One of the many advantages of plastics is the ease with which they can be coloured. Whether for cosmetic or functional purposes, it is possible to match any colour to achieve the desired result in a finished part. There are a range of possibilities when it comes to colouring the material depending on the stage that the colour is added. Options include compounding, where the base material is specifically coloured in a controlled process prior to moulding, and masterbatch, concentrated coloured pellets or liquid that are mixed with the base material as it melts during the injection moulding process. Both methods can achieve great results, but designers should take into consideration that the same colour may appear very different depending on the surface finish and wall thickness of the part.
Compounding provides the greatest level of consistency for a chosen colour. To maintain the purity of the material and maximise the performance of structural parts, the pigments are polymerised with the base polymer. As a result, pellets arrive coloured, requiring no premixing and measuring of material.
Widely used Pantone® and RAL number compounds are readily available or alternatively a specific colour can be matched. Where a custom colour is required, the mix is carefully controlled to ensure repeatability between runs. Compounding also helps to achieve uniformity in base polymers that are strongly self-coloured or don’t mix well with pigments.
In small quantities, compounded material will increase a part’s unit price as it introduces an additional process. However, this becomes more economical as volumes increase.
Masterbatches are most commonly produced as concentrated pigments held in a solid carrier resin and cut into granules, but liquid masterbatches are also available. The granules containing the pigment melt and mix with the plastic as it moves through the barrel of the injection moulding machine before the cavity is filled.
Like the previous method discussed, colours can be selected from a range, or a specific match chosen. Unlike compounding, colour is measured out by the plastic moulding processor, requiring the weighing and mixing of masterbatch and base polymer at a specific ratio to control the final colour.
Masterbatches can be universal, where the same product is used with a broad range of plastics, or polymer-specific so as not to adversely affect the physical properties of the material. They can therefore be adapted for use with commodity and most engineering plastics, as well as transparent polymers.
Masterbatches are not only limited to a single colour. Combinations of colours, metallic and sparkle effects are all possible. Paul Norman Railways’ sleepers use a combination of two different masterbatch colours which deliberately do not fully mix, creating a marbling effect to replicate a wooden railway sleeper.
To enhance or add desirable properties to a moulded part, additives can be introduced prior to or during the moulding process and are as straightforward to incorporate as colour. Quantity is controlled in the same way as a colour masterbatch with the additive measured and included at the required ratio per weight of base material. These can be added independently or incorporated as part of the colour masterbatch granules. Options available include flame retardant, UV resistance, anti-static, anti-sink and even fragrances.