Controlling Centre-to-Selvedge Colour Variation in Sheet Dyeing of Denim
In denim manufacturing, colour variation is one of the most visible and commercially sensitive problems. A small shade difference that may look harmless on dyed yarn can become very obvious after weaving, garment washing and finishing.
Among the different types of shade variation, one important problem in sheet dyeing or slasher dyeing is centre-to-selvedge colour variation. This happens when the yarns in the centre of the warp sheet dye slightly differently from the yarns near the two selvedges.
After weaving, this may show as darker or lighter bands running lengthwise in the denim fabric. In garment form, it may further become visible as panel-to-panel shade difference, side shading, streakiness or inconsistent washing response.
The problem is not caused by one factor alone. In sheet dyeing, centre-to-selvedge variation is usually born at the intersection of three controls: liquor pick-up, warp-sheet mechanics and indigo bath chemistry.
Central idea: In sheet dyeing, shade is not controlled only by the dye recipe. Shade is controlled by the complete process — yarn preparation, liquor pick-up, nip pressure, tension, oxidation, washing and monitoring.
Table of Contents
- What is centre-to-selvedge colour variation?
- Why sheet dyeing is sensitive to this problem
- Main causes of centre-to-selvedge shade variation
- How to control centre-to-selvedge variation
- Practical troubleshooting table
- A practical control plan for mills
- Conclusion
- General disclaimer
What is centre-to-selvedge colour variation?
In sheet dyeing, warp yarns are spread side-by-side in open sheet form and pass through dye boxes, squeeze rollers, oxidation zones and sizing units. Ideally, every yarn from the left selvedge to the right selvedge should receive the same dyeing treatment.
In practice, the centre yarns and edge yarns may not behave exactly alike. Centre-to-selvedge variation means that the yarns near the centre of the sheet show a different depth, tone or brightness compared with the yarns near the selvedges.
The difference may be visible immediately after dyeing, but sometimes it becomes clearer only after weaving, finishing or garment washing. This is especially important in denim because washing partly removes and modifies the indigo surface, making earlier shade differences more visible.
Denim is a highly visual fabric. The indigo shade is not only a colour; it is part of the identity of the fabric. Buyers expect a controlled blue, black, grey, sulphur-bottom or topping shade. Any side-to-side difference reduces the acceptability of the fabric.
Why sheet dyeing is sensitive to this problem
In rope dyeing, warp yarns are gathered into ropes, dyed, oxidised and later opened during long-chain beaming. Because the yarns are rearranged during subsequent processing, some shade variation may get distributed.
In sheet dyeing, however, yarns remain in sheet form. The position of the yarn across the width is more directly related to its final position in the fabric. This makes sheet dyeing efficient and compact, but it also makes it more sensitive to width-wise variation.
If the left edge, centre and right edge do not receive the same liquor pick-up, pressure, tension, immersion or oxidation, the variation can directly appear in the woven denim. In simple words, sheet dyeing gives less room to hide width-wise mistakes.
Main causes of centre-to-selvedge shade variation
1. Uneven nip pressure across the width
The padding or squeezing system is one of the most important areas to examine. When yarns come out of the dye box, the squeeze rollers control how much dye liquor remains on the yarn. If nip pressure is not uniform across the full width, liquor pick-up will also not be uniform.
If the centre pressure is higher, the centre yarns may carry less liquor. If the edge pressure is higher, the selvedge yarns may carry less liquor. In both cases, the shade can change across the width.
This may happen because of roller deflection, roller hardness variation, poor roller grinding, incorrect loading, worn bearings, improper alignment or uneven pneumatic or hydraulic pressure. The problem may become more serious on wider machines because roller deflection becomes more difficult to control.
The first rule of centre-to-selvedge control is therefore simple: do not blame the dye before checking the padder or squeeze roller.
2. Variation in liquor pick-up
In indigo sheet dyeing, liquor pick-up determines how much reduced indigo solution is carried by the yarn before oxidation. Any variation in pick-up becomes a variation in available dye.
