How do printing and dyeing mills solve the problem of one piece of fabric having two colors? What are the causes of color difference and dyeing defects?

In the dyeing and finishing process of textiles, there are many overall influencing factors on the final results. Therefore, various quality problems frequently occur in actual mass production, among which color-related issues are the most common, and color difference ranks first. The following explains the causes and solutions for color difference and dyeing defects.

Color differences are categorized into the following types:

a. Sample release color difference (color deviation between lab sample and bulk production);

b. Batch color difference: including inter-batch color difference, within-batch vat difference, and within-vat tube difference;

c. Other color differences: front-to-back color difference, left-middle-right color difference, and discoloration color difference (color change after post-treatment and finishing).

Current dyeing and finishing production is generally carried out strictly according to customer-specified color standards, with extremely high requirements for color matching grade; some customers even hold overly stringent standards.

Sample release color difference refers to the color deviation between laboratory small samples and bulk production large samples. It is the first problem that must be addressed and resolved in mass production. The higher the consistency of dyeing methods and processes between lab samples and bulk machines, the smaller the sample release color difference.

In practice, perfect consistency is rare; only a certain degree of similarity can usually be achieved. For some dyeing methods such as jigger dyeing, the similarity between small samples and bulk products is even quite low. Hence, a successful lab trial does not guarantee a successful mass production run.

This is especially true when developing or trialing new products, new materials and new processes. Laboratory trial methods and procedures cannot always be directly copied to workshop mass production, and the gap between lab trials and bulk production may be substantial.

Therefore, apart from being equipped with advanced and matched lab sampling machines and supporting facilities, it is more beneficial and stable to conduct a bulk trial of one fabric piece (or 1–2 cones) first before full-scale mass production when conditions permit. The effect of pilot bulk trials is far closer to formal bulk machine production than laboratory small trials.

One-time accurate sample release has always been a technical pursuit, and it is fundamental to preventing vat difference. It relies not only on advanced testing and laboratory equipment but also on technical proficiency and technical management level. Undoubtedly, it is one of the core technologies of printing and dyeing enterprises, and also a prerequisite for the widely advocated concept of one-time dyeing success.

The premise of achieving one-time accurate sample release is to shift from traditional post-adjustment to pre-judgment and full-process control. Meanwhile, mastering the evolution rules of color throughout the entire production process requires substantial detailed basic technical work.

Batch color difference includes color deviation between different production batches and batch-based color difference within the same production batch. It has long been a persistent problem in color production with wide-ranging impacts, and its prevention and resolution require systematic management.

The main factors causing batch color difference are summarized into four aspects:

a. Fiber raw materials from different origins, batches and lots have different dye absorption rates;

b. Dyestuffs, auxiliaries and chemical materials from different origins, batches and lots vary in active ingredient content;

c. Irrational vat allocation and production scheduling plans (dyeing machines of different models have inherent shade and tone differences; improper vat grouping and scheduling will cause inter-batch and vat color difference);

d. Non-standard implementation of process conditions (determined by staff professionalism and management level), covering not only the dyeing stage but also pre-treatment, post-finishing and all other production links.

A single flaw in any of the above four points will trigger batch color difference. If an enterprise has deficiencies in both technology and management, multiple influencing factors may overlap, making it far more complex and difficult to identify the root cause.

Vat difference is another frequent stubborn problem in batch dyeing production.

First, reasonable grey fabric grouping in the pre-preparation stage is the prerequisite. The biggest influencing factor is the overall professionalism of front-line workshop operators, namely whether process operation and implementation are strictly standardized and highly unified among individual operators, machines and shifts.

Accurate weighing and dosing of dyestuffs and chemicals is a basic requirement. However, errors often occur in actual warehouse operations, so review and recheck systems are essential for full guarantee. Even with modern automatic weighing and dosing systems, absolute zero risk cannot be ensured. System failures after long-term operation or manual operation errors are inevitable. Lack of daily maintenance, inspection and management will lead to large-scale defective batches once problems occur in continuous production lines.

