PETpla.net Insider 11 / 2014

PREFORM PRODUCTION 39 PET planet insider Vol. 15 No. 11/14 www.petpla.net Injection Parameters, part 16 Practical Guide to Injection Moulding of Preforms PETplanet insider is publishing extracts from successive chapters of Ottmar Brandau’s “Bottles, Preforms and Closures”, which was first published by hbmedia. A newly revised version is reissued under the Elsevier imprint. The parameters operators can control are:  Injection pressure  Injection speed  Transition point  Hold time  Hold pressure  Material “cushion” Most modern machines allow the operator to set all relevant values on the screen, whereas older machines may feature manual hydraulic and posi- tion controls. For this discussion it has no relevance how the pertinent values are controlled. It is important that read- ers understand the concepts. 3.2.1 Injection Pressure Many operators are under the mis- conception that the injection pressure can be dialed in because there is a field on the screen with this name. However, this is not the case. Injection pressure is the result of how hard the machine pushes the resin and how hard the resin resists this pressure. The value on the screen merely determines the pressure at which the machine hydrau- lics will stop pushing and drain oil to the tank instead of sending it to the injec- tion cylinder. The factors that determine injection pressure are:  Injection speed settings: The faster the machine injects, the higher the pressure.  Melt viscosity of the material (see the last chapter): This in turn depends on  Temperature: The lower the resin temperature, the higher the required pressure.  Intrinsic viscosity (IV) of the mate- rial: The higher the IV, the higher the required pressure. Pressures over 100 bar (1500 psi) are not recommended for PET. They tend to shear the material too much and this can lead to burn marks. Very thin (<2.3 mm) preforms may require higher pressures, and this is one of the challenges in thin-wall moulding. Use of higher pressures during startup is a common practice when the mould is still colder compared to continuous production conditions. The hydraulic pressure controlled on the screen is not the pressure the material is subjected to. This is because the injection piston that is pushing the screw or shooting pod for- ward has a five to seven times larger area than the screw or shooting pod. This leads to pressure intensifica- tion by the same ratio. Actual mate- rial pressure should not exceed 700 bar (10,000 psi), and this equates to