How is clamp force calculated for injection moulding?

Clamp force (tonnes) ≈ projected area of parts plus runner (cm²) × a material/depth factor of roughly 0.3-0.8 t/cm². A part projecting 100 cm² typically needs a 30-80 tonne machine. Deep-draw parts and high-viscosity resins use the upper end.

What determines injection moulding cycle time?

Cooling dominates — typically 50-70% of the cycle — and scales roughly with the square of the thickest wall. Filling takes 0.5-3 s for small parts; packing, mould open/close and ejection add fixed seconds. Wall thickness is the designer's main cycle-time lever.

How long is a typical injection moulding cycle?

Small thin-walled parts run 10-20 s; general housings with 2-3 mm walls run 25-45 s; thick-section parts exceed 60-90 s. Thin-wall packaging runs below 5 s on dedicated machines. Every second is machine-hour cost divided across cavities.

What is shot size and why does it matter?

Shot size is the maximum melt volume the injection unit delivers per cycle. Total shot (parts + runner) should use roughly 20-80% of capacity: too little means long residence time and degradation; too much means inconsistent plasticising and short shots.

Can a mould run on a smaller machine to save money?

Only if clamp force, shot size, platen size and tie-bar spacing all fit. Undersizing clamp force causes flash; undersizing shot capacity causes fill inconsistency. Machine-hour rates rise with tonnage, but running a 300T job on a 100T machine is false economy — the defects cost more.

How do cycle time and machine size affect part price?

Unit machine cost = (cycle seconds ÷ 3600 × hourly rate) ÷ cavities. A 30 s cycle on a €35/h 100T machine with 4 cavities costs about €0.073 per part. Halving wall thickness may roughly quarter cooling time, but filling, packing and mould movements remain. Nordmould confirms the complete cycle assumptions for the project.

Injection Moulding Cycle Time & Clamp Force: How Machine Size Is Chosen

Two numbers decide which machine moulds your part and what each part costs: clamp force keeps the tool closed, and cycle time meters out machine hours. Both are calculable from the part before any quote — and both are strongly influenced by design decisions made in CAD.

Halving wall thickness may roughly quarter cooling time under otherwise comparable conditions, but filling, packing and mould movements remain. The complete cycle and machine cost therefore do not fall to one quarter.

How is clamp force estimated?

The melt enters the cavity at pressure and tries to blow the mould open. The clamp must resist: clamp force ≈ projected area × cavity pressure factor.

Projected area is the shadow of parts plus runner in the opening direction — a 100 × 100 mm housing projects 100 cm². The factor accounts for material and geometry: roughly 0.3-0.5 t/cm² for easy-flow resins in shallow parts, 0.5-0.8 t/cm² for viscous resins (PC, filled grades), deep draws and thin walls needing high injection pressure.

Example part	Projected area	Factor	Clamp needed	Typical machine
Small clip, 20 cm²	20 cm²	0.4 t/cm²	~8 t	50T press
Housing, 100 cm²	100 cm²	0.5 t/cm²	~50 t	80-120T press
Large panel, 400 cm²	400 cm²	0.6 t/cm²	~240 t	250-350T press
4-cavity housing tool	4 × 100 + runner ≈ 450 cm²	0.5 t/cm²	~225 t	250-350T press

Note the last row: cavitation multiplies projected area, so a 4-cavity version of a part can need a bigger, more expensive machine than a single-cavity one — part of why cavitation is an economic decision, not just an output multiplier.

What makes up the cycle?

Phase	Typical share	Controlled by
Filling	0.5-3 s	Gate size, melt temperature, flow length
Packing/hold	2-10 s	Wall thickness, gate freeze time
Cooling	50-70 % of cycle	Wall thickness², mould temperature, cooling layout
Open/eject/close	3-8 s	Machine, part release, robot

Cooling is the giant, and its scaling law is the most useful rule in part design: cooling time grows roughly with the square of the thickest wall. A 3.0 mm section cools about 4x slower than 1.5 mm; a 4.5 mm section about 9x slower. The thickest feature — often a boss junction, not the nominal wall — sets the whole cycle.

What does this mean for part cost?

Machine-hour rates rise with tonnage: roughly €25-40/h at 50T, €30-50/h at 100-150T, €40-80/h at 200-350T in typical European budgeting. The unit machine cost is then arithmetic: (cycle ÷ 3600 × rate) ÷ cavities.

Worked example: a housing needs ~50 t clamp → 100T machine at €35/h. Cycle 30 s, 2 cavities: (30/3600 × 35)/2 ≈ €0.146/part. Cut the max wall from 3.0 mm to 2.0 mm: cooling drops ~55%, cycle to ~20 s, machine cost to ≈ €0.097/part — a third off, forever, from one design decision. That is why Nordmould's DFM review reports wall-thickness-driven cycle impact alongside tooling price.

What else constrains machine selection?

Clamp force is necessary but not sufficient. Shot size: total shot should sit in the 20-80% band of machine capacity. Platen and tie-bar dimensions: the mould must physically fit. Opening stroke and ejection: deep parts need daylight to drop. A competent quote states the assumed machine class — if yours doesn't, ask; it determines whether the unit price survives contact with reality.