Lateral Variance
Visual defect characterized by a gradual shift in hue or depth from the middle of the fabric to the edges. When center to selvedge shading appears, it signifies a non-uniformity in the dyeing or finishing process across the machine width. The discrepancy is typically measured using a spectrophotometer to compare the color values between the middle and the edges of the web.
Roller Pressure
Variations in the nip force of the padder often cause the edges to receive more or less liquor than the center. If center to selvedge shading is present, it might be due to a deflection in the padder rollers where the pressure is higher at the ends than in the middle. Modern padders use swimming rollers or specialized crowning to maintain a flat pressure profile.
The mechanical state of the bearings and the alignment of the frame also contribute to how the rollers meet under load. A single millimeter of deviation in the gap can lead to measurable differences in the wet pick up of the fabric.
Migration Effect
Drying speed differences across the width of the stenter can pull dye molecules toward the area that dries first. This form of center to selvedge shading occurs when air nozzles are blocked or if the exhaust system does not pull evenly across the chamber. Temperature probes must verify that the heat profile is consistent from side to side to prevent this movement of solids.
Batch Limit
Total fabric width determines the risk level for this specific quality issue. While center to selvedge shading is common in wide-width home textiles, it is less frequent in narrow-width apparel fabrics where roller deflection is minimal. The tolerance for this variance is usually set at a delta E of one or less for premium garment production.
