Defect analysis and elimination of black specks

What is black specks?

Black specks are dark or dark streaks on the surface of products, and black streamline appears along the flow direction near the gate, which is a surface quality problem, as shown in the figure

Black spot and black stripe

Defect analysis and elimination of black specks

1) The melt temperature is too high

If the material temperature is too high, the melt will overheat and decompose, forming carbides. In order to avoid the overheat decomposition of the melt, the temperature at the end of the barrel must be strictly controlled for PVC and other heat sensitive thermoplastic materials. When black spots and stripes are found on the surface of the plastic parts, the temperature controller of the barrel should be checked immediately to see if it is out of control, and the temperature of the barrel and die should be reduced appropriately. However, it should be noted that if the material temperature and die temperature are too low, bright streaks will also appear on the surface of plastic parts.

2) Too large gap of material cylinder

If the wear gap between the screw and the barrel is too large, the converging melt will stay in the barrel, resulting in local overheating decomposition of the retained melt and black spots and stripes. In this regard, you can first slightly reduce the barrel temperature, observe whether the fault can be eliminated. Secondly, check the barrel, nozzle and die to see if there is any dead angle and smooth grinding.

After taking the above measures, if the fault is not eliminated, the equipment should be repaired in time, and the gap between screw and barrel should be apintejinusted.

3) Overheat due to friction between melt and die wall

If the injection speed is too fast, the injection pressure is too high, and the relative movement speed between the melt and the cavity wall is too high during mold filling, it is easy to cause friction overheating, which will cause the melt to decompose and produce black spots and wrinkles. Therefore, the injection speed and pressure should be reduced appropriately.

4) Poor exhaust of barrel and die

If the cylinder or die is not well vented, the residual gas in the melt will burn due to adiabatic compression, which will cause the melt to overheat and decompose and produce black spots and stripes. Therefore, the injection speed can be reduced appropriately, and the exhaust port structure of the barrel can be improved when the particle size and uniformity of the raw material are suitable.

For the poor exhaust of the mold part, check whether the gate position and exhaust hole position are correct, whether the gate type is suitable, remove the rust inhibitor and other volatile substances adhered in the mold, and reduce the amount of mold release agent. On the premise of no flash, the clamping force can be reduced and the exhaust clearance can be increased. In addition, check the barrel and thimble for oil leakage.

5) Bulk coking

When the nozzle does not match with the main channel of the mold, the accumulated material will be coking near the gate and injected into the mold cavity with the flowing material, forming black spots and stripes on the surface of the plastic parts. Therefore, the relative position of the nozzle and the main channel of the die should be apintejinusted in time to make it fit well.

In addition, if the hot runner of the mold is not well designed or manufactured, the molten material will not flow smoothly in the runner, stay and coke, which will also cause black spots and stripes on the surface of the plastic parts. Therefore, the surface luminosity of hot runner should be increased and the heating temperature of hot runner should be reduced.

6) The raw material does not meet the molding requirements

If the content of volatile matter in raw materials is too high, the drying of water sensitive resin is poor, the amount of recycled materials is too much, the fine powder is too much, the raw materials are not evenly colored, the lubricant type is not selected correctly or used too much, it will lead to black spots and stripes on the surface of plastic parts in varying degrees. In this regard, we should take corresponding measures according to different situations and eliminate them separately.

About PTJMould Industrial Company Limited

PTJmould is a China mold maker of plastic molds- injection mold, die casting moulds, plastic blow molding, rotational molding, medical plastic injection molding, two shot plastic injection molding, insert molding, overmolding, metal injection molding, micro injection molding, powder injection molding, ceramic injection molding, liquid injection molding, husky injection molding, household mold, casting mold, die mold tool, custom molds, china moulds, rapid prototyping tooling, plastic prototyping tooling, punch press tooling, die and tooling for mobile/ cell phone parts, automotive parts, vacuum cleaners, rechargeable tools, telephones, copiers, computers, multimedia speakers, and many other electronic products and household appliances. And also a plastic product manufacturer, mold manufacturer China– plastic parts, plastic water tank, plastic balls, plastic containers, plastic buckle, plastic anchor, plastic hanger, plastic spoon, plastic pipe fitting, plastic tumble, plastic tableware, plastic cups, plastic bottles, plastic tray, plastic cosmetic container, plastic case, plastic food container, plastic chairs, plastic caps, plastic cap closure, plastic tubes, plastic water pipes, plastic knobs, plastic tubing, plastic utility boxes, plastic racks and so on.