How clamp force is worked out
Clamp force (t) = total projected area × material factor × (1 + safety). The projected area is the shadow of the parts and the runner on the parting line — cavities × part area + runner area. The factor is the clamping pressure the material needs, in tonnes per square inch of that shadow (1 in² = 6.45 cm²), and it rises with the material's viscosity and the injection pressure it needs.
| Material | Factor, t/in² | Density, g/cm³ |
|---|---|---|
| LDPE, TPE | 1.5 | 0.92 – 1.0 |
| PP, HDPE, GPPS, HIPS | 2.0 | 0.91 – 1.05 |
| ABS, PVC rigid, PP GF30 | 3.0 | 1.05 – 1.40 |
| PA6, PA66, POM, PBT, PET, PMMA, PC/ABS | 3.5 | 1.14 – 1.41 |
| PC | 4.0 | 1.20 |
| PA66 GF30 | 4.5 | 1.37 |
The result is rounded up to the next standard press size — 80, 100, 120, 150, 180, 200, 250, 300, 350, 400, 450, 500, 650, 800, 1000 tonnes — because that is what sits on the floor.
Shot weight and the barrel
Shot weight = part weight × cavities + runner weight. Barrels are rated in polystyrene (density 1.05), so a shot of another material is converted: PS-equivalent = shot × 1.05 ÷ density. A press should use between 20% and 80% of its rated shot — below 20% the melt sits too long in the barrel and degrades, above 80% the screw cannot keep up and the cushion disappears. The sweet spot is about half.
Why the press matters for cost
A 350 T press running a job that needs 150 T costs roughly twice the hour rate for the same parts. On most Indian floors the biggest press is the busiest one, not because the jobs need it, but because nobody checked. Cystems keeps each mould's area and material on the mould record and each press's clamp force on the press, so the match is checked every time a job is planned.