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How to solve the problem of poor gloss on injection molded parts?
- Sep 22, 2018 -

Generally, the surface gloss defects of products can be divided into three types: mold failure, improper control of molding conditions, improper use of molding materials.

Mold failure

Because the surface of the plastic part is the reappearance of the cavity surface of the mold, if the surface of the mold has surface defects such as scars, corrosion, micro-pores, etc., it will be copied to the surface of the plastic part to produce poor gloss. If there is oil stain on the surface of the cavity, water, too much dosage of mold release agent or improper selection, it will also make the surface of the plastic parts dark. Therefore, the mold cavity surface should have a good finish, it is best to take polishing treatment or surface chromium plating. The surface of the cavity must be kept clean and timely to remove oil stains and water stains. The variety and dosage of the release agent should be appropriate.

Mold temperature also has a great impact on the surface quality of plastic parts. Generally, different kinds of plastics have different surface gloss 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 melt solidifies immediately after contacting with the mold cavity, which will reduce the reproducibility of the mold cavity surface. In order to increase the gloss, the mold temperature can be raised appropriately. It is better to use the method of introducing warm water into the cooling loop of the mold to make the heat transfer rapidly in the mold cavity, so as to avoid prolonging the molding cycle. This method can also reduce the residual stress in the mold.

In addition, mold faults such as too small stripping slope, abrupt change of cross-section thickness, too thick ribs, too small or sudden change of gate and runner cross-section, too large shearing effect of gating system, turbulent flow of melt, poor mold exhaust and other mold faults will affect the surface quality of plastic parts, resulting in poor surface gloss.

Two. Improper molding condition control.

If the injection speed is too fast or too slow, the injection pressure is too low, the holding time is too short, the pressure of the supercharger is not enough, the cushion pad is too large, the nozzle hole is too small or the temperature is too low, the filler dispersion performance of fiber reinforced plastics is too poor, the filler is exposed or the aluminum foil filler has no directional distribution, the temperature of the barrel is too low, the melt plasticization is not good and so on. Insufficient supply will result in poor gloss on the surface of the plastic parts. In this regard, we should adjust the situation.

If there is a dark area near the gate or at the variable section, it can be eliminated by reducing the injection rate, changing the gate position, enlarging the gate area 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 surface gloss is poor due to poor dispersion of the filler, the resin with better flowability or the screw with stronger mixing ability should be replaced.

Three, molding materials do not meet the requirements.

Raw materials which do not meet the requirements of application will also cause poor gloss on the surface of the plastic parts. The causes and treatment methods are as follows:

1. The content of water or other volatile substances in the raw materials is too high, and the volatile components condense between the cavity wall and the melt during the molding, resulting in poor surface gloss of the plastic parts. Raw materials should be pre dried

2. discoloration of raw materials or coloring agents causes poor gloss. High temperature resistant raw materials and coloring agents should be selected.

3. the poor fluidity of raw materials makes the surface of plastic parts undense resulting in poor gloss. We should replace the resin with better fluidity, increase the amount of lubricant and improve the processing temperature.

4. raw materials mixed with mixed or insoluble raw materials. New materials should be replaced.

5. raw material particle size is uneven. The raw materials with different grain sizes should be sieved.

6. crystalline resin causes poor gloss due to uneven cooling. The mold temperature and processing temperature should be controlled reasonably. For thick-walled plastic parts, if the cooling is insufficient, the surface of the plastic parts will be hair and gloss will be dark. The solution is to take the plastic parts out of the mold and put them into the cold-pressed mold immersed in cold water to cool and set.

7. the proportion of recycled materials used in raw materials is too high to affect the even plasticizing of the melted materials. The dosage should be reduced.