A custom got a problem of breakage in service, under very moderate effort, of articles made of POM. Investigation has shown that:
- the fracture surface doesn’t show any flaw (Bubbles, impurities)
- treatment with sulphuric acid causes breaks after some time, and this seems indication of residual tension
- the breakage happens in some markets and not in others
POM is sensitive to chemical attack by both strong and weak acids, also bases. There are many household products that are either acidic or basic. Coupled with manufacturing defects, if the part is used in this type of environment, it would certainly compound the problem. POM because of its very high crystallinity, undergoes a high coefficient of thermal expansion and high mold shrinkage. Once again the high thermal expansion in the presence of frozen-in stresses would certainly contribute towards part failure. The breakage occurred only in some markets and not in others. It would be very educational, if you can investigate in which markets it did not fail – usage conditions etc.
ACO Mold recommendation is to do some basic root cause analysis before you do anything else. Try and look for patterns in the failures; production lots, material shipments, processing conditions, etc. Isolate and mitigate all variances found. If the problem isn’t on the production side, THEN look at the marketplace side; distribution conditions, inventory practices, failure regions, use cases, chemical exposure, etc. Find yourself a six sigma black-belt to help guide your investigation. Until you understand why, you will never know what to do.
Make sure the mold temperature (steel temperature) is above the glass transition point during injection molding and make sure there are no sharp corners on the parts as actual has somewhat of a low notched izod. Another problem could be shearing of the material during injection. Make sure the injection molding machine has no problems (worn screw, barrel or tip assembly), the nozzle orifice is large enough and the sprue runner and gates are large enough for the correct molding of the part. Also make sure the venting is adequate. Residual stress can be caused by overpacking or underpacking of the parts during the molding cycle. If running regrind make sure the regrind and virgin are properly blended as the two run quite differently.
