The tool trial process is vital for the repeatability and on-going success of plastic injection moulding production, ensuring ROI on mould tool investment.Process optimisation in plastic injection moulding is therefore crucial. But what does it involve? Pentagon Plastics Group explains.

To help understand why process optimisation in plastic injection moulding is so important, here is a simplified version of what takes place during the trial process.

First, the tool is mounted into the required press size; this will vary dependent on the tonnage pressure required to hold the tool closed. The thermoplastic material of choice is then injected into the tool. This starts with a small amount, not enough to fill the cavities (approx. 50%), with the holding phase set to 0 pressure. The volume of plastic is then increased, shot by shot, while the operator examines the pattern of fill and, in the case of multi-impression tools, checks that all impressions fill evenly.

During this process, the speed and pressure of the 1st phase fill will be broadly established, driven by the visual aspects of the moulding(s). When the part(s) are 95% full, the holding phase is introduced to pack the part out fully. Once the part looks full, a weight check is completed. This involves gradually increasing the holding pressure and time while recording the weight of the product.

At the point where the weight no longer increases the gate “freeze time” is established to prevent more material being introduced into the cavity. If the parts aren’t fully packed at this point during the trial, modifications will need to be made to increase the gate and/or feed to allow more material to enter before freezing off; if they are fully packed out, then further optimisation takes place.

First, the cooling time is reduced until just before the parts are sufficiently solidified to eject without distortion; it is then gradually increased back to the shortest time, allowing good parts to be ejected with a safety cushion.

Finally, the ejection speeds, pressures and the number of strokes are refined, with the aim being to eject the parts in the most efficient manner and achieving the best quality at the shortest cycle time. The parts produced will then be released to quality control for First Article Inspection Reports (FAIRS).

As this demonstrates, process optimisation can be labour intensive to complete effectively, even when it is a simple open/shut mould tool, particularly when processing some of the more challenging thermoplastic materials.

Pentagon Plastics Group recommends that the process takes place while the Toolroom are on hand to support, and that the Toomaker is involved, as their insight on how the tool is constructed can be enormously beneficial to the optimisation phase.

This structured optimisation process ensures a plastic injection mould tool is fully optimised before mass production, reducing risks of future tool repair and production delays.