Liquor pick-up can vary due to nip pressure, yarn absorbency, yarn tension, bath level, viscosity, wetting, foam, contamination or uneven yarn sheet density. Even if the dye bath recipe is correct, poor pick-up control can still produce shade variation.
Liquor pick-up may be expressed as:
\[ \text{Liquor Pick-up \%} = \frac{\text{Wet Weight} - \text{Dry Weight}}{\text{Dry Weight}} \times 100 \]
A practical mill should not depend only on visual judgement. Width-wise pick-up should be checked at the left selvedge, left-middle, centre, right-middle and right selvedge. If the values are not consistent, shade variation is almost expected.
3. Uneven warp tension across the sheet
Warp-sheet tension is another major factor. If some sections of the sheet are tighter than others, the yarns may pass through the bath, squeeze rollers and oxidation zone differently.
Higher tension may flatten the yarn, reduce penetration, alter squeeze-out and change the way the yarn opens during oxidation. Lower tension may allow the yarn to carry more liquor or behave differently at the nip.
Uneven tension can also create small differences in yarn path, contact angle and residence time. Centre-to-selvedge variation should therefore be investigated together with tension variation.
The sheet should enter the dye box evenly and should not show slack edges, tight centre, uneven spreading, crowding or bowing.
4. Uneven wetting and pre-treatment
Before indigo dyeing, cotton warp yarn must be properly prepared. Cotton contains natural waxes, pectins, oils, size residues and other impurities. If these are not removed uniformly, the yarn will not absorb dye liquor uniformly.
Poor wetting is especially dangerous in sheet dyeing. If the centre yarns wet more slowly than the selvedge yarns, or if the selvedge yarns contain more residual wax or size, the dye uptake will differ.
Trapped air in yarns can also reduce liquor contact and create uneven dyeing. Good pre-scouring, wetting-agent control, washing and yarn absorbency testing are therefore essential.
In many mills, the dyeing department tries to correct shade variation that actually started in preparation.
5. Indigo bath instability
Indigo is not applied like many other dyes. It must first be reduced into a soluble leuco form so that it can enter or deposit on the cotton yarn. After dipping, the yarn is exposed to air, where the reduced indigo oxidises back to its insoluble blue form.
Because of this chemistry, the final shade is affected by several variables: indigo concentration, caustic level, reducing-agent level, pH, oxidation-reduction potential, temperature, immersion time, number of dips, oxidation time and wetting agent.
If the bath is unstable, the shade may vary over time. But if bath circulation is poor across the width, or if chemical distribution is not uniform, width-wise variation can also appear.
In a good denim range, indigo bath control should not be based only on recipe addition. The mill should monitor pH, redox condition, temperature, circulation, bath level and concentration at regular intervals.
6. Non-uniform oxidation or skying
After each dip, indigo needs controlled oxidation. Oxidation develops the blue colour and influences brightness, tone and fastness. If oxidation is incomplete or uneven, the shade will vary.
In sheet dyeing, the centre and edge portions of the sheet must receive similar exposure to air. Variation in airflow, sheet spreading, roller path, moisture level or dwell time can create width-wise differences.
If the centre portion remains wetter or less exposed, oxidation may be different from the selvedge portions. Indigo dyeing is not only a dipping process; it is a repeated dip-and-oxidise process.
7. Edge effects and selvedge behaviour
The selvedge side of the warp sheet often behaves differently from the centre. Edge yarns may experience different airflow, drying, tension, guiding pressure or contact with machine elements.
They may also be more exposed to side evaporation, splash, dripping or mechanical disturbance. In some cases, the selvedge becomes lighter because it carries less liquor or oxidises differently.
In other cases, it becomes darker because of higher liquor retention or local accumulation. The exact direction of shade difference depends on the process condition.
Therefore, the question should not be only “Why is the selvedge lighter?” or “Why is the centre darker?” The better question is: Which width-wise process variable is different at that position?
How to control centre-to-selvedge variation
1. Start with width-wise measurement
The first correction is measurement. The mill should build a habit of checking left, centre and right positions. Ideally, five positions should be used: left selvedge, left-middle, centre, right-middle and right selvedge.
