PREFORM PRODUCTION PETplanet Insider Vol. 24 No. 10/23 www.petpla.net 20 BOTTLE MAK NG ‘Stretch Blow Molding’, part 16 Injection stretch blow moulding machines by Ottmar Brandau Ottmar Brandau has newly revised his book ‘Stretch Blow Molding’, first published by hbmedia / PETplanet Publisher, and now re-issued in a third edition under the Elsevier Imprint. PETplanet Insider is publishing extracts from successive chapters in a serie of articles. 8.3 Injection Controls All machines on the market use reciprocating screws. That is, the screw turns and pushes material to the front of it while retracting backward in the barrel. This is called recovery. During injection the screw moves forward pushing material into the hot runner while a check valve located at the front of the screw prevents material from moving back. The parameters that can be adjusted on the machine are discussed as follows: injection pressure injection speed transition point or switchover point hold pressure hold time cooling time cushion length Injection pressure and speed Processors can dial in injection pressure either from the screen or manually on a pressure relief valve. A maximum of 100bar (1,500psi) is recommended for PET to avoid shearing the material too much. This is hydraulic, not material pressure and the difference should be explained. The hydraulic injection piston has a 5–7 times bigger area than the screw area leading to pressure intensification. For example, a 200mm injection piston has an effective area of 31.4cm2. The 80mm screw is connected to have an area of 5.024cm2. Using 100bar in the piston results in a force of 3,140kg acting now on the smaller screw area. This leads to the screw experiencing a pressure of 3,140bar/5.024cm2 or 625bar. This is the pressure the material is subjected to and 700bar (10,000psi) is the recommended maximum for PET. It is possible to increase this pressure slightly during start-up to get the process going but many machines run well below the maximum. A notable exception is very thin preforms (<2.3mm), as these require fast injection and with it high pressure to prevent freezing off in the cold tool. Pressure is also not actually regulated by the pressure setting. Instead, this setting is just limiting the pressure to the dialled-in value, draining oil to tank when the actual pressure reaches the selected threshold. What actually creates the pressure is the speed of injection. The faster the speed the higher is the resulting pressure. Speed should be chosen so that the maximum pressure is not reached. Most machines show the actual pressure on the screen while on some it can be read on a dial gauge at the extruder. Figure 8.5 Recommended injection fill time versus preform weight. This graph should only be taken as a general guideline. Other process considerations may force processors to change the time significantly. Injection time is the time between start of injection and when the screw reaches the transition or switchover point. The actual injection time is not a time that can be dialled in. Instead, it is the result of the speed setting and how long it takes to move the material from start of injection to the transition point. This in turn depends on the chosen speed setting and melt viscosity. As discussed earlier, melt viscosity is a function of the material intrinsic viscosity (IV) and melt temperature. Fig. 8.5 gives a guideline what injection time should be for PET.
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