Understanding Custom Rubber Molding: A Guide by FromRubber
- Share
- Issue Time
- Jan 5,2026
Summary
Custom rubber molding is a process choice before it is a material choice. This guide compares compression, transfer and injection moulding on the parts each one suits, sets out the tooling economics that decide the route, and shows how to write a specification that survives the first production run.
Process guide
Custom rubber molding covers three processes that are not interchangeable: compression, transfer and injection. The choice is decided by part geometry, tolerance, insert content and volume — and it changes the tool cost, the cycle and the defects you will spend the first month chasing. Choose the process before the compound, not after.
- Compression suits large, simple, low-volume parts and inserts; it is slow but cheap to tool.
- Transfer moulding handles complex shapes and bonded inserts better, at the cost of more scrap in the pot.
- Injection gives the tightest cycle and the best dimensional repeatability, but only pays above a volume threshold.
The three processes inside custom rubber molding
Compression moulding places a preformed blank in a heated cavity and closes the press. The material flows a short distance, the tool is simple and inexpensive, and the process tolerates large parts and heavy inserts. Its weakness is cycle time and flash: the blank has to be sized generously, so flash lands at the parting line and has to be trimmed, often by hand.
Transfer moulding moves the compound into a pot and forces it through a runner into the closed cavity. That gives better fill on complex geometry, better bonding around inserts and less trapped air. The price is a pot of cured scrap per shot and a more expensive tool. Injection moulding goes further: a screw plastifies the compound and injects it under pressure, which gives the shortest cycle and the most repeatable dimensions, with the highest tool cost.
Matching the process to the part
Start with geometry. A flat gasket, a simple boot or a large mat is a compression part. A keypad with thin walls, deep cores and a bonded insert is a transfer or injection part, because the compound has to travel and the air has to leave. Anything with a metal insert that must not shift during cure pushes you toward transfer, where the closed cavity holds the insert in position.
Then look at tolerance and volume. Injection holds dimensions more consistently because every shot is metered, which matters when a seal has to sit in a groove with a narrow compression window. Below a few thousand units a year, the tooling premium of injection rarely repays itself; above it, the shorter cycle usually does.
- Compression — large, simple, low volume, heavy inserts, loose tolerance.
- Transfer — complex shape, bonded insert, moderate volume, tighter tolerance.
- Injection — thin walls, high volume, tight tolerance, best repeatability.
Process comparison for a custom rubber part
| Criterion | Compression | Transfer | Injection |
|---|---|---|---|
| Tool cost | Low | Medium | High |
| Cycle time | Long | Medium | Short |
| Dimensional repeatability | Moderate | Good | Best |
| Inserts and bonding | Manual placement | Held by closed cavity | Automated possible |
| Flash and trimming | Significant | Moderate | Minimal |
| Best volume band | Hundreds to low thousands | Thousands | Tens of thousands |
Writing a specification that survives production
A moulding specification needs four things beyond a drawing: the compound by hardness and class, the process, the acceptance criteria and the packing. Hardness alone is not a compound — two 70 Shore A materials can differ by a factor of two in compression set and in oil resistance, and the difference only appears after ageing.
Acceptance criteria should name what is measured and how often: dimensions on a first article and at a sampling frequency, hardness on every batch, compression set on a periodic basis, and a visual standard with a boundary sample for flash and flow marks. Without a boundary sample, every inspector judges flash differently and every shipment argues about the same defect.
How a custom moulding programme is quoted and run
The quotation follows the four decisions above; each one changes both the tool line and the part price.
Inputs
3D model and 2D drawing, compound requirement, insert drawings, annual volume and the service environment.
Process and tool
Compression, transfer or injection chosen with a cavity count, plus a prototype or bridge step if the design is new.
Validation
First article report, hardness and compression set on production parts, and a functional test in the assembly.
Volume and packing
Amortisation period, colour and marking, and packaging that does not deform the part in transit.
Tooling, cavitation and the numbers behind a moulding quote
A custom rubber molding quote has three lines that move independently: tool cost, cycle cost and finishing. Tool cost scales with cavity count and with how much steel has to move; cycle cost scales with cure time, which follows section thickness far more than it follows the shape. Finishing is the line that surprises buyers, because deflashing and inspection are labour and labour does not fall with volume.
The practical consequence is that the cheapest quote is often the one with the highest cavitation and the most automation, and the most expensive one is often a low-cavity tool with hand deflashing carried in the piece price. Ask for the cavity count, the cure time and the deflash method in writing, and the comparison becomes arithmetic instead of intuition.
Questions buyers ask about rubber moulding
How do we choose between silicone and EPDM or NBR?
By the environment: temperature range, fluid contact and UV. Silicone covers the widest temperature band and the weakest oil resistance; EPDM handles water and steam; NBR handles oil at moderate temperature.
What is a realistic tolerance for a moulded rubber part?
A few tenths of a millimetre on a moulded dimension, wider across a long span, and tighter only where the drawing specifies it. Tolerances tighter than the process can hold add cost through sorting, not through capability.
Why does flash appear on one side only?
Usually a tool that is not seating evenly, or a press with a temperature gradient. It is a tool and process issue rather than a material one, and it should be corrected rather than trimmed harder.
Can we add a bonded insert later?
Bonding after cure works for some assemblies but is weaker and less consistent than moulding the insert in place. If the insert carries load, mould it in.
Send the drawing and the annual volume
FromRubber runs compression, transfer and injection moulding in Dongguan, with in-house toolmaking and a stated process recommendation in every quotation. Send the part drawing, the environment and the volume; we will tell you which process fits, what the tool costs and where the tolerance is realistic.
Request a moulding process reviewRelated reading: prototype tooling for moulded parts and custom silicone seals.