(1) Defect characteristics
The main manifestation of plastic parts is: there are many dark black stripes or black spots on the surface or inside of the plastic product.
(2) Causes of defects and countermeasures
The main cause of black spots in plastic manufacturing is that the plastic melt is overheated and decomposed under high temperature and high pressure conditions, and the carbonized carbon is mixed in the melt to form a black spot.
1) Injection mold
(1) If the injection mold produced by the injection molding manufacturer has poor venting, the residual gas in the cavity will be compressed adiabatically to produce high temperature, which will cause the melt to overheat and carbonize to form a black spot.
(2) If the mold cavity surface produced by the plastic factory is not smooth enough, it may be easy to adhere to a small amount of plastic, which is easy to be carbonized at high temperature, and then mixed with the melt to form a black spot.
(3) If the form and position of the gate are not properly arranged, the melt flow will be blocked or the venting will be poor, which will easily cause local carbonization of the melt and produce black spots.
(4) If the nozzle does not match well with the main runner of the mold, carbonization will occur near the gate, and these carbonized materials will be injected into the cavity with the melt to form a black spot.
2) Injection molding process
(1) If the melt temperature is too high, it is easy to cause the melt to decompose and carbonize to produce black spots.
(2) If the injection pressure is too high, the flow rate will also be high, and the relative speed between the melt and the cavity wall will be high, which will produce a lot of friction heat, causing the melt to produce local carbonization and produce black spots.
(3) If the injection speed is too fast, the flow of the melt will change from laminar to turbulent, which is easy to form black spots and bubbles.
3) Plastic molding equipment
(1) If the temperature at the nozzle is too high, it is easy to cause the melt to overheat and carbonize to produce black spots.
(2) If there is accumulation and dead angle in the barrel and nozzle, these accumulations will carbonize due to overheating.
(3) If the barrel venting is not good, the gas accumulated in the barrel will cause the melt to overheat locally, decompose, carbonize, and produce black spots.
(4) If the capacity of the injection molding machine is too large, or the geometric dimensions of the injection molding screw are not compatible with the molding material, it is easy to cause overheating and carbonization, and produce black spots.
(5) If there is melt retention at the check valve, it is easy to overheat and carbonize to produce black spots.
(6) If the injection screw rotates too fast and the back pressure is too large, it will produce a lot of friction heat with the melt, causing the melt to decompose and carbonize, and produce black spots.
(7) If the wear between the barrel and the screw, piston is too much, the gap is too large, the melt is retained in the barrel and overheats and carbonizes to produce black spots.
4) Raw materials
(1) If the powder material in the raw material is too much, it is easy to be overheated and carbonized to produce black spots.
(2) If the recycled material is added too much, the recycled material contains more impurities, which are easy to be carbonized and appear black spots, so the amount of recycled material should be strictly controlled.
(3) If the melting index of the resin is too large, it is easy to cause local carbonization of the melt and produce black spots.
(4) If the water content and volatile content of the raw material are too high, and the large amount of gas generated cannot be discharged in time, it will cause local carbonization and produce black spots.
(5) If the release agent is used too much and the coating is not uniform, it will cause the surface of the plastic part to be carbonized and produce black spots.