General product surface gloss defects can be divided into the following three types: mold failure, improper control of molding conditions, and improper use of molding materials.

A mold fault

Because the surface of the plastic part is the reproduction of the cavity surface of the mold, if the surface of the mold has surface defects such as scars, corrosion, micro-holes, etc., it will be copied to the surface of the plastic part and have poor gloss. If there is oil stains, moisture on the surface of the cavity, too much amount of release agent or improper selection, it will also make the surface of the plastic part dark. Therefore, the surface of the cavity of the mold should have a good finish, preferably polished or chrome-plated. The surface of the cavity must be kept clean, and oil and water stains must be removed in time. The type and amount of release agent should be appropriate.

Mold temperature also has a great influence on the appearance quality of plastic parts. Generally, different varieties of plastics have great differences in appearance under different mold temperature conditions. Too high or too low mold temperature will lead to poor gloss. If the mold temperature is too low, the molten material will solidify immediately after contact with the mold cavity, which will reduce the reproducibility of the mold cavity surface. In order to increase the luster, the mold temperature can be appropriately increased. The best way is to introduce warm water into the mold cooling circuit, so that the heat can be quickly transferred in the cavity, so as not to prolong the molding cycle. This method can also reduce the residual stress during molding.

In addition, the demolding slope is too small, the thickness of the section is suddenly changed, the ribs are too thick, and the gate and runner section is too small or changed suddenly, the shearing effect of the pouring system is too large, the melt moves in a turbulent state, and the mold exhaust is poor, etc. Mold faults will affect the surface quality of plastic parts, resulting in poor surface gloss.

2. Improper control of molding conditions

If the injection speed is too fast or too slow, the injection pressure is too low, the pressure holding time is too short, the booster pressure is not enough, the buffer is too large, the nozzle hole is too small or the temperature is too low, the filler dispersing function of the fiber reinforced plastic is too poor, Exposed fillers or non-directional distribution of aluminum foil-like fillers, too low barrel temperature, poor melt plasticization and lack of material supply will all lead to poor surface gloss of plastic parts. This should be adjusted according to the specific situation.

If a dark area occurs near the gate or at the variable cross-section, it can be eliminated by reducing the injection rate, changing the gate position, expanding the gate area, and adding an arc transition ratio at the variable cross-section.

If there is a thin layer of milky white on the surface of the plastic part, the injection speed can be appropriately reduced. If the dispersing function of the filler is too poor, the surface gloss is poor, and the resin with better mobility should be replaced or the screw with stronger mixing ability should be used.

3. The molding material does not meet the application requirements

Materials that do not meet the application requirements will also lead to poor surface gloss of plastic parts. The reasons and solutions are as follows:

1. The content of moisture or other evaporable substances in the molding material is too high, and the evaporative components condense between the cavity wall of the mold and the molten material during molding, resulting in poor surface gloss of the plastic part. The raw materials should be pre-dried.

2. The material or colorant is decomposed and discolored, resulting in poor gloss. Materials and colorants with higher temperature resistance should be used.

3. The movement function of the material is too poor, so that the surface of the plastic part is not dense, resulting in poor gloss. The resin with better performance should be replaced or an appropriate amount of lubricant should be added and the processing temperature should be increased.

4. The raw materials are mixed with different materials or incompatible materials. New materials should be used.

5. The particle size of the material is uneven. Materials with too large particle size differences should be screened out.

6. Crystalline resin has poor gloss due to uneven cooling. The mold temperature and processing temperature should be reasonably controlled. For thick-walled plastic parts, if the cooling is insufficient, the surface of the plastic parts will be hairy and the gloss will be darker. The treatment method is to take the plastic parts out of the mold and put them in cold water immediately. Cooling and shaping in the cold press mold.

7. The proportion of recycled materials in the raw materials is too high, which affects the uniform plasticization of the melt. Its dosage should be reduced.