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How to solve the poor surface gloss of plastic parts?

Jan. 01, 70
What is poor surface gloss?
Poor gloss means that the surface is dim and has no gloss, and the transparency of transparent products is low. There are many reasons for poor gloss. Other injection defects are also the cause of poor gloss.
Analysis and elimination methods of poor surface gloss defects
1. Mold failure
Since the surface of the plastic part is a reproduction 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 reflected on the surface of the plastic part to produce poor gloss. If the surface of the cavity is oily, moisture, excessive amount of release agent or improper selection, it will also make the surface of the plastic part dark.
Therefore, the surface of the mold cavity should have a good finish, and 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 must be removed in time. The type and amount of release agent should be appropriate.
Mold temperature also has a great influence on the surface quality of plastic parts. Generally, different types of plastics have a large difference in surface gloss under different mold temperature conditions. Too high or too low mold temperature will cause poor gloss. If the mold temperature is too low, the melt will solidify immediately after contact 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 use warm water in the mold cooling circuit to quickly transfer heat in the cavity to avoid prolonging the molding cycle. This method can also reduce the residual stress during molding. . In general, except for polystyrene, ABS, AS, the mold temperature can be controlled above 100 degrees. But it should be noted that if the mold temperature is too high, it will also cause the surface of the plastic part to become dark.
In addition, the draft angle is too small, the section thickness changes suddenly, the ribs are too thick, and the gate and runner sections are too small or suddenly change, the gating system shears too much, the melt flows in a turbulent state, and the mold exhaust is poor, etc. Mold failure 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 holding time is too short, the booster pressure is not enough, the cushion is too large, the nozzle hole is too small or the temperature is too low, the filler dispersion performance of the fiber reinforced plastic is too poor, Exposed fillers or non-directional distribution of aluminum foil-like fillers, too low barrel temperature, poor plasticization of the melt, and insufficient material supply will cause poor surface gloss on the plastic parts. In this regard, adjustments should be made to specific circumstances.
If a dark area is generated 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 layer of milky white on the surface of the plastic part, the injection speed can be appropriately reduced. If the dispersing performance of the filler is too poor and the surface gloss is poor, you should switch to a resin with better flowability or a screw with a stronger mixing ability.
3.The molding materials do not meet the requirements for use
Raw materials that do not meet the requirements for use will also lead to the poor surface gloss of the plastic parts. The reasons and treatment methods are as follows: the moisture or other volatile content in the molding raw material 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 part. The raw materials should be pre-dried. Raw materials or colorants decompose and change color, resulting in poor gloss. Materials and colorants with higher temperature resistance should be selected.
The fluidity of the raw materials is too poor so that the surface of the plastic part is not dense and the gloss is poor. It should be replaced with a resin with better flow properties or an appropriate amount of lubricant should be used and the processing temperature should be increased. The raw materials are mixed with foreign materials or incompatible materials. New materials should be used. The particle size of the raw material is not uniform. Raw materials with too large difference in particle size should be screened out.
Crystalline resins have 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 not enough, the surface of the plastic parts will be hairy and the gloss will be dark. The solution is to immediately put the plastic parts in cold water after taking them out of the mold. It is cooled and shaped in the cold press mold. The ratio of recycled materials in the raw materials is too high, which affects the uniform plasticization of the melt. Its dosage should be reduced.

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