Colouring of natural materials is achieved by the use of granular masterbatch, liquid colour concentrate or powdered dye or pigment. To achieve a consistent colour it is essential that the concentrated colour is used in the correct proportions and to ensure that the mixing within the cylinder of the injection moulding machine is effective. It is usually recommended that back pressure be applied to the melt as previously discussed. This promotes more even dispersion of the colorant in the material.
Additives
Incorporation of other additives, to improve flow or to overcome mould release problems, is sometimes necessary and it is usual to incorporate additional lubricant such as zinc stearate. On occasions it is necessary to produce parts with a nonstandard filler or reinforcement content, and this can be achieved by blending the appropriate proportions of unfilled material with a more highly filled grade. For example, 60% glass fibrereinforced nylon can be diluted with unfilled nylon of the same type to give lower levels of reinforcement. However, just as with colouring of materials, care must be taken to ensure uniform mixing of the correct proportions of the constituents.
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