PETpla.net Insider 11 / 2019

BOTTLE MAKING 27 PET planet Insider Vol. 20 No. 11/19 www.petpla.net ever, in the case of the smaller bottle 73% more blow air is needed than would be if just the bottle needed to be supplied. Smaller dead air volumes will save significant money over the lifetime of the machine. Small machines particularly often fea- ture high dead air losses and users should carefully examine manufactur- ers’ offerings (Fig. 4.19). There is one more source of needed air volume that is not men- tioned yet. On some machines that use pneumatic cylinders for stretch rod activation, this air is actually high- pressure blow air that is reduced to the 8 bar or so that the stretch rod cyl- inders are using. While compressing air to 35 or 40 bar and then reducing it to 8 or 10 bar is an expensive under- taking, this is done to avoid adding another line to the distributor inside the blow machine as this distributor for air, water, and electrical power is one of the most expensive parts of the blow moulding machine. The newest machines with servo or electrome- chanically controlled stretch rods do of course not use any air and will save money in the long run. Because of the cost of air con- sumption, most companies now offer air-recycling systems. These sys- tems, instead of exhausting blow air into the atmosphere, pipe it back to a storage tank where it can then be used for preblow and stretch rod air, the machine air circuit, or other plant air requirements. Of course, pres- sure reducers in the systems allow the proper pressure for the vari- ous uses. In this manner, savings of 25–50% can be achieved. This not only reduces operational costs but also helps with capital expenses as fewer or smaller compressors may be needed. A small cycle time penalty is unavoidable because the air does not move as fast to the pressurised storage tank as it does to the lower atmospheric pressure. 5. Blow Moulds Blow moulds play a large part in making high-quality bottles. While the machine has to deliver preforms at the right temperature, it is the blow moulds that give containers repeat- able features and a brilliant appear- ance (Fig. 5.1). Figure 5.1 Blow mould halves in and out of mould base and base insert. Highly polished aluminium is the most com- monly used material for standard bot- tles. Photo courtesy of SIPA. 5.1 Design Neck and thread finish are already formed in the preform; so blow moulds form only the body and base of the bottle. In the reheat stretch blow mould- ing (RSBM) process they consist of three parts: two mould halves and one base insert (also called push-up). The base insert is necessary because the walls at the base of the concave con- tainer could not slide over the mould halves during mould opening if these were forming them. Instead, the verti- cally moving base insert is drawn out of the way before, or as, the mould opens (Fig. 5.2). Figure 5.2 Typical mould design for linear RSBM machine. The given details are followed by their descriptions: 1, Preform Retainer Insert; 2, S.S. Insert; 3, Mould Body; 4, Back Plate; 5, Base Insert/Push- up; 6, Locating Ring; 7, Push-up Holder; 8, Taper Lock Pins and Bushings; 9, Guide Finger. Drawing courtesy of Hallink Molds Ltd. While the three-piece design is common to all moulds, they are manu- factured quite differently depending on the type of machine to which they will be fitted. Linear machines have all mould cavities mounted within two blocks where the cavities sit side by side. In rotary machines each blow mould is mounted to a separate car- rier, opening and closing individually. Modern machines use so-called shell moulds whereby the actual mould halves are only 5mm thick and are assembled onto bases that are all the same for a family of containers. These bases carry all water connections and need not be touched during a changeo- ver, thus reducing valuable time. Moulds are usually built from alu- minium, which is chosen for its high heat transfer rate, easy machinability, and lightweight. The types of aluminium used are typically those used in the aircraft industry. AL 7075 T6, T-2024, or Alumenec 89 are some of the grades used worldwide for this application. Base inserts may be of the same mate- rial or made from beryllium–copper. Another material in use is stain- less steel for hot-fill applications. While a polish to mirror-like quality is still the norm, some companies have quite successfully tried to leave moulds at a much rougher polishing state. This saves cost because the mirror polish is still applied manually while detract- ing only very slightly from the expected bottle surface appearance. The internal pressure of the blow air results in a considerable force against the closing mechanism of the blow mould. Blow moulds have guide pins and bushings as well as taper locks in the base insert that keep the mould in position during the blow process. Mould carriers feature locking mechanisms that keep them closed against the blow pressure. To alleviate these stresses and also make ever-lighter frames pos- sible, “pancake” cylinders have become increasingly popular. These cylinders are very thin shells behind the blow moulds and are filled with the same air that blows the bottle. Since pressure is equal both inside and outside the mould there is no resultant force acting against the mould halves (Fig. 5.3). Figure 5.3 Blow mould with locking mechanism mounted inside a rotary machine. Photo courtesy of SIPA.