Injection Moulding Clamp Force

Melt goes into the cavity under pressure, and pressure on an area is a force trying to push the two halves of the mould apart. The clamp has to beat it. That is the whole physics of this page, and everything difficult about it lives in the two inputs rather than the multiplication.

The projected area is a drafting job and the calculator cannot do it for you. It is the shadow the entire shot casts on the parting plane, viewed along the direction the mould opens — not the surface area of the part, not the volume, and not the footprint of the cavity block. Everything that sees cavity pressure counts: every cavity, the full runner system, the sprue, and any cold slug wells. On a multi-cavity tool the runner is regularly a fifth of the total, and a hot runner changes the answer again because the manifold is not opening the tool. Get the layout, project it, and add it up. Nothing on this site can see your drawing.

The cavity pressure is not the injection pressure. The gauge on the machine reads hydraulic pressure or nozzle pressure, and the melt loses a large fraction of it — commonly half or more — passing through the sprue, the runner and the gate before it reaches the cavity at all. Using nozzle pressure here sizes a press twice as big as the job needs. The honest ways to get the cavity figure are a pressure transducer in the tool, a filling simulation, or experience with the same material at the same wall thickness. And it is not a single number even then: pressure is highest at the gate and lowest at the last place to fill, so what goes into this equation is an average standing in for a field that varies across the part.

The shop factor is practice, not physics, and it belongs on top. Working moulders size the press at 1.1 to 1.3 times the calculated figure. That margin is not a fudge — it is carrying the knowledge of tools that flashed. It covers the non-uniformity of the pressure field, the extra pressure a thin-wall or long-flow part needs at the far end, the fact that a press held at its rated tonnage all day does not stay at its rated tonnage, and the tendency of a tool to want more pack pressure than anyone planned once it is in production. The equation has no term for any of that and should not pretend to.

Flash is what happens when the calculation was wrong, and it announces itself at the parting line as a thin fin of plastic where the two halves lifted. It is not only a trim cost: once a tool flashes it peens the parting-line steel, and the tool then flashes at lower pressures than it used to, which is a slow expensive spiral. Under-clamping is the more common error, but over-clamping is not free either — an oversized press costs more per hour, holds more tonnage against the mould faces than they need, and can crush venting, delicate cores and the parting line itself.

Two last practical notes. Tonnage is not the only thing that has to fit: shot size, plasticising rate, platen dimensions, tie-bar spacing and daylight all limit a job independently, and a tool that fits the tonnage can be wrong on any of them. And in North America presses are sold in US tons of clamp, at 2000 lbf to the ton — so the imperial answer on this page comes back in tonf and the metric one in kilonewtons, which are the two numbers a press is actually specified by.