Clamp Force Tonnage for Injection Molding — How Do You Calculate the Required Tonnage, and What Happens When the Machine Is Too Small or Too Large?
Last updated: 2026-09-03 · Reading time: about 6 min
Clamp force holds the mold shut against injection pressure. Size it as projected area × cavity pressure × cavity count × a 1.1 safety factor. Get it wrong in either direction and it costs you: too small and the mold flashes open, too large and you crush the parting line, seal the vents and burn the part. Land the requirement at 60–80% of the machine’s rated tonnage.
See also related article.
Key facts
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How do you size the clamp force — and what drives the number?
Clamp force is the tonnage the machine applies to keep the mold shut while the melt is injected and packed. It is not a guess — it comes from three numbers you already have: the projected area, the cavity pressure and the cavity count. See also related article.
F_clamp = A_projected × P_cavity × N_cavities × 1.1 — projected area in cm², cavity pressure in bar, and 1 t ≈ 10 kN. The 1.1 covers the ±5–10% shot-to-shot pressure swing so the mold stays shut at the peak, not the average.
Cavity pressure is the part you estimate, and it tracks the resin’s viscosity. Use the class below, then adjust for wall thickness and flow length.
| Resin class | Cavity pressure (bar) | Why |
|---|---|---|
| PP, PE — semi-crystalline, low viscosity | 300–500 | Flows easily, fills at low pressure |
| ABS, PS — amorphous, medium viscosity | 400–700 | Standard engineering resins |
| PC, POM — medium-high viscosity | 500–900 | Needs more pressure to fill thin walls |
| PA66, PBT — glass-filled | 700–1,200 | Glass raises viscosity 2–3× |
| PPS, PEEK — high-performance | 900–1,500 | Very high viscosity, high injection pressure |
Worked example: a PC part, 120 × 80 mm, single cavity. Projected area = 12 × 8 = 96 cm². PC at a 2.5 mm wall runs about 700 bar. F = 96 × 700 × 1 × 1.1 = 73,920 bar·cm²; since 1 bar·cm² ≈ 0.00102 t, that is about 75 t — so a 100–130 t press.
What happens when the machine is too small — or too large?
Wrong tonnage fails in two opposite directions, and both cost money. Here is what each edge does to the part, the mold and the bill.
| Condition | Symptom | Mechanism | Cost |
|---|---|---|---|
| Under-clamped | Flash, 0.05–0.5 mm at the parting line | Melt pressure beats clamp force; the halves open 0.01–0.05 mm | Trim $0.01–0.05/part; parting-line repair $500–2,000 |
| Over-clamped | Burn marks at end of fill | Platens over-compress and seal the vents; trapped air compresses and ignites | Mold teardown and repair $1,000–5,000 |
| Over-clamped | Ejector binds, short shots | Excess tonnage deflects the platens and pinches the ejector pins | Component replacement and re-test |
| Over-clamped | Higher energy bill | The clamp motor works past what the job needs | $100–500 per 100,000 shots |
Size to the band, not to the maximum. The required tonnage should sit at 60–80% of the machine’s rated clamp force. An 80 t requirement belongs on a 100–130 t press — not a 200 t machine, where 80 t is only 40% of rated and the mold runs over-compressed.
“Clamp force is the first number a buyer asks and the easiest to get wrong in both directions. Too little tonnage and we chase flash all run; too much and we crush the parting line, seal the vents and hand back burned parts. We size the mold to the projected area, then pick the machine so the job lands mid-range — and we would rather reorient the part to cut tonnage than quote a bigger press.”
— Project Engineer, aCatMould
Frequently asked questions
How do you reduce the tonnage requirement to fit an available machine?
Three mold-side levers, no bigger press needed.
Reorient the part so its smallest face points along the closing direction — a 120 × 80 × 30 mm part drops from 96 cm² to 36 cm² projected, cutting tonnage 62%, if ejection and side-actions allow it.
Run a hot runner to delete the cold-runner projected area, worth 10–30% of the total on a multi-cavity mold. Or enlarge the gate to lower cavity pressure 100–250 bar, at the cost of a bigger gate vestige. Fewer cavities also cut tonnage — but they cut output with it.
How do you measure the real cavity pressure — and why does the estimate miss?
A cavity pressure sensor — piezoelectric (±1% full scale, 0.001 s response, to 400°C, $2,000–5,000) or strain-gauge ($500–1,500, slower) — reads the melt pressure inside the cavity through fill and pack. Kistler and Priamus build these into production monitoring.
It matters because the clamp-force calc uses an estimated cavity pressure that can be 20–50% off. The measured waveform shows whether packing is right and lets the machine adjust in real time, so the tonnage you sized on paper matches the shot.
Can you fix flash just by adding clamp force?
Not always — first confirm the flash really is under-clamping. Flash also comes from a worn or damaged parting line, over-packing, excessive melt temperature, or vents cut too deep. Cranking tonnage on a worn parting line only crushes it further.
Diagnose first: check the parting line for damage, back off packing pressure and melt temperature, and measure the actual cavity pressure. Only if the halves are genuinely opening under pressure do you need more tonnage — a press one size up, not a doubled setting.
Key takeaways
- Size the tonnage from the physics — projected area × cavity pressure × cavities × 1.1 — not from the nearest round number. A 96 cm² PC part at 700 bar needs about 75 t.
- Respect both failure edges: under-clamping flashes the parting line, over-clamping seals the vents, burns the part and deflects the platens. Land the requirement at 60–80% of rated tonnage.
- Before you quote a bigger press, try to shrink the number — reorient the part, run a hot runner, enlarge the gate. Fitting an available machine beats waiting on a larger one.
Clamp force is a window, not a single number: too little tonnage leaks flash, too much crushes the mold and burns the part. Calculate the requirement from projected area and cavity pressure, land it at 60–80% of the machine’s rated force, and if the number is too big for the press you have, change the mold — not the machine.
Sources
Kistler — cavity pressure sensing and clamp-force monitoring
Priamus — cavity pressure technology for process control
Plastics Technology — clamp force and machine sizing
MoldMaking Technology — projected area, gating and mold design
MatWeb — resin viscosity and melt-flow data
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet