PETpla.net Insider 08 / 2017

BOTTLE MAKING 32 PET planet Insider Vol. 18 No. 08/17 www.petpla.net Stretch Blow Molding, part 5 Material Basics / Reheat 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 second edition under the Elsevier imprint. PETplanet Insider is publishing extracts from successive chapters in a series of articles. The relationship between resi- dence time and AA level is linear. For the preform moulder this means that extruder size and preform weight should be closely matched. One intriguing detail of PET pre- form manufacture is that cycle times do not vary significantly depending on weight but rather on wall thick- ness. Two preforms with the same wall thickness where preform A is twice the weight of preform B may differ in cycle time by only one or two seconds. This means that the residence time of the lighter preform B is almost twice as long as that of preform A with corresponding effect on AA content. Over the last few years, water bottles especially have become lighter with the negative effect that their residence time has increased if they are produced in the same extruders as they were before light weighing. The relationship between AA gen- eration and temperature is exponen- tial (Fig. 2.19). AA creation In the initial stages of PET resin manufacture, AA levels may be as high as 150 ppm. What happens is that –OH end groups combine with water, glycol, or oxygen that may come in contact with the resin at that stage to form AA whose formula is CH 3 CHO. During solid-state polycon- densation, the material is heated and AA is removed by nitrogen gas to a level of <1 ppm. Drying is again critical to AA generation during preform injection moulding as moisture present in the resin will not only break the molecular chains but also create AA. A mate- rial temperature of 165°C (329°F), as measured at the extruder throat of the injection machine, has been proved to be optimal to minimise AA generation. Residence time and temperature are the other crucial factors (Fig. 2.18). Figure 2.18 Residence vs. AA Level. A linear relationship exists. Diagram cour- tesy of Shell.