Uneven color means that color of surface of injection molded part is not uniform, with different shades and hues, including mixed colors, discoloration, color difference, etc. Uneven color is a common defect in injection molding. It is not uncommon for injection molded parts to be scrapped in batches due to color differences of matching parts. There are many factors affecting uneven color, including raw material resin, masterbatch, mixing of masterbatch with raw materials, injection molding process, injection molding machine, mold, etc. Following is a brief analysis of causes and improvements of uneven color.

 

(1) Volatile content in raw material is too high, mixed with foreign materials or poor drying will affect material processing performance.
(2) After fiber-reinforced raw materials are molded, fiber filler is unevenly distributed, and fibers on the surface of plastic parts are exposed, affecting appearance of product.
(3) Crystallization performance of resin is too poor, which affects transparency of plastic parts and causes uneven color on the surface of plastic parts.
(4) After materials such as high-impact polystyrene and ABS are molded, internal stress is large, which will also cause stress discoloration.
(5) Check thermal stability of raw material resin. It is recommended to replace material with poor thermal stability.
(6) Strengthen inspection of raw materials entering warehouse to ensure that performance of raw materials meets requirements.

(1) Performance of colorant is directly related to color quality of plastic parts after molding. If dispersion performance, thermal stability and particle shape of colorant cannot meet process requirements, it is impossible to produce products with good color.
(2) Some colorants are in the form of thin flakes. After being mixed into melt and molded, they will form a directional arrangement, resulting in uneven color on the surface of plastic parts.
(3) Colorant is mixed with raw material by dry mixing and adheres to the surface of pellets. After entering barrel, dispersion is poor, resulting in uneven color. As shown in Figure 3-21

 

Figure 3-21 Uneven dispersion of masterbatch
(4) If thermal stability of colorant or additive is poor, it is easy to be decomposed by heat in barrel, causing discoloration of plastic part.
(5) Colorant is easy to float in air, deposit in hopper and other parts, contaminate injection molding machine and mold, and cause uneven color on the surface of plastic part. If injection molding equipment mold is contaminated by colorant, hopper, barrel and mold cavity should be thoroughly cleaned.
(6) When selecting colorant, it should be screened according to process conditions and color requirements of plastic part, especially for important indicators such as heat resistance temperature and dispersion characteristics, which must meet process requirements.
(7) Most injection molding factories do not produce plastic masterbatches or masterbatches themselves. Control of plastic masterbatches or masterbatches should be strengthened in production management and raw material inspection.
(8) Before masterbatch enters factory and goes into mass production, a random color test should be conducted. Masterbatch should be compared with color plate provided by customer and with masterbatch used last time and this time.
(9) When comparing color difference, it is best to use a colorimeter for testing. If color difference is within allowable range or visual color difference is not large, it can be judged as qualified.
(10) Many companies now use masterbatch machines to add masterbatch, which provides great help for color difference control. Following issues should be noted when using masterbatch machine:
a) Determine amount of masterbatch to be added through experiments, then adjust speed of masterbatch machine screw to match plasticizing time to ensure that masterbatch is added when plasticizing time is completed.
b) When using masterbatch machine, it should be noted that because masterbatch machine has a small outlet, after a period of use, raw material powder particles accumulated in masterbatch machine screw may cause inaccurate feeding or even stop during production. Therefore, masterbatch machine screw needs to be cleaned regularly.

(1) In production, it is often encountered that barrel temperature changes drastically due to damage of a heating coil or loss of control of heating control part, resulting in color difference due to poor plasticization or material decomposition. Method of judging color difference caused by this kind of reason is: Generally, color difference caused by damage and failure of heating coil is accompanied by uneven plasticization, while loss of control of heating control part is often accompanied by product gas spots, severe discoloration, and even coking. Therefore, heating part should be checked frequently during production, replaced and repaired in time when heating part is damaged or out of control to reduce this kind of color difference problem.
(2) When there is coked melt accumulation in barrel or nozzle, barrel and nozzle should be thoroughly cleaned.

(1) When injection molding process parameters need to be adjusted for reasons other than color difference, try not to change plasticization temperature, back pressure, injection molding cycle and masterbatch addition amount. If adjustment is required, observe impact of process parameter change on color. If color difference is found, it should be adjusted back in time.
(2) Avoid using high injection speed, high back pressure and other injection molding processes that cause strong shearing effects as much as possible to prevent color differences caused by thermal decomposition and other factors.
(3) Reasonably set temperature of each heating section of barrel, especially heating temperature of nozzle and section close to nozzle.
(4) Poor plasticization, that is, melt cannot be completely and evenly melted, will also cause uneven product color. As shown in Figure 3-22

 

Figure 3-22 Uneven color

(1) Mold release agent in mold and dirt caused by friction between ejector and ejector hole mixed into melt will cause surface of plastic part to discolor. Mold cavity should be kept clean before injection molding.
(2) Mold venting is poor. Clamping force can be appropriately reduced, gate can be repositioned, and venting hole can be set at last filling point.
(3) Mold temperature has a great influence on crystallinity of melt during cooling. Mold should be cooled evenly, especially when molding crystalline plastics such as polyamide. If mold temperature is low, melt crystallizes slowly and surface of plastic part is transparent; if mold temperature is high, melt crystallizes quickly and plastic part becomes translucent or milky white. In this regard, surface color of plastic part can be controlled by adjusting mold and melt temperature.