Causes of colour variation in injection moulding

When a molded plastic part shows uneven colour or gloss across the same surface, the defect is known as colour variation. Uneven distribution of colourant or its alignment along the melt flow can cause it. Heat damage and severe deformation during demolding can also leave visible differences.

Changes in resin, colour powder or masterbatch, uncontrolled regrind, and changes to melt temperature, back pressure, cushion, injection speed or screw speed can all affect colour. Different machines, mixing and drying times, mould sets, sample ageing and stock colour are further sources of variation. The following checks can help:

1. Resin grade or batch: Different grades of the same resin can have different base colours. For one order, use resin from the same supplier and batch where possible.

2. Colourant batch: Powder and masterbatch can vary slightly between batches. Use one batch for a production run and avoid buying many small batches when consistency matters.

3. Melt temperature: A large temperature change can cause yellowing or degradation in sensitive resins. Set an appropriate temperature and keep it stable.

4. Regrind: Keep the number of reuse cycles and the proportion of sprue or runner material consistent.

5. Residual material: Excess material left in the barrel may overheat and degrade. Reduce the residue.

6. Back pressure and screw speed: Shear affects how powder and masterbatch disperse. Fix these process settings and avoid arbitrary changes.

7. Machine size: Machines with different plasticising capacities can produce different results. Use the same machine or model where possible.

8. Mixing: Keep preparation time and the amount of dispersing aid consistent.

9. Drying: Control drying time; do not extend it unnecessarily or vary it between batches.

10. Colourant distribution: If using powder, first mix the resin with white mineral oil, then add the powder so that it coats the pellets evenly. Control the masterbatch density and addition ratio.