PETpla.net Insider 05 / 2021

BOTTLE MAKING 18 PETplanet Insider Vol. 22 No. 05/21 www.petpla.net Stretch Blow Molding, part 14 Fundamentals of the Blow Process 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. In this chapter, we will be exam- ining what actually happens inside the blow mould during the blowing of bottles. This description is based on materials science, practical experi- ence gathered by interrupting the blow process at various stages, and blow mould simulation software. 7.1 Reheating preforms Throughput rate, air flow, and lamp settings all determine the tempera- ture of the preforms before blowing. Throughput rate may be dependent on preform wall thickness if the heating capacity of the machine is limited, or by the required blow process time. For example, a 2 l bottle requires a longer venting time than a 500ml bottle. Although throughput rate can be adjusted while the machine is in pro- duction, it is easier to set the machine at a speed derived from experience with similar bottles before starting up. On rotary machines blow wheel movement and mandrel chain speed are coupled and output rate deter- mines process time that the processor can then subdivided into the different process stages within the confine- ments of the machine cycle. On some linear machine a process time can be selected and the machine adjusts the output to suit. Air flow is best left at a medium level and adjusted once some bottles have been blown. The first step in lamp adjustment is to adjust the height of the oven bank to suit the particular neck design. For this purpose a cold preform is placed on a mandrel and the oven heat shield positioned such that it is just above the neck support ring of the preform. Most oven banks are mounted on adjust- ment blocks that can be vertically moved with threaded rods (Fig. 7.1). Figure 7.1 Modern ovens allow easy access for adjustment and mainte- nance. Photo courtesy of Krones AG. The next step is to examine the preform to be blown. Preform wall thickness ranges from 2 to 5mm (0.080–0.200 in.) for standard bot- tles up to 5 l and might not be evenly distributed. Preform designers strive to create an even body wall thickness but bottles such as those with a cham- pagne style base (e.g., beer bottles) need extra material close to the base. These preforms have 4–5mm wall thickness in the lower fifth of the body, requiring extra heating there. The initial heat profile will always have a high setting for the first, and sometimes the first two lamps. This will depend on where the transition area between the neck of the preform and body ends, and the shoulder angle of the bottle. Steep angles in the bottle shoulder require less mate- rial and the preform must be heated more in the area that will form them. The remaining lamps will be adjusted to suit the preform body wall thickness. As stated earlier, there are rotary and linear ovens. Rotary and single-line ovens heat up preforms