What is compression moulding?
Compression moulding is a process where a measured charge of thermoset material is placed in a heated, open mould cavity, the mould is closed under a hydraulic press, and heat and pressure make the material flow into every detail and cure into a rigid, finished part.
It is the standard way to make parts from thermoset compounds such as SMC (sheet moulding compound), DMC and BMC (dough and bulk moulding compound), Bakelite (phenolic), melamine and urea-formaldehyde. Once these materials cure, they cannot be melted again, which is why they hold their shape at high temperatures, resist fire and stay electrically insulating for decades.
That makes compression moulding the backbone of the electrical industry: energy meter boxes, junction and distribution boxes, switchgear insulators, socket and switch housings, and outdoor enclosures that face sun and rain every day. The quality of every one of those parts starts with the mould. As a specialist compression mould manufacturer in India, we build only thermoset tooling, so every mould we make is designed around how these materials flow and cure.
Compression moulding diagram
A compression mould has two halves mounted between the heated platens of a press. The charge is loaded into the lower half (the cavity); the upper half (the force or plunger) closes on it and squeezes the material into shape.
The compression moulding process, step by step
- First, the charge is prepared. SMC sheet is cut and stacked to a set weight; DMC and BMC are weighed as dough; phenolic and melamine powders are often pre-formed into tablets and pre-heated. A correct charge weight is the first defence against short shots and heavy flash.
- Next, the charge goes into the heated mould. It is placed in the cavity at the position worked out in the DFM, usually where it covers 40–70% of the projected area for SMC, so the material flows evenly to the edges.
- The press closes the mould at a controlled speed. The shear edge traps the material while it flows into ribs, bosses and grooves, and air escapes through the vents.
- The part cures under heat and pressure. The thermoset cross-links, and the cure time depends mostly on the thickest wall in the part. That is why uniform wall sections matter at the design stage.
- The mould opens and the part is ejected. Ejector pins push the part out square, without distortion.
- Finally, the part is deflashed and checked. The thin flash at the parting line is removed and the part is measured against the drawing. A well-made mould keeps that flash small and consistent for years.
Compression moulding materials and typical settings
We build compression moulds for all the main thermoset families. The table gives typical starting ranges; the final settings always come from your material supplier's datasheet and the trial.
| Material | Mould temperature | Moulding pressure | Typical parts | What the mould needs |
|---|---|---|---|---|
| SMC | 140–160 °C | 40–100 kgf/cm² | Meter boxes, junction boxes, enclosures, covers | Hardened, polished steel for glass fibre; tight shear edge |
| DMC / BMC | 140–170 °C | 50–150 kgf/cm² | Switchgear parts, insulators, lamp housings | Wear-resistant inserts, good venting for detailed ribs |
| Bakelite (phenolic) | 150–185 °C | 150–350 kgf/cm² | Wiring devices, handles, electrical bases | High pressure rating, chrome plating, steady heat |
| Melamine | 145–170 °C | 150–400 kgf/cm² | Switch plates, electrical fittings, tableware | Mirror finish for gloss, careful gas venting |
| Urea-formaldehyde | 135–165 °C | 150–400 kgf/cm² | Sockets, switches, toilet seats, closures | Polished, chrome-plated cavities; precise temperature |
SMC moulds: the compression mould we build most
An SMC mould has to handle a glass-fibre compound that is abrasive, flows a long way and cures fast. Our SMC moulds use hardened, polished cavities, a tight vertical shear edge and a charge pattern worked out in the DFM, so the fibres reach every rib and boss without dry spots. We build single and multi-cavity SMC moulds for meter boxes, junction boxes, enclosure bases and lids. See our SMC moulds.
Choosing between thermoset and thermoplastic? Thermosets cure once and keep their strength at high heat, resist fire and electrical tracking, and do not creep under load, which is why they win for electrical and outdoor parts. Thermoplastics re-melt, which suits fast injection moulding but not every electrical job.
What makes a good compression mould
Most compression moulding problems (flash, short fills, blisters, cracks and warpage) are designed in long before the first trial. These are the points our engineers settle before any steel is cut.
Shear edge and flash land
A vertical shear edge with the right clearance traps the material and keeps flash thin. Too loose, and you trim every part; too tight, and the edge galls.
Draft on every wall
Walls, ribs and bosses need draft to release cleanly. We map draft face by face and flag every near-zero area.
Uniform wall sections
Thick sections slow the cure and cause sink and blisters; thin ones risk non-fill. We study the wall map and suggest changes early.
Even heating
Heater layout and thermocouple positions keep the mould surface within a few degrees, so the whole part cures at the same rate.
Venting and ejection
Trapped gas leaves burns and porosity. Vents, ejector positions and stripping are designed around how the material flows.
Steel, hardness and finish
Glass-filled compounds are abrasive. Cavities are made in hardened steel, polished, and hard chromed where the material demands it.
How much press force does a compression mould need?
Press force (tonnes) = projected area (cm²) × moulding pressure (kgf/cm²) ÷ 1000
For example, a cover with a projected area of 300 cm² moulded in SMC at 80 kgf/cm² needs about 300 × 80 ÷ 1000 = 24 tonnes, before a safety margin. We do this check for every part, so the mould suits the press you already have.
Mould steels we use
| Steel | Where we use it | Why |
|---|---|---|
| H13 (hardened) | Cores, cavities and inserts | Toughness and wear resistance at moulding temperature |
| Böhler W302 ESR | Cavities for long runs and abrasive compounds | Premium, electro-slag remelted H13-type steel: cleaner, takes a better polish |
| P20 (pre-hardened) | Plates, holders and low-volume tools | Machinable, stable and cost-effective |
