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TPGOA Technical Insights

Controlled Process Problems and Part-Sticking

Several processing missteps can cause sticking of the sprue or part. Unbalanced filling, especially in a multi-cavity tool, can cause over- or under-packing and subsequent part sticking. A test of mold-filling balance is one way to confirm or eliminate this factor.

Overpacking is the major process-related cause of sprue sticking. If some plastic remains stuck in the sprue channel, the next injection cycle will cause extremely high packing pressures, preventing normal shrinkage of the sprue that allows for release. So more sticking will result. The sprue can also be overpacked by high backpressure during screw recovery.

If overpacking is the cause, run tests to determine the gate-seal time and then run the part with the gate sealed and gate unsealed. Determine whether the difference in pack/hold time makes a difference in sticking.

Packing can also be related to part sticking. Overpacking the mold with melt can result in too little shrinkage of the part, making ejection difficult. Reducing the pack or hold pressure (or time) is one solution to try. Pressure and time taken from the second-stage pack and hold should be added to the cooling or cure.

Note that underpacking the mold can cause excessive shrinkage onto cores, which can also result in sticking. Here, molders should increase the second-stage pack/hold time and pressure while decreasing cooling time to maintain the same overall cycle time. Sometimes it helps to reduce the cooling time to minimize shrinkage onto a core. But extending the overall cycle time by lengthening the cooling time can work also if you need more time for the part to shrink away from a cavity onto a core.

Mold or material temperature can also be a source of sticking problems. Degraded materials tend to stick, so check to see that the process temperatures are correct, especially at the nozzle, where both setpoint and actual temperatures need to be controlled tightly. Molders have considerable leeway to raise or lower the mold temperature, as long as it does not cause mold damage. But also check the water flow rate in mold-cooling lines to make sure that you achieve a Reynold’s number of 5000 or greater for turbulent flow and optimal cooling. The temperature difference between the inlet and outlet water lines should be less than 4° F.

Crazing of the part surface can also be a sign of the cause of sticking. Look for symptoms in the area of the part that is sticking, such as stress-whitening near ejector pins. Then check the tool for a source of stress on the part in those areas. Make sure the crazing is not caused by residual cleaning solvents or mold sprays that can attack certain resins.

A sprue that is too soft or not frozen can cause sticking. You can try to downsize the sprue, or cool the sprue bushing, or try lowering the nozzle or melt temperature. A last resort is to increase the cooling time.

Last, but not least, check whether there is mold release agent in the resin, and what kind. Try adding more or changing to a different type or grade.

TPG makes every effort to insure that the information contained herein is accurate - however, we accept no liability for the content of this piece, or for the consequences of any actions taken on the basis of the information provided.
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