Analysis and solution of black spot defects in injection molded products

Black spots refer to dark or striped black spots on the surface of the product, usually appearing near the gate along the flow direction. This defect is a surface quality problem and is common in the injection molding process. The following is an analysis and elimination method for black spot defects.

Analysis and elimination method of black spot defects

1. The melt temperature is too high

– When the melt temperature is too high, it will cause overheating and decomposition to form carbides, which will then produce black spots and black stripes. For heat-sensitive thermoplastic materials such as polyvinyl chloride, the temperature of the barrel tail must be strictly controlled to avoid overheating. Once black spots and stripes are found on the surface of the plastic part, immediately check whether the temperature controller of the barrel is out of control, and appropriately reduce the temperature of the barrel and mold. However, it should be noted that if the material temperature and mold temperature are too low, bright stripes may also appear on the surface of the plastic part.

2. The barrel gap is too large

– If the wear gap between the screw and the barrel is too large, the melt may be retained in the barrel, causing local overheating and decomposition, forming black spots and stripes. In this regard, you can try to slightly reduce the barrel temperature and observe whether the fault is eliminated. At the same time, check whether there are dead corners of material stagnation in the barrel, nozzle and mold, and ensure that they are smooth. If the problem persists, the equipment should be repaired in time and the gap between the screw and the barrel should be adjusted.

3. Overheating of the melt and the mold wall due to friction

– When the injection speed is too fast or the injection pressure is too high, the relative movement speed of the melt during the filling process is too fast, which may cause frictional heat, thereby decomposing the melt and producing black spots and stripes. In this regard, the injection speed and injection pressure should be appropriately reduced.

4. Poor exhaust of the barrel and mold

– If the exhaust of the barrel or mold is poor, the residual gas in the melt may cause combustion under adiabatic compression, causing the melt to overheat and decompose, forming black spots and stripes. In this regard, the injection speed can be appropriately reduced, and the barrel exhaust port structure can be optimized under the condition of ensuring the particle size and uniformity of the raw materials. For the mold part, check whether the gate position and the exhaust hole position are correct, and confirm whether the gate type is appropriate; remove the residue of volatile substances in the mold, and reduce the amount of release agent used. Under the premise of not causing overflow and flash, the clamping force can be appropriately reduced, the exhaust gap can be increased, and the barrel and ejector pin can be checked for oil seepage.

5. Accumulated material coking

– When the nozzle and the main channel of the mold do not match well, accumulated material coking will occur near the gate and will be injected into the cavity with the flow material, thus forming black spots and stripes on the surface of the plastic part. In this regard, the position of the nozzle and the main channel of the mold should be adjusted in time to ensure a good fit. In addition, if the hot runner of the mold is poorly designed or manufactured, the molten material does not flow smoothly in the runner, and it is retained and coked, which may also cause black spots. At this time, the smoothness of the hot runner surface should be improved and the heating temperature of the runner should be reduced.

6. The raw materials do not meet the molding requirements

– If the raw materials contain too much volatile matter, poorly dried water-sensitive resin, too much recycled material, too much fine powder, uneven raw material coloring, improper selection of lubricants or excessive use, black spots and stripes may appear on the surface of the plastic part. For this, corresponding measures should be taken for different situations to eliminate them one by one.

Through the above analysis and measures, the occurrence of black spot defects in the injection molding process can be effectively reduced, the surface quality of the product can be improved, and the overmolding for injection molded parts can be ensured.