PETpla.net Insider 10 / 2013

BOTTLE MAKING 46 PET planet insider Vol. 14 No. 10/13 www.petpla.net Bottles, preforms and closures, part 6* Moulds: Concepts, design and manufacture Depending on customer-specific criteria, such as the filling method (hot fill, aseptic, etc.), the general size of the cus- tomer’s bottles, the market positioning of the customer or the new bottle, the frequency of product changeovers, etc., there are different mould concepts that can be adopted. Some of the more common mould concepts are described below. After the bottle geometry and the mould concept have been deter- mined, the focus shifts to the mould. *Ottmar Brandau’s “Bottles, Preforms and Closures” was first published by hbmedia. A newly revised version is reis- sued under the Elsevier imprint. PETplanet insider is pub- lishing extracts from successive chapters. Shell moulds The most universal concept is the shell mould. With bottle capaci- ties from 0.1l to about 2l, this type of mould is suitable for a very wide range of sizes. It comprises a mould housing that also incorporates the cooling channels, into which a shell, with the cavity itself, and the base mould are inserted using appropri- ate spacers as required by the bottle height. The base mould is cooled separately. These bottle-specific parts can be exchanged very quickly. Usually only a few screws have to be released to remove a shell. Hot-fill moulds Although shell, full-body, and small-cavity moulds are usually made of aluminium, hot-fill moulds are made principally of steel. The reasons are to be seen in the thermal elongation of the mould material and the prob- lem of adhesion. Moulds for hot-fill bottles are heated to higher process temperatures (120–140°C) in the body area to permit a certain relaxation and so partly eliminate the internal ten- sions set up in the bottle during the stretching process. The neck area is always held at 20°C or lower to avoid deformation of the precision injection- moulded preform neck finish, whereas the base mould temperature is around 60–80°C. This relaxation in the blow mould at inflation pressure prevents a subsequent relaxation, i.e., shrink- ing, when the bottle is in contact with the hot-fill contents, which sometimes have a temperature of up to 95°C. The higher mould temperatures result in a higher thermal load and thus a more pronounced elongation of the mould. Additionally, the PET mate- rial has a tendency to stick to alu- minium at such high temperatures. As an alternative to steel, an aluminium mould with an anti-stick coating could be used but this would not solve the problem of thermal elongation. As a rule, hot-fill bottles have the so-called vacuum panels that, through con- trolled deformation, compensate for the partial vacuum that results from the cooling of the contents. Instead of such clearly visible panels, design ele- ments such as grip areas have been developed that perform the same function but are not readily identifiable as vacuum panels. Another variant is the design of the bottle base such that it is able to compensate for the partial vacuum, again through controlled deformation. Under certain circum- stances, moulds for bottles that will be filled with products at temperatures of up to 85°C may still be made as genu- ine aluminium shell moulds. In con- trast to the true “hot-fill” process, this is referred to as the “relax” process. Other variants of heat-resistant bot- tles are used for applications where the bottles are sterilised with heated steam before filling or bottles are con- veyed through a pasteurising system after filling. The related temperature load (temperature and duration) may result in other requirements for the selection of the mould concept. Mould design After the bottle geometry and the mould concept have been determined, as described in the previous sections, the focus shifts to the mould. At this stage, it is important that full use has been made of all the technical pos- sibilities during the design phase to ensure a common understanding of design-related issues between the supplier and the customer. Because a 2D drawing can never give a 100% true impression of the subsequent appearance of the con- tainer, a photorealistic rendering will be much more meaningful. Depend- ing on the selected mould concept, the complete mould will consist of different elements or assemblies that are usually available as ready-to-use