PETpla.net Insider 10 / 2013

BOTTLE MAKING 47 PET planet insider Vol. 14 No. 10/13 www.petpla.net modules for the respective make or type of stretch blow moulding machine and are combined to create a com- plete mould. The following discussion will thus be limited to the bottle-spe- cific parts of a blow mould. Although the data on the bottle geometry will be used as a basis for the design of the mould in the CAD system, other aspects must also be considered. For example, a blow mould will usually comprise two cavity halves (which map the area below the bottle neck down to just above the base) plus the related base mould or insert. The first thing that must be decided is the position of the mould parting lines. In the case of a rotation- ally symmetrical design, the position of the vertical mould parting line in the body of the container is not a critical issue. But for design elements such as logos, recessed handles, etc., the position of the parting line must be agreed in detail with the customer. There may also be technical reasons that require a certain position of the mould parting line, e.g., if the number of venting bores is to be reduced or if such bores are to be fully avoided. To a certain extent, an appropriate positioning of the mould parting line may also be used to accommodate inevitable undercuts. Also the position of the parting line between the bottle body and the base must be carefully selected, which again is not only a matter of styling but also of engineering. For example, the height of the base and/ or the tapering between the feet of a petaloid base will clearly have an impact on the properties of the base of the bottle. Finally, the maximum bottom stroke of the blow moulding machine must be taken into account when determining the position of this parting line. Even if the spacer for the base mould is a relatively simple com- ponent, it must still be constructed specifically for each new container depending on the bottle height. One of the key aspects that should be considered when designing a blow mould based on the geometrical data of the bottle is shrinkage. Like all thermoplastics, PET will shrink when it cools down, so the cavity must be designed with an appropriate allow- Please order your copy at the PETplanet insider book shop: https://www.petpla.net/books Bottles, Preforms and Closures A Design Guide for PET Packaging Second Edition by Ottmar Brandau 115,00 € 180 pages © Copyright Elsevier 2012 ance. The actual shrinkage, which can amount to several millimetres in height and diameter, depends not only on the material and the geometry of the preform but also on the mould concept, the bottle size, the design elements, etc. These shrinkage fac- tors are experimental values and part of the skill of the mould maker and ideally should be stored in digital form in an expert system. Also the design of the neck cool- ing must be taken into account for the mould design. In this respect, a difference is made between active neck cooling, with cooling bores in the neck plate, and passive neck cool- ing. For both passive and active neck cooling, a neck plate made of steel is used. For lower mould temperatures, passive cooling by a neck plate with- out cooling bores will be sufficient. If temperatures become higher, active cooling may become necessary. Here, the neck plate is cooled with water to keep the zone immediately below the neck ring as cool as possible. Other items that should be incorporated in the design of the mould cavity may include logos, the cavity number for quality control, country-specific PET recycling logos, etc., which are usually integrated in the bottom area of the bottle. Mould making With integrated CAD/CAM, the mould design data can be used to gen- erate a full set of instructions for an NC milling machine for the production of the bottle-specific parts of the mould. In this respect, it will be necessary to decide whether design elements such as logos, textures, etc. permit milling the entire contour or whether certain parts may have to be produced by laser engraving or etching. State of- the-art manufacturing methods and appropriate CAM systems today make it possible to create most textures and logos by milling. This eliminates the need for additional processes that would increase not only the costs but also the time needed to produce the mould. Ultimately, the mould maker will also consider his or her own capa- bilities, such as the available equip- ment, when making the decision. After the cavities have been produced by milling and/or other appropriate tech- niques, they are cleaned and finally polished, which can be done manually or by machine. Even today, most mould makers polish their moulds by hand because it is generally the less costly method. Machine polishing is expensive because for each individual bottle design a separate polishing body made of polycarbonate or a similar plastic must be produced. On the other hand, machine polishing will yield a higher surface quality, is a repeatable and dependable process, and is faster. Hence, for larger mould makers, where ancillary wage costs may be higher, an investment in a polishing machine would be attractive once annual production reaches about 2,000 – 3,000 moulds. After polish- ing, the complete mould is assembled and then fitted to the blow moulding machine for mould trials (see part 7). *This article was published in Bottles, Preforms and Closures, Ottmar Brandau, Chapter 1.6/1.7., Copyright Elsevier 2012