In a blow molding machine, air has multiple purposes: pre-blow and inflation air expands the parison until it contacts the mold wall to reproduce the shape of the blow mold. The inflation air presses the material against the mold wall to reproduce surface details such as lettering, surface textures, etc. Furthermore, the inflation pressure promotes good and permanent contact between the mold wall and the polymer, since the polymer starts to shrink during the cooling period. In addition to these functions required directly with the actual blow molding process, air is required for pneumatically activated functions in the machine, such as the insertion of a blow needle to move molded parts (core pulls) or doors in the machine’s safety guard.

Wherever possible, the blow air should be introduced into the parison through the largest possible opening. In this way, sufficiently large airflow can be provided to quickly inflate the parison. In most cases, this is done using this blow pins which are inserted into the parison from above after the blow mold has closed, or are inserted into the open parison from the below before the mold closes.

However, not all applications have openings as large as most packing applications, so that in such cases so-called blow needles are inserted into the parison. This can be done during or after the mold is closed.

In most cases, compressed air systems with 8 to 10 bar air pressure with clean and oil-free air are sufficient for the applications mentioned above. In some cases, for example for safety reasons or to reduce cycle time, blow molders install two different compressed air circuits: one compressed air circuit for the blow air with 16 bar and a second circuit, not necessarily absolutely oil-free, with 6 to 8 bars for the pneumatic functions.

In contrast to injection molding, blow molding only allows heat to be dissipated from the molten plastic on side via the cooled blow mold surface. To keep the cycle time as short as possible, there are several methods of dissipating additional heat from the plastic via the blow air.

One possibility is the so-called “interval blowing”. This means that in a sequence of short intervals, blowing air is blown into the blow mold, followed by short intervals in which the heated air is released (vented) from the blow mold. The volume of air exchanged is relatively small here, another cooling method using a larger volume of air is the so-called “reverse purging”, where air can flow continuously through the blow molded part. The air blown into the hollow part is vented through a second opening, creating a constant exchange of blow air inside the mold and cool air from outside of it. In any case, it is important to design the outlet opening so that a certain internal pressure is maintained in the blow molded part. The advantage of this method is the doubling is the cooling front. The heat does not have to be discharged exclusively to the outside. The method is also limited in the volume of the articles. For an article the size of a bung drum, approx. 210 m³ air per minute would have to be compressed and introduced from ambient pressure (1 bar) to 5 bar blowing pressure to achieve an average heat transfer coefficient of α¹ = 50W/m² K.

Another way to reduce the cycle time is to use dry, cryogenic air with temperatures as low as -45⁰ C. With e.g. Beko Blizz air treatment units the air is dried and cooled. In combination with water cooling of the mold, a considerable amount of heat can be removed from the blow molded part by using cryogenic air in combination with interval blowing or reverse purging. The use of this cryogenic purge air achieves a cycle time reduction of 20 to 30%. Since the blow mold surface tends to “sweat” (condensation of humidity on the mold surface) when using very cold water and cryogenic blow air, the use of dry air curtains is recommended. Another variant is the use of cold and humid air, as the heat capacity of humid air is higher than that of dry air.