Injection Molding Clamp Tonnage Calculator

Even though the runner system isn't part of the finished part, it's still inside the mold and pressurized by the same injected resin, so it pushes against the parting line just like the part cavities do. The calculator adds a runner projected area (default 10% of part area) to the total projected area before multiplying by cavity pressure, because a mold sized only for the part area would flash open along the runner. Hot runner systems that don't have a cold runner channel can set this to 0%.

Each additional cavity adds its own projected area to the mold, and the code multiplies part area by the number of cavities before computing force, so a 4-cavity mold roughly needs 4x the clamp tonnage of a single-cavity mold at the same part size and pressure. This is why higher-cavitation tooling requires a bigger, more expensive press even though each individual part is unchanged.

The calculator computes exact required tonnage, then walks a list of real standard injection press sizes (50, 80, 100, 150, up through 6000 tonnes) and returns the smallest one that's still at or above your required force. It rounds up rather than down because a machine below your calculated tonnage can't safely hold the mold shut, so the recommendation is always a real, quotable machine size rather than a fractional number you can't purchase.

Cavity pressure is a process outcome rather than a fixed resin property, but different resins do tend to need different ranges: PE/PP typically 200-400 bar, ABS 300-600 bar, and PC or nylon 400-700 bar because of their higher viscosity. The 350 bar default sits in the low-middle of that spread as a generic starting point, but you should replace it with the actual packed-and-held cavity pressure for your specific resin and part geometry once you have real process data.