Color Difference Control in Dyeing and Printing Processing

The concept of color difference in continuous pad dyeing within the dyeing and printing industry covers a wide range, including shade variation against the standard sample, batch-to-batch color difference, face-to-back color difference, left-middle-right color difference, and within-batch color difference. This has long been a complex and intractable quality issue faced by professionals in the dyeing and printing sector. With the expansion of China’s textile exports and the increasingly fierce international competition, the requirements for product quality have become more stringent.

Currently, for exported dyed products, the standard requirement for left-middle-right color difference and within-batch color difference is above Grade 4.5, which is far higher than China’s national standard. Meanwhile, the evaluation of color difference has shifted from traditional visual grading to computerized color measurement and assessment. In other words, the traditional craftsmanship has evolved into scientific and technological methods. Color transmission has started to transition from physical color swatches to digitalization, with computer color matching controlled by CMC and DE values—within 0.6 for color matching against standards and within 0.9 for within-batch color difference. In addition, the light sources used for color matching have developed from common office lights such as D65, CWF, TL84, and D75 to specialized ones like U30, HORIZON, and INCA. Moreover, color matching is no longer conducted under a single light source but requires consistent shade under two or even three different light sources.

To meet customers’ quality requirements and enhance enterprises’ competitiveness in export markets, controlling, reducing, and ultimately eliminating dyeing color difference has become a crucial task for dyeing and printing professionals. This paper focuses on approaches to control and reduce left-middle-right color difference (commonly known as edge color) and within-batch color difference (commonly known as LOT color), based on the author’s years of practical experience, for reference only.

The raw materials for dyed products mainly refer to greige fabric and dyeing and finishing chemicals. Regardless of the batch size, ensuring consistent quality of raw materials is an important prerequisite for eliminating LOT color. Even if there are objective quality variations in raw materials, it is essential to identify these variations clearly, adopt differentiated processing methods, and strictly control LOT color within the minimum allowable range. Specific measures are as follows:

To address this issue, we adopt computer color matching. Based on pre-selected dye combinations, we select six formulation options with optimal DE values. Then, the dyeing supervisor selects one formulation that meets the criteria of high quality, strong stability (consistent dye uptake curves), and low cost. The selected formulation is then sent to the lab for sample dyeing. This approach ensures that the lab sample not only meets customer requirements but also guarantees stable color difference control during mass production.

The difficulty in producing dyeing and printing products and the operational challenges faced by dyeing and printing mills stem from the characteristics of the production process. From greige fabric to finished products, dyeing and printing typically involves over 10 production processes, which are influenced by a combination of chemical, physical, mechanical, environmental, and human factors. Any problem in any of these processes can affect the final product quality. Therefore, continuous reform and strengthening of production management for dyeing and printing products are essential to control and reduce color difference and ensure product quality.

Traditional enterprise management in dyeing and printing mills suffers from the drawback of fragmented responsibilities among planning, technology, production workshops, and equipment departments, which is detrimental to prioritizing process technology and product quality. Several years ago, we integrated the technology and planning departments into a Production and Engineering Department, which takes full charge of the entire factory’s production operations. Meanwhile, we reformed the production operation management model to a technology-led and quality-focused one.

On the premise of minimizing color difference, specific pre-treatment processes are designed for colors and varieties with high dyeing difficulty, replacing the one-size-fits-all scouring and bleaching process previously applied to multiple varieties. We emphasize that technology plays a leading role in production operation management, while planning, workshops, and equipment departments serve as supporting roles. In addition, the procurement of dyeing and finishing chemicals is placed under the unified management of the Production and Engineering Department. During production, whether it involves production plan adjustments or problem-solving, and regardless of the shift (day, swing, or night), the decision-making and command power lie with the technical supervisor. This avoids conflicts such as mutual buck-passing and prioritizing output over quality, embodies the core principle of “process-centricity”, and ensures product quality.

Given the characteristics of dyeing and printing production, the vast majority of color difference and other quality issues arise during the production process. For a long time, dyeing and printing mills have emphasized the principle of “prevention first”. But how to implement this principle in actual production? We believe that on-site monitoring, quality tracking, and nipping potential problems in the bud in dyeing and printing mills are the most practical ways to practice “prevention first”. Over the years, we have attached great importance to on-site production management—all production planning staff, process technicians, and their offices are located directly in the production workshop.

