Production Process

The special forming technique with which PET is almost invariably associated, injection-stretch blowing, is a combination of two longer-established processes, offering the best benefits of each. A small thick-walled 'preform' is injection-moulded, with the neck zone full size and with the final finished rim or thread to take any type of closure. This ensures that the neck is extremely accurate, allowing a perfect seal, even with a wadless closure. In a second stage (which may be separate or, as with our Aoki machines, carried out sequentially on the same machine) the hot preform is mechanically stretched by a powered rod, then blown into a second mould to the finished dimensions. One effect of the stretching is to orientate (align) the polymer molecules down the length of the container, and the blowing stage aligns them around the girth. In this way the pack is biaxially stretched, which greatly enhances its strength, clarity and barrier properties. The high strength of PET means that very low wall thicknesses can be used. These, although physically adequate, may have a soft 'feel' to them. This can be minimised by clever design and the optimum distribution of material over the entire surface of the container. Achieving this is part of our skill based on years of experience, backed by computerised design facilities.

Preforms are usually characterised by the diameter of the finished neck zone, and the total weight. Unless the preform has been designed for a very specific pack, most can be used to produce different sizes and shapes of finished containers. Normally they are up to 150 mm in diameter, and weights of up to 300g have been used. From the latter, spherical bottles of 30-litre capacity have been produced. At the other extreme, tiny preforms allow the production of containers as small as 10 ml. Wide and narrow-necked jars or bottles can be produced on the same machines (although not at the same time). Even larger containers can be, and are produced, but the practical, and economic penalties inherent in making these very large sizes have to be justified. There are many special situations where PET containers in the size range 5 to 20 litres are ideal or even essential. Agrochemicals represent a prime example; another is heavyweight returnable bottles used in water dispensing systems. These size ranges are well within the capabilities of our SB 250 AOKI machines.