Compression moulding shapes and cures a rubber part in one step: place a weighed charge of uncured compound in the cavity of a two-part steel mould, close it in a press, heat the mould to about 150 °C and hold pressure and temperature for the cure time, then open it and trim the thin flash. A hydraulic press is ideal, though any press that can hold the mould shut will serve. The clamping force is the moulding pressure times the projected area of the part, and the weighed charge must exceed the cavity volume slightly.
Part of Rubber Processing
Press-curing rubber in metal moulds for gaskets, O-rings, seals and solid tires.
The Press Holds The Mould Shut Against The Rubber Pushing Out
When the compound softens under heat it flows to fill the cavity, and the excess squeezes out through the parting line as flash. If the press cannot hold the mould shut, the cavity does not fill and the part is undersized or porous. The carrier gives the process:
- A two-part steel mould, machined or cast.
- A weighed charge of uncured compound in the cavity.
- Close the mould in a press; the carrier says a hydraulic press works perfectly, see Hydraulic Systems.
- Heat the mould to curing temperature (150 °C) by oven, hot plate or steam.
- Hold pressure and temperature for the cure time.
- Open the mould, extract the part and trim flash.
What You Need
- A mould with a release agent (chalk, talc or light oil).
- A press and a heat source that reaches 150 °C and holds it.
- A balance to weigh the charge.
- Tongs and gloves for the hot mould.
- Cure schedule: Curing Temperature and Time.
Method: An O-Ring Mould
The carrier’s example: machine a groove in a steel plate with the cross-section profile of the O-ring, machine the matching groove in the top plate, lay a rubber cord in the groove, close, heat and cure, then open and trim. For making the cord and the groove sizes, see Gaskets and Seals and the corpus’s Gaskets and Seals article.
Step 1: Weigh the charge to slightly more than the cavity volume.
Step 2: Coat the mould with release agent.
Step 3: Load and close, so a thin ribbon of flash shows at the parting line.
Step 4: Heat to 150 °C and start the clock when the centre of the mould is at temperature.
Step 5: Open, extract and trim.
Worked Example
Charge for an O-ring of 3 mm cross-section and 50 mm inside diameter (an illustrative size). The ring’s centre-line diameter is 50 + 3 = 53 mm, so its length is π × 53 = 166.5 mm. Cross-section area is π ÷ 4 × 3² = 7.07 mm². Volume is 166.5 × 7.07 = 1,177 mm³, or 1.18 cm³. At a compound density of about 1.1 g/cm³ (recalled; the corpus gives none) the charge is about 1.3 g, plus a little extra for flash.
Clamping force for a flat gasket mould, projected area 100 mm × 100 mm = 0.01 m². The corpus gives no moulding pressure; take 5 MPa as an assumption for the arithmetic (not a corpus figure). Force is pressure times area: 5,000,000 Pa × 0.01 m² = 50,000 N, or 50 kN. That is about 50,000 ÷ 9.81 ≈ 5,100 kgf, about 5.1 tonnes of force, which is why the carrier points to a hydraulic press. The right pressure is the one that closes the mould with a thin flash; find it by trial.
Common Mistakes
- Under-weighing the charge. The cavity does not fill.
- Over-weighing, which makes thick flash and keeps the mould from closing.
- Opening the mould hot without a release agent, which tears the part.
- Confusing newtons with kilograms. 50 kN is about 5.1 tonnes of force, not 50 tonnes and not 50 kg.
Safety
Press crush and hot mould
A press closing on a mould crushes a hand instantly and a hydraulic press cannot be pushed back by the operator. Keep both hands outside the platens when the press is moving, put in a stop or locking bar that holds the upper platen up while the mould is loaded, and decide who can reach the release before pressure is applied. A mould at 150 °C looks the same as a cold one: lift it only with tongs.
What’s Next
- Learn next: Gaskets and Seals — the products this makes
- Combine with: Tire Basics — solid tires