PETpla.net Insider 10 / 2020

BOTTLE MAKING 40 PETplanet Insider Vol. 21 No. 10/20 www.petpla.net the amount of crystallisation reaches about 4%. Since crystalline material reduces many bottle properties, haze always causes bottles to be rejected and must therefore be avoided. As mentioned earlier crystalline haze may already be present in the preform and operators need to check their supply of preforms first when this defect occurs. Fig. 6.8 Thermo image of preform at the oven exit. Lower blowing temperatures yield better performing bottles. Photo courtesy of Husky IMS. Temperatures below 85°C (185°F) cause microcracks in the PET structure. These cracks show up as whitish rings, usually in areas with the highest stretch ratios and also cause bottle rejection. This type of failure is called pearles- cense because of the pearl-like appear- ance of the defect (Fig. 6.8). Making bottles that show neither crystalline haze nor pearlescense still leaves a process window to work with. At very high stretch ratios there is often only one temperature the preform can be successfully blown at but most other times operators can blow at a certain temperature range. Bottles produced in this range might all look alike but when it comes to bottle properties such as car- bonation retention, the bottles blown at the lowest possible temperature before pearlescense sets in are the ones that will perform best. This is because - as shown in chapter 3, section 3.2 - by low- ering the preform temperature the maxi- mum strain within the natural stretch ratio is reduced and the material is forced to orient more. It should now also be appar- ent that the optimal temperature profile within the preform wall (higher inside than outside temperature) leads to lower overall temperatures since the onset of pearlescense can be delayed: The fur- ther stretching inner parts do not fracture as early at the higher temperature. Please order your copy at the PETplanet Insider book shop: www.petpla.net/book3 Stretch Blow Molding Third Edition by Ottmar Brandau € 130,00 374 pages © Copyright Elsevier 2017, 2012 I should mention at this point that most bottles blown in this matter, that is, at the lowest possible temperature, require a minimum blow air pressure of 35bar with bar as a maximum. Com- pressed air takes up approximately 50% of bottle production energy cost (without the cost of preforms) and higher air pressure increases overall bottle cost. Sometimes a compromise between optimal bottle quality and eco- nomic considerations must be found. Recycling high-pressure air for preblow pressure is available on some machines and can reduce these costs.