setting problems. A good compression mould gets these right:

Steel and hardness. Abrasive compounds need hardened, through-hardening tool steel for cavity and core, and often surface treatment on the moulding faces.

Shear edge. The shear edge clearance decides flash thickness. It must be machined and fitted accurately, hardened, and designed so it can be reworked when it wears.

Heating. Cartridge heaters and thermocouples are placed so the whole moulding surface stays at an even temperature. Hot and cold spots cause uneven cure.

Venting and ejection. Gases from curing must escape. Ejector pins are placed for clean release without marking cosmetic faces.

Guiding and support. Guide pillars and bushes keep the halves aligned under load. Support pillars stop the plates from bending under press tonnage.

Polish. A good polish helps release and gives a better part surface. High-gloss faces are polished and, for long runs, often chrome plated.

Common compression moulding defects and causes

If you are seeing these on an existing mould, our mould repair team can usually diagnose it from photos and a defective part.

Frequently asked questions