PETpla.net Insider 05 / 2015

PREFORM PRODUCTION 36 PET planet insider Vol. 16 No. 05/15 www.petpla.net Cooling issues, part 20 Practical guide to injection moulding of preforms PETplanet Insider is publishing extracts from successive chapters of Ottmar Brandau’s “Bottle, Preforms, Closures”, which was published by hbmedia. A newly revised version is reissued under the Elsevier imprint. 3.8 Cooling issues PET preforms feature relatively thick walls, and the material melts at a high temperature. The combination of these two attributes requires a mas- sive amount of chilled water in order to cool preforms down to demolding temperature. As shown in Chapter 2.4, rapid cooling is also paramount to avoid crystallisation of the material. To better understand how to best cool, we need to look at how the cooling medium flows through the various channels to the machine and in the tool. Process heat flows to the chilled water in three different ways: 1) PET to mould metal 2) Mold metal to coolant in cooling channel 3) From outside of the mold (shop air) to the non-PET side of the cooling channel (so-called “mould sweat- ing”). The first step is controlled by the heat conductivity of the mould mate- rial. The second step depends on how much contact the cold cooling agent makes with the PET side of the cool- ing channel. This in turn is controlled by the way the cooling agent flows and its distance from the hot PET. We distinguish two flow modes in liquids: laminar and turbulent. Laminar flow is characterised by the highest speed in the center of the channel, where there is little friction, decreasing to zero at the wall of the cooling chan- nel, where friction is highest Heat exchange between cooling agent and Figure 3.24 Velocity profile in laminar flow. Longer arrows indicate higher speeds.