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Description:contamination-injection-molding-defects.jpg


Contamination can be defined as an imperfection in a molded part caused by the presence of a foreign object or material that is not part of the original molding compound.



Possible Causes & Remedies:


Machine Cause


◇ OIL LEAKS AND GREASE DRIPS


Remedies: Fix oil leaks as quickly as possible. Clean up grease drips as they occur and do not use the same cleanup rags to wipe out the hopper between material changes.


◇ UNCOVERED HOPPER


Remedies: Use the hopper lid. Do not improvise with flattened cardboard boxes, as the paper particles will cause contamination. If the original lid is lost buy a new one. They are designed to do a specific job well.


◇ IMPROPERLY CLEANED HOPPER


Remedies: Clean the hopper thoroughly between material changeovers. This may require wiping the inside with a cloth slightly dampened with denatured alcohol to remove all traces of fines.


Mould Cause


◇ EXCESSIVE LUBRICATION


Remedies:  Optimize the use of lubricants and minimize the use of mold release sprays. Clean up any excess lubricants and use only the amount needed for a specific application. A little lubricant goes a very long way. Investigate the reason for using mold release. Usually it is a temporary approach to a more severe problem, and the problem should be solved to eliminate the need for the release agent.


Material Cause


◇ CONTAMINATED RAW MATERIAL


Remedies: This type of contamination can be minimized by dealing with high quality, reputable suppliers and by using good housekeeping practices. Properly trained material handlers will also help reduce contamination.


◇ IMPROPER REGRIND


Remedies: Grinding the plastic in a granulator that is specially designed for the purpose creates the regrind. The material is ground until it falls through a screen with specifically sized openings. The larger the openings, the larger the regrind pellet. If the granulator is not cleaned out between changes (including the screen), pellets of incompatible materials may be mixed with the next material and this causes contamination. Granulators must be completely taken apart to perform a thorough cleaning between uses. Do not assume that the granulator will always be used only for a certain material. Plans have a way of changing. As soon as a run is completed, the granulator should be removed from service and cleaned. Use of compressed air, combined with wiping with clean rags will suffice, as long as every component is cleaned, including rotor, blades, container, screen, sidewalls, and feed throat.


◇ EXCESSIVE MOISTURE


Remedies: Although it is commonly understood that non-hygroscopic material does not require drying, do not take chances. Dry all materials. It may be that fillers used in the material are hygroscopic and they will absorb moisture. Every plastic material requires specific drying conditions. And each material should be dried according to the material suppliers recommendations. The desired moisture content is between 1/10th of 1 percent and 1/20th of 1 percent by weight. This means the dry air being used to take moisture from the material should have a dew point of -20 to -40 degrees F.


Operator Cause


◇ POOR HOUSEKEEPING


Remedies: If possible, run the machine on automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is really necessary. And, instruct all employees on the importance of maintaining contaminant-free areas.


(Source: plastictroubleshooter.com)

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