PETpla.net Insider 11 / 2013

BOTTLE MAKING 21 PET planet insider Vol. 14 No. 11/13 www.petpla.net a single station and a heating box, the preforms are fed manually. A more sophisticated variant will have auto- matic parison feeding and a continu- ous oven, although it will still have only a single blowing station. This setup allows several preforms to pass through the oven and be inflated suc- cessively. The performance of the blowing station can even be as high as that achieved with rotary machines. How- ever, even smaller rotary machines with two to four blowing stations and a continuous oven section are used as laboratory machines. Wherever pos- sible, the laboratory machine should be of the same design as the machine that will later be used for volume pro- duction. Important aspects to be consid- ered include whether the preform will be transported with the neck up or down and which kind of drive for the stretching movement, which kind of blowing station, and which mould car- riers will be used. The same applies to the process parameters such as the dwell time in the oven. Such labora- tory trials should mimic the subse- quent volume production as closely as possible. In certain cases, especially for hot-fill or returnable bottles, it may be advantageous to use a rotary machine with, for instance, eight stations, even for the mould trials. In such a case even if only a single mould is actu- ally fitted and running, the trial results can be transferred to the production machine with a significantly higher level of reliability. One important aspect concerns the time required to remove the bottle from the mould after the pressure has been relieved and the bottle has been cooled with the scavenging air. On rotary machines, this will be a CAM-controlled and exactly timed process, so the transferability of the results to the production environ- ment is extremely important. What 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 is more, with rising mould tempera- tures, the findings from mould trials on a single-station laboratory machine will be more difficult to transfer to the production machine. The timing and the relative movements, and thus also the heat transfer between mould and bottle, will have a major impact on shrinkage. Process finding during mould trials During the mould trials, the pro- cess engineer will adjust the initial parameters for heating and blowing based on his or her experience so that at least a fully blown bottle leaves the machine. These bottles will then be verified against individual ele- ments of the specification, such as wall thickness distribution, top load, burst pressure, etc., and the process parameters will then be optimised successively until the key milestone values are achieved. Then a larger sample run is started to produce a larger number of bottles for more detailed laboratory testing (typically 50 - 100 bottles). For these mould trials and the subsequent laboratory tests normally about 500 preforms will be required. If one of the prescribed labo- ratory tests reveals that the sample bottles do not conform to the specifi- cations, the process parameters must be adapted in an iterative fashion until the bottles meet the specification. If this cannot be achieved through changes in the process parameters, it may become necessary to modify the mould or even use a different preform. Yet, thanks to the available expert knowledge or the use of expert sys- tems and FE simulations in product development, this rarely happens in practice. (The most important of these laboratory tests will be described in part 8.) Sometimes laboratory machines are also used to produce larger quantities of sample bottles that are made available to the customer. Such quantities of sample bottles may be needed for market tests, packaging tests, or also setting up and testing downstream equipment, such as con- veying systems, filling lines, labellers, packers, and palletisers. *This article was published in “Bottles, Preforms and Closures”, Ottmar Bran- dau, chapter 1.8.1. and 1.8.2., Copyright Elsevier 2012