TABLE OF CONTENTS Thermoplastic materials, in general, can be moulded in a wide range of colours. The colour can be provided in a pre-coloured base plastic or by adding solid or liquid colour concentrates at the machine just prior to plasticisation.
Colour concentrates are high pigment content dispersions of colorants in carrier resins. The concentrate supplier matches the desired colour with a blend of colorants. The supplier then compounds a concentration formula, typically 20-60%, in a carrier resin. Additives such as antioxidants, stabilisers and anti-blocking agents are often co-blended at this point. A recommendation is then made as to the amount of concentrate required to blend with the base resin to obtain the desired colour. This is referred to as the let down ratio.
Example: if 45 kg of resin is mixed with 0.45 kg of concentrate the ratio is 100:1
Generally, the lower the ratio (25:1, 30:1) the easier it is to disperse the colour accurately. Factors such as screw L/D ratio and screw speed may also require different percentages of pigment to those originally suggested. The reason for this is the pigments need to be mixed in the screw, the less time it spends in the screw, the less mixing takes place.
The carrier resin is preferably the same as the let down resin for good compatibility. The melt flow of the carrier resin should be high enough that it will mix readily and uniformly throughout the let down resin.
The important quality check for colour concentrates...
