Causes and Solutions for Warped Parts

Entec Rotational Molding materials are designed for optimized processability and properties in rotational molding. Our materials are designed to yield consistent material, box-to-box and shipment-to shipment.

Because of the diversity of items made by the rotational molding process and the different types of rotational molding machines and molds used, it is impossible to establish a universal, optimum molding condition that is perfect for all circumstances. Entec can provide recommendations for initial starting conditions and guidelines for adjustments to the process conditions to help optimize the process settings for the particular part being molded. In order to optimize the properties, it is necessary to use proper molding conditions to mold the parts.

Mold venting is critical to good part quality. Proper venting will allow the part to be heated and cooled at atmospheric pressure which will eliminate blowholes at the parting line. Proper venting will prolong the contact of the cooling part with the inner surface of the mold, resulting in a shorter cooling cycle and lower post-mold warpage. Lower pressure differential also results in longer mold life. In general, mold vents of sufficient size and number will allow equalization of the inner mold pressure. Vent tubes should extend to the center of the mold. The inside end of the vent tube should be loosely packed with a filter media to prevent the uncured powder from exiting the mold early in the heating cycle. In order to be efficient, the vent tubes must be kept clean. If the filter media becomes clogged with plastic the vent tube will no longer function to equalize pressure. This can cause parts to warp. Below is a more detailed list of possible causes and solutions to warpage in rotational molded parts:

Possible Causes and Solutions for Warped parts:

1. Inadequate venting: Provide adequate venting - 3/8”-1/2” diameter vent per ft3 of molded volume is suggested for thin walled parts.
2. Non-uniform cooling of part caused by resin pulling away from mold: Rotate mold during cooling cycle. Provide adequate venting and make sure vents are not clogged. Use less mold release. Check for too effective mold release agent. Avoid large flat panels in part design if possible. Reduce cooling rate during initial part cooling cycle. Increase the cooling medium temperature, air cool, and then water cool. Apply low psi air pressure through spine during cooling.
3. Non-uniform cooling caused by sections of the mold being shielded from heat and cooling medium: Mount mold to eliminate shielding problems, add baffles to direct heat and cooling into recessed or shielded areas.
4. Uneven cooling caused by clogged water nozzles: Check and clean nozzles on a periodic schedule.
5. Over-cured part. Degradation of the resin due to high temperature and/or excessively long heating cycle: Decrease oven temperature or heating time.
6. Highly under-cured part. Some degree of under-cure is advisable especially in the case of low melt-index resins to prevent degradation, however highly under cured parts can cause significant loss of impact strength: Increase oven temperature or total heating time. Increase heat transfer coefficient, e.g. steel or aluminum
7. Improper coloring: Select pigment and pigment loading that does not affect resin.
8. Resin type: Use proper resin having adequate melt index and molecular weight distribution for application.
9. Moisture on resin or pigment: Only use dry powder and/or pigment.
10. Mold Parting Line Buildup: Poor parting line fit of the mold will cause plastic to build up between the surfaces and lock molded parts to the mold in those specific areas and not other areas during cooling. This causes the part to be held in place during the cooling cycle in that location and in other areas the part is allowed to release properly. The result is a part that has warped areas and is dimensionally off as it’s not allowed to evenly release.

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