What is the injection moulding cycle?
The injection moulding cycle describes all the successive stages that an injection moulding machine goes through to produce a plastic product: dosing, clamping, injection, holding pressure, cooling and ejection. The total duration of a complete cycle – the cycle time – directly determines the production capacity and the cost per part.
Measuring and closing
The cycle begins whilst the previous product is still cooling in the mould. The screw in the barrel retracts and, at the same time, measures out the next batch of plastic granules. The granules melt due to friction and the heat from the heated barrel wall.
As soon as the dosing movement is complete and the previous product has been ejected, the mould closes. The clamping force, expressed in tonnes, holds the two mould halves closed against the injection pressure that will soon be generated.
Injection and holding pressure
The screw moves forwards like a piston and injects the molten plastic into the mould cavity under high pressure. The injection speed and pressure profile are precisely controlled to prevent filling defects, flash and inclusions.
The holding pressure phase follows immediately after filling. The machine maintains a lower pressure to compensate for shrinkage: as the plastic cools, it contracts in volume, and the holding pressure continuously makes up for this loss until the gate solidifies. The distinction between injection pressure and holding pressure determines the dimensional accuracy and weight of the end product.
Cooling
Cooling is usually the longest phase and largely determines the total cycle time. Cooling channels in the mould dissipate heat via temperature-controlled water. The mould temperature and cooling time are set according to the material used and the wall thickness of the product.
Cooling too quickly causes internal stresses and deformation. Cooling too slowly extends the cycle time without improving quality. Well-designed cooling geometry in the mould is an investment that pays for itself with every cycle, as every second saved on cooling time counts when producing large volumes.
Ejection
Once the product has solidified sufficiently, the mould opens and ejector pins release the product from the core side. A robot or conveyor belt collects it for further processing or packaging. The ejection force and the number of pins depend on the geometry. Products with deep walls, ribs or undercuts require a well-designed ejection system.
As soon as the product has been removed, the cycle starts again. Many phases overlap to some extent, meaning that the measured cycle time is shorter than the sum of all the individual phase times.
How can you reduce the cycle time?
The cooling phase generally offers the greatest scope for reduction. Improved mould cooling, conformal cooling channels or insert plates reduce the cooling time without compromising product quality. In-house engineering at Didak Injection makes it possible to optimise cooling, ejection systems and screw geometry simultaneously for each project.
The result is shorter cycles, less material waste and greater dimensional accuracy. That is what 24/7 production with over 42 machines actually means: every hour counts, and every second saved per cycle translates into direct cost savings for your project.
See how Didak Injection optimises production processes in-house.
