BOTTLE MAKING 23 PETplanet Insider Vol. 22 No. 11/21 www.petpla.net Thermal effects or flow patterns of the melt generate different AA values at varying positions within a shot. As a result, cavities with higher AA values are formed whose positions remain reproducible during manufacture. The outer edge layers of the melt experience higher shear than the inner core. Consequently, the layers close to the wall have a higher temperature and always follow the inside of the manifold. In the further development of the optiRun line, the focus has been on mixing the middle and edge layers through mechanical intervention. This is achieved by, for example, manufacturing the manifolds using alternative production processes. Furthermore, thermal imbalances are reduced by the number of heaters. The new PET line has numerous control points thus providing ideal and precise interaction between machine and hot runner. In order to be able to draw definite conclusions about the level of AA values, tests were carried out on Netstal’s new PET line side-entry in conjunction with a 128-cavity mould from MHT. The mould includes a high-performance hot runner from the optiRun range. New virgin PET material Lighter S93 from Equipolymers was processed. The preform produced has a wall thickness of 2.6mm, a weight of 18.58g and a PCO 1881 thread. The booster temperature was 170 °C, with a residual moisture level of 34ppm. In order to measure the AA value, it is best to use a method based on the gas chromatography principle. Netstal measures the AA values of the preforms produced according to ASTM F2013 standards using gas chromatography methodology. Firstly, a whole shot was measured during production with a cycle time of 8.2 s to obtain an overview of the distribution of AA values and to find high and low cavities. In any new mould configuration, a whole shot is initially analysed to determine high and low cavities, i.e. those in which AA values are highest and lowest. AA is not formed homogenously in the melt but shear-induced temperature peaks and, above all, differences in the residence time of the melt result in locally high and low values. When analysing the shot, it was noticeable that the two mould quarters on the non-operator side have generally higher values than on the non-operator side itself. The limit value of 4 ppm is not exceeded, although one of the values is exactly 4 ppm. In Figure 1, the proximity to the limit value can be explained by the residence time, which is relatively long for an 8.2 s cycle compared to the target cycle time design. In order to avoid variations in data, each preform was analysed in two readings. It became clear Figure 1: A fully measured shot of 128 cavities Figure 2: Distribution of AA values - One preform analysed using two measurements
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