Plastic injection molding (commonly referred to as injection molding) is a molding method which combines injection with molding processes. Injection molding provides many advantages over its alternatives: fast production speed and high efficiency; automated operation; variety in designs, colors and shapes from simple to complex; size possibilities from large to small products with accurate dimensions that make replacement easy; accuracy when it comes to product dimensions as well as complex shape products with accurate dimensions easily replaceable by other products made using injection molding; as well as other molding processing fields being appropriate applications of this technology.

Plastic injection molded parts have become essential in numerous industries ranging from automotive and consumer goods production, to electronics and consumer durables. Unfortunately, however, manufacturers sometimes encounter black spots on these components which obstruct aesthetics and functionality of products; thus prompting further investigations to understand root causes of black specks issues as well as effective solutions.

Four major causes of black spots

I. Causes of mold

1. Full pre-production oil removal inspection can effectively prevent oily and rusty products and runner materials from being mixed into the crushed materials and used (especially during the production process rather than during the initial trial production);

2. If the gates of the mold are too small or too rough, a large amount of friction heat will be generated. If the mold is poorly vented, it will cause gas combustion to produce obvious black stripes (black spots).

II. Reasons for equipment

3. The dryer motor has been running for a long time and has carbon buildup deposits, dirt and evil spirits, so it must be cleaned regularly;

4. The dryer filter must be cleaned and replaced regularly, otherwise foreign matter may enter or the material may become discolored or agglomerated due to poor ventilation;

5. The barrel of the dryer is not (completely) sealed due to operator negligence or insufficient operating equipment, causing dust in the air to enter; or the dust scatters, contaminating surrounding products and materials in the crusher; or the barrel is too large. The operation is not easy, and factors such as the angle (the feeding pipe and its interface) are ignored, resulting in the barrel not being completely cleaned during the material changing process.

6. Long-term production operation without proper protection, resulting in scattered dust mixed into the equipment;

8. The crusher should be used in black and white colors as much as possible

9. When the barrel or the glue head is worn, cracked and bent, part of the polymer will be overheated and produce black lines (black spots);

10. When the rubber ring is worn, the melt cannot advance smoothly and becomes extruded, which will cause excessive gas in the material tube. When the injection is smooth, black lines will be produced due to sharp friction;

11. The tolerance of the gap between the outer diameter of the apron ring and the cylinder wall is too small, which causes the screw melt to produce a large resistance surface, thereby generating frictional heat and causing black lines (the outer diameter of the apron ring can be disassembled and ground a few strips smaller or replaced if the tolerance is larger) The new product enables the screw to melt glue smoothly);

13. The eccentricity between the screw and the material tube generates very large friction heat;

14. The compression ratio of the screw is too large, and the groove in the feed part is shallow, which creates a large friction force between the inner wall of the barrel and the raw material, causing the raw material to overheat and decompose to produce black lines (black spots);

15. The melt temperature range is incorrect. The actual temperature of the electric heating plate and the thermometer are too different. The temperature of resin thermometer in a certain section is too hot and it is difficult to intuitively detect it. You need to use a measuring device to check it.

III. Materials

16. Raw materials have black spots which exceed the acceptable range for customer incoming material inspection or allowable standard of finished product; these black spots exceed allowable standards of the finished product as inspected;

17. Raw material bags become damaged or contaminated during transportation, handling, or storage; foreign matter mixes in with them and causes further issues;

18. Materials are combined manually during recycling process by mixing or shaking;

19. Certain raw materials require the addition of lubricant; insufficient quantities could create severe friction heat generation and in the event that too much exhaust gas escapes, it will accumulate to cause black lines (black spots) caused by gas combustion;

20. The raw material itself has insufficient heat resistance and cannot withstand the operating temperature, and the raw material (additive) is thermally cracked;

21. A material storage bag contains foreign matter that ends up polluting its contents when used again, leading to contamination when being recycled plastics reused by future users.

IV.Other reasons

22. Different materials are produced in the barrel for a long time. Due to temperature changes, oxidation and degradation occur. Factors related to the different adhesion of various materials to the metal surface accumulate over time, resulting in oxidation and degradation on the screw surface, tube wall, nozzle, etc. At this point, a carbonized layer is gradually formed, which is separated when encountering materials with higher viscosity and gradually brought out. In addition, if there are scratches or gaps in parts such as the screw and pipe wall, it will be easier for the molten material to stay, and frequent shutdowns and starts will make the above problems more serious;

23. People who don’t understand the materials crush defective products, resulting in confusion of materials;

24. The technician will confirm whether defective products are crushed or discarded;

25. When replacing materials, in order to facilitate the operation, only use an air gun to blow away the remaining material chips, but this method cannot completely clean the barrel; or use an air gun to blast wildly on site, causing dust to permeate the entire workshop;

26. The floor products or runner materials are directly put into the crusher;

27. Defective products will be shattered just by blowing them with an air gun a few times;

28. When processing the products in the red box, pour them directly into the crusher;

29. When dealing with foreign matter mixed in, if only one cause is found, it is considered that all real causes have been found;

30. The grinder has a scraper blade phenomenon, and the material is not adjusted and controlled in time;

31. The shutdown method of special materials such as PP, PMMA, and ABS is not accurately grasped;

32. The feeding method is wrong. For example, if you add black material and then add white material directly, black chips may fall into the white material due to static electricity or other reasons;

33. The low-temperature materials originally used in the discharge pipe were not completely replaced when cleaning the machine. When the operating temperature rises, the low-temperature materials (fire retardant, additives, etc.) cannot withstand the high temperature ranges and degrade due to heat. This situation usually occurs in the nozzle. (especially if it occurs during the production process rather than during the initial trial run).


When production involves injection molding, and black spots appear on a product, solutions should include the following.

Check Raw Materials: Before selecting plastic raw materials for use, always inspect them thoroughly for impurities or contaminants such as impurities. Should these be found, clean raw materials must be replaced promptly to restore production processes and meet quality goals.

Clean Mold: Check whether there is dirt, oil stains and/or blocked mold holes present on the surface of the mold to see whether its holes need clearing out; any dirt or oil stains found should be eliminated to maintain an unblemished mold surface. If necessary, these should also be cleaned accordingly in order to preserve its appearance.

Adjust injection parameters appropriately: When setting injection process parameters such as injection temperature, speed and holding pressure time according to actual situation, optimize them so as to optimize plastic material melting and flow evenly while avoiding black spot formation.

Install filters or purification equipment during injection molding process in order to filter impurities or pollutants out in order to preserve purity of materials while also preventing formation of black spots.

Strengthen environmental controls: Keep the injection molding workshop clean and hygienic to avoid dust, pollutants and other sources infiltrating product surfaces. Adjust environmental factors like workshop temperature and humidity appropriately in order to guarantee stable quality plastic materials.

Maintain and Clean an Injection Molding Machine: For proper functioning and purity of plastic material purity, regularly maintain and clean an injection molding machine by wiping down its parts such as the nozzle, barrel and other. Regular inspection should also take place so any build up on parts such as the nozzle barrel remains removed to maintain normal operation of this equipment.

You can choose a suitable solution based on the actual situation. If the problem cannot be solved by yourself, it is recommended to seek help from professional technicians.

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