Short answer
In most plants shade drift is a process problem before it is a pigment problem. The usual causes are colorant dosing that is not held constant (pump calibration, temperature-driven viscosity change, settled colorant in the day tank), changes in polyol or catalyst, and different reaction temperatures. Measure first, then change one variable at a time.
Likely causes
Dosing ratio drifting
Worn seals or check valves, an uncalibrated pump, or colorant that is thicker on a cold morning so less is delivered per stroke.
Colorant not homogeneous at the day tank
Pigment settles in drums and tanks left unagitated, so the top of the drum is weaker than the bottom.
Base chemistry changed
A new polyol lot or source, or a change in catalyst or amine package, shifts both reaction temperature and background colour.
Exotherm differences
Higher or lower core temperature changes how the foam looks, especially in pastel shades on large slabstock blocks.
Judging method
Comparing skin to cut surface, fresh foam to cooled foam, or viewing under different light sources.
How to diagnose it
- Dose ten consecutive shots by weight and compare to the set point.
- Cut samples at the same position and the same age, and compare under one light source.
- Check colorant from the top and the bottom of the drum with a drawdown against the approved standard.
- Lay polyol lot records alongside the dates the drift started.
How to fix it
- Agitate or recirculate the colorant before and during use.
- Calibrate dosing by weight at working temperature and hold the day tank at a constant temperature.
- Agree a ΔE tolerance and a physical master standard, and measure under daylight (D65) plus one other illuminant.
- Change one variable at a time and record the result.
How to stop it coming back
- Keep an approved standard plaque cast in your own PU system.
- Log dosing as parts per 100 parts polyol on every batch.
- Ask your supplier for batch values of tinting strength and viscosity on the certificate of analysis.
Where this problem appears most
Send us your batch number, PU system and a photo of the defect and our technical team will help you trace the cause. Ask a technical question.
