Compression moulding places a measured charge of material into an open heated tool, closes it under pressure, and holds it while the material flows to fill the cavity and cures or consolidates. It is the older sibling of injection moulding and it is still the right answer for thermosets, for fibre reinforced compounds and for large parts where injection pressures would be impractical.
How the process runs
The charge is weighed and placed, the tool closes, heat and pressure do the work, and the part is removed once the material has cured enough to hold shape. Cycle times are long compared with injection moulding because cure is chemical rather than simply thermal, but the tooling is simpler, the pressures are lower and there is no runner system throwing material away.
What it is for
Thermoset compounds that cannot be re melted, so injection is awkward. Sheet and bulk moulding compounds with long glass fibres, where injection would break the fibres and lose the strength you are paying for. Rubber parts and seals. And large flat or gently curved panels where an injection tool would need a very large press to hold the clamp force.
Designing the tool
The cavity has to allow for the charge to be placed and for excess material to escape in a controlled way, so flash lines are a design decision rather than a defect. Heating has to be even, because uneven cure means uneven properties. Ejection has to work on a part that may still be warm and soft. And venting matters, since the cure reaction can release volatiles that must have somewhere to go.
Against injection moulding
Compression tooling is cheaper and simpler, cycle time is longer, and part price falls more slowly with volume. Fibre length and therefore strength are preserved. Very high cavity counts and very fine detail belong to injection moulding. The honest comparison is per programme rather than per part, because the two processes have different cost curves.
Questions people ask about compression molding
What materials suit compression moulding?
Thermosets, rubber compounds, and fibre reinforced sheet or bulk moulding compounds. Thermoplastics can be compression moulded but are usually better served by injection.
Is compression moulding slower?
Per cycle, yes, because the material has to cure. Whether that makes it slower per programme depends on cavity count, tooling lead time and volume.
Why does it preserve fibre strength?
Because the material is not forced through a screw, a runner and a gate, all of which break long fibres. In compression the charge flows a short distance under pressure and the fibres survive.
Does compression moulding produce flash?
Usually yes, by design, because excess material must escape as the tool closes. Flash removal is a planned secondary operation rather than a fault.