Cooling time is part of what determines how many acceptable parts an injection moulding cell produces in a shift. Reduce it safely and you may shorten your lead times, increase capacity and lower the production cost per part. But reduce it too far and your components may deform after ejection or fall outside tolerance.
With that in mind, your objective isn’t to make the cooling time as short as possible. You’re looking for the shortest validated cooling time that still produces repeatable parts and reduces the total cycle time. So, if cycle time is limiting your output, talk to us about your injection moulding requirements.
What is cooling time in injection moulding?
In thermoplastic injection moulding, the molten polymer must lose enough heat inside the tool to become rigid enough for ejection and handling. The component doesn’t need to reach room temperature before it leaves the mould, but it must reach a suitable ejection condition.
While the component cools, the machine normally prepares the material for the next shot by rotating and retracting the screw. These processes, cooling and preparing the next shot of molten plastic, overlap for part of the cycle.
The mould can’t move on to opening and ejection until the required cooling time has elapsed. If screw recovery takes longer than cooling, however, the machine may still have to wait for the next shot to be ready.
How does cooling time affect injection moulding cycle time and output?
Consider a single-cavity tool running with a 30-second total cycle. It can complete 120 cycles per hour before allowing for stops or rejects. Reducing that cycle to 27 seconds increases the theoretical output to around 133 parts per hour, an 11% gain.
Across a long production run or a multi-cavity tool, a few seconds can make a surprising difference to overall output. Shorter cycles may also reduce the energy used per component, helping us lower the environmental impact of production.
However, output alone doesn’t measure efficiency. The useful measure is how many conforming parts you can produce per hour.
What happens if the cooling time is too short?
A component that leaves the tool while it’s too hot may not resist the forces from the ejector system or downstream handling. It might develop ejector marks, distort or continue shrinking outside the tool. These effects increase the likelihood of dimensional variation, affecting how the part fits and functions.
If a faster cycle causes more rejects, sorting or rework, it won’t improve production efficiency. Our guide to common injection moulding defects explains why cooling must be considered alongside component design, material and the other process settings.
Cooling for too long creates the opposite problem. Once the part meets its ejection and quality requirements, additional time doesn’t add any value.
What factors affect injection moulding cooling time?
Wall thickness is one of the main influences. In simplified heat-transfer models, cooling time rises roughly with the square of the controlling wall thickness. This means doubling the thickest section can increase the cooling time to roughly four times its original value (when the other conditions stay the same).
The controlling section may be a local boss, junction or other concentration of material rather than the component’s general wall thickness. More uniform walls and cored-out thick sections can help, provided the design still meets its strength and mould-filling requirements.
Then there’s also the polymer itself to consider. Different materials transfer heat at different rates and require different melt, mould and ejection temperatures. A lower mould temperature isn’t always the answer because it can affect filling, surface finish, shrinkage or material behaviour. Read more about temperature control and injection moulding quality.
Finally, tool design determines how quickly and evenly the component loses heat. Cooling-channel position, circuit balance, coolant temperature and flow all affect heat removal. One hot area can hold back the complete cycle or create uneven shrinkage.
How can you reduce injection moulding cooling time?
The best opportunity to reduce injection moulding cooling time comes before the tool is manufactured, when wall sections, material build-up and cooling provision can still be optimised. If the tooling already exists, reviewing the component, tool and process may still identify opportunities to reduce cycle time.
If you’re still developing the component, talk to us about design and tooling before committing to production. We use CAD, 3D modelling and simulation to assess component geometry and mould flow. We also test and approve tooling before release to production. This allows us to consider cycle time alongside the component’s tolerances, function, production volume and material.
During production, we use programmable controllers to maintain the agreed process settings and keep each cycle consistent. Robots and sprue pickers can then support repeatable part removal and handling. Alongside this process control and automation, we use machine-side specifications, first-off and last-off inspections and dimensional measurement to make sure higher output hasn’t come at the expense of quality.
Improve injection moulding efficiency without compromising quality
This cooling-time also discussion applies to thermoplastics. Thermosets cure in a heated tool, so cure time presents a different process consideration. As we provide both thermoplastic and thermoset moulding, we can help you select a suitable process for your component.
So, if you’re developing a new moulded part or reviewing your current production methods, send us your drawing, material requirements, tolerances and expected volumes. We’ll assess how the component, tooling and process affect cycle time, quality and cost. Contact Talisman Group to discuss your next injection moulding project.