Engineering tools

Clamping force calculation in injection moulding — calculator and tonnage factors

The clamping force calculation: required clamp force (tonnes) = total projected area (cm²) × a material tonnage factor (t/cm²). Projected area is the shadow the part throws on the mould parting plane — all cavities plus runners — and typical factors run from about 0.32 t/cm² for free-flowing polypropylene to 0.7 t/cm² and beyond for glass-filled engineering polymers. Add roughly 10% headroom, and the machine’s rated clamp must exceed the result.

Worked example

An ABS housing with a projected outline of 75 cm² in a 2-cavity mould: total projected area = 150 cm². Required clamp = 150 × 0.47 = 70.5 tonnes; with 10% headroom, about 78 tonnes. A machine rated 80–110 T runs this comfortably — on a floor like ours, that part books the 180 T machine with room to spare.

Why the factor varies — and when the rule of thumb fails

The tonnage factor is a stand-in for cavity pressure, and cavity pressure depends on how hard the melt must be pushed: thin walls raise it, long flow paths raise it, viscous and glass-filled materials raise it, and a badly placed gate raises it more than any of those. That is why two parts with identical projected areas can need very different clamp — and why the honest answer to “what tonnage does my part need?” is a mould-flow analysis before steel is cut, not a factor from a table. Use the calculator for budgeting; use the analysis for the tooling decision.

Clamp sizes the mould; shot sizes the barrel

Clamping force and shot weight are independent checks, and a machine must pass both: a large flat part can demand high clamp with a small shot, and a thick compact part the reverse. Run the second check with our shot-size calculator.