Whether it is the manager of the Production and Engineering Department, the workshop director, or the technical supervisors and technicians responsible for scouring and bleaching, jigger dyeing, pad dyeing, and post-finishing, their daily work focuses on the workshop floor, making the workshop their office. This enables technicians to have a thorough understanding of product processing, machine operation, employee performance, and quality changes. Meanwhile, they are required to adhere to the “three on-site requirements” (being present at machine startup, when changing product varieties, and when production is suspended due to faults), inspect the “four first pieces” (the first piece of fabric after shift change, after changing product varieties, after machine cleaning, and after equipment maintenance), and earnestly implement the “three standardizations” (standardization of technical operations, institutionalization of process compliance inspections, and standardization of semi-finished product quality upon exiting the process). By doing so, the “three key process parameters” (temperature, speed, concentration) during production can be maintained accurately and consistently, ensuring stable and uniform quality of semi-finished products at each process step, which is the key to controlling and reducing color difference.

The most fatal flaw in dyeing and printing production is loose management and lax quality control, which often leads to massive quality problems before root causes are investigated. To effectively control color difference, in addition to strict management, it is imperative to implement rigorous quality checks at all levels. Our practice of implementing quality veto power at all levels has proven to be another effective guarantee for reducing color difference:

① If the user department discovers quality variations in raw materials, it has the right to order returns or replacements.

② Process technicians must establish a rigorous inspection system for equipment conditions. If equipment problems are found to affect product quality, they have the right to suspend production and notify the equipment department to carry out maintenance until the equipment is fully functional.

③ Dyeing technical supervisors have quality veto power over pre-treatment quality. If scoured and bleached semi-finished products are found to affect dyeing quality, they have the right to order rework until the quality meets the required standards.

④ Quality inspection personnel are responsible for checking edge color and LOT color of dyed products; products that fail to meet the standard must be returned to the dyeing supervisor for reprocessing until they pass inspection.

⑤ Technical supervisors at all levels have the power to reward, punish, transfer, or dismiss employees under their supervision based on their operational skills and performance. Any employee who fails to meet the technical operation requirements of their position must be dealt with promptly and reassigned appropriately.

⑥ Customers are “king”. Any request for product rework due to quality issues must be accepted unconditionally, without any buck-passing or delays.

Edge color and LOT color in dyeing are not solely caused during the dyeing process. Their control involves not only enterprise management and raw materials but also processes, technologies, equipment, operations, and control methods. The entire production process is intricate—errors in any single link can have a “domino effect” and lead to color difference.

When edge color and LOT color occur in dyeing, most people focus on investigating causes within the dyeing process while ignoring pre-treatment, which is the fundamental stage where color difference can originate. This one-sided approach is clearly incorrect. It can be said that consistent capillary effect and whiteness after pre-treatment play a pivotal role in preventing color difference. Therefore, to control and reduce color difference, it is necessary to prioritize and strengthen pre-treatment work.

① Thorough Desizing and Scouring:

Due to equipment limitations or considerations of production planning and costs, many enterprises often combine desizing and scouring into a single process. However, for products with high size content (such as corduroy and greige fabric from Pakistan and India), this combined process results in highly unstable capillary effect after scouring and bleaching. Through practice and exploration, we have concluded that desizing is crucial for the pre-treatment of pure cotton fabrics—only thorough desizing can ensure complete scouring, laying a stable foundation for subsequent pre-treatment processes. In practice, we have found that the optimal process sequence is: sewing → desizing → singeing → scouring and bleaching. For dyed products, the capillary effect is generally required to be above 8 cm, with the variation in capillary effect between the left, middle, and right sections of the fabric, as well as between the front and back ends, controlled within 2 cm.

② Consistent Pick-up Rate of Padding Machines:

Whether the pick-up rate of padding machines in pre-treatment is consistent across the left, middle, and right sections of the fabric, as well as between batches, is a problem that is easily overlooked in dyeing and printing mills. In particular, for padding machines used in scouring, bleaching, and mercerizing processes, unstable pick-up rates will inevitably lead to dyeing color difference. Therefore, the maintenance and upkeep of key pre-treatment padding machines should be prioritized to the same level as dyeing padding machines. The equipment department should conduct key maintenance during weekly downtime, and technicians should measure the left-middle-right pick-up rate of padding machines at the start of the first shift each week; production can only commence if the pick-up rate meets the required standards.

③ Sudden mechanical and electrical failures are inevitable in dyeing and printing production, and they have a significant impact on continuous desizing, scouring, and bleaching machines with large fabric capacity (600–800 meters). In the event of such failures, if the fabric residence time in the scouring and bleaching process exceeds the specified limit, each factory should formulate rigorous rework measures based on actual conditions. The quantity of reworked fabric and corresponding box numbers must be accurately notified to the dyeing supervisor, so that trial dyeing can be conducted separately before mass production, avoiding color difference.

