PETpla.net Insider 01+02 / 2015

PREFORM PRODUCTION 35 PET planet insider Vol. 16 No. 01+02/15 www.petpla.net Injection parameters, part 17 Practical Guide to Injection moulding of Preforms PETplanet insider is publishing extracts from successive chapters of Ottmar Brandau’s “Bottles, Preforms and Closures”, which was first published by hbmedia. A newly revised version is reissued under the Elsevier imprint. 3.2.3 Transition Point As discussed in Chapter 2.4.2 the purpose of the injection phase is to fill the cavity with molten material, at which point the machine switches over to hold (also called packing). The point at which this happens is called the transition point. During the hold phase the material cools and shrinks, thereby changing density. Density is a measure of mass per volume and is dependent on how energetic molecules of a given material are under given circumstances. For plastics (and most other materials) density changes with temperature. At higher temperatures plastic molecules take up more room and density decreases. As the plas- tic cools down, the opposite happens and the material shrinks. Typical melt density for PET is 1.15g/cm 3 , whereas For PET, only the last option is used as it has been proven that it leads to the most stable process. Once the operator has established the injection parameters, the transition point is calculated as follows: TP =(SS - CU) × 13% + CU where TP = transition point, SS=shotsize, and CU = cushion. SS - CU is the distance the screw actually travels, 13% of it should happen after injection and during hold. The cushion has to be added at the end as the transition point is a fixed value. solid, amorphous density is 1.335g/ cm 3 with solid crystalline density being slightly higher. At around 100°C (212 °F) demolding temperature density is about 1.3g/cm 3 . The difference between the melt and the demolding density is about 13%. This means that 87% of the injection stroke should happen during injection and 13% during the subse- quent hold phase. The machine does not set the transition point automatically. Instead, it gives the operator three choices at which point hold should begin:  When the pressure reaches a cer- tain value  After a certain injection time  When the screw reaches a certain position