Black specks and/or streaks are one of the most common processing defects faced by molders, extruders, and blow molders alike. They are different from black spots which are caused by burns on the parts themselves. Black specks or streak can cause a mountain of scrap and leave even the best processors of clear or colored parts scratching their head. Why is this happening? Where did they come from? How do we solve this issue quickly?
The most common cause of black specks and streaks is material degradation within the plastics processing system itself. This can be due to excessive temperatures, excessive residence time or trouble pre-existing contamination on the screw or barrel. It's also commonly caused as a result of using a multi generation regrind, or a compound that has a additive subject to degradation (such as flame retardant or susceptible pigments). Then there is also the chance of a foreign contaminant finding its way into the hopper or feed system.
As is always the case, prevention is the best measure. This includes carefully setting up the temperature profile to meet the needs of the resin or compound being processed. Make sure any additives are compatible within the primary resin. Minimize residence time of any material, especially regrind. Check for and redesign any known dead spots within the system. Use and maintain good housekeeping practices, make sure material is always kept covered when in storage or in process.
Even when all the above measures are taken, and the processor is still faced with defect causing black specks or streaks, one of the best remedies is to use a specialized purging compound for maximum cleaning power. A glass filled grade will aggressively clean the screw and barrel by letting the machines power do the work. Effectively removing even, the most stubborn residue and contaminants. As an added benefit, if the problem is so severe that an overhaul and system inspection is needed, products such as Asaclean EX Grade have a special additive package that will greatly assist with the screw removal and cleaning of the screw and barrel.
If the contaminant causing the defect is found in an area of low pressure or a dead spot, a chemical type purging compound is a likely solution. Working by the principles of increased temperature and time, these types of purging compounds are excellent at expansion and removal in the out of the way places where this type of defect causing contamination often builds up.
Using a purging compound is a common and reliable step most processors will consider before other alternatives such as sending out tooling or components to a specialized cleaner or performing manual cleaning tear down. Both of which can incur considerable costs in terms of expense and machine downtime.
Using a purging compound as described should be a high priority and always an up-front consideration when needed for effective removal of these defect causing black specks or streaks.
If you would like additional information and some useful tips on maintaining machinery and best practice purging methods, please contact your Asaclean Technical Sales Representative.
Learn about the importance of implementing proper purging procedures by downloading this free industry guide.
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