Case Study: Brown Streaking from Screw DesignIn this case study a machine was running a PC/ABS material with a colorant concentrate added at 4%. The job was running well but another screw manufacturer wanted to evaluate their mixing screw in our process. The original screw was replaced with the new screw and immediately scrap from brown streaking started. A variety of process adjustments were made working with the new screw, but nothing seemed to resolve the problem. After running for about 2 weeks with excess scrap the screw was finally replaced with the original screw, which resolved the scrap problem. Examination of the new screw showed that the root of the screw fights seemed to have gotten very hot (steel changed color). It appeared that the mixing zone on this screw was creating way too much shear. The screw designer had some ideas on changing geometry but they ended up missing their opportunity, because no one wanted to continue trying to “fix what wasn't broken”, so the original screw stayed in the machine.