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, micropores on the surface of the mold, it will be reflected on the surface of the plastic part to cause gloss poor.
If there is oily, moisture on the surface of the cavity, too much of the mold release agent or improper selection, it will also cause the surface of the plastic parts to become dark.
Therefore, the cavity surface of the mold should have a good finish, and it is best to use polishing or chrome plating on the surface. The surface of the cavity must be kept clean and oil and water stains must be removed in time. The type and dosage of mold release agent should be appropriate.
The mold temperature also has a great impact on the surface quality of plastic parts. Usually, different types of plastics have a large difference in surface gloss under different mold temperature conditions.
Too high or too low mold temperature will lead to poor gloss. If the mold temperature is too low, the melt solidifies 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 the method of injecting warm water into the mold cooling circuit to quickly transfer heat in the cavity to avoid extending the molding cycle.
This method can also reduce residual stress during molding. Generally speaking, except for polystyrene, ABS, and AS, the mold temperature can be controlled above 100 degrees. However, it should be noted that if the mold temperature is too high, it will also cause darkening of the surface of the plastic parts.
In addition, the mold release slope is too small, the cross-sectional thickness changes suddenly, the rib strips are too thick, the cross-section of the gate and runners are too small or suddenly change, the casting system has too much shearing effect, the melt flows in a turbulent state, and the mold failures such as poor mold exhaust will affect the surface quality of the plastic parts, resulting in poor surface gloss.
Improper molding conditions control
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 exposed or aluminum foil-like filler has no directional distribution, the barrel temperature is too low, the melt plasticization and insufficient supply will all lead to poor gloss on the surface of the plastic parts. In this regard, adjustments should be made to the specific situation.
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 arc transitions 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 surface gloss is poor due to poor dispersion performance of the filler, you should replace the resin with good flow performance or the screw with strong kneading ability.
The molding materials do not meet the usage requirements
If the raw materials do not meet the usage requirements, it will also lead to poor gloss on the surface of the plastic parts. The causes and treatment methods are as follows:
The moisture or other volatile content 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 gloss on the surface of the plastic parts. The raw materials should be pre-dried.
The decomposition and discoloration of raw materials or colorants lead to poor gloss. Raw materials and colorants with higher temperature resistance should be selected.
The flow performance of the raw materials is too poor, which makes the surface of the plastic parts not dense, resulting in poor gloss. Resin with better flow properties or additional amount of lubricant should be used and processing temperature should be increased.
The raw materials are mixed with heterogeneous or incompatible raw materials. New materials should be replaced. The raw material particle size is uneven. Raw materials with too large particle size differences should be screened out.
The crystalline resin has poor gloss due to uneven cooling. The mold temperature and processing temperature should be reasonably controlled. For thick-walled plastic parts, if they are not cooled enough, the surface of the plastic parts will become whispered and the luster will be darker.
The solution is to take out the plastic parts from the mold and put them in a cold press mold immersed in cold water to cool and set.
The reuse ratio of recycled materials in the raw materials is too high, which affects the uniform plasticization of the melt. The amount should be reduced.
