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Analysis and Elimination of Surface Gloss Defects of Products
- Jul 27, 2018 -

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Poor gloss (Low Gloss) refers to the dark and ungloss of the surface, the low transparency of the transparent products, and many reasons for the poor gloss. Some other injection defects are also the cause of poor gloss.

1. Die failure

Because the surface of the plastic part is the reproduction of the mold cavity surface, if the surface of the mould has the surface defects such as scar, corrosion and micropore, it will be reflected to the surface of the plastic parts to produce a bad gloss. If the surface of the cavity has oily dirt, moisture, too much amount of mold release agent or improper selection, it will make the surface of plastic parts darker.

Therefore, the mold cavity surface should have a good finish, preferably by polishing or chrome plating. The surface of the cavity must be kept clean and timely to remove oil stains and water stains. The variety and dosage of demoulding agent should be appropriate.

The mold temperature has a great influence on the surface quality of the plastic parts. Usually, the surface luster of different kinds of plastics varies greatly under different mold temperature conditions, and the high or low mold temperature will lead to the bad luster. If the mold temperature is too low, the melt will solidify immediately after contacting 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 improved properly. It is better to use the method of warm water in the mold cooling loop, so that the heat transfer at the speed of the cavity, so as not to prolong the molding cycle. This method can also reduce the residual stress in the molding. In general, except for polystyrene, ABS and AS, the mold temperature can be controlled over 100 degrees. However, it should be noted that if the mold temperature is too high, it will also cause the surface of plastic parts to become dark.

In addition, the mold failure of the casting system will affect the surface quality of the plastic parts, and the surface luster will be bad.

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 pressure of the supercharger is not enough, the cushion is too large, the nozzle is too small or the temperature is too low, the dispersion performance of the filler of the fiber reinforced plastic is too poor, the filler is exposed or the aluminum foil like filler has no direction distribution, the barrel temperature is too low, and the plasticization of the melt material is poor and Inadequate material supply will lead to poor surface gloss of plastic parts. In this regard, the specific circumstances should be adjusted.

If the dark area is produced near the gate or in the variable section, it can be eliminated by reducing the injection rate, changing the gate position, expanding the area of the gate and increasing the arc transition at the variable section.

If there is a thin layer of milky white on the surface of the plastic parts, the injection speed can be reduced appropriately. If the poor dispersion of filler results in poor surface luster, replace the resin with better flow performance or replace the screw with stronger mixing capacity.

3. The raw material for molding is not in conformity with the requirements

The content of water or other volatile matter in the molding material is too high. The volatile components of the molding are condensed between the mold cavity wall and the molten material during the molding, which leads to the poor gloss on the surface of the plastic parts. The raw materials should be pre dried.

The decomposition and discoloration of raw materials or colorants results in poor gloss. High temperature resistant raw materials and coloring agents should be selected.

Poor fluidity of raw materials causes poor gloss on the surface of plastic parts. We should replace the resin with better fluidity, increase the amount of lubricant and improve the processing temperature.

The raw materials are mixed with different materials or insoluble materials. New materials should be replaced.

The grain size of the raw material is not uniform. It is necessary to screen out the material with too much difference in particle size.

Crystalline resins have poor gloss due to uneven cooling. The mould should be properly controlled and the processing temperature should be properly controlled. For the thick wall plastic parts, if the cooling is insufficient, the surface of the plastic parts will be hairy and the gloss is dark. The solution is to remove the plastic parts from the mold and immediately put them into the cold pressing die immersed in cold water.

The reuse ratio of recycled materials in raw materials is too high, which affects the even plasticizing of the melted materials. Its dosage should be reduced.