What should be done if there are black streaks in injection molded products? 

I. Causes of black streaks:
1-1 Thermal decomposition of the material
1-2 Damage to the injection molding machine screw
1-3 Carbonization of the material due to friction and compression of air 

II. Related Knowledge
2-1 The most common cause of black streaks is the residue of material decomposition remaining in the injection molding machine's barrel.
2-2 In plunger injection molding machines, residues tend to accumulate in various parts.
2-3 For injection molding machines with valve-controlled nozzles, residues are also prone to accumulate in the valve control section.
2-4 When air in the barrel is compressed to high temperatures, combined with the shear heat at the gate, it can easily lead to the formation of black streaks. 

III. Solutions
3-1 Immediate: When replacing materials, reduce the injection pressure and lower the temperature of the heating barrel.
3-2 Short-term: Clean the heating barrel (using a copper wire brush).
3-3 Long-term: Repair the heating barrel. 

4. Due to material differences
4-1 In the case of mixed resins, the overheating of pigments, paints, lubricants, UV absorbers, anti-static agents, etc. in the barrel and their prolonged retention can lead to thermal decomposition of these materials, which is also a cause of black streaks. This tendency is particularly pronounced when certain types of additives have poor heat resistance.
4-2 When molding with polycarbonate (PC), black streaks are also prone to occur with valve-controlled nozzles.
4-3 Screws with backflow prevention washers are also prone to having charred residues. 

V. Reference Matters:
5-1 When replacing materials, if no special cleaning materials are used, ordinary soft polystyrene (GPPS) should be employed.