Clamping Force in Silicone Compression Moulding: Pressure Is Not the Setting

What clamping force is for

In silicone compression moulding the clamp does two jobs. It keeps the mould closed while the compound expands and cures, and it sets the thickness of the flash that escapes at the parting line.

Those two jobs want different things. Enough force to keep the mould closed is a floor, and more force to thin the flash is a slope that turns into a cliff once the flash is thin enough that the force starts acting on the mould rather than on the material.

That is why clamping force has an optimum rather than a maximum. Below it, flash and thick parts. Above it, crushed inserts, damaged parting lines and a mould that needs rework.

The number that matters is not the gauge reading. It is the gauge reading converted to force, divided by the projected area of the cavity and the flash land.

Converting pressure to force

The press gauge shows hydraulic pressure in bar. Multiply by the ram area in square centimetres and divide by a thousand to get kilonewtons, then divide by the projected area in square centimetres to get specific pressure in bar.

For a press with a 200 millimetre ram the area is 314 square centimetres. At 150 bar on the gauge that is 4,710 kilonewtons, which sounds enormous and is not, because it is spread over the whole mould.

On a 300 by 300 millimetre plate with a 250 by 250 millimetre cavity the projected area is 625 square centimetres, so the specific pressure is 7.5 bar. That is well below what silicone needs, which is why the same gauge reading on a bigger plate produces flash.

The practical consequence is that the gauge setting is not transferable between moulds. Every mould needs its own number, calculated once and written on the tool card.

How much is enough

Silicone compounds typically need 50 to 120 bar of specific pressure to close a mould and hold a thin flash. The lower end suits soft compounds and thick parts, the upper end suits hard compounds and thin sections with a large projected area.

The chart on the left is flash thickness against specific pressure for a 70 Shore A compound in a 300 millimetre plate. At 20 bar the flash is 0.35 mm, at 60 bar it is 0.13, at 100 bar it is 0.05 and beyond 120 bar it stops improving at 0.04.

The right hand panel is the other side of the same curve. Rejects fall from 12 percent at 20 bar to 2 percent at 100 bar, and then rise again to 9 percent at 150 bar as inserts crush, parting lines take damage and parts tear on ejection.

The optimum for this mould is around 100 bar. That is not a universal number, but the shape of the two curves is: a knee on the way down and a wall on the way up.

What too much force does

It crushes inserts. A metal insert in a silicone part is held by the compound around it, and excessive force closes the cavity onto the insert before the compound has flowed, which produces a part with a distorted insert and a stress concentration at the bond line.

It damages the parting line. The parting line on a silicone mould is a narrow, accurately machined face, and every cycle at excessive force peens it slightly. After a few thousand cycles the flash is thick in one place and thin in another, and no pressure setting fixes it.

It changes dimensions. Silicone is nearly incompressible, so once the cavity is full the force has nowhere to go except into elastic deformation of the mould and the press. A part moulded at 150 bar is measurably thinner than the same part at 100 bar.

And it wears the press. Platens deflect, guide pillars take side load and the hydraulic system runs hot, all of which shorten the life of equipment that is usually the bottleneck in a silicone shop.

Table 1 — Specific pressure against flash and risk.

What too little force does

Thick flash is the visible cost. Flash has to be trimmed, and trimming is labour that scales with flash thickness. A part with 0.3 mm of flash costs more to finish than a part with 0.05 mm.

Thick parts are the hidden cost. If the mould does not close fully, the cavity volume is larger than the design, and the part comes out over thickness with the wrong compression ratio and the wrong hardness.

Porosity is the third. Insufficient pressure lets the compound expand against a light constraint, and volatile products from the cure form voids that are only visible after trimming or on a section.

And flash line weakness is the fourth. A thick flash line is a thin web of material that tears away from the part and leaves a notch, which becomes a crack initiation point in a dynamic application.

Setting it on the press

Calculate the specific pressure for the mould once, from ram area, gauge pressure and projected area, and write the gauge value on the tool card. Do not let operators tune the clamp by feel.

Set the press in force control where it is available. A press that holds force rather than position compensates for compound variation and holds a consistent flash, and it removes the most common cause of cycle to cycle variation.

Check the flash at four points around the part, not one. A flash that is thick on one side and thin on the other is a parallelism problem, not a pressure problem, and raising the pressure will just damage the high side.

And check it after maintenance. A platen that has been resurfaced, a mould that has been reworked or a ram that has been resealed all change the relationship between gauge and force.

Table 2 — What to check and how often.