At each position, the mill can check shade, liquor pick-up, pH, moisture, tension and yarn appearance. For shade, visual assessment should be supported by spectrophotometer readings wherever possible.
A small colour difference may become commercially significant after garment washing. The colour difference can be expressed using \(\Delta E\), where:
\[ \Delta E = \sqrt{(\Delta L^*)^2 + (\Delta a^*)^2 + (\Delta b^*)^2} \]
Here, \(L^*\) represents lightness, \(a^*\) represents the red-green axis and \(b^*\) represents the yellow-blue axis. Without width-wise data, the discussion remains subjective.
2. Check padder and squeeze roller condition
The padder or squeeze roller system should be checked for uniformity across the width. Important checks include roller hardness, roller surface condition, roller grinding accuracy, nip impression, pressure balance, loading system, bearing condition and roller parallelism.
A simple carbon paper or nip impression test can sometimes reveal what the eye cannot see during running. If the nip is not uniform, the shade cannot be expected to remain uniform.
For wider machines, deflection-controlled or specially designed padders are especially useful because normal rollers may bend under pressure, creating different squeezing behaviour at the centre and edges.
3. Standardise liquor pick-up
Liquor pick-up should be treated as a critical process parameter. It should be measured and recorded, not assumed. If the target pick-up is 70%, the left, centre and right should not show large deviations.
Pick-up control depends on nip pressure, machine speed, yarn absorbency, bath temperature, wetting-agent level, yarn tension, bath level and roller condition. Whenever centre-to-selvedge variation is noticed, pick-up testing should be one of the first diagnostic steps.
4. Maintain uniform warp-sheet tension
The warp sheet should run flat, straight and evenly spread. The machine operator should check whether the sheet is tighter at the centre, looser at the edges, or unstable during running.
Important controls include uniform let-off tension, correct guiding, proper sheet spreading, avoidance of slack selvedges, equal loading across beams, proper alignment of guide rollers and avoidance of yarn crowding or overlapping.
If the sheet itself is mechanically unstable, dyeing uniformity becomes difficult.
5. Improve pre-treatment and wetting
Before dyeing, the yarn should be uniformly absorbent. A simple drop test or absorbency test across width can reveal whether the preparation is consistent.
Good preparation includes removal of wax and impurities, removal or control of previous sizing materials, proper wetting, control of water hardness, effective washing, avoidance of oil or grease contamination and prevention of trapped air.
If yarns do not wet evenly, they cannot dye evenly.
6. Control indigo bath chemistry
The indigo bath should be controlled for concentration, pH, caustic, reducing agent, redox potential, temperature and bath circulation. Operators should avoid large corrections made only after shade variation becomes visible.
A stable bath gives the process a stable base. But stability should mean both length-wise and width-wise stability. The bath should be well circulated, and chemical additions should be properly mixed before they affect the yarn sheet.
Important controls include regular pH checking, ORP monitoring, indigo concentration control, hydrosulphite or reducing-agent control, caustic control, temperature control, foam control, bath level control, filtration and circulation.
7. Ensure uniform oxidation
Oxidation should be uniform across the full sheet width. The yarns should not be crowded, stuck together or unevenly spread during skying. Air movement should not favour one side of the sheet.
Important checks include adequate skying length, uniform airflow, proper yarn separation, consistent machine speed, avoidance of wet patches, no side dripping and stable roller path.
The shade after indigo dyeing is not created inside the dye box alone. It is created by repeated dipping and oxidation. If oxidation is uneven, the shade will also be uneven.
8. Use left-centre-right shade control after washing
Indigo shade should be assessed after proper washing and drying, not only in the wet state. Wet yarns and wet fabric can mislead the eye.
A proper comparison should be done under standard light conditions after the sample reaches a stable state. For better control, mills may maintain a record of left-centre-right shade reading, \(\Delta E\), K/S value, pick-up percentage, bath pH, ORP value, machine speed, nip pressure, oxidation length, lot number and beam number.
