Perfecting your Purging Procedures

Determining the source of streaking or contamination in your molded parts is a critical step in perfecting your purging procedures, ultimately saving you time and money. We take a closer look at the nitty-gritty details of purging.

- Kruti Bharadva

Part contamination in the form of streaking and/or black specks is a common problem in injection molding. Once you find contamination, the next task is to determine the source of the problem. In this article, we will address how to locate the problem area and then provide some purging procedures to deal with specific areas of concern. This will streamline the purging process and allow you to utilise your purging compound in the most cost-effective manner.

Determining the Source

If the streak or contamination shows up randomly, it’s likely to be in the barrel or feed system. Colour or degraded resin may be trapped in a negative-flow area. There may not be enough physical turbulence in the area to remove all the contamination and the next material only picks up trace amounts that show up intermittently

At this point, don’t discount resin issues. Examine your resin for specking and be sure that the drying system is working reliably. Dirty filters in a feed system can wreak havoc. Regrind—if used—is another possible culprit, caused by poorly cleaned grinders, fines, and dust. If the cause is not in the feedstock, then it may be in the barrel. Long residence times can lead to degradation and cause black specks. Remember this rule of thumb: Use 25 per cent to 65 per cent of the barrel capacity when choosing the best-sized machine for your job.

TO DETERMINE IF THE BARREL IS THE ISSUE, RUN SHOTS OF PURGE COMPOUND THROUGH THE BARREL. BE SURE THAT THE BARREL IS FULL TO ENSURE THAT ANY MATERIAL LEFT BEHIND IN A DEAD SPOT IS ALSO BEING PICKED UP