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How do I estimate the cycle time of an injection moulded part?
Cycle time is the sum of mould open and close time, injection and packing time, and cooling time, and cooling is usually the largest part. Cooling time rises with the square of the wall thickness: a published worked example for a 1.5 mm ABS wall gives about 3.6 seconds of cooling, so the same calculation for a 3 mm wall gives about four times as long, roughly 14 seconds.
Cycle time matters because the press is charged by the hour. A part that cools in half the time can cost noticeably less per piece and needs fewer cavities for the same output.
What makes up the cycle
Toray's technical guide splits the cycle into three parts:
- Intermediate (dry cycle) time: opening and closing the mould, removing the part, loading any inserts.
- Injection time: filling the cavity and packing in extra material to compensate for shrinkage.
- Cooling time: until the part is rigid enough to be pushed out without deforming.
BASF's guide to estimating cooling times states that most of the cycle is taken up by cooling. In one study described by RJG, about 85 percent of the cycle was spent cooling the part.
The cooling time equation
For a flat wall, the published estimate (used by BASF, Toray and the MISUMI tutorial) is:
t = s² ÷ (π² × α) × ln[ (8 ÷ π²) × (Tmelt − Tmould) ÷ (Teject − Tmould) ]
where s is the wall thickness, α the resin's thermal diffusivity, and the temperatures are the melt, the mould surface and the temperature at which the part can be ejected. BASF notes that it uses an effective diffusivity matched to moulding trials, because the textbook value changes with temperature.
A worked example
| Wall | Cooling estimate | Source |
|---|---|---|
| 1.5 mm | about 3.6 s | MISUMI worked example |
| 2.0 mm | about 6.3 s | Same inputs, our calculation |
| 3.0 mm | about 14.3 s | Same inputs, our calculation |
The MISUMI tutorial uses ABS with α = 0.0827 mm²/s, a 230 °C melt, a 50 °C mould and an ejection temperature of 90 °C. Changing only the thickness shows the square law at work: double the wall, about four times the cooling.
Adding it up
- Estimate cooling from the thickest wall that matters, not the average.
- Add a safety margin; RJG suggests about 20 percent on the cooling timer to allow for variation in material and machine.
- Add injection and packing time, which the moulder can estimate from the shot size.
- Add the dry cycle for opening, ejection and closing, which depends on the press and mould.
- Check the estimate against the moulder's experience with similar parts.
Designing for a shorter cycle
- Keep walls thin and uniform; a single thick boss can set the cooling time for the whole part.
- Core out thick sections and use ribs for stiffness instead.
- Avoid thick runners in cold-runner moulds; BASF notes the feed system should be matched to the part's cooling time.
- Ask about cooling channel layout near hot spots such as deep cores.
These are estimates from published equations. Real cycle times come from the moulder's trials on the actual mould and press.
Questions buyers ask
Why is my quoted cycle longer than the calculation?
The equation covers a flat wall cooling evenly. Real parts have thick spots, cores that are hard to cool, and handling time. Moulders also add margin to keep the process stable.
Does resin choice change cycle time?
Yes. Diffusivity, melt and mould temperatures and the safe ejection temperature differ between resins, so the same wall cools at different rates.
Can the moulder just cool faster?
Colder moulds or shorter cooling can cause warp, sink or stress. There is usually a minimum stable cooling time for a given part.
How does cycle time affect cavity count?
Directly. The cavity count formula multiplies demand by cycle time, so halving the cycle can halve the cavities needed for the same output.