PETpla.net Insider 11 / 2018

BOTTLE MAKING 14 PET planet Insider Vol. 19 No. 11/18 www.petpla.net Strech Blow Molding, part 10 - Blow wheel / blow clamp, blow moulds Machine details by Ottmar Brandau Ottmar Brandau has newly revised his book “Strech 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 chaperts in a serie of articles. 4.3 Blow wheel / blow clamp Rotary machines Blow clamps are highly engi- neered, complex devices contribut- ing significantly to the rather large price premiums that rotary machines demand when compared with linear machines of the same cavitation. Each clamp has its own valves, water, and electrical connections supplied from the center of the blow wheel. As mentioned before cams control all blow mould movements. While the blow wheel turns carrying the blow clamps, stationary cams engage through rollers and levers guiding the linkages into the desired movements. Figure 4.15: Drawing numbers and their respective functions: 1. cam “opening/ closing of the mould”; 2. cam “lifting the base mold”; 3. cam “locking the mould”; 4. cam “unlocking the mould”; 5. cam “lifting/lowering the stretch rod.” Moveable mechanisms deflect cams and levers in the case of a malfunc- tion. Blow clamps are built to accept the most common bottle sizes ranging from about 200ml to 2.5 or 3l in volume. There are also minimum and maximum preform and bottle dimensions that must be respected. Over the years engineers have sig- nificantly lightweighted blow clamp com- ponents. This in turn has allowed the increase in throughput of the machines. To achieve this the closing mechanisms do not themselves hold the moulds closed, which would require them to resist the large forces created by the high pressure. Instead, locking mecha- nisms of various designs engage after mould closing. To reduce the stress on these, blow air in small quantities acts on the outside of the mold thus balanc- ing the forces inside and outside of the blow clamp (Figs. 4.15 and 4.16). Figure 4.16: Blow clamp and components. Diagram courtesy of Krones AG. 4.4 Machine timing In rotary machines the control of the various machine functions is facilitated by using the 360 degrees of the turn- ing wheel as trigger points rather than timers. Typically, timing starts when the mould is fully open. Each move- ment then happens with a start and end degree. For example, the mould may close between 14 and 44 degrees. How long this actually is depends on the machine speed. Table 4.2 lists the converting times in degrees. Table 4.3 is the reverse table, con- verting degrees into times. This now has the peculiar effect that process time cannot be simply calcu- lated. Here is an example to demon- strate the following. Assuming that a machine runs at 2,200b/cph for a lightweight water bottle and the mould closing takes 30 degrees. It would therefore take 0.136s to complete. When the machine is then slowed down to 1500b/cph to make a thicker-walled bottle the same move- ment now takes 0.2s. In most rotary machines process time (preblow, blow, and exhaust) is about 70% with 30% necessary to complete the mechanical machine functions. A 30% dead time or unproductive time translates into 108 degrees and at 2,200b/cph into 0.49s