Injection Molding Defects And Solutions -- Unmelted Lumps

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Injection Molding Defects and Solutions -- Unmelted Lumps 2025-10-20

Cold Slugs / Unmelted Lumps:
Unmelted or insufficiently plasticized material enters the mold cavity along with the melt flow, causing hard plastic lumps to become embedded inside or on the surface of the molded part.

20.1 Causes	20.2 Solutions
20.1.1 Material temperature, mold temperature, and nozzle temperature are too low, or injection speed is too slow.	20.2.1 Properly increase the material, mold, and nozzle temperatures, and raise the injection speed.
20.1.2 Back pressure too low, resulting in uneven plasticization.	20.2.2 Increase back pressure appropriately and adjust screw speed to ensure uniform plasticization.
20.1.3 Insufficient plasticizing capacity of the injection machine; machine capacity is close to the part weight; molding cycle too short.	20.2.3 Use an injection molding machine with higher plasticizing capacity.
20.1.4 Contamination or mixing of materials of different grades.	20.2.4 Replace material and ensure it is free from impurities.
20.1.5 Uneven or oversized pellets.	20.2.5 Replace material with uniform and appropriately sized pellets.
20.1.6 Absence of a main runner or cold slug well, or improper position of the cold slug well.	20.2.6 Add a cold slug well in the mold and polish the runner.
20.1.7 Poor fit between mold gate and nozzle.	20.2.7 Repair or adjust nozzle radius or diameter.
20.1.8 Low hot runner temperature or heater malfunction.	20.2.8 Increase hot runner temperature or replace the heater.