BOTTLE MAKING 24 PETplanet Insider Vol. 22 No. 11/21 www.petpla.net that the largest measurement fluctuation was +/-0.1 ppm (see error bars). As a rule, variations within the range of 0.01 ppm can be assumed. In order to show the distribution and frequency of individual AA values, a different approach was taken. In Figure 2, the frequency of the individual preforms was plotted against the AA value. It is noticeable that around 60% of the preforms are between 2.6 and 3.2 ppm. Again, each preform was measured twice with the values plotted on the graph. The differences in the distribution of the preforms between the first and second measurement result from rounding errors as the AA values using this approach were rounded up and down to the nearest 0.5 ppm. The cycle time was shortened to 7.5 s to further reduce the AA values. Figure 3 shows the corresponding values for individual cavities. Thus, a reduction in AA values by up to 1 ppm can be achieved by reducing the residence time. However, individual cavities respond only slightly to a reduction in cycle time. These are cavities located on the operator side where AA values were already low at the longer time cycle. In order to show the variation from shot to shot, four consecutive shots were taken from the injection moulding machine. The average value is shown as a grey bar in Figure 3, with variation represented by the error bars. Fluctuations of -0,15/+0.3 ppm can be observed. The processed PET will vary slightly from shot to shot due to their treatment in the dryer (position of the granulates in the hopper, heat load), the different shear and temperature conditions in the plasticising process, and lastly the different conditions inside the hot runner. However, there is certainly some measurement variation in this as shown by the error bars for the measurement of one and the same preform at a cycle time of 8.2 s. Summary An indication of an AA value is always merely a snapshot of a machine’s condition, the material and measurement. Fluctuations occur which do not lead to limit values being exceeded if the basic value is justifiable. It could be shown that AA values on the new PET line side-entry are in a low range without the use of any AA blockers. www.netstal.com www.mht-ag.com Figure 3: AA values at different cycle times GAWiSD featuring AutoJob™–job set up in seconds +1.724.482.2163 AGRINTL.COM ACCURATE • FAST • REPEATABLE SEE GAWIS4D IN ACTION Dimensional and thickness measurements for plastic containers and preforms. ®
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