1 Foreword <br> Knitted fabrics are popular among consumers because of their good dimensional stability, elasticity, water absorption, softness and breathability. Nowadays, people in the market pursue leisure for clothing, emphasizing comfort and individuality. Many knitted fabrics have been developed into underwear, making the number of knitted fabric printing products increasing year by year.
In general, the equipment that can be used for knitted fabric printing is: flat screen printing, rotary screen printing, roller printing, and manual plate printing. The printing process used is mainly paint printing and reactive dye printing, and a small amount of transfer printing. This article mainly describes the practice of paint printing process in the past eight years.
The key issue for coatings used in knitted fabric printing is that the components of the printing paste (including binders, paint pastes, and pastes) must be properly selected to maximize the balance between hand and fastness. Since 1997, we have cooperated with Zhejiang Silk Academy, and have used ultra-soft TYS-1 printing paste made of imported raw materials and some domestic coatings. Direct printing coating/reactive printing and coating special printing on the rotary screen printing machine. (including three-dimensional foam printing, gold and silver powder printing, pearl printing, glittering printing) and the newly emerging paint discharge printing. So far, more than 2,000 tons of knitted printed fabrics have been produced, and some practical experience of rotary screen paint printing has been accumulated. Now I will write a written experience and discuss it with colleagues in the printing and dyeing industry.
2 Knitted fabric pretreatment
2.1 Before the printing of knitted fabrics, it is necessary to pre-treat and bleach the knitted fabrics to improve their taking performance and appearance quality, and prepare for the subsequent printing. Good pre-processing is knitted fabrics. The basis of rotary screen printing.
Due to the structure of the grey fabric, the processing quality requirements and the processing equipment used, different pre-processing processes can be used, and the process flow is generally:
Rough cloth → (alkali shrinkage (silk light)) → scouring → bleaching (chlorine bleaching or oxygen bleaching) → whitening → soft treatment → dehydration → drying → calendering (double-sided fabric is not alkali-shrinking)
Knitted fabric pretreatment mainly consists of three parts: alkali shrinkage (silk), scouring and bleaching (whitening). Alkali shrinkage is a process in which a cotton knit fabric is stacked in a flat (cylinder), tension-free state, padded with a caustic soda solution, and then washed with water to remove alkali. By alkali shrinkage, the fabric shrinks, increasing density, elasticity and gloss, increasing the smooth hand, reducing the shrinkage rate, and improving the dimensional stability of the finished product. In recent years, the use of mercerizing fabrics has gradually increased, and the quality of products after mercerizing is much higher than that of alkali shrinking products.
The function and principle of scouring and bleaching of knitted fabrics are the same as that of woven fabrics, but because of its loose structure, the processing conditions (mainly the working fluid concentration) are lower than that of woven fabrics. The processing methods of scouring and bleaching are batch type and continuous type, and the batch