PETpla.net Insider 04 / 2014

PREFORM PRODUCTION 29 PET planet insider Vol. 15 No. 04/14 www.petpla.net Our guess would be that to this day no 48 or more cavity system is built without first performing a trial run with a single-unit cavity. This is despite the fact that we understand material properties fairly well, and resin companies as well as inde- pendent laboratories offer material characterisations that detail stress/ strain graphs for a variety of material conditions. The unpredictable factor is that the formation of a bottle from a preform is literally explosive, and even tiny temperature variations affect the out- come. In addition, the parameters that control the inflation characteristics of a particular preform are manifold, still too many to make perfect predictions. Here is a partial list of the parameters that make the difference between fail- ure and success:  The preform temperature: PET can be blown at a range of 95–115°C (203–239°F).  The temperature distribution: The profile of temperatures both in the vertical axis as well as through the preform wall is the result of many factors characterising the oven system in a reheat stretch blow machine or the hot runner system in a single-stage machine. We do not yet have models that can truly reflect them.  Process conditions, such as stretch rod speed, and timing of primary and secondary air pressure result in different bottle wall thickness outcomes.  Venting and the shape of mould corners affect preform inflation as well. Needless to say, even the best preform designs need experienced processors to dial in a specific blow machine. Two machines of the identi- cal model using the same preforms and tooling will require slightly dif- ferent setups to account for subtle differences in the machine character- istics, including such things as screw wear and infrared lamp age. This can only be accomplished by experimen- tal development, and experience is invaluable. 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 In practice, less than perfect preforms are often used to make an acceptable bottle because preform designers typically add a few grams of material to allow for the factor of uncertainty and still make bottles which can be sold. The cost of the added weight is offset by a wider pro- cess window and lower scrap rates. Our guide sheds some light on the methodology that a designer might go through to come up with a preform suitable for a given bottle shape and wall thickness distribution. There are other ways of getting the same or sim- ilar results, and our methodology is by no means the only one. What most experienced designers do is to look up a similar bottle and then modify the preform to adjust for the slightly dif- ferent shape of the new bottle. Rather unscientific but practical! Newcom- ers in the field do not have this luxury and are often stuck with whatever preforms they can buy from vendors. They then try a preform with the right or a similar weight and make adjust- ments to the design as necessary. With the help of our guide this task should become easier. We will explore all relevant material characteristics that are paramount to understand the inflation behaviour of the preform. We then take you through the design process step by step and point out differences between preforms for the single-and two-stage process. In any case, you should always make a trial cavity and perform blow mould- ing trials to determine whether your design works before committing to a multicavity tool! The next section gives some intro- ductory information about PET that will help understand the terms used in the following sections. The sec- tion on behaviour in the blow mould is critical to understand the factors at play when designing preforms. The remaining sections in this chapter will help to give the designer and proces- sor a feeling of the overall steps of the bottle-making process, which will be of practical use when developing or troubleshooting a container. The best practitioners of PET design and pro- cess development have to understand all the steps because every step has an influence on the properties of the final container. * This article was published in Bottles, Preforms and Closures, Ottmar Brandau, Chapter 2.1. Copyright Elsevier 2012