by: Jeff Rondeau, Senior Technical Service Engineer, Northeast Region
“Look at this… have you ever seen something so awful in a molded part?”
This is a question that technical service engineers are frequently asked – usually accompanied by many pictures with appropriate circles and arrows, highlighting relatively large black chunks of contamination in an otherwise good looking plastic part.
To answer the opening question: Yes, tech service representatives have seen this in plastic parts – and not infrequently. The customer’s first instinct is to blame this on a dead spot in the molding process, such as a worn check ring area on the barrel, a worn check ring itself, or possibly a hot manifold – and, usually, their instinct is correct. Most hot manifolds have some dead spots where material can aggregate and grow chunks of black, degraded materials and colors.
When a customer first notices black specks in their plastic parts, the first things a technical service engineer should consider are:
Assuming there is a hot manifold, are there hot drops with spreaders, torpedos or probes in the tips? Material usually builds up at the back of these probes, degrading over time to form brown to black deposits.
The manifold itself should have smooth ends, and contoured end-plugs with no dead spots. Sometimes, an end plug gets moved, or rotated wrong, or simply lost during mold maintenance. This can be a serious dead spot for degradation build-up.
Check the barrel and screw thoroughly. Pull the screw and check the flights for damage and screw-to-barrel clearances. Examine the check ring and associated surfaces. Clean all surfaces with wire brushes, being careful not to scratch the finish.
If the clearances are starting to exceed 0.002-0.003” per side – and especially if there is greater than 0.005” per side – then the barrel itself could be the location of the degraded material build-up.
Each time you shut the machine down fully, a layer (or “skin”) of degraded material can pull away from the walls, and can be molded into plastic parts for some time. At the proper clearances, this is minimal and should clear up quickly at start-up. However, if the clearances are out of tolerance, then the skin is thicker, causing the black chunks to come for hours.
If the black specks are sudden and large, and appear fibrous in nature, the cause may be related to a shop towel or other contamination.
We have not addressed colorants, because a bad color concentrate usually manifests itself in a different manner. The size and shape of the specks or streaks, swirls, and a trained eye can usually differentiate the two. For this article, we are not referring to bad color concentrate. Also, this does not take into account the occasional tiny black specks that can be present in some prime polymers direct from the manufacturer. These are usually barely visible – perhaps the size of the small end of a pin – and are generally acceptable in most plastic applications.
While this article covers most of the early to check, easy stuff, it does get tougher from here. Most likely, it has probably just been a while since the machine was cleaned and the whole mold needs a tear-apart and thorough cleaning, along with the barrel and screw checks we have previously noted.
If you’ve gone through this list and you are still seeing black specks, or you need any other assistance, call your M. Holland Technical Service Representative.
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