Uneven color analysis and solution for injection molded parts

1. Colorant quality does not meet the use requirements

The performance of the coloring agent is directly related to the color quality of plastic parts after molding. If the colorant dispersion performance, thermal stability, and particle form can not meet the process requirements, it is impossible to produce products with good color.

Some colorants are in the form of aluminum foil and flakes, which will form a directional arrangement after mixing into the melt, resulting in uneven surface color of plastic parts.

Some colorants with dry mixing method, and the raw material stirring adhered to the surface of the material particles, into the barrel after the dispersion is not good, resulting in uneven color.

If the colorant or additive has poor thermal stability, it will be easily decomposed by heat in the barrel, resulting in discoloration of plastic parts. In addition, the coloring agent is easy to float in the air, deposited in the hopper and other parts, polluting the injection molding machine and mold, causing uneven color on the surface of the plastic parts.

Therefore, the selection of coloring agent should be carefully screened against the process conditions and color requirements of plastic parts, especially for heat-resistant temperature, dispersion characteristics, and other more important indicators that must meet the process requirements, coloring agent is best to use the wet mixing method.

If the injection molding equipment and molds are contaminated by colorants, should thoroughly clean up the hopper, barrel, and mold cavity.
 

2. Raw materials do not meet the requirements of use

If the volatile content of raw materials is too high, mixed with foreign materials or poor drying; fiber-reinforced raw materials after molding fiber filler distribution are uneven, the accumulation of exposed or plastic surface and solvent contact with the resin loss, fiber exposed; resin crystallization performance is too poor, affecting the transparency of plastic parts, will lead to uneven surface color of plastic parts.

In addition, high impact polystyrene and ABS, and other raw materials after molding the internal stress are large, will also produce stress discoloration.

For the above failures.

Removal of foreign matter from raw materials, purification of raw materials, and pre-drying of raw materials to reduce the moisture in raw materials.

Improving the distribution state of the fibers in the resin by adjusting the process parameters and sending as much as possible to reduce the amount of lubricant and release agent.

change the resin with better crystallization performance or improve the crystallization performance of the melt by controlling the cooling conditions of the plastic part.

For raw materials that are prone to internal stress in molding should be used to reduce the process conditions of internal stress in molding.
 

3. unreasonable molding conditions

Uneven color is often reflected by different phenomena, and its causes are different. If the color is uneven near the feed opening or the fusion part, it is generally due to the uneven distribution of the colorant or the nature of the colorant does not meet the requirements of use.

If the whole plastic part is discolored or unevenly colored, it is often related to the molding process conditions. When the barrel temperature is too high, the high-temperature molten material in the barrel is easy to overheat and decompose, making the plastic part discolored. If the temperature at the nozzle is too high, the melt will be coked at the nozzle and remain, which will also cause the uneven color on the surface of the plastic parts.

In addition, screw speed, injection back pressure, and injection pressure are too high, injection and holding time is too long, injection speed is too fast, poor plasticization, dead space in the barrel, and too much lubricant will lead to uneven surface color of plastic parts.

To prevent the molten material from overheating and decomposition in the high-temperature barrel for too long, the injection volume should not exceed two-thirds of the injection capacity of the injection molding machine.

When there is scorched melt accumulation at the barrel or nozzle, clean the barrel thoroughly, remove and clean the nozzle, check whether the nozzle tip is aligned with the sprue sleeve, and lower the nozzle temperature appropriately.

For the adjustment of process parameters such as screw speed, back pressure, injection pressure, injection, and holding time, fine adjustment can be made according to the actual situation and according to the principle of item-by-item adjustment.
 

4. Mold problems

If the oil in the mold, mold release agent, or dirt from the friction of the top pin and pinhole are mixed into the melt, the poor exhaust of the mold and uneven cooling of the mold will lead to discoloration of the surface of the plastic parts. Therefore, the mold cavity should be ensured to be clean before injection.

To reduce the effect of poor exhaust, reduce the closing force appropriately, or reposition the gate and set the exhaust hole at the last mold filling.

Since the mold temperature has a large influence on the crystallinity of the melt when it cures, the mold should be cooled evenly. For example, when molding crystalline plastics such as polyamide, if the mold temperature is low, the melt will crystallize slowly and the surface of the plastic part will be transparent; if the mold temperature is high, the melt will crystallize faster and the plastic part will become translucent or milky white. In this regard, the mold and melt temperature can be adjusted to control the surface color of the plastic parts.