Machine Sizing
Clamping force in injection molding is the tonnage a machine holds on the mold during filling. It keeps the tool shut while molten plastic tries to push it open. The standard estimate multiplies projected area by cavity pressure, divides by 1,000 for metric tonnes, and adds a 10–30% margin. Too little means flash; too much wears the mold.
The Formula Behind Clamping Force Tonnage
Clamping force (tonnage) is the clamping unit’s push that holds the two mold halves together during injection and packing. Machines are rated by it.
A 180-ton press holds the mold shut with up to 180 metric tonnes of force.
The math fits on one line. Clamping force in tonnes equals projected area in cm² times cavity pressure in kg/cm², divided by 1,000.
Projected area is the shadow the part and runner cast as seen from the clamp direction. It is not the total surface of the part.
Cavity pressure is the pressure the melt exerts inside the closed mold, usually expressed in bar or kg/cm².
For a 200 cm² PC enclosure at 350 bar, the math gives 70 tonnes of theoretical force. Add a 15% margin and the job sizes at about 80 tonnes. A 100-tonne machine covers that with room to spare.
Why Projected Area Is Almost Always Underestimated
The most common sizing mistake is measuring the part and forgetting the runner. Your gate and runner layout decisions change the area the clamp must hold shut.
On a multi-cavity tool, the runner alone can rival the area of several parts.
Projected area counts only the footprint facing the clamp direction; vertical walls add nothing while shutoff surfaces add their full share.
Deep containers are a trap. The footprint looks small, but the rim shutoff still feels the full cavity pressure.
A hot runner tool has no cold runner to count. That quietly lowers the tonnage when buyers switch to hot runner tooling.
The tonnage saving costs more in tooling and maintenance, which pays off at volume but not for short runs.
Cavity Pressure Is the Half You Cannot Read Off a Drawing
Projected area you can measure; cavity pressure you have to estimate.
| Resin | Reference cavity pressure range [VERIFY] |
|---|---|
| PP, PE | 300–500 bar |
| PC, PA | up to 600–800 bar |
Cavity pressure, not part size, is what makes two similar-looking parts need different tonnage.
Thinner walls and longer flow paths push pressure up; a well-vented mold with a generous gate pushes it down.
Packing gets overlooked at quoting time. The clamp must hold through the packing phase, when pressure on thick sections can exceed filling pressure.
Melt temperature and injection speed both move cavity pressure, so process settings and tonnage stay linked. Our injection molding process guide shows how the cycle stages interact.
How Much Safety Margin Is Enough?
Margin is where confidence gets priced in. Too little and the first trial flashes; too much and you rent a bigger machine for nothing.
A 20% safety margin is cheaper than one extra mold trial.
For a single-cavity tool with a stable resin, 1.1–1.15 covers normal variation [VERIFY]. For multi-cavity or tight-tolerance tools, use 1.2–1.3 [VERIFY].
The margin also absorbs resin batch variation. Regrind content and melt flow differences shift cavity pressure more than most buyers expect.
A bigger margin buys process safety and costs machine capacity; the right size sits between first-trial flash and wasted tonnage.
When a quote lists a machine tonnage with no supporting math, ask for the numbers. It is the same discipline as vetting any tooling partner in China.
Over-Clamping Damages Molds More Than You Would Expect
When flash appears, the instinct is to crank the clamp up. That instinct shortens mold life.
Excess clamp crushes vent depth, pounds the parting line, and loads tie bars and platens beyond what the job needs.
Crushed vents stop venting. That invites burn marks and short shots, the exact defects the extra clamp was supposed to prevent.
Clamp force hides bad venting; fixing the vent beats raising the tonnage every time.
Flash has its own root-cause list, and our flash and weld line fixes separate mold causes from process causes.
Start trials at the lowest clamp that holds, then add only what stops the flash.
I’ve seen shops run far more clamp than a mold needed, then blame the toolmaker when the parting line wore out early.
Calculate Your Own Clamping Force
The calculator below runs the same math on your numbers. Enter projected area, cavity pressure, resin, and cavity count to get a recommended tonnage.
Clamping Force FAQ
How do I calculate clamping force for my part?
Multiply the total projected area in cm² by the cavity pressure in kg/cm², then divide by 1,000 to get metric tonnes. Total projected area includes every cavity, the runner system, and shutoff surfaces, not just the part. Apply a safety factor of 1.1 to 1.3 depending on cavity count and tolerance. Round up to the nearest standard machine size.
Should the runner count toward projected area?
The runner fills with melt at the same pressure as the cavities, so its projected area adds directly to the clamp requirement. Cold runner systems on multi-cavity tools contribute a meaningful share. Hot runner tools skip this addition, since the melt stays inside the heated manifold. Forgetting the runner is the most common reason calculated tonnage comes in low.
What happens if clamping force is too low?
Cavity pressure pushes the mold halves apart and melt escapes at the parting line. The visible result is flash, and the hidden costs run deeper. Dimensional variation, inconsistent part weight, and short shots follow where plastic escaped before packing finished. Cutting injection pressure to stop flash usually trades one defect for weak parts. The fix is recalculating the required tonnage, not starving the process.
Can too much clamping force damage the mold?
Yes, and it does so quietly. Over-clamping pounds the parting line, crushes vent depth, and loads tie bars and platens beyond the job’s needs. Crushed vents stop venting, which brings burn marks and short shots. The extra tonnage also burns energy and masks the real cause of flash, such as poor venting or excessive injection pressure. More clamp is not a fix for a bad mold.
How does cavity count change the tonnage I need?
Multiply the single-cavity projected area by the number of cavities, then add the runner. Eight cavities need roughly eight times the clamp of one, which is why multi-cavity tools land on bigger machines. Use the higher end of the safety margin, 1.2 to 1.3, because fill imbalance between cavities raises local pressure. Balanced filling matters more as cavity count grows.
Does parting line direction change the calculation?
Yes, because projected area is always measured perpendicular to the clamp direction. A part molded on its side can show a smaller footprint than the same part molded flat. Designers sometimes reorient a part to cut tonnage, but the trade is a more complex parting line or extra side actions. Tonnage, mold cost, and parting line complexity get decided together.
Get the area right, estimate the pressure honestly, and let the margin do the worrying. The cheapest tonnage is the tonnage that holds the mold shut through packing and not a tonne more. What number did your supplier quote for your part, and can they show the math behind it?