Practical troubleshooting table
| Observed problem | Possible cause | What to check first | Corrective action |
|---|---|---|---|
| Centre darker than selvedge | Higher pick-up at centre or lower squeeze pressure at centre | Nip impression and pick-up test | Correct roller pressure, alignment or deflection |
| Selvedge darker than centre | Higher pick-up at edges or edge liquor accumulation | Edge yarn wetness and squeeze condition | Check edge pressure, dripping and guiding |
| One side darker than the other | Left-right pressure imbalance or poor machine alignment | Left vs right nip and tension | Balance pressure and align rollers |
| Shade changes after every few hundred metres | Bath instability or poor chemical dosing | pH, ORP, indigo concentration | Stabilise dosing and circulation |
| Variation increases after washing | Uneven ring dyeing or oxidation | Oxidation and washing uniformity | Improve skying and washing control |
| Random bands across width | Yarn preparation or absorbency variation | Width-wise absorbency test | Improve scouring and wetting |
| Thick counts show more variation | Poor penetration and higher sensitivity to tension or pick-up | Count-wise process settings | Adjust dip time, wetting, pressure and speed |
A practical control plan for mills
A mill can control centre-to-selvedge variation through a simple but disciplined routine. First, check the machine. The padder, squeeze rollers, guide rollers and tension system should be mechanically sound.
Second, check the yarn sheet. The sheet should run evenly from left to right. There should be no crowding, slack edges, tight centre, broken yarn disturbance or uneven spreading.
Third, check liquor pick-up. Measure it across the width. Do not assume that the centre and selvedge are carrying the same amount of dye liquor.
Fourth, check bath chemistry. Maintain pH, reducing condition, temperature, dye concentration and circulation within the required range.
Fifth, check oxidation. Ensure that the yarn sheet gets uniform exposure to air after every dip.
Sixth, check shade with data. Use left-centre-right readings, \(\Delta E\), K/S values and proper production records.
Related Reading on Dyeing, Cotton Yarn Quality and Textile Processing
References and Further Reading
- Xin, J. H., Chong, C. L., & Tu, T. M. (2000). Colour variation in the dyeing of denim yarn with indigo. Coloration Technology, 116, 260–265. View source
- Cotton Incorporated. Open Width Pad-Batch Dyeing of Cotton Fabrics, Technical Bulletin TRI 3007. View source
- EFI Mezzera. Indigo Dyeing and Finishing Ranges / Denim Line Brochure. View source
- Textile Commissioner, Government of India. Semi-continuous Openwidth Dyeing Machines. View source
- Paul, R. (Ed.). (2015). Denim: Manufacture, Finishing and Applications. Woodhead Publishing / Elsevier. View source
Conclusion
Centre-to-selvedge colour variation in denim sheet dyeing is not a mysterious defect. It is usually the visible result of invisible process differences across the width of the warp sheet.
The most important causes are uneven nip pressure, unequal liquor pick-up, non-uniform tension, poor wetting, unstable indigo chemistry and uneven oxidation. Among these, nip pressure and liquor pick-up deserve special attention because they directly decide how much dye liquor each yarn carries.
In sheet dyeing, the yarns remain spread in open-width form. This gives the process speed, compactness and flexibility, but it also makes width-wise control critical. A well-controlled sheet dyeing range must therefore be managed not only from lot to lot, but also from selvedge to centre to selvedge.
The best approach is not to correct shade variation after it appears, but to prevent it through systematic control of machine condition, yarn preparation, bath chemistry, oxidation and left-centre-right monitoring. In denim, shade is not only a recipe. Shade is a result of the whole process.
General Disclaimer
This article is for educational and general textile knowledge purposes only. Actual denim dyeing results depend on yarn quality, cotton fibre properties, machine design, indigo chemistry, reducing system, process route, water quality, operator skill, maintenance condition, testing method and buyer requirements.
Mills should validate all process changes through laboratory trials, pilot runs and controlled bulk trials before implementing them in commercial production. The author does not accept responsibility for production losses, shade rejections or process failures arising from direct application of this educational material without mill-specific technical verification.
