Coloring with masterbatch: a dosing and consistency guide
What should you watch for when dosing color masterbatch at the right ratio and consistently, and why does batch-to-batch color variation happen?
Masterbatch is a coloring concentrate with a high pigment load, pelletized with a carrier resin (usually compatible with the base material). It's safer and cleaner to dose than raw pigment powder.
What determines the dosing ratio?
- Typical ratio is 1-4% by weight; it increases for dark/opaque colors and decreases for pastel/transparent shades.
- Carrier resin compatibility: The masterbatch's carrier must be compatible with the base material (PP masterbatch → PP, PE masterbatch → PE); an incompatible carrier can change melt flow (MFI) and mechanical strength.
- Wall thickness and light transmission: The same ratio can look lighter or different in thin-walled or semi-translucent parts; color approval should be done on actual part thickness, not just a plaque sample.
Typical causes of batch-to-batch color variation
- Dosing equipment calibration: Volumetric dosers can drift when pellet size/density changes; gravimetric (weight-based) dosing is more consistent.
- Base material lot changes: Recycled/regrind blends or base resin from a different supplier can give a different base color/opacity even at the same masterbatch ratio.
- Melt temperature and residence time: High temperature or long residence time can shift the color of some pigments (especially organic ones), causing heat-related yellowing or dulling.
- Drying/moisture: A moist carrier or base material can create gas/micro-bubbles in the melt, giving a dull/matte surface appearance that changes color perception.
Control recommendations
- Record the dosing ratio, equipment settings and material lot number for the first color-approved batch, and compare subsequent batches against them.
- For critical (visible/corporate) colors, ΔE measurement with a spectrophotometer is more reliable than visual judgment.
- Before switching masterbatch suppliers, a trial run in the same mold and machine settings is recommended.
Practical note
When a color complaint comes in, check the dosing equipment's calibration first; in the field, the most common cause is dosing drift, not an actual pigment problem.
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