Techie-talk – Venting an Injection Mould Tool
Why do we need to vent an Injection Mould Tool?
When a mould tool shuts during its normal cycle on the machine a volume of air is entrapped in the cavity, this volume of air is compressed as the hot melt plastic is injected into the tooling. As the hot melt enters the cavity it also gives off gasses (sometimes called volatiles) which also get compressed as the cavity starts to fill. This mixture of hot gas given off by the plastic and the oxygen in the entrapped air, can prove to be literally explosive and will burn within the mould tool.
Centre fed Polypropylene moulding showing the devastating effects of gassing at the outer edge of the part.
The conventional answer for the moulder is to introduce localised venting, sometimes known as ribbon venting at the periphery of the cavity, or to add in extra ejector pins where the gas trap is deeper in the cavity. The moulder may also slow down the injection rate as this allows the compressed gasses a longer period to escape from the tool.
The down sides of these strategies are the hidden costs that they bring to the project. Adding extra vents and ejector pins, post the initial trialling of the tool, means that there will be extra toolmaking costs, time to market penalties etc, always assuming that other issues such as water cooling channels, feed systems amongst others will allow for these extra vents to be added.
OK then, just slow the injection rate down, no problems, right? Well no, best practice is generally to inject the melt into the cavity as fast as possible. The science behind this is that a fast injection rate for most plastic materials helps to orientate the structure of the plastic properly, thus it reduces the stresses that can be built into the finished component. A fast injection rate also means that the melt gets to all areas of the cavity in the shortest possible time, meaning that it is cooling at the same rate across the cavity, also reducing possible stresses being set into the finished moulding. In-built stress in the finished part may well lead to a failure in the field as the stress works its magic, leading to premature failure. This can be both costly in pound notes to fix the specific component, but also costly for your good reputation.
At Plasmotec we build-in positive venting of the tool at the concept stage. Using our Mould Flow Analysis software, we model the plastic flow into the cavities, looking to see where gas trap issues are likely, and then build the tool to cope with removal of the gas in an efficient and effective manner. Through our tool trialling procedures, we ensure that the tooling goes on the right size of machine and that the clamp tonnage is flexed to allow the tooling to “breath” properly as the melt is injected. Just part of the “hidden” benefits of sourcing your plastic components from Plasmotec.
For more information on our Custom Moulding Services, please call us on 01280 701 335 to talk to one of the team. We'll do the teckie stuff, so you get the moulded components that you want. We'd love to bring another moulding project to life with you.