Injection Molding Defects And Solutions -- Black Spots

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Injection Molding Defects and Solutions -- Black Spots 2025-10-17

Black Spots / Black Streaks:
These defects occur when the plastic decomposes or contains volatile substances, air, or other gases that carbonize under high pressure. The carbonized particles are carried by the molten resin into the mold cavity, appearing on the surface of the molded part as black spots, black streaks, or burnt-like marks along the surface.

14.1 Causes	14.2 Solutions
14.1.1 Plastic decomposition (especially for heat-sensitive plastics).	14.2.1 Control melt temperature and residence time in the barrel for heat-sensitive plastics.
14.1.2 Plastic debris stuck between the screw and barrel.	14.2.2 If the screw-barrel clearance is too large, replace the screw and barrel.
14.1.3 Residual decomposed material in the barrel, nozzle, check ring, or mold dead zones.	14.2.3 Clean decomposed material from the nozzle and check ring; control processing conditions to prevent decomposition.
14.1.4 Barrel not cleaned properly.	14.2.4 Clean the barrel thoroughly; disassemble the screw if necessary.
14.1.5 Poor mold venting or excessive clamping force.	14.2.5 Reduce clamping force or add venting slots in the mold.
14.1.6 Gate too small or improperly located.	14.2.6 Change gate position or enlarge gate size appropriately.
14.1.7 Flammable volatiles in the plastic or on mold surfaces.	14.2.7 Replace with qualified material or clean mold surfaces thoroughly.
14.1.8 Poorly dried hygroscopic materials, leading to decomposition.	14.2.8 Dry materials thoroughly according to process conditions.
14.1.9 Non-uniform pellets, high barrel temperature at feed port, high screw speed, low back pressure, excessive air in melt; decomposition occurs at high temperatures.	14.2.9 Replace material, control barrel temperature, increase back pressure appropriately to prevent air entrainment, and use well-dispersed pigments or masterbatch.
14.1.10 Poor pigment dispersion, uneven color, presence of dark particles, pigment degradation.	14.2.10 Reduce screw speed, increase back pressure appropriately, prevent air entrainment, or change masterbatch.
14.1.11 Dye with low heat resistance decomposes in barrel.	14.2.11 Use heat-resistant dye.
14.1.12 Melt temperature too high, screw speed too fast.	14.2.12 Reduce melt temperature and screw speed.
14.1.13 Rough runner surface.	14.2.13 Polish runners.
14.1.14 Small nozzle diameter.	14.2.14 Use a larger open-type nozzle.
14.1.15 Hot runner temperature too high or thermocouple failure; material decomposes.	14.2.15 Reduce hot runner temperature, replace thermocouple.

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Injection Molding Defects and Solutions -- Cracks
Следующая :
Injection Molding Defects and Solutions -- Wrinkles

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