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.
I. mold failure
Because the surface of the plastic part is the reappearance of the mold cavity surface, if there are surface defects such as scars, corrosion and micropores on the surface of the mold, it will be copied to the surface of the plastic part to produce poor gloss. If there is oil stain, moisture, too much amount of release agent or improper selection on the cavity surface, it will also make the surface of plastic parts dark.
Therefore, the cavity surface of the mold should have a good finish, and it is best to adopt polishing treatment or chrome plating. The cavity surface must be kept clean, and oil and water stains should be removed in time. The variety and dosage of release agent should be appropriate.
Mold temperature also has great influence on the surface quality of plastic parts. Generally, different kinds of plastics have great differences in surface gloss under different mold temperatures, and too high or too low mold temperature will lead to poor gloss. 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 gloss, the mold temperature can be appropriately increased. It is best to introduce warm water into the mold cooling circuit, so that the heat can be quickly transferred in the mold cavity to avoid prolonging the molding cycle. This method can also reduce the residual stress during molding.
In addition, mold failures such as too small demoulding slope, sudden change of section thickness, too thick ribs, too small or sudden change of gate and runner section, too large shearing action of gating system, turbulent flow of melt, and poor exhaust of mold will affect the surface quality of plastic parts, resulting in poor surface gloss.
How to eliminate surface defects of injection molded products.
Second, 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 supercharger pressure is not enough, the buffer 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 barrel temperature is too low, the melt plasticization is poor and the supply is insufficient, which will lead to poor surface gloss of plastic parts. Therefore, it should be adjusted according to the specific situation.
If there is a dark area 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 increasing the arc transition at the variable cross-section.
If there is a thin milky white layer on the surface of the plastic part, the injection speed can be appropriately reduced. If the surface gloss is poor due to poor dispersion performance of filler, resin with better fluidity or screw with stronger mixing ability should be used.
Third, the molding raw materials do not meet the use requirements
Raw materials that do not meet the use requirements will also lead to poor surface gloss of plastic parts. The causes and treatment methods are as follows:
1. The content of water or other volatile substances in the molding raw materials is too high, and the volatile components condense between the cavity wall of the mold and the melt during molding, resulting in poor surface gloss of the plastic parts. The raw materials should be pre-dried.
2. The raw materials or colorants decompose and discolor, resulting in poor gloss. Raw materials and colorants with high temperature resistance should be selected.
3. The poor fluidity of raw materials makes the surface of plastic parts not dense, resulting in poor gloss. The resin with better fluidity should be used, or a proper amount of lubricant should be added and the processing temperature should be increased.
4. The raw materials are mixed with foreign materials or immiscible raw materials. New materials should be used.
5. Uneven granularity of raw materials. Raw materials with too large difference in particle size should be screened out.
6. Poor gloss of crystalline resin 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 hairy and the gloss will be dark. The solution is to take the plastic parts out of the mold and immediately put them into a cold press mold soaked in cold water for cooling and shaping.
7. The proportion of recycled materials in raw materials is too high, which affects the uniform plasticization of molten materials. The dosage should be reduced.
