PETpla.net Insider 09 / 2014

PREFORM PRODUCTION 40 PET planet insider Vol. 15 No. 09/14 www.petpla.net Please order your copy at the PETplanet insider book shop: https://www.petpla.net/books Bottles, Preforms and Closures A Design Guide for PET Packaging Second Edition by Ottmar Brandau € 115,00 180 pages © Copyright Elsevier 2012 Hot runner controls In order to keep temperatures in the hot runner and the injection noz- zles stable, companies go to great lengths designing control algorithms that not only react to temperature variations but also are able to pre- dict them and compensate for them before any fluctuations may occur. To do this, temperature is measured up to 20 times per second and control- lers change heating current by very small amounts when variations of as little as 0.1°C are detected. Control loops measure the rate of change and adjust the necessary current changes continuously. Melt temperatures may change when operators change dryer settings or screw speeds, for exam- ple, and hot runners must be able to cope with these in order to deliver stable injection profiles (Fig. 2.27). Figure 2.27 Sophisticated control algo- rithms and an easy-to-use interface are characteristics of modern hot runner controllers. (Picture courtesy of Mold- Masters) Gate mechanism There are two ways to separate the hot melt coming through the hot runner from the cooled preform: ther- mal and mechanical. Thermally gated hot runners are most prevalent in the one-stage process, whereas the mechanical ones are most prevalent in two-stage injection moulding. In a thermally gated hot runner the break point between the hot and the cold melts is controlled by tem- perature alone. As seen in Fig. 2.28, the temperature difference between the cold cavity and the hot melt is substantial at around 255 °C (491 °F). A suitable insulation made from stainless steel or any other material with insulating properties separates the two sections. When the machine ejects the preforms, the melt breaks at the point where the cold gate ves- tige connects with the hotter mate- rial inside the nozzle. This material is already partly cooled down and so has a higher viscosity than melt. This prevents it from seeping into the cavity or leads to “stringing,” a common defect where small strands of PET from the melt stream are pulled out with the preform during ejection. An air gap may also assist in the separation process. (Fig.2.28) Figure 2.28 The temperature difference between the cold cavity and the hot melt is used to break the preform free off the melt. (Diagram courtesy of Syn- ventive Molding Solutions) Valve-gated hot runners (see also Chapter 3.2.7) use a mechanical seal between the hot and cold areas by means of a pin called the valve stem. Usually around 3mm (1/8 in) to 5mm (0.2 in) in diameter, this pin moves back allowing melt flow into the cavity, stays back during hold time, then moves forward controlled by a timer that energizes with the end of hold time. Gate vestiges made with valve-gated hot runners are on aver- age shorter than those with thermally gated ones, and the cutoff is more precise. Properly operated they also tend to give less problems with string- ing and gate crystallinity. Disadvan- tage of valve gates is the higher main- tenance requirement for the pins and air cylinders driving them (Fig. 2.29). Figure 2.29 Typical difference in length of gate vestige between preforms of thermally gated (left) and valve-gated (right) hot runners. * This article was published in Bottles, Preforms and Closures, Ottmar Brandau, Chapter 2.5. Copyright Elsevier 2012