Compression or injection moulding for a low-volume rubber part

The difference is where the cost sits

In compression moulding, a pre-weighed slug of uncured rubber is placed into an open die, the press closes, and heat and pressure cure it to shape. The tooling is comparatively simple: two halves, no runner system, no injection barrel.

Injection moulding forces heated rubber into a closed mould through a runner system. Cycle times are shorter and the process is more repeatable at volume, but the tooling is far more complex and correspondingly more expensive.

Where the crossover falls

For most industrial components the crossover is somewhere in the low thousands of parts per year. Below that, the injection tooling cost dominates and never gets amortised. Above it, the shorter cycle time starts to pay.

Replacement and maintenance parts — which is much of what this industry actually supplies — sit firmly on the compression side. So do large-section parts such as heavy mounts and thick diaphragms, where injection can struggle with cure through the section.

Transfer moulding sits between them

Transfer moulding loads rubber into a chamber and transfers it into a closed cavity. It costs more than compression tooling and much less than injection, and it earns its place in two specific situations: when metal inserts must be held precisely in position, and when flash must be kept off a sealing face.

For bonded components with critical insert location, transfer is often the right compromise.

What this means for you

If a supplier quotes you a large tooling charge and a long lead time for a few hundred parts, ask what process they are quoting. There is a good chance the answer is injection moulding on a job that does not need it.

We run compression and transfer moulding and cut our dies in-house, which is why our tooling quotes are measured in days and why a single replacement part is a job we will take.