Black spots and carbonized stains are common appearance defects appearing during plastic injection molding. Such flaws not only damage product surface quality, but also lead to mass rejection, especially for precise medical and electronic shells. Systematic inspection shall be conducted from raw material, equipment condition, mold status, molding parameters and production environment aspects to eliminate black spot failures efficiently.
1. Inspection and Improvement of Raw MaterialsMaterial carbonization is the leading cause of black spots. Long-time high-temperature heating, mixed impurities and damp raw materials easily produce carbon residues attaching to product surfaces. Plastic pellets shall be sealed and stored with ambient humidity controlled below 60%. Damp materials need sufficient drying treatment according to material types to avoid pyrolysis blackening. Random mixing of recycled scraps and foreign plastic materials is forbidden, as different melting points easily cause high-temperature carbonization. Inner walls of feeding hoppers and packaging bags are cleaned regularly to remove dust and aged adhesive leftovers, cutting off external impurity sources fundamentally.
2. Troubleshooting and Maintenance of Injection Molding MachinesCarbonized deposits accumulating inside barrel, screw and nozzle repeatedly generate black spot defects. Residual plastic sticking on internal parts ages and carbonizes under continuous heat, mixing into molten plastic during injection. Professional cleaning materials are adopted to flush internal carbon dirt during regular equipment maintenance. Heating temperature is properly adjusted to avoid overheating which accelerates material deterioration. Worn screw and barrel inner walls shall be replaced timely, since stuck residual materials keep forming carbonized impurities. Oil leakage of hydraulic systems is checked to prevent oily stains mixing into raw plastics.
3. Mold Structure Optimization and Internal CleaningPoor exhaust performance and internal dirt accumulation inside molds also result in black spots. Trapped air inside cavities compresses and heats up rapidly, scorching molten plastic and forming dark stains. Exhaust grooves are added at melt filling terminals and wall thickness mutation areas to discharge internal air smoothly. Complete mold cleaning is implemented periodically to wipe off dust, scraps and oil stains on parting surfaces and cavities. Cooling pipelines are kept unobstructed to maintain even mold temperature and avoid local material stagnation and carbonization. Hidden narrow gaps that gather contaminants are reduced in structural design. Molds remain closed when idle to prevent workshop dust falling into cavities.
4. Adjustment and Optimization of Injection Molding ParametersUnreasonable injection speed, back pressure and material residence time intensify shear heat and trigger carbonization. Proper reduction of back pressure lowers friction heat generated by repeated material compression. Staged slow injection speed avoids excessive heating caused by fast melt flow. Material standing time inside high-temperature barrel is shortened to prevent aging blackening. Holding pressure and duration are reasonably controlled to avoid excessive material compaction and residual carbon formation. Stable parameter setting effectively reduces internal stress and black spot generation chances.
5. On-site Environment Control and Operational StandardizationFloating dust and irregular manual operation introduce external impurities forming surface black spots. Molding workshops keep tidy with regular cleaning to reduce suspended particulate matters. Workers wear dust-proof gloves and uniforms to prevent skin debris and stains contaminating products. Feeding ports are equipped with dustproof covers, and waste materials are placed in designated sealed containers to avoid stray scraps mixing into raw materials. Finished products are prevented from contacting greasy and dusty platforms during taking and transmission procedures.
6. Rapid Defect Judgment and Emergency HandlingBlack spots distributing all over products mainly relate to raw material and equipment carbonization. Fixed-position dark stains are caused by mold dirt and poor exhaust conditions. Burnt stripes result from excessive injection speed and trapped air. Once defects appear, production shall suspend for targeted inspection. Staff clean accumulated carbon, check material purity and adjust molding parameters step by step. Small-batch trial production is carried out after modification confirmation, and formal mass production restarts only after defects disappear completely. Defect records are kept to avoid repeated similar failures.
