Quick Answer
Colour variation in injection molding happens when moulded parts do not match the approved colour, or when the shade changes from shot to shot, batch to batch, or across one part. The most common causes are an inconsistent masterbatch dosing ratio, poor mixing or dispersion, changes in the base resin or regrind, too much heat or residence time in the barrel, and differences in part thickness or mould temperature.
Most cases can be fixed by calibrating the dosing unit, using a masterbatch with a compatible carrier and good dispersion, keeping resin and regrind ratios steady, and holding process settings stable. This guide walks through each cause and its fix.
What Does Colour Variation Look Like?
Colour problems in moulded parts usually show up in one of these forms:
- Shot-to-shot variation: parts from the same run look slightly different in shade.
- Batch-to-batch variation: a new lot of resin or masterbatch gives a different colour from the last one.
- Streaks and swirls: lines or patches of lighter or darker colour, often near the gate.
- Specks or spots: undispersed pigment particles visible on the surface.
- Yellowing or darkening: a shift in shade caused by heat damage to the polymer or pigment.
- Uneven colour across one part: thick and thin sections, or glossy and matte areas, look different.
Knowing which type you have narrows down the cause quickly. Streaks point to mixing; batch-to-batch changes point to material; yellowing points to heat.
Common Causes and How to Fix Them
Plastic color inconsistency almost always traces back to one of five areas: the base material, the masterbatch, the machine, the process, or the part and mould design.
| Area | Likely cause | How to fix it |
|---|---|---|
| Base material | Different resin grade or lot, varying base colour or melt flow (MFI), uncontrolled regrind ratio, moisture | Fix the resin grade and supplier, check incoming lots, keep regrind at a steady percentage, dry hygroscopic resins |
| Masterbatch | Wrong or changing let-down ratio, poor pigment dispersion, carrier not compatible with the base resin, low heat stability | Calibrate dosing, use a masterbatch made for your polymer, match carrier MFI, choose heat-stable pigments |
| Machine | Poor mixing in the screw, worn screw or barrel, inaccurate or unstable dosing unit | Use a mixing section or nozzle mixer, maintain screw and barrel, use a gravimetric doser |
| Process | Barrel temperature too high, long residence time, low back pressure, changing cycle time | Lower melt temperature within the resin range, right-size the shot, raise back pressure moderately, keep cycle time steady |
| Part and mould | Uneven wall thickness, gate location, mould temperature differences, surface finish | Balance wall thickness, review gate position, keep mould temperature even, standardise surface texture |
Masterbatch Colour Variation: The Most Common Culprit
Masterbatch colour variation is one of the most frequent reasons parts drift off-shade. Here is what to check.
1. Let-down ratio and dosing accuracy
The let-down ratio (LDR) is the percentage of masterbatch added to the base resin, for example, 2% or 3%. Even small changes in this ratio change the shade. Volumetric dosers can drift as granule size or bulk density changes, so a gravimetric dosing unit gives far more consistent results. Calibrate the doser regularly and check it whenever you change material.
2. Pigment dispersion
If pigments are not fully broken down and spread through the carrier, you get specks, streaks, and weak colour strength. Good dispersion depends on how the masterbatch is manufactured. A well-made masterbatch disperses evenly at the recommended dosage without extra processing effort.
3. Carrier compatibility
The masterbatch carrier should suit the base polymer. A carrier with a very different melt flow from the base resin may not mix evenly, which causes swirls and uneven colour. Always use a masterbatch designed for your polymer, such as PP, PE, ABS or PC.
4. Batch-to-batch consistency
Reliable suppliers check every masterbatch batch against an approved standard using a spectrophotometer. Ask your supplier for colour test data with each lot, so new batches match the last one.
5. Heat stability
Some pigments shift colour or degrade at high processing temperatures. Engineering plastics such as PC, PA or PBT run hot, so they need heat-stable pigments rated above your actual melt temperature.
Step-by-Step Troubleshooting Checklist
Work through these steps in order. Change one thing at a time, so you know what fixed the problem.
- Compare against the approved standard under the same light source, ideally in a light booth.
- Check the materials: confirm the resin grade, lot number, masterbatch batch, and regrind percentage match the approved settings.
- Verify the dosing: weigh the masterbatch output over a set time and compare it with the target let-down ratio.
- Look at mixing: streaks or swirls suggest poor mixing; increase back pressure moderately or add a mixing nozzle.
- Check temperatures and residence time: yellowing or darkening suggests too much heat; reduce melt temperature or residence time.
- Keep the cycle steady: stops, starts, and changing cycle times change how long material sits in the barrel.
- Review the part: if only thick sections or certain areas differ, look at wall thickness, mould temperature and surface finish.
- Talk to your masterbatch supplier: share sample parts and settings; they can check dispersion and colour strength in their lab.
How to Measure Colour and Prevent Variation
Judging colour by eye is unreliable, because lighting and individual perception change what people see. Most moulders measure colour with a spectrophotometer and express the difference from the standard as Delta E (ΔE). A smaller ΔE means a closer match, and each customer sets its own acceptable limit.
Also watch out for metamerism: two parts can match under shop lighting but look different in daylight. Check colour under a standard daylight source such as D65, and under any lighting the customer specifies.
To prevent colour variation in the first place:
- Approve a physical colour standard and a ΔE tolerance with the customer before production
- Use a consistent resin grade and keep regrind at a fixed, recorded percentage
- Use gravimetric dosing and calibrate it on a schedule
- Record and lock process settings for each mould and colour
- Store masterbatch sealed and dry, and use older stock first
- Buy masterbatch from a supplier that tests every batch against your standard
Why Choose Surya Compounds & Masterbatches
Surya Compounds & Masterbatches has manufactured masterbatches and compounds in India since 1986, from its plant in Kundli, Haryana. For injection moulders who need consistent colour, Surya offers:
- Color Masterbatch, White Masterbatch and Black Masterbatch for a wide range of shades
- Polymer-specific grades such as PP Masterbatch, ABS Masterbatch and PC Masterbatch, so the carrier suits your resin
- In-house R&D support for colour matching and troubleshooting
Frequently Asked Questions
1. What causes colour variation in injection moulding?
The main causes are inconsistent masterbatch dosing, poor mixing or dispersion, changes in resin or regrind, excessive heat or residence time, and differences in part thickness or mould temperature.
2. How do I fix streaks of colour in moulded parts?
Streaks usually mean poor mixing. Increase back pressure moderately, add a mixing nozzle, and check that the masterbatch carrier is compatible with your base resin.
3. Why does colour change between masterbatch batches?
Small differences in pigment content or dispersion can shift the shade. Ask your supplier to test each batch against the approved standard and share the colour data.
4. Does regrind affect colour?
Yes. The regrind has been heated before, so it can be slightly darker or yellower. Keep the regrind percentage fixed and recorded to avoid shade changes.
5. What is Delta E in colour matching?
Delta E (ΔE) is a number that shows how different a sample colour is from the standard. The lower the ΔE, the closer the match.
6. Can high barrel temperature change plastic colour?
Yes. Too much heat or a long residence time can degrade the polymer or pigment, causing yellowing, darkening or a dull shade.
7. What dosing method gives the most consistent colour?
Gravimetric dosing, which weighs the masterbatch, is generally more accurate and stable than volumetric dosing.
Get Consistent Colour in Every Shot
Struggling with plastic colour inconsistency on your moulding line? Share your polymer, part sample, and target shade with the Surya Compounds & Masterbatches team for a colour match and a masterbatch recommendation.