④ Adhere to Pre-treatment Quality Inspection and Clarify Handover Standards:

The inspection of the appearance quality of pre-treated semi-finished products is relatively straightforward, and the testing of internal quality indicators such as capillary effect, pH value, and size content is also not complicated. As mentioned earlier, the “standardization of semi-finished product quality upon exiting the process” must be persisted in and earnestly implemented. However, the above inspections are still insufficient to meet the actual requirements for controlling dyeing color difference. Our approach is: in the process immediately before dyeing, sample 1 meter of fabric from each box, sew the samples together continuously, and send them to the dyeing supervisor for inspection and trial dyeing with the same or similar colors for quality assessment.

If the shade depth and left-middle-right color consistency of each trial-dyed sample are uniform, the pre-treated semi-finished products are deemed qualified; otherwise, they are unqualified, and corrective measures must be taken in the pre-treatment process. If edge color is detected in the trial-dyed samples, the pre-dyed white fabric samples can be sewn at a 90-degree angle, converting the weft direction to the warp direction, and then re-dyed. This method can help verify whether the edge color is caused by the dyeing process or the pre-treatment process.

⑤ Maintain Stability of Process and Equipment Routes:

Production planning and scheduling must unconditionally comply with process and technical requirements. That is, products of the same batch or reprinted orders of the same color must be processed using the same process and equipment.

If it is indeed necessary to adjust the production machine, prior approval from the technical department must be obtained before implementation; the practice of making adjustments without prior approval or failing to report adjustments after implementation is strictly prohibited.

In addition, the demand for wide-width dyed products (greige fabric of 60–63 inches) is increasing. However, due to equipment limitations, many dyeing and printing mills are forced to use the same machine for both wide-width and narrow-width products. How to reduce color difference in this situation? Our solution is to adopt a technology-led approach, requiring the planning department to arrange centralized production of wide-width products. Before production, the equipment department is required to replace the padding machines at key positions in the pre-treatment process, and an additional uniform padding machine is installed before the pad dyeing machine, dedicated to wide-width product production. This approach achieves ideal control of edge color. Of course, it is optimal to use dedicated machines for wide-width product production.

⑥ Color Difference Control for Brushed Products:

Driven by the demand of the garment market, the number of products undergoing brushing post-treatment is increasing. Currently, brushing is not only applied to pure cotton fabrics but also extended to elastic twill, elastic poplin, and nylon-cotton blended fabrics. Inconsistent brushing intensity between the front and back ends of the fabric, as well as between the two edges, can easily lead to edge color and LOT color during dyeing. To ensure consistent quality of brushed semi-finished products, it is essential to strictly control the fabric moisture content, brushing pressure, tension, and speed during the brushing process. During long-term production, brushing sandpaper wears out as production volume increases; therefore, the brushing pressure must be adjusted appropriately to ensure uniform brushing intensity across the entire fabric length.

To check whether the brushing intensity is consistent between the left, middle, and right sections of the fabric, in addition to manual touching and visual inspection, the best method is to conduct sampling dyeing to assess edge color caused by brushing. In addition, since most fabrics are treated with softeners before brushing, it is advisable to re-conduct scouring and bleaching before dyeing to remove surface lint and residual softeners, ensuring consistent shade throughout the dyeing process.

As the name suggests, color difference refers to variations in color. It is only through the dyeing process that color difference in dyeing and printing products becomes apparent. Therefore, the dyeing process is both a key area where color difference arises and the core stage for color difference control. Thus, it is necessary to focus resources, uphold a rigorous and meticulous work style, and adopt scientific and reasonable methods to manage this critical stage and control color difference effectively.

① Material Preparation:

For a specific color, regardless of the batch size, dyes must first meet the criteria of the same batch, shade, and concentration. Otherwise, the dye formulation must be re-adjusted, and trial dyeing must be conducted before mass production can proceed.

② Material Weighing:

Color difference within batches caused by weighing errors is a low-level mistake that dyeing and printing mills should never make. A reliable method is to assign dedicated personnel to weigh dyes by color category, with a second person responsible for rechecking. In addition, the regulations on weighing instrument usage must be strictly followed based on the weight of the dyes to ensure accurate weighing.

③ Dye Dissolving:

The optimal capacity of each dye dissolving tank is 800–1000 liters, with three tanks configured for each pad dyeing machine (for the pad-steam fixation method, the volume of each tank should be limited to 300–500 liters). For large-batch color production using processes such as reactive dye pad-steam fixation, vat dye dry reduction, sulfur dyeing, and pigment dyeing, 2–3 dye tanks can be filled and connected for simultaneous use. This method can reduce color difference caused by dye or operational errors. In addition, after each tank of dye liquor is prepared, filter paper can be used for random sampling to analyze the dye diffusion rate and shade depth variation, serving as the final quality check before dyeing.

