PETpla.net Insider 10 / 2016

CAPPING / CLOSURES 42 PET planet Insider Vol. 17 No. 10/16 www.petpla.net Different applications, neck finishes for various, part 9 Closure for PET bottles & Resins PETplanet Insider is publishing extracts from successive chapters of Ottmar Brandau’s “Bottles, Preforms and Closures,” which was published by hbmedia. A newly revised version is reissued under the Elsevier imprint. 4.6.2 Compression Moulding CM machines work very differ- ently from IM machines. There is of course an extrusion screw, but it is running continuously because the screw has no part in the transfer of the molten plastic into the mould. Single-cavity tools are mounted on a rotary table with practically no limita- tion as to maximum cavitation. Figure 4.36 shows the various stations of the machine with the extruder on the left and the wheel turning counterclock- wise. A drop of precisely measured resin, cut from a continuous extruded stream, is placed into a transfer wheel. The same wheel also takes finished caps out of the main wheel. At position (1) the transfer wheel comes to a position above an empty cavity of the main wheel and drops the plastic into it. At position (2) the cavity lifts vertically up into the core and the actual moulding takes place. In position (3) the plastic now spreads between core and cavity, climbing up the cap sidewalls under relatively low pressure. It therefore leaves no sprue point, which may be a great advantage should the cap surface need decorating. During position (4) the fully formed cap is now cooling. In position (5) the cavity moves down leaving the cap on the core. In posi- tion (6) a stripper ring pushes the fin- ished cap onto the transfer wheel. An additional transfer wheel then places the finished caps onto a conveyor (Fig. 4.37). Figure 4.36 Schematic of compression machine cycle (picture courtesy of Sacmi)