Plastics processors who run multiple colors and materials on the same processing equipment will agree that color streaking in their molded or extruded parts is an unfortunate likelihood. Streaking is a problematic defect that can amount to costly, accumulated scrap. While the intentions are good to maximize utilization of your equipment, color streaks in your production parts certainly are not!
These color streaks are often caused by overlooking steps in procedure, and specific areas within the machine or tooling during the changeover process. If you are experiencing streaking or just want to avoid it going forward, here are some tips on troubleshooting for the (common) root causes, and related solutions.
1.) For every changeover, check and thoroughly clean the hopper, or any additive feeders; anything that leads into and including the machine feed throat. As simple as this seems, it is amazing how a few overlooked stray pellets of a concentrate or pre-colored resin can make a costly interruption into what was otherwise perceived to be a good start up and production run.
2.) Use consistent and proper cleaning methods for the screw and barrel between changeovers. This really should include a commercial purging compound. Which will help tremendously to insure you don’t have color streaks rearing upon the finished goods.
3.) Understand that natural layering of material upon the metal surfaces occurs during extended production runs. If there is screw and barrel wear, this exacerbates the problem. Use a purging compound to start, but if the problem becomes persistent, it’s time to pull or push the screw to see first hand where the hang up is and clean it thoroughly. Take measurements and record. Replace any worn components over time as needed.
4.) Know that there is no hope for a clean manifold in injection molding and likewise in extrusion system components, such as an intricate die without a clean screw and barrel first! Get aggressive purging your screw and barrel for routine color and material changes. Do so by using a glass-filled type grade of purging compound.
For injection molders, always consider the check ring! The check ring is often the bane of all things color streak related. To remedy the issue, be sure to finish out your purge procedure by using a series of short, high velocity air shots. Doing so will focus agitation here, which greatly increases the effectiveness of deposit or hang up removal. If the nozzle area is also suspect, raise its temperature by up to 50 degrees F to remove any layered resins within during the purge procedure.
For extrusion processors, look at what comes next following the screw and barrel! Common areas for color hang ups can be screen packs, melt pumps and other areas of low pressure and flow, including specialized tooling or outside edges of the dies. Use a chemical grade purging compound, or a mechanical grade with some emphasized foaming and expansion. Doing so will remove and prevent buildup of difficult deposits in these places.
Ready to reduce your production downtime to protect your profits? Learn more about how purging compounds and process efficiency work in tandem.