Analysis and Elimination Method of Poor Gloss Defects on Injection Surface

Time:2020-05-21 10:43:56 / Popularity: / Source:

poor gloss 
Low gloss means that surface is dark and dull, and transparency of transparent products is low. There are many reasons for poor gloss, and some other injection defects are also reasons for poor gloss.

1. Mold failure

Since surface of plastic part is reproduction of cavity surface of mold, if mold surface has flaws, corrosion, micropores and other surface defects, it will be reflected on the surface of plastic part to produce poor gloss. If surface of cavity has oil stains, moisture, too much amount of release agent or improper selection, it will also make surface of plastic parts dark.
Therefore, cavity surface of mold should have a good finish, and it is best to adopt polishing treatment or chrome plating on the surface. Surface of cavity must be kept clean to remove oil and water stains in time. Variety and dosage of release agent should be appropriate.
Mold temperature also has a great influence on surface quality of plastic parts. Generally, different types of plastics have a large surface gloss difference under different mold temperature conditions. If mold temperature is too high or too low, it will cause poor gloss. If mold temperature is too low, melt will solidify immediately after contact with mold cavity, which will reduce reproducibility of mold cavity surface. In order to increase gloss, mold temperature can be appropriately increased. It is best to use warm water in mold cooling circuit to transfer heat rapidly in cavity to avoid prolonging molding cycle. This method can also reduce residual stress during molding. In general, except polystyrene, ABS, AS, mold temperature can be controlled above 100 degrees. However, it should be noted that if mold temperature is too high, it will also cause surface of plastic part to become dark.
In addition, too small stripping slope,sudden changes in cross-sectional thickness, excessively thick ribs, too small or sudden changes in gate and runner cross-sections, too much shearing effect of pouring system, turbulent flow of melt, poor mold exhaust and other mold failures will affect surface quality of plastic parts, resulting in surface gloss bad.

2. Improper control of molding conditions

If injection speed is too fast or too slow, injection pressure is too low, pressure holding time is too short, pressure of booster is not enough, cushion is too large, nozzle hole is too small or temperature is too low, filler dispersion performance of fiber-reinforced plastic is too poor, filler is exposed or aluminum foil-shaped filler has no directional distribution, temperature of barrel is too low, melt plasticization is poor and supply is insufficient, which will cause surface gloss of plastic parts to be poor. In this regard, adjustments should be made for specific situations.
If a dark area is generated near gate or at variable cross-section, it can be eliminated by reducing injection rate, changing gate position, expanding gate area, and increasing arc transition at variable cross-section.
If there is a thin layer of milky white on the surface of plastic part, injection speed can be appropriately reduced. If poor gloss of surface is caused by poor dispersion of filler, resin with better flow properties or screw with stronger mixing ability should be replaced.

3. Molding materials do not meet requirements for use

Content of moisture or other volatile materials in molding raw material is too high, volatile components condense between cavity wall of mold and melt during molding, resulting in poor gloss on the surface of plastic part. Raw materials should be pre-dried.
Raw material or colorant decomposes and changes color, resulting in poor gloss. Materials and colorants with higher temperature resistance should be selected.
Flowability of raw materials is too poor, so that surface of plastic parts is not dense, resulting in poor gloss. It should be replaced with a resin with better flow properties or an appropriate amount of lubricant, processing temperature should be increased.
Raw materials are mixed with foreign materials or incompatible materials. New materials should be used instead.
Raw material particle size is not uniform. Materials with too large a difference in particle size should be screened out.
Crystalline resins have poor gloss due to uneven cooling. Mold temperature and processing temperature should be reasonably controlled. For thick-walled plastic parts, if cooling is insufficient, surface of plastic parts will be fluffed and gloss will be dark. Solution is to remove plastic parts from mold and immediately put it in a cold compression mold immersed in cold water to cool and set.
Proportion of recycled materials in raw materials is too high, which affects uniform plasticization of the melt. Amount should be reduced.

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