PETpla.net Insider 08 / 2014

PREFORM PRODUCTION 29 PET planet insider Vol. 15 No. 08/14 www.petpla.net PREFORM PRODUCTION 29 hopper with the time noted. The coloured pellets will eventually show up in the preforms, and the time can then be measured. Depending on the position of the resin in the hopper, drying times differ with the resin in the centre of the hopper travelling up to 20% faster. Therefore, a median residence time must be chosen. Once this residence time has been estab- lished, the proper drying temperature can be chosen from the graph toward the left (Fig. 2.11). Maximum drying temperature is 171°C (340°F). Higher temperatures lead to oxidation, which shows up as yellowing of the resin. Improper drying and the resultant drop in IV change the inflation behavior of the preform in that the preform will inflate under lower pressure because the NSR is greater. In turn, this will lead to less orientation and weaker bottles. Preform designers should know this connection in case problems arise during production, which are all too easily blamed on the preform design. The theory of Injection moulding of preforms We will not discuss the melting and viscoelastic flow of the material in the extruder barrel as they do not pertain as much to the preform design. However, the injection part is important for designers to under- stand because of the particular opportunities and process limits as well as the possible defects that will then affect the blown bottles (Fig. 2.12). Injection moulds consist of the male core, the female cavity, and the neck insert. The last has to move during ejection of the part to release the undercuts created by the thread beads. For this 4 5 6 7 8 9 10 11 350 340 330 320 310 300 290 280 171°C 160°C 149°C Peller Dwell Time. Hr Fig. 2.11 Fig. 2.12 The various components of a typical injection mould.