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Colour Batch Consistency in Footwear: Why Batches Don't Match and How to Fix It
Colour batch consistency in footwear depends on how well you control five inputs: pigment lot, dose, mixing, mould temperature and sole density. Change any one of them and the same shade comes out different.
A QC manager sees this as a pour on Tuesday that no longer matches Monday's approved sample. This guide covers what causes batch-to-batch colour variation, how to measure it, and a step-by-step method to fix it and reduce colour rejection.
What Causes Batch-to-Batch Colour Variation in Footwear?
Something in the process changed. The job is to find out what. Look at these places first.
Pigment and raw materials. Paste strength can differ between lots. Pastes also settle in the drum, so paste taken from the bottom of an unstirred drum carries more pigment than paste from the top. Polyol and isocyanate lots have their own base tone, and that shifts the final shade too.
Dosing. Scoops and estimated pours cause most shop-floor variation. Say a 5 kg polyol shot takes 100 g of paste. A 5 g weighing error changes the pigment load by 5% on that pour.
Mixing. A short mix leaves pigment unevenly spread, which shows up as streaks or a paler skin. Mixing time and temperature need to be identical on every pour.
Mould and sole density. Mould temperature changes surface gloss, and gloss changes how a colour looks. Density and thickness have the same effect.
Process drift. An operator raises the mould temperature to fix a demoulding problem, and nobody sets it back. Two weeks later the shade has moved and nobody knows why.
What Does Batch Colour Mismatch Cost a Footwear Factory?
Cured PU cannot be recoloured. A mismatched sole is scrapped, painted over or sold as a lower grade.
The losses add up:
- Material, labour and mould time are spent twice on every rejected pair.
- A rerun pushes the dispatch date back.
- Cartons end up with left and right soles from different runs.
- Buyers spot the shift at inspection or on the shelf, and the next order goes elsewhere.
Our guide to cutting colour rejection covers this in more detail.
How Do You Measure Colour Batch Consistency in Footwear?
Judging colour by eye stops working the moment two people disagree. Use a standard, a number and a fixed light.
- Colour standard. Keep the approved sample, dated and stored away from light. Your buyer should hold a matching copy.
- Delta E. This is a number for the distance between two colours. Zero means identical. Below about 1, most people cannot see a difference. Buyers set their own limit, so get yours in writing.
- Spectrophotometer. This instrument reads colour as numbers, so the result does not depend on who is looking. A colour-matching lab uses it to compare every sample against the standard.
- Fixed lighting. Two colours can match under a tube light and differ in daylight. This is called metamerism. Check under the same light source every time.
- QC checkpoints. Test each incoming pigment lot, the first pour of every shift, one pour mid-run and the last pour.
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How Do You Fix Inconsistent Colour Batches? Step by Step
- Find the source. Lay a good batch beside a bad one. Compare pigment lot, dose, polyol lot, mixing time, mould temperature and sole density from your records. Change one variable per trial. If you change three, you will not know which one fixed it.
- Standardise pigment and dosing. Buy from one source. Record the lot number on every pour. Stir the paste before each use and weigh every dose.
- Control the process. Set the mould temperature and mixing time, then log both every shift.
- Re-match to the standard. Measure against the approved sample and change the recipe one step at a time. Lock it once it holds across several pours on different days.
How to Prevent Colour Mismatch in Footwear
- Write a recipe card for every shade: dose, mixing time, mould temperature and sole density.
- Route recipe changes through QC. Operators should not adjust a recipe on their own.
- Calibrate scales and metering units on a fixed schedule.
- Cast a test piece from every new pigment lot and compare it with the last lot before it reaches the floor.
- Keep a retained sample from every batch. When a complaint arrives, you have something to compare it against.
Does Pigment Quality Affect Colour Batch Consistency?
Yes. When pigment strength changes between lots, operators adjust the dose to compensate, and that is where drift begins. Poor dispersion does the same thing, because specks and pale patches vary from pour to pour.
A pigment made to a fixed strength lets your recipe stay the same. Our guide on how to choose the right pigment lists what to check before you approve one.
How Mittal Chemplast Helps with Colour Batch Consistency in Footwear
Mittal Chemplast has worked on footwear colour since 1982. The Ansh range of PU colourants is made for footwear components, including soles.
- Pigment paste supplied to hold its shade from lot to lot, so your recipe card keeps working
- In-house colour matching and shade cards, with custom shades matched to your sample
- Technical support when a batch drifts and you need to trace the cause
If your rejections trace back to shade, talk to us about colour matching and batch consistency. Bring one good sample and one bad sample, and we can work backwards from there.
FAQs
Why does my sole colour change between batches?
Sole colour changes between batches when the pigment lot, dose, mixing, mould temperature or base PU system differs between runs - inconsistent pigment and dosing errors are the most common causes.
How do I fix colour mismatch in footwear production?
Fix colour mismatch by comparing records from a good and a bad batch, changing one variable per trial, standardising your pigment and dosing, then re-matching to the approved shade standard.
Does pigment quality affect batch consistency?
Yes. Pigment quality directly affects batch consistency, because pigment that varies in colour strength or dispersion between lots shifts the shade even when your process stays the same.
What is Delta E in colour matching?
Delta E is a number that measures the difference between two colours in footwear colour matching - the lower the Delta E, the closer the match to your target shade.
How can I prevent colour variation in footwear?
Prevent colour variation by using one consistent pigment source, weighing every dose, keeping a recipe card per shade, logging process settings and testing every new pigment lot against the last.
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