PETpla.net Insider 03 / 2015

PREFORM PRODUCTION 47 PET planet insider Vol. 16 No. 03/15 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 * This article was published in Bottles, Preforms and Closures, Ottmar Brandau, Chapter 3.2. Copyright Elsevier 2012 on the dashboard of a car for a few weeks during the summer months. The water in this bottle will taste stale and have a light, unpleas- ant sweetness to it. Water bot- tlers have decreased the amount of AA they accept in their bottles. While 10 years ago 6 ppm was widely accepted some companies now demand levels as low as 1.5 ppm. Machine manufacturers have responded by fine-tuning screw and hot runner design and are able to deliver preforms at those levels. Besides the previously mentioned design optimizations a resin inlet temperature at the extruder throat of the injection machine is also impor- tant. Best results are achieved by having the resin at 164 °C (327 °F) at the inlet. Because resin in the lower part of the dryer is some¬what cooler than resin in the upper parts, and may also cool down on its way from the dryer to the extruder throat, the dryer should be set to 170°C (338 °F). Ideal placement of the dryer is directly above the extruder rather than standing beside the machine. This will minimize the cool- ing down effect during transport. 3.6.3 AA-level Measurements There are two ways to measure AA level. Most common is the ground- comparison method, bottles, or resins are cut into small pieces and ground up toparticles smallerthan 1mm. Inorderto avoidcreatingAAduringgrind- ing, specimens are cooled down with liquid nitrogen before entering the grinder. The ground material is then placed in a closed glass vial and heated to 150°C (302 °F) for 30min- utes. The thus-prepared sample now contains headspace withAA in it, and it is this gas that is being measured in a gas chromatograph. The other method uses blown bot- tles. These are purged with nitrogen and stored for 24hours.Then a fixed gas volume is extracted with a syringe and measured in the gas chromato- graph. It is apparent that both proce- dures are cumbersome and time- consuming. With the development of 144-, 192-, and even 216-cavity preform injection tools, it is outright daunting in its scope.A more efficient method is to check only the so-called “hot cavities.” In Chapter 2.5.3 the mechan- ics of viscous heating has been explained, the fact that the resin heats up unevenly as it moves through the barrel and hot runner channels. Although this has undesired effects in injection stretch blow molding with respect to the bottles, in injection molding the effect is that the preforms in some cavities are always warmer than the majority of the others. This higher temperature also increases the AA level in the affected cavities, and fortunately it is always the same cavities that show this behavior. After establishing the performance of all cavities, labora¬tory personnel can concentrate on the “hot cavities” and still make valid assumptions about the AA level of the entire tool.