Plastic molded products can be colored by painting after molding or by using colored molding materials themselves. In order to omit the painting process when molding a certain quantity of products in bulk, it is common practice to use colored materials.
There are four methods for coloring molded products, as described below.
(1) Powder coloring “dry coloring”: Powder coloring agents are mixed into the material pellets.
(2) Liquid coloring: Liquid coloring agent is dropped into the screw cylinder of an injection molding machine and mixed into the material.
(3) Master batch coloring: Highly concentrated coloring pellets called ・・・・・・・・・ master batch are mixed into the main material pellets.
(4) Colored pellets ・・・・・・・・・・ pre-colored pellets are used.
The above coloring methods are selected according to the molding quantity and coloring quality requirements.
Colored pellets are the best in terms of coloring quality, but they are costly.
Powder and masterbatch methods are widely used. These methods can be performed at the molding site as autocoloring.
However, they are sometimes plagued by uneven coloring.
Color irregularities may occur in individual molded products, or they may be caused by a change in the color tone of the molded product during the molding process, resulting in a different color than the target color (color shift), even if there are no irregularities in individual molded products. Color shift is one of the molding defects.
The key to avoiding color irregularities in self-coloring is to mix thoroughly in a screw-type injection molding machine.
Use heat-resistant colorants, and do not allow the molding material to remain in the cylinder for long periods of time.
Prevent thermal decomposition of the colorant without allowing the molding material to remain in the cylinder for a long period of time.
Colorants can be pigments or dyes. Pigments are colored compounds that are insoluble in water or organic solvents. Inorganic pigments are resistant to heat, weather, solvents, and chemicals, and there is no risk of pigment migration. Organic pigments are better for dispersion and clarity.
When used with olefin resins, pigment migration may occur, and dyes are the best in terms of dispersion and vividness. However, dyes are weakest in terms of color durability as described above, and there are concerns about migration, so they are rarely used except for styrenic, acrylic, and thermosetting resins.
Dyes are used for coloring fabrics and paper and are organic compounds soluble in water and solvents.
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