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I have seen operators make several thicknesses of sheet, film, pipe, etc. from the same die gap using drawdown as an adjustment. By the same token, I have seen those same parts distort or even crack from exposure just to sunlight when placed under mild strain due to excessive orienta- tion caused by drawdown. The tendency for polymers to experience post-extrusion warpage and distortion from orientation is related to their glass-transition temperatures (Tg). Glass transition is the temperature at which the polymer changes from a rubbery consistency to a rigid struc- ture. Molecular chain movement is greatly restricted below the Tg, so that any orienta- tion largely remains in place. Once the Tg temperature is reached, the molecules have sufficient freedom of movement to reorder themselves into their more relaxed configu- ration to relieve the orientation. The level of temperature above the Tg largely determines the rate at which the relaxed state is reached. The accompanying table lists commonly extruded polymers and their Tg. It shows whether orientation is expected to be a problem, depending on the service tempera- ture expected for the part. On this list there are four polymers whose Tg is at or below the freezing point of water, meaning they will relax their orientation even at room temperature. Others with higher Tg will retain much of their orientation without appreciable shrinkage or distortion until raised to their Tg. However, almost all have a Tg below the temperature of boiling water. ABOUT THE AUTHOR: Jim Frankland is a mechanical engineer who has been involved in all types of extrusion processing for more than 40 years. He is now president of Frankland Plastics Consulting, LLC. Contact jim.frankland@comcast.net or (724)651-9196. work becomes. It used to be done by the "cut-and- try" method but is now executed mostly by computer simulation along with practical experience. That said, much of the undesirable orienta- tion occurs after the extrudate exits the die and is stretched between the die exit and the hauloff. At this point the polymer is going through the temperature range where orientation effects are at their maximum, with immediate cooling locking in the orientation. This can be avoided simply by minimizing drawdown or neck-in as the extrudate leaves the die. This means setting a die gap as close to the finished product dimensions as possible, with precise takeaway velocity to minimize any stretching by pullers or rolls until the polymer sets up. Precise speed control of the downstream pulling/cooling equipment is the key to minimizing machine-direction orientation. This portion of the orientation is within the control of the operator and does not need special expertise. Excessive drawdown after the die exit is mostly due to operators' lack of knowledge of the effects and their reluctance to reset all the operating conditions for every profile change. Commonly Extruded Polymers and Their Glass-Transition Temperatures Polymer Tg, °F PP 32 ABS 221 Nylon 66 158 PLA 145 PET 158 PMMA 117 LDPE -193 PVdF -31 PS 203 Nylon 6 158 HDPE -110 Source: Wikipedia Much of the undesirable orientation occurs after the extrudate exits the die and is stretched between the die exit and the hauloff. Here's a Better Melt Filter from a Reliable U.S. Supplier F R O M S C R A P T O P E L L E T The CFO melt-filtration system is better built and better supported than any filter of its kind. The automatic, self-cleaning design is ideal for heavy contamination – even tolerates stones and metal. • Higher operating pressures – up to 3500 psi • Filtration up to 80 mesh • Service and parts based in U.S.A. • Cost-effective; quick ROI For more information, visit www.adgs.net/CFO 203.255.9444 www.adgs.net 36 AUGUST 2016 Plastics Technology PTonline.com K now How