TROUBLESHOOTING

Pressures in the mold work to increase the contact between the shot material and the mold surface. Generally, high injection pressures maintain part properties, improve part appearance and reduce molding cycle times. However, if there is too much pressure in the cavity, it can cause the material to stick to the mold wall. Also, as parts cool and solidify they shrink away from the mold walls. If the part is not sufficiently cooled, then the part can stick in the mold. Other causes of this problem are associated with mold and part design.

Part Is Over-Packed
Over-packing can lead to parts sticking in the mold. Over-packing, however, will do more than cause sticking parts; it will increase processing and maintenance costs. The parts will consume an excessive amount of material, increase rejection rates (flashed parts) and cause damage to the parting line. Please refer to the Start Up Procedures for the injection molding process.

Part Is Not Sufficiently Cooled
If there is not enough time to cool the part to a point where it can pull away from the mold wall, it could stick to the mold cavity. Increasing cooling time will help. Reducing melt and mold temperatures will decrease the time needed to cool the part.

Mold Design Is Incorrect
The part may be sticking in the mold because either the sprue is sticking or due to mold design and polish. If the sprue is sticking in the tooling, check that the nozzle is correctly aligned and sealed with the sprue bushing. If it is not aligned properly, it can cause material seepage, which will act like an undercut and hold the material in the cavity. If this is not the case, try reducing the length of the sprue to decrease the surface area/hold that the sprue cavity has on the material. If the part is sticking to the mold surface, it could be that the part/mold design is incorrect.

Tool Steel
We recommend having a minimum draft angle of 0.5° on mold walls to reduce surface area drag of ejecting parts on the mold wall. We also recommend using an SPI/SPE-B #3, #4, #5 or #6 finish (VDI 30-36 / Ra 3-6) for good mold release. For higher polished surfaces, air assisted ejection is required for easy part removal and minimum distortion. We suggest a minimum draft of 0.5° per each 0.025 mm (0.001") depth of texture used. When polishing the mold, always draw polish. Polishing in the direction the part will be ejected will reduce the amount of small ridges that can hold back the part. Finally, make sure that any cores in the mold are strong enough to handle the injection pressure. If the core bends slightly from injection pressure, it will create an undercut in the part and hold the part in the mold. Thickening core areas and using stronger steel in these areas is recommended.

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