Rubber is given its shape while the compound is still soft and is then cured, so the forming method has to hold the shape through the cure. Three methods cover most products: compression moulding in a heated two-part mould for gaskets and O-rings, extrusion through a die for tubing and profiles, and dipping a former in liquid latex for gloves, balloons and other thin items. The product decides the method.
Part of Rubber Processing
Choosing how to give a rubber compound its shape and hold it there during the cure.
The Cure Fixes The Shape, So Forming Comes First
Before vulcanisation the compound is soft, tacky and will take a shape. After vulcanisation the shape is permanent, so a mistake in shaping cannot be reworked. The carrier’s subtitle says “shaping vulcanized rubber”, but the process in practice runs the other way: form the uncured compound and cure it in that form, as Sulfur Vulcanization sets out.
| Product | Method | Article |
|---|---|---|
| Gaskets, O-rings, seals, solid tires | Compression moulding | Compression Molding |
| Tubing, hose, profiles, cord | Extrusion, or wrapping on a mandrel | Extrusion |
| Gloves, balloons, membranes, inner tubes | Dip moulding in latex | Dip Molding (Latex) |
What You Need
- A compound ready to cure; see Compound Mixing and Compounding Recipe.
- Moulds, dies or formers made of metal, ceramic or glass.
- A heat source that holds the cure temperature.
- A release agent: the chemistry heat-treatment article uses chalk powder, talc or light oil on mould surfaces.
Method: Choose, Shape, Cure
Step 1: Name the product and pick the method from the table.
Step 2: Shape the uncured compound, slightly overfilling a mould, since the Heat Treatment article notes rubber compresses during curing.
Step 3: Hold the shape through the cure. Clamp, press or bind it.
Step 4: Cure and test. See Curing Temperature and Time and Quality Testing.
Common Mistakes
- Cutting or reshaping after cure. The shape is fixed; only trimming of flash is possible.
- Underfilling the mould. It leaves voids and thin spots.
- No release agent. The part tears when the mould opens.
Safety
Hot moulds and presses
Moulds are cured at about 150 °C and look identical to cold iron. Lift them with tongs, never by hand; keep fingers out of the press while it closes; and open a mould away from the face, since hot rubber and trapped steam can spit. A hot oil bath or sealed steam pot adds the hazards described in Curing Temperature and Time.
What’s Next
- Learn next: Compression Molding — the commonest method
- Combine with: Essential Rubber Products — what to make first