Short answer
Dosing error comes from viscosity swings, air in the line, abrasive pigments wearing soft parts, and very small volumetric doses. Check dosing by weight at working temperature, bleed air, use hardened wear parts for abrasive colours, and ask for a dosing colour engineered for your machine type.
Likely causes
Viscosity swings with temperature
Cold colorant delivers less per stroke.
Air in the line
Compressible air gives erratic shot weights.
Abrasive pigments
Titanium dioxide and some inorganic pigments wear soft metal parts.
Resolution of the dose
A very concentrated colorant at a fraction of a part per 100 polyol is hard to meter accurately.
How to diagnose it
- Weigh a series of shots at working temperature.
- Inspect valves, seals and filters for wear and blockage.
How to fix it
- Temperature-control the day tank and lines.
- Bleed air and fit hardened or ceramic wear parts for abrasive colours.
- Move to a lower-concentration dosing colour so each dose is a larger, more accurate volume.
How to stop it coming back
- Scheduled calibration by weight.
- Share the machine make and dosing range with your supplier so viscosity and concentration are engineered to suit.
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.
