Many times I hear molders complain that their screw is “windmilling” or not picking up the material. This phenomena is due to poor solids conveying. Solids conveying occurs when the plastic material “sticks” to the barrel and “slips” on the screw. This is necessary for the material to be moved forward.
Normally you can solve this problem by adjusting your barrel temperature profile, particularly if you are running high-temperature engineering resins. Try switching from the commonly used “flat” profile to a reverse temperature profile, where the barrel temperatures are higher in the rear and lower at the discharge end of the barrel.
For example, if your current profile, starting in the feed zone, is 450F, 450F, 450F, and 450F, adjust that to 500F, 480F, 460F, and 440F.
Some molders believe that higher settings in the rear zones will increase their melt temperature. Not so—the plastic is still in pellet form in the feed section of the screw; increasing the heat there will improve the coefficient of friction at the barrel wall between the pellets and barrel. and therefore improve the solids conveying and eliminate “windmilling.”
Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He was inducted into the Plastics Hall of Fame in 2012. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com
