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Among the benefcial properties of PET is its self-leveling ability. This means that as one part of a preform starts to stretch frst, the very act of stretching induces strain- hardening, making it tougher to stretch as a result. This forces adjacent parts that were maybe a little cooler to stretch, and in this back-and-forth of stretching and pausing, the preform develops into a bottle that may show as little as 0.001 in. diference in wall thickness around the circumference of a round bottle. However, many bottles show much greater diferences than that. As with all plastic processes, there are a number of conditions that must be maintained in order to achieve the described result. Following are the various issues that can skew wall thickness, in order of relevance: OFF-CENTER GATE This is by far the most common cause of wall-thickness problems. The vestige of the injection molding process must be centered to the blow mold and held there frmly when high-pressure air infates the pre-infated preform bubble into a bottle. Otherwise, wall thickness will go where the gate goes and no amount of self- leveling can prevent this from happening. Here are the most common causes for this defect: • Stretch rod does not pin the preform down enough. Typically, the distance between stretch rod and blow mold bottom should be adjusted to 0.040 in. less than the preform gate wall thick- ness. This will ensure that the preform cannot slip during high- pressure blow. As preforms have become thinner, these gaps have to become smaller. • Preblow pressure is too high and/or too early. The stretch rod has to connect with the preform before preblow pressure has a How to Address Uneven Wall Thickness in Stretch-Blow Molding chance to blow the preform of-center. When it is connected, the preblow cannot be so high as to blow the preform of the rod. The amount of preblow pressure required depends on the preform thickness and temperature. Therefore, operators may have to experiment with diferent pressures to test their efect. The same is true for when to engage the preblow. In two-stage (reheat) stretch-blow molding this is most often controlled by position rather than time, and often delaying the onset of pressure keeps the gate in the center. • High pressure is too early. High pressure may not be energized before the stretch rod has frmly pressed the preform against the blow mold bottom. Otherwise, even small diferences in temperature or preform wall thickness will invariably move the gate of-center. Again, in most machines this is controlled by position, and the operator has to make sure this is adjusted correctly. Rotary machines do this automatically but many linear machines do not. • Blow mold bottom is not machined correctly. In order to assist the stretch rod in pinning down the preform, a small well is Many conditions must be met to get the best wall distribution in a PET bottle. Skilled operators are, as always, indispensable. By Ottmar Brandau Get more insights on Blow Molding at: short.ptonline.com/blwmldKH Learn more at PTonline.com KNOW HOW BLOW MOLDING When the injection gate goes off-center as shown here, bottle wall thickness becomes uneven. 32 DECEMBER 2014 Plastics Technology PTonline.com K now How BLOW MOLDING