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Plastic parts discoloration and uneven color, how to solve?
Nov 03, 2017

1, colorant quality does not meet the requirements

The performance of the colorant is directly related to the color quality of the molded part after molding. If the colorant dispersion, thermal stability and particle morphology can not meet the process requirements, it is impossible to produce good color products.


Some colorants in the form of aluminum foil and flake, mixed into the melt after forming a directional arrangement, resulting in uneven color castings surface.


Some colorants with dry mixing method, agitated with the raw materials attached to the surface of the pellets, poor dispersion into the barrel, resulting in uneven color.


If the colorant or additive has poor thermal stability, it can easily be thermally decomposed in the barrel, causing the part to discolor. In addition, the colorant is very easy to float in the air, deposited in the hopper and other parts, polluting injection molding machines and molds, causing the uneven surface color of plastic parts.


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Therefore, the selection of colorants should be carefully controlled according to process conditions and color requirements of plastic parts, especially for the heat-resistant temperature, dispersion characteristics and other more important indicators must meet the process requirements, the best colorant wet mixing method.

 

If injection molding equipment and molds are contaminated with colorants, the hoppers, cartridges and mold cavities should be thoroughly cleaned.


2, raw materials do not meet the requirements


If the raw material volatile content is too high, mixed with different materials or poor drying; fiber-reinforced raw materials after the uneven distribution of fiber filler, plastic accumulation exposed or after contact with the solvent on the surface of the resin resin loss, fiber bare; crystallization of the resin Poor, affecting the transparency of plastic parts, plastic parts will result in uneven color.


In addition, high impact polystyrene and ABS and other raw materials after forming a larger internal stress, but also produce stress discoloration.


For the above fault,

First, remove foreign matter in raw materials, purification of raw materials, pre-drying the raw materials to reduce moisture in the raw materials;


Second, by adjusting the process parameters, to improve the distribution of resin fibers, try to reduce the amount of lubricant and release agent;


The third is to change the crystallization of better performance of the resin or plastic parts by controlling the cooling conditions to improve the melt properties of the crystal;


Fourth, the raw material should be used to reduce the stress of forming the process conditions for the easy formation of internal stress.


3, Molding conditions are not reasonable


Uneven color often reflect the phenomenon of different, its causes are also different. If the feed inlet near the weld or uneven color, usually due to uneven distribution of colorant or the nature of the colorant does not meet the requirements caused by the use.


If the entire plastic color or uneven color, often associated with the molding process conditions, when the cylinder temperature is too high, the high temperature melt in the barrel easy overheating decomposition, the plastic parts discoloration. If the nozzle temperature is too high, the melt coking in the nozzle accumulation, can cause uneven surface color of plastic parts.

 

In addition, the screw speed, injection back pressure and injection pressure is too high, injection and dwell time is too long, the injection speed is too fast, poor plasticization, barrel dead angle and the amount of lubricant too much, will lead to the plastic surface color is not all.


In order to prevent the melt in the high temperature barrel residence time too long produce overheating decomposition, the injection amount should not exceed injection molding machine injection capacity of two-thirds.

 

When the barrel or nozzle coking melt accumulation, the barrel should be thoroughly cleaned, remove and clean the nozzle, check the nozzle tip and sprue sleeve alignment, and appropriate to reduce the nozzle temperature.


For screw speed, back pressure, injection pressure, injection and dwell time adjustment process parameters, according to the actual situation, in accordance with the principle of one-by-one fine-tuning.


4, mold problems


If the mold oil, mold release agent or pin and hole friction dirt mixed into the melt, mold exhaust and mold cooling uneven, will lead to discoloration plastic parts. Therefore, before injection molding should ensure that the cavity clean.


In order to reduce the adverse effects of exhaust gas, mold clamping force may be appropriately reduced, or the gate may be repositioned and the exhaust hole may be disposed at the final filling mold.


As the mold temperature for the solidification of the melt greater impact on the crystallinity, the mold should be uniform cooling. For example, when forming a crystalline plastic such as polyamide, if the mold temperature is low, the melting of the melt is slow and the surface of the plastic is transparent; if the mold temperature is high, the melt crystallizes rapidly and the plastic becomes translucent or milky white . In this regard, by adjusting the mold and melt temperature to control the surface color of plastic parts.


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