Clamping Force and Tonnage Selection: The Five-Step Calculation

Why tonnage is the first number to get right

Every injection moulding quotation starts with a machine size, and that size is set by clamping force. Get it too low and the tool flashes at the far corner of the parting line, no process setting fixes it, and the moulder spends a week blaming the tool. Get it too high and the mould vents are crushed shut, parts burn, and the tool wears out years early.

The number is not a matter of judgement. It is projected area multiplied by cavity pressure, with a small safety margin. The difficulty is that cavity pressure is not the same as injection pressure, and the projected area is not the same as the part footprint.

The arithmetic

Clamping force has to exceed the separating force that the melt exerts on the mould faces. That force is cavity pressure times the projected area of everything the pressure acts on.

F (kN) = A (cm²) × P (bar) ÷ 100

Where A is the total projected area — parts plus runner plus overflow — and P is the peak cavity pressure during packing, not during fill. Then multiply by a safety factor of 1.1 to 1.2.

Table 1 — The five steps of a tonnage calculation. Steps 1 and 3 are where the errors live; the arithmetic itself is trivial.

A worked example. A flat ABS housing, 180 × 120 mm outline, one cavity, a runner with 40 cm² of projected area. Total projected area is 216 + 40 = 256 cm². ABS packs at around 500 bar. Separating force is 256 × 500 ÷ 100 = 1,280 kN, or 128 tonnes. With a 15% margin, the machine needs 150 tonnes.

Cavity pressure by material

Cavity pressure is a material property as much as a process setting. Viscous, fast-freezing, glass-filled or semi-crystalline resins need more pressure to fill and pack the same geometry.

Table 2 — Cavity pressure ranges. These are pack pressures, not fill pressures; a thin-walled part can need double the table value.

The table values assume a reasonably balanced wall thickness and a flow length to thickness ratio under about 150. A part with a 0.8 mm wall running 120 mm from the gate has a ratio of 150 and sits at the limit; a 0.6 mm wall over the same distance needs pressures well outside the range and is usually a tooling problem rather than a tonnage problem.

Projected area: what actually counts

Projected area is the shadow the mould cavity casts on the parting plane. It is not the surface area of the part, and it is not the area of the part's largest flat face.

Table 3 — What belongs in the projected area. The runner is the item most often left out, and on small parts it is often larger than the part itself.

On a 16-cavity tool making a 3 cm² part, the parts contribute 48 cm² and the runner can contribute 30 cm² or more. Ignoring the runner understates the tonnage by 40%, which is exactly the margin that separates a clean part from a flashed one.

What too little clamping force looks like

Flash is the obvious symptom, but it is the last one to appear. The earlier signs are more diagnostic.

Table 4 — Reading the symptoms. Flash on one side only is a parallelism problem and adding tonnage will not fix it.

The trap is that raising pack pressure to fix a short shot increases the separating force, so a marginal tool starts flashing. The operator backs off the pressure, the short shot returns, and the job runs in a narrow band between two defects. That is the signature of a machine that is one size too small.

What too much clamping force costs

Oversizing is the more common error because it feels safe. It is not free.

Table 5 — The cost of oversizing. Vent crushing is the one that turns a good tool into a tool that burns parts for the rest of its life.

A mould built for 100 tonnes run on a 250-tonne press will usually work, because most presses can be set to a lower clamp force. The real risk is a moulder who does not set it down, or a press without a clamp force display, where the machine simply closes at full tonnage.

Verifying the number on the machine

The calculation gives a starting point. The machine gives the answer. Three checks confirm it.

First, the flash test: reduce clamp force in 10% steps until flash just appears at the furthest point, then add back 15–20%. That is the true minimum for the tool at that pressure.

Second, the vent check: with the calculated tonnage applied, inspect the vents. If they are bright and polished, they are being crushed. If they hold a faint witness mark, they are working.

Third, the platen check: on a large mould, place pressure-sensitive film or lead wire at the four corners of the parting line and close at production tonnage. Uneven impressions mean the platens are not parallel and the tonnage is not distributed, whatever the display says.

Choosing the machine, not just the tonnage

Tonnage is necessary but not sufficient. Three other numbers decide whether a machine can run a tool.

Table 6 — Machine selection beyond tonnage. Shot size below 25% of barrel capacity is the most common hidden problem on small parts run on large presses.

The shot size rule is worth emphasising. A 20 g part on a machine with a 500 g barrel spends twenty-five times as long in the barrel as it should, and the material degrades. The part may pass inspection and still fail in service because the molecular weight has dropped. A smaller barrel on the same tonnage is almost always the better machine.