Defect analysis and solution for dull surface of injection molded products

Poor surface gloss of AR glasses injection molded products means that the surface of the plastic parts is dim and lacks gloss, and the transparency of transparent products is reduced. There are many reasons for poor gloss, and other injection molding defects may also cause this problem. The following is a defect analysis of poor surface gloss and corresponding solutions.

1. Mold failure

Since the surface of the plastic part is a reproduction of the mold cavity, if there are defects such as scars, corrosion or micropores on the mold surface, it will directly affect the gloss of the plastic part. In addition, oil, moisture, excessive or improper use of release agent on the mold surface will also cause poor surface gloss. Therefore, the surface of the mold cavity should maintain good finish. It is recommended to polish or chrome-plated, keep it clean, and remove oil and water stains in time. The type and amount of release agent should also be appropriate.

Mold temperature has a significant impact on the surface quality of plastic parts. Different types of plastics show different gloss under different mold temperature conditions. Too high or too low mold temperature will lead to poor gloss. When the mold temperature is too low, the molten material solidifies quickly after contact with the mold, which will reduce the reproducibility of the mold cavity. Properly increasing the mold temperature, especially by passing warm water into the cooling circuit, can speed up heat transfer, reduce the molding cycle, and reduce residual stress. In general, except for polystyrene, ABS and AS, the mold temperature should be controlled above 100 degrees, but it should be noted that too high a mold temperature may also cause the surface to darken.

In addition, factors such as insufficient demoulding slope, sudden changes in cross-section thickness, too thick ribs, too small or sudden changes in gate and runner sections, excessive shearing of the pouring system, turbulent flow of the melt, and poor mold exhaust will affect the surface quality of the plastic part and lead to poor gloss.

2. Improper control of plastic injection molding conditions

Too fast or too slow injection speed, too low injection pressure, too short holding time, insufficient booster pressure, too large cushion, too small nozzle aperture or too low temperature, poor filler dispersion of fiber-reinforced plastics, exposed fillers, non-directional distribution of aluminum foil-like fillers, too low barrel temperature, poor plasticization of the melt, and insufficient feed, etc., may all lead to poor surface gloss of the plastic part. Adjustments should be made accordingly for specific situations.

If dark areas appear near the gate or at the cross-section, the problem can be solved by reducing the injection rate, changing the gate position, expanding the gate area, and adding arc transitions at the 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 poor dispersion of the filler leads to poor gloss, the resin with better fluidity should be replaced or a screw with stronger mixing ability should be used.

3. The molding raw materials do not meet the use requirements

The raw materials that do not meet the use requirements will also lead to poor gloss on the surface of the plastic parts. The main reasons and treatment methods are as follows:

1. Excessive moisture or volatile content: During molding, the volatile components condense between the mold cavity wall and the melt, resulting in poor gloss. The raw materials should be pre-dried.

2. Raw materials or colorants decompose and discolor: Select raw materials and colorants with higher temperature resistance.

3. Poor flow performance: Poor flow performance will lead to loose surface and poor gloss of the plastic parts. Resins with better fluidity should be replaced or an appropriate amount of lubricant should be added, and the processing temperature should be increased.

4. Raw materials mixed with foreign materials or incompatible materials: should be replaced with new materials.
5. Uneven particle size: Screen out raw materials with large particle size differences.
6. Uneven cooling of crystalline resin: Reasonably control the mold temperature and processing temperature. Insufficient cooling of thick-walled plastic parts will cause the surface to become hairy and the gloss to be dark. The plastic parts can be immediately placed in cold water to cool and shape after being taken out of the mold.
7. The proportion of recycled materials reused is too high: The amount of recycled materials used should be reduced to ensure uniform plasticization of the melt.

Through the above analysis and solutions, the surface gloss problem of plastic parts can be effectively improved and the quality of products can be improved.