What cavity pressure should I use?

Use the average cavity pressure from a mold-flow analysis if you have one. Otherwise take it from the resin supplier's design guide for your resin and wall thickness. Stiff, glass-filled and thin-wall parts need more pressure.

Why is the answer in US tons?

North American presses are rated in US tons of 2,000 lb force. Metric presses are rated in kN or tonnes; one US ton of force is about 8.9 kN or 0.91 tonne.

Is a bigger press always safer?

No. Extra rating guards against flash, but clamping far harder than the mold needs can hob the parting line, and a bigger press costs more per hour. Set clamp force near the requirement plus your margin.

Does projected area include the runner?

Yes for a cold runner: it holds melt at or above cavity pressure and pushes the mold open like a part does. A hot runner manifold sits inside the mold plates and adds no parting-line area, but a cold sub-runner it feeds does.

Injection Molding calculator

Clamp Tonnage Calculator

Find the clamp force a mold needs from projected area, cavity count and cavity pressure. Compare it with a press rating before you quote the job or hang the tool.

What this calculator does

Clamp force a mold needs, and whether your press can hold it shut.

Formula used

Projected area = area per cavity × cavities
Required force (lb) = projected area × average cavity pressure × (1 + safety margin)
Required tonnage = required force ÷ 2,000 lb per US ton
Press load = required tonnage ÷ press clamp rating
Headroom = press clamp rating − required tonnage

Inputs explained

Projected Area per Cavity: Part outline on the parting plane from CAD, plus its runner share.
Cavities: Cavities filled every shot.
Average Cavity Pressure: From mold-flow results or the resin supplier's design guide.
Safety Margin: Extra force held back for packing pressure spikes.
Press Clamp Rating: Nameplate clamp force of the press you plan to use.

How to use the result

Best suited to choosing a press for a new mold, moving a tool to another press.
Thin walls and long flow lengths raise cavity pressure; use mold-flow results for those parts. Does not add slide locking loads or check platen size, tie-bar spacing or shot capacity.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 280.569 (BLS, Aug 2026), up 6.6% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

What cavity pressure should I use? Use the average cavity pressure from a mold-flow analysis if you have one. Otherwise take it from the resin supplier's design guide for your resin and wall thickness. Stiff, glass-filled and thin-wall parts need more pressure.
Why is the answer in US tons? North American presses are rated in US tons of 2,000 lb force. Metric presses are rated in kN or tonnes; one US ton of force is about 8.9 kN or 0.91 tonne.
Is a bigger press always safer? No. Extra rating guards against flash, but clamping far harder than the mold needs can hob the parting line, and a bigger press costs more per hour. Set clamp force near the requirement plus your margin.
Does projected area include the runner? Yes for a cold runner: it holds melt at or above cavity pressure and pushes the mold open like a part does. A hot runner manifold sits inside the mold plates and adds no parting-line area, but a cold sub-runner it feeds does.

Related guides

Last reviewed 2026-10-01.