Summary of solutions for the poor surface gloss of injection molded parts

General product surface gloss defects can be divided into the following three kinds: mold failure. Improper control of molding conditions. Improper use of molding materials.

I. Mold failure

As the surface of the plastic part is the reproduction of the mold cavity surface, if the surface of the mold has scars, corrosion, microporosity, and other surface defects, it will be copied to the surface of the plastic part to produce bad gloss. If the cavity surface has oil, moisture, too much mold release agent, or improper selection, it will also make the surface of the plastic parts dark.

Therefore, the cavity surface of the mold should have a good finish, it is best to take polishing treatment or surface chrome plating. The surface of the cavity must be kept clean, and oil and water stains should be removed in time. The variety and amount of mold release agents should be appropriate.

Mold temperature also has a great impact on the surface quality of plastic parts, usually, different kinds of plastics in different mold temperature conditions surface gloss varies greatly, mold temperature is too high or too low will lead to bad gloss. If the mold temperature is too low, the molten material will be cured immediately after contact with the mold cavity, which will make the reproducibility of the mold cavity surface decrease.

In order to increase the luster, the mold temperature can be properly increased, preferably by using the method of passing warm water in the cooling circuit of the mold, so that the heat is rapidly transferred in the cavity to avoid prolonging the molding cycle, and this method can also reduce the residual stress in the molding.

In addition, the mold slope is too small, the thickness of the section is abruptly changed, the rib is too thick and the gate and sprue cross-section is too small or suddenly changed, the shearing effect of the pouring system is too large, the melt is flowing in a turbulent state, the mold exhaust is bad and other mold failures will affect the surface quality of the plastic parts, resulting in poor surface gloss.



How to eliminate the surface defects of injection molded products? 

II. Improper control of molding conditions

If the injection speed is too fast or too slow, the injection pressure is too low, the holding time is too short, the booster pressure is not enough, the cushion is too big, the nozzle hole is too small or the temperature is too low, the filler dispersion of fiber-reinforced plastic is too poor, the filler is exposed or the foil-like filler has no directional distribution, the barrel temperature is too low, the molten material is poorly plasticized and the material supply is insufficient, all of which will lead to the poor surface gloss of plastic parts. In this regard, adjustments should be made to the specific 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, expanding 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 part, the injection rate can be reduced appropriately. If the dispersion performance of the filler is too poor to lead to poor surface gloss, you should replace the resin with better fluidity or use a screw with a stronger mixing ability.

III. Forming materials do not meet the requirements of use

Raw materials that do not meet the requirements of the use of plastic parts can also lead to poor surface gloss. The causes and treatment methods are as follows.

1. molding raw materials in the moisture or other volatile content is too high, molding volatile components in the mold cavity wall and melt between the condensation, resulting in plastic parts with poor surface gloss. Raw materials should be pre-drying treatment. 

2. raw materials or coloring agent decomposition discoloration leads to bad luster. Should use higher temperature-resistant raw materials and coloring agents. 

3. The liquidity of raw materials is too poor, so the surface of the plastic parts is not dense, resulting in bad luster. Should be replaced with better flowability of the resin or increase the number of lubricants and improve the processing temperature. 

4. Raw materials mixed with foreign materials or insoluble raw materials. Should be replaced with new materials. 

5. Uneven particle size of raw materials. Should be sieved to remove the particle size difference is too large raw materials. 

6. Crystalline resin due to uneven cooling leads to bad luster. Should be reasonable control of mold temperature and processing temperature, for thick-walled plastic parts, if the cooling is not enough, will also make the surface of the plastic parts hairy, glossy dark, the solution is to take out the plastic parts from the mold, immediately into the cold press mold immersed in cold water cooling and shaping. 

7. The proportion of recycled material in raw materials is too high, affecting the uniform plasticization of molten material. The amount should be reduced.