Molders quite often call me and say their screw is “windmilling” or isn’t picking up the material. This phenomena is due to poor solids conveying. Solids conveying occurs when the material “sticks” to the barrel and “slips” on the screw. This has to occur in order for the material to be moved forward.
Most often this problem is occurs when processing high-temperature engineering resins. You can solve it simply by changing the temperature profile 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.
Say, for example would be as follows, your profile, starting at the feed zone of the screw: 450F, 450F, 450F, and 450F. Try this instead—500F, 480F, 460F, and 440F—and see what happens.
Some molders believe that running hotter in the rear zones will increase their melt temperature. Not so; higher temperatures in the rear will not affect the melt temperature because the plastic is still in pellet form. But it 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 now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com