④ Trial Dyeing:

The length of fabric used for pad dyeing trials should generally be controlled at around 20 meters per trial to ensure accuracy while minimizing waste. The process parameters for trial dyeing (including pressure, machine speed, temperature, and concentration) must be identical to those used in mass production. Only with accurate trial dyeing can color difference be minimized. During trial dyeing, in addition to adjusting the formulation for color matching, adjustments should also be made to optimize left-middle-right color consistency to an ideal level, ensuring accurate color matching and good edge color control during formal mass production.

⑤ Pre-drying:

Whether infrared pre-drying or transverse guide roll pre-drying ovens are used, the heating temperature on the fabric surface must be kept uniform. Otherwise, dyes will migrate with moisture, resulting in color difference. In particular, it is necessary to establish a system to strengthen the inspection of air distribution uniformity in pre-drying ovens to prevent errors. Additionally, enterprises with sufficient conditions can install online moisture meters at the exit of the pre-drying process to check the moisture consistency of the fabric after pre-drying, preventing color difference between the front and back ends of the fabric caused by uneven moisture content.

⑥ Steaming Chamber:

The speed, temperature, and concentration in the reduction steaming chamber must be kept consistent throughout production. In particular, caustic soda is an essential component in both reactive dye fixation liquor and vat dye reduction liquor. Practice has proven that shade is highly sensitive to the concentration of caustic soda in the fixation or reduction liquor. To reduce the impact of caustic soda concentration errors, the total amount of caustic soda required for each color should be prepared in advance and used only after accurate titration.

The control of steaming chamber temperature and pressure can be regulated by monitoring the pressure and temperature of the steam entering the chamber. In addition, a U-tube manometer can be installed on the wall of the steaming chamber, with specified control points set for the liquid level height of the U-tube (which corresponds to the pressure inside the steaming chamber). Since the U-tube manometer responds more quickly and sensitively than temperature and pressure gauges, controlling the temperature and pressure inside the steaming chamber using the U-tube manometer ensures higher accuracy and more consistent shade across batches.

⑦ Monitoring:

With the continuous development of science and technology, instruments for monitoring edge color and LOT color are constantly advancing. For example, padding machines equipped with computerized systems for automatic control of left-middle-right pick-up rates, online color difference measurement devices, portable colorimeters, automatic temperature control systems, and automatic pH control systems are now available. For enterprises with the means, gradually investing in advanced and practical scientific instruments will undoubtedly provide significant assistance in reducing color difference. At present, dyeing production in most enterprises still relies on strict quality control and careful inspection by employees. Samples should be cut and compared against the standard at least three times per box of fabric to check for color difference between the front and back ends. After dyeing, fabric strips (generally 20 cm wide) should be cut, and the left, middle, and right sections of the strips should be sewn together to inspect edge color. Any issues detected should be addressed promptly with appropriate corrective measures.

⑧ Post-finishing:

The post-finishing of dyed and printed fabrics can cause color difference due to the influence of post-finishing agents, which must be taken seriously and prevented. Specific measures are as follows: during pad dyeing trials, based on the post-finishing requirements of the product (such as softening, sizing, water repellency, stain resistance, coating, and resin finishing), the trial-dyed samples should undergo the specified post-finishing process before color matching. This avoids the problem where the shade matches the standard after dyeing but changes after post-finishing. Therefore, the selection and formulation of post-finishing agents should be determined by the pad dyeing supervisor. After post-finishing, samples should be cut and compared against the standard color sample (provided by the dyeing supervisor), and the left-middle-right color difference of each box of fabric should be inspected to nip potential color difference issues in the bud.

⑨ Final Inspection:

Currently, the final inspection process in our factory is as follows: roll inspection → color sorting → packaging. Roll inspection mainly involves checking for surface defects on the fabric and classifying it into Grade A and Grade B. Two fabric samples (approximately 10 cm long each) are cut from the start of each roll as required—one sample is provided to the customer, and the other is retained in the factory for future reference.

Color sorting should be assigned to dedicated personnel, who are primarily responsible for classifying fabrics based on LOT color and inspecting edge color according to the fabric strip samples. Only qualified products are allowed to be shipped.

Packaging should be carried out separately according to the LOT color classification after color sorting, with the fabrics stacked accordingly. Prominent color labels in both Chinese and English should be attached to clearly distinguish different LOT colors, facilitating color-sorted cutting in garment factories.