PETpla.net Insider 01+02 / 2015

36 PET planet insider Vol. 16 No. 01+02/15 www.petpla.net In our example TP should be (assuming a 5-mm cushion): (65 -5) × 13% +5 = 12.8 or 13mm This is a good starting point that may need adjustment to account for changes in temperature or viscosity. If the transition point value is set too high, the cavity is not filled when the machine goes into hold. This could result in sink marks or even short shots as there may not be enough resin in the mold to account for sub- sequent shrinkage during hold. If the transition point value is set too low, the cavity is completely filled during injection and the pressure will rise steeply as the resistance of the mate- rial in the cavity increases. This could lead to overpacking of the cavity and gate problems. Very thin (<2.5mm) and very thick (>4.5mm) preforms deserve special considerations. Melt injected into thin mold channels cools down quicker than when it is traveling in thicker channels. Therefore, its density increases during injection and the transition point for these preforms should be set much lower, as far down as 6–8%. Very thick preforms show the opposite behavior. They cool down very slowly and must therefore be run with a transition point set at a higher percentage, up to 18%. When in doubt it helps to study the injection graph (or watch the injection pressure gauge) and note the point at which the pressure increases. This is the point when the cavity is filled and where the transition point should be. 3.2.4 Hold time and pressure During the hold or packing phase the material shrinks and density increases. If no more material was added after injection, this shrinkage would result in voids that are called sink marks. The operator has full control over the pressure, whereas the machine adjusts to a low speed setting without operator input. Hold time is dependent on the wall thickness of the preform and tempera- ture of the melt. The chart in Fig. 3.10 gives values for different melt tem- peratures. This chart assumes certain cool- ing water conditions that may not be available on every machine. It should therefore be taken as a starting point only. Most machines feature three hold times, and the overall time should be equally divided among them. Also available are three hold pressures. The first is used to pack the material in the neck finish area. The material has had longer time to cool, and its viscosity has therefore increased. It requires the highest hold pressure. The second controls the body of the preform and is set to a lower value. The third controls the gate area and may be again lower but features the largest difference in pos- sible values because wall thickness in the gate area, gate diameter, and gate cooling all come together to demand a particular setting for optimal gate uniformity and appearance. The maxi- mum injection pressure can be read either on the screen or on a gauge mounted on the injection cylinder. It can be used to calculate hold pres- sures 1, 2, and 3 as 60%, 50%, and 40% of this pressure, respectively. These values should only be used as starting points and may need adjust- ments to overcome sink marks or gate problems. 3.2.5 Decompression In either injection system, whether reciprocating screw or shooting pot, the pressure that is present in the hot runner and the cavities must be relieved when hold is finished to allow easy demolding and prevent the material to drool into the cavity once the preform has been removed. This is done by moving the screw or shoot- ing pot backward (“screw pull-back” on the screen) and leaving it there for a period of time (“pullback dwell time”). A distance of 10mm is usually enough, whereas the time depends on the last hold pressure used. Higher Figure 3.10 Recommended hold times versus performed wall thickness. Figure3.11 This machine uses a graphical user interface for the control of injection parameters. (Picture courtesy of Netstal-Maschinen AG).