Cumsa, has developed a system for eliminating the air trapped within a mould cavity before the injection cycle begins. Because the system works on a vacuum principle, the solution to the problem does not require altering injection parameters.
Conventional means of removing air trapped in the cavity when the mould closes—air that is heated and compressed by the plastic—involve using the plastic melt to exert positive pressure on the air pocket. The injected material pushes the air out of the cavity through the clearances around ejectors, through inserts created for the purpose, through porous-material inserts, through air vents on the periphery of the cavity, or by other methods. In each of these cases, injection parameters stray from the optimal and complete filling of the cavity becomes more difficult to achieve.
The most effective solution to the problems arising from this would seem to be to remove the air from the cavity before injection takes place. That is what Cumsas system does, by applying a Venturi system that functions with a standard air pressure of 6 bar. It requires that the mould be hermetic, with no air vents. The system translates the 6-bar pressure into negative energy: when the air is sucked out of the cavity, the resulting vacuum allows the plastic mass to flow freely and fill the whole cavity easily.
Using this system, moulders can produce parts with uniform colour, with few sink marks and invisible weld lines, an even structure and dimensional stability, and strong mechanical resistance.
Cumsa has created two models of the system—VacuumJet and Vacuum&Blow—which have different capacities in order to cover the requirements of the majority of moulds. If necessary, two or more systems may be grouped together to achieve higher performance. This system is suitable for all thermoplastic injection moulds and for all materials injected. The system should be installed at any mould area not exceeding 110°C of temperature.