Sizing
How to choose injection molding machine tonnage
One formula, one factor table, and the three things that make the formula wrong. Enough to size a machine from the part rather than from the last machine you happened to buy.
What tonnage actually is
Clamping force — quoted in tons — is how hard the machine holds the mould shut while molten plastic is injected into it. The plastic pushes back, hard: injection pressure acting over the projected area of the part tries to open the mould. If the clamp loses that argument, the mould separates by a fraction of a millimetre and you get flash on every shot.
Undersize and you cannot make the part. Oversize and you have paid for a bigger machine, a bigger motor and a bigger footprint, and you heat and power all of it every day for years. Sizing is not a formality.
The formula
Required clamping force (t) ≈ projected area (cm²) × material factor (t/cm²)
- Projected area is the shadow the part casts when you look along the direction the mould opens — not the surface area. For a multi-cavity mould, multiply by the number of cavities and add the runner system’s projected area, which people routinely forget.
- The material factor follows how hard the melt is to push. Free-flowing PP, PE and PS: 0.3–0.45. ABS: 0.45–0.55. PC, PA and filled compounds: 0.55–0.7.
- Add 10–20% margin. For process drift, for the next part you have not designed yet, and because running a clamp at 95% of rating all day is how toggles wear out.
| Material | Factor (t/cm²) | Notes |
|---|---|---|
| PP, PE, PS | 0.30 – 0.45 | Free flowing; use the low end for thick sections |
| ABS, SAN | 0.45 – 0.55 | General-purpose starting point |
| PC, PA, POM | 0.55 – 0.70 | Stiffer melt, higher injection pressure |
| Glass-filled compounds | 0.60 – 0.80 | Add for filler content and abrasion |
| Thin wall / long flow | +15 – 30% | Flow length to wall thickness ratio above ~150:1 |
Two worked examples
A stacking chair. Projected area roughly 1,600 cm², moulded in PP. 1,600 × 0.35 = 560 t. Add margin: a 650 t machine. A 470 t machine will flash the seat edge on a hot day, and no process adjustment fixes it.
A four-cavity closure mould. Each cap projects 12 cm², so 48 cm² of part, plus roughly 10 cm² of runner = 58 cm². In PP at 0.4 that is 23 t — which means the clamp is not what sizes this machine at all. Shot volume, plasticising rate and, above all, cycle time are what you should be selecting on. A 90 t machine is the practical floor of the market.
The clamp is not always the binding constraint. Check three others: shot weight (aim to use 30–70% of rated shot capacity), plasticising rate against the cycle you need, and the physical mould — its height, its width between tie-bars, and its minimum mould height.
The mould has to physically fit
- Distance between tie-bars — the mould has to pass between them, in both axes, plus clearance for the clamps.
- Minimum and maximum mould height — a mould thinner than the machine’s minimum daylight cannot be clamped without a spacer plate.
- Mould-opening stroke — must exceed part depth plus runner plus enough clearance for the part to fall or be taken out. For deep parts this, not the clamp, decides the machine.
- Platen bolt pattern and locating ring — check against your existing tooling before you buy, not after.
- Ejector pattern and stroke, and whether you need hydraulic core pulls, air valves or a robot interface.
A starting table
| What you are moulding | Typical clamping force |
|---|---|
| Closures, cutlery, small housings, precision small parts | 90 – 160 t |
| Small appliance housings, small automotive parts, PET preforms | 160 – 250 t |
| Crates, helmets, medium housings | 320 – 470 t |
| Chairs, large appliance housings, larger automotive parts | 650 – 800 t |
| Pallets, bins, bumpers, large industrial mouldings | 1000 t and above |
Treat this as a sanity check on your arithmetic, not as a substitute for it. Two parts of the same description can differ by a factor of two in projected area.
Questions
Common sizing questions
Is it safer to buy a bigger machine than I need?
A little headroom is sensible; a lot is expensive. A machine two classes too large costs more to buy, uses more energy per part, heats a barrel you are only partly filling — which degrades material — and takes floor space you will want later. Aim to land in the middle of the machine’s capability rather than at either end.
Can I calculate tonnage from part weight instead of area?
Weight sizes the shot, not the clamp. They are different constraints and either can be the binding one. Use projected area for clamping force, and part weight against rated shot capacity to check the injection unit — a part that needs more than about 70% of rated shot, or less than about 25%, points to a different machine.
What if I do not know the projected area?
Send a photograph of the part with a ruler in frame, its weight and its material. Working the area out from a photograph and a dimension is routine and takes a few minutes. We do it free and with no obligation.
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