PETpla.net Insider 10 / 2020

BOTTLE MAKING 37 PETplanet Insider Vol. 21 No. 10/20 www.petpla.net acteristics. Only then are they able to find the optimal solution to a multifac- eted problem. 6.2 Stretch ratios To better understand what the process needs to accomplish, we need to study how stretch ratios between bottle and preform are calculated and how temperature differences in the preform wall affect them. While there is no accepted standard in the industry how stretch ratios are calcu- lated, it is safe to say that they represent the ratio of bottle dimensions to corresponding preform dimensions. There are two types of stretch ratios (Fig. 6.2):  Axial stretch ratio dividing the length of the bottle by the length of the preform as measured from under- neath the neck support ring (NSR) to the end of bottle and preform.  Hoop stretch ratios can be measured in three different ways:  1) Dividing the outside bottle diameter by the corresponding outside pre- form diameter.  2) Dividing the inside bottle diameter by the cor- responding inside preform diameter.  3) Dividing the average bottle diameter by the corre- sponding average preform diameter. Taking Fig. 6.2 as an exam- ple, here are three calculated stretch ratios:  Outside hoop stretch ratio (max.): 74.98/22.2 = 3.38  Inside hoop stretch ratio (max.): 74.58/15 = 4.97  Axial stretch ratio: 199.63/92.05 = 2.17 It becomes apparent that the inside stretch ratio is sig- nificantly (47% in this particular case, others may be lower or higher depending on the wall thickness of the preform) higher than the outside stretch ratio. This is the result of the bottle wall thickness being so much smaller than the preform wall thickness, so there is only a small dif- ference between inner and outer bottle diameter but a large difference between inner and outer preform diameter. Intui- tively, it is understood that warmer areas stretch easier than colder ones. Since the inner preform walls must stretch fur- ther, we can postulate that they should be at a higher temperature than the outside walls. However, heat enters the preform from the outside by means of the infrared lamps, so it seems a rather difficult challenge to get the inside wall to POWER UP YOUR LINE! READY TO SHINE? To make the most of what differentiates you, our complete solutions are based on future-proof technology, disruptive concepts and human ingenuity. Our goal: to bring you performance all along the line. You are unique, so are our solutions. sidel.com Fig. 6.2: Stretch ratios can be calculated from preform and bottle dimensions.