It is also worth noting that sample release is closely related to vat difference. If color correction by supplementary dye addition is frequently required, vat difference problems will only become more severe.

Tube difference mainly occurs in large-capacity jet and overflow dyeing machines with multiple parallel vat bodies.

Large-capacity dyeing machines are designed to reduce or even eliminate vat difference risks, but they inevitably bring about tube difference due to their operating principle. The more parallel vat bodies equipped, the larger the single loading capacity and the higher the probability of tube difference. Therefore, blindly expanding single-batch capacity by adding excessive parallel vats is inadvisable and should be kept moderate. Essentially, tube difference is another manifestation of vat difference in this type of dyeing machine.

Key measures to prevent tube difference include: avoiding grey fabric differences through scientific grouping in pre-preparation; maintaining normal equipment operating status (more critical than conventional single-vat machines); and implementing strict operating standards and real-time tracking systems.

Inspection, identification, color correction and rework for tube difference are far more complicated and inconvenient than for vat difference. Especially for blended fabrics such as polyester-acrylic, polyester-cotton and viscose-nylon that require compound dyeing, tube difference may reoccur. If detected only after full dyeing is completed, color correction and rework will incur extremely high costs.

Front-to-back color difference commonly occurs on continuous pad dyeing ranges and large batch jigger dyeing machines, determined by the working principle of such equipment. It rarely happens on intermittent loose immersion dyeing machines. However, if operators heat up rapidly while adding dyestuffs for efficiency, front-to-back color difference will still occur on loose dyeing machines.

In addition, during heat setting of polyester fabrics, incomplete removal of sublimated dyestuffs inside the setting machine will cause sublimation staining on subsequent fabrics. This is particularly common when producing light shades after dark shades or complementary color arrangements, known as setting machine cross-staining. Residual dyestuffs inside the setting machine being gradually carried away will also lead to front-to-back color deviation.

Left-middle-right color difference mainly appears in pad dyeing and jigger dyeing, mostly caused by mechanical structural factors rather than other reasons.

In pad dyeing: Uneven left-right pressure and asymmetry of padding rollers lead to inconsistent left-right pick-up rates; malfunction of padding tank liquid replenishment system causes uneven dyestuff concentration on the left and right sides.

In jigger dyeing: Worry roller deformation and asymmetric left-right radian result in inconsistent liquid scraping rates on the two sides. Thin fabrics with low water absorption are more prone to left-middle-right color difference when dyeing sensitive shades dominated by three primary colors. Non-standard manual feeding of dyestuffs and improper tension adjustment for thin fabrics will also cause uneven left-right dyestuff concentration and subsequent color deviation.

Strictly speaking, loose immersion dyeing machines theoretically do not produce left-middle-right color difference. However, edge-middle color difference occasionally occurs on warp-knitted fabrics. Some warp-knitted varieties are prone to edge curling; severe curling that cannot be unfolded automatically during dyeing will cause edge-middle color difference. This is caused by fabric structural characteristics, which is completely different in nature and cause from the left-right difference in pad and jigger dyeing, requiring targeted solutions from weaving and dyeing process optimization.

Some dyestuffs have varying degrees of thermal discoloration, so dyestuff screening is essential in the first place.

In terms of auxiliaries, post-treatment and finishing agents such as color fixatives, cross-linking agents, softeners, water/oil repellents and coating agents will inevitably cause more or less color change to dyed fabrics. The longer the post-treatment and finishing process, the higher the finishing temperature, and the more auxiliaries applied, the greater the possibility of color deviation.

Even mechanical finishing processes such as napping and sanding, which have no direct relation to dyestuffs and auxiliaries, will affect the apparent fabric color and cause color difference. Such color difference is not caused by chemical factors but by changed fabric surface structure and light reflection performance, creating a visual color perception deviation. The hue remains basically unchanged, while shade depth and tone are obviously affected. Although it is a physical color change, it still needs pre-control and preventive measures.

Complete dyeing is only achieved after all post-finishing processes are finished, the finished fabric is fully cooled, and the color stably meets customer standards for garment making and subsequent production. It is unrealistic to require post-treatment and finishing auxiliaries to cause zero color change (at least under current technical conditions). This issue also falls into the scope of sample release color difference, with similar deviation problems between lab trials and bulk production (focusing on color stability).

Technical solutions must be adopted in sampling and sample release: lab sampling must fully simulate bulk production procedures, methods and process conditions, with deviation correction verified through pilot and mass trials.

It should be clarified that post-treatment and finishing auxiliaries mainly affect intrinsic fabric quality (such as color fastness), hand feel, style and special functional properties. Dyestuffs remain the dominant factor affecting color deviation and correction. Mastering color evolution rules is the premise of solving color difference, and pre-control via technical means is the fundamental solution.

Special attention should be paid to the yellowing problem of amino silicone softeners. Dominated in softener applications due to excellent hand feel and durability, this type of softener has inherent performance drawbacks: higher amino value brings better hand feel but greater yellowing tendency. Higher finishing temperature and longer processing time will further aggravate oxidative yellowing.

Although chemical manufacturers keep improving formulations via modification, inherent contradictions remain. Therefore, strict selection is required for pure white fabrics and bright light shades (especially purple tones). If necessary, partial softness requirements should be compromised to maintain whiteness stability.

Continuous technological research and optimization are key driving forces for industry upgrading. However, every new breakthrough in technology, dyestuffs, auxiliaries and equipment has a relatively stable service life and presents periodic changes.

On the other hand, to maximize enterprise production efficiency and benefits, production must consume raw materials, energy and water in a necessary, reasonable and highly efficient manner under current technical levels. The concept of one-time dyeing success is the concrete embodiment of this principle in process implementation.

One-time dyeing success means no abnormal rework or redyeing beyond standard processes throughout the entire production flow. Achieving this standard requires systematic comprehensive strength, including mature technology, standardized management, complete equipment configuration and high overall professionalism of front-line employees.

Undoubtedly, one-time dyeing success greatly improves labor productivity and realizes full utilization of raw materials, energy and water. It is not only a means to upgrade product quality but also an important way to save resources, achieve energy conservation and emission reduction, and increase enterprise benefits.

Traditional quality management relies on final finished product inspection and grading. By then, product quality grades (first-class, second-class, substandard) are already finalized with little room for remedy.

The epidemic prevention mechanism in the medical industry provides valuable inspiration: Prevention is better than remedy is not only an effective way to control dyeing and finishing defects but also crucial for minimizing production costs and resource consumption.

As the guiding ideology of modern quality management systems, “Prevention is better than remedy” adopts a proactive source-control approach. Based on historical production data, preventive measures are implemented in all links with potential quality risks. Efforts are made to avoid quality degradation of semi-finished products and eliminate defect factors in all production stages, ensuring perfect final results. Therefore, it is also an indispensable theoretical foundation for realizing one-time dyeing success.

Color difference is caused by multiple factors: quality differences in raw and auxiliary materials (fiber, grey fabric, dyestuffs and chemicals), equipment operating status, process technology, management norms and staff professionalism.

Every link from pre-preparation, pre-treatment, dyeing, post-treatment to post-finishing has potential risks of color deviation, making color difference the most frequent quality problem and the top three chronic defects in printing and dyeing production.

Although color correction, rework and redyeing are unavoidable in actual mass production, they are only passive remedial measures. Moreover, some textiles do not allow any rework at all. Therefore, a prevention-centered strategy should be adopted for all potential color difference factors, which is the leading philosophy of production management, technical management and quality management in the printing and dyeing industry.