Three ways to fill the mold
Compression. A preform of uncured rubber is placed directly in the open cavity and the press closes the mold with heat and pressure. The material flows inside the cavity and the excess escapes at the parting line as flash. It is the simplest and most economical mold.
Transfer. The rubber is loaded into a chamber or pot above the closed mold and a plunger pushes it through channels into the cavities. It allows parts with metal inserts and more complex geometry, with less flash on the part.
Injection. A rubber injection machine heats the compound and injects it into the closed mold. It offers shorter cycles and good repeatability, in exchange for a higher investment in mold and machine.
How to choose the process
| Criterion | Compression | Transfer | Injection |
|---|---|---|---|
| Mold cost | Low | Medium | High |
| Production volume | Low to medium | Medium | High |
| Parts with inserts | Limited | Suitable | Suitable |
| Flash on the part | More | Less | Less |
| Material waste | Flash | Flash and pot residue | Runners |
The table shows general trends. The final decision also depends on the compound, the part tolerance and, above all, the press or machine available in the plant.
Flash and venting
In vulcanizing, flash is not a defect that can be eliminated completely but something to be managed. The mold design decides where it forms, how thick it is and how much work it takes to remove. Well-placed flash grooves and a stable closure reduce trimming and waste.
Air and the gases released by the compound during curing need a way out. Without venting you get unfilled areas, bubbles or surface marks, which are especially serious in a seal, where a bubble can become a leak.
Shrinkage of the actual compound
As it cools after curing, the rubber part shrinks. How much depends on the formulation, hardness, type of elastomer and curing conditions. That is why the cavity is made larger than the final part, and the shrinkage factor must come from the compound that will actually be used.
Using a typical shrinkage value instead of the actual compound value is one of the most common causes of out-of-size seals. If you change rubber supplier, it is worth validating again.
Press, temperature and demolding
The mold is designed for the available press: platen size, clamping force, heating system and opening method. A cavity layout that makes poor use of the platens, or that needs more force than is available, limits productivity from day one.
Heat must reach every cavity evenly for the cure to be uniform. Demolding must allow soft and sometimes tacky parts to be removed without tearing, with draft, textures and extraction aids suited to the compound and the working temperature.
Checklist before requesting a quote
☐ Drawing, 3D model or sample
☐ Functional tolerances
☐ Metal inserts
☐ Surface finish
☐ Elastomer type and hardness
☐ Supplier shrinkage value
☐ Cure temperature and time
☐ Data sheet
☐ Process: compression, transfer or injection
☐ Platen size
☐ Clamping force
☐ Heating system
☐ Annual volume
☐ Number of cavities
☐ Trimming method
☐ Part families
Common questions.
Which process suits a seal with a metal insert?
Usually transfer or injection, because the closed mold holds the insert in place while the rubber enters. In compression molding, the insert can shift as the preform flows.
Can you reproduce a mold from a rubber part?
Yes, by measuring or 3D scanning the part and compensating for the shrinkage of the compound that will be used. If a drawing exists, the result is more accurate than copying a part that is already worn.
Can one mold produce several part numbers?
For compatible part families, yes: with interchangeable inserts or cavities for different part numbers in the same mold, as long as they share the compound and curing conditions.
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