Cracking and/or Crazing can be defined as a fracture or surface breakage in the material of a molded part, usually found in weld line areas, but also on the surface in general.
Possible Causes & Remedies:
◇ MOLDED-IN STRESSES
Remedies: Reduce packing and fill rates by adjusting until the part is properly filled with minimum stresses. Maintain a holding pressure that is no more than 1/2 the primary injection pressure.
◇ COOLING TIME TOO SHORT
Remedies: Increase the cooling time portion of the cycle. This holds the mold closed longer and allows a thicker skin to form on the molded part. The skin will be strong enough to keep crazing or cracks from forming.
◇ UNDERCUTS OR PARTING LINE BURRS
Remedies: Inspect the sidewalls and edges of the cavity. Make sure there is adequate draft (see the next paragraph) and that there are no burrs or other undercut conditions. If there are they must be removed by stoning or machining.
◇ INSUFFICIENT DRAFT ALLOWANCE
Remedies: As a general rule-of-thumb, a draft angle should be at least 1 degree per side to facilitate easy ejection. This does result in a dimensional change in the part and must be considered in the mold design phase. To minimize future problems, the product designer should be made aware of this requirement.
◇ USE OF MOLD RELEASE
Remedies: The remedy is to keep the mold as clean as possible and make every effort to eliminate the use of external mold releases.
◇ IMPROPER EJECTOR DESIGN
Remedies: Any evidence of cracking in ejector pin areas indicate the pins are too small, or the ejection speed was too great, or there was too much injection pressure used to fill the mold. Injection pressure was covered earlier. Ejector pins should be resized, or relocated. They need to be located such that they are close to side walls or under bosses or other strong areas of the part that can absorb the ejection stresses being produced. They need to be as large as possible in diameter to distribute the ejector forces over a large area.
◇ DEGRADED MATERIAL
Remedies: Use only high grade regrind and use it only once. Mix regrind with virgin at a level of approximately 15% regrind by weight to minimize the tendency to degrade. If this is still a problem, eliminate the use of regrind altogether.
◇ EXCESSIVE MOISTURE
Remedies: Although it is commonly understood that non-hygroscopic material do 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.
◇ 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 consistent cycles.
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