PETpla.net Insider 11 / 2019

BOTTLE MAKING 26 PET planet Insider Vol. 20 No. 11/19 www.petpla.net Stretch Blow Molding, part 12 Machine Details, Blow Moulds 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. Air Consumption Another important feature is air consumption and more specifically the so-called dead air volume. Air is the single-most costly part of blowing bot- tles besides the preforms and users do well to pay close attention to this issue. Air consumption is driven by the air pressure, the actual volume of the bottles produced, and the air that is not used in the blow process but needs to be exhausted every blow cycle. This air “hides” in the connec- tion pieces between the blow valves and the cavity. When the exhaust valve is energised, all air that is between it and the cavity must be exhausted besides the actual bottle volume to make sure there is no pres- sure left in the blown bottle. Many manufacturers of blow machines do not publish this important information but users can calculate it by looking at the machine specifications. Let us assume a manufacturer specifies blow air consumption of 1,300 Nm 3 /h for the production of 20,000 b/h of 1.5 l volume at 35 bar. The volume that is actually inside the bottles is 20,000 b/h × 1.5 l × 35 bar/1,000 l/m3 = 1,050 Nm 3 /h. The difference of 1300-1050 = 250 Nm 3 /h is the dead air volume. This in turn is 367ml/bottle (250 Nm 3 /h × 1,000 l/ Nm 3 /h/20,000 b/h/35 bar). It is important to calculate this as a fixed number rather than a percentage as this lost air is present for bottles of each size. A 500ml bottle has the same losses as a 1.5 l bottle; how- Figure 4.19 During the “airback” phase, blow air is redirected to a distributor panel instead of exhausted to the atmosphere. Diagram cour- tesy of KHS Corpoplast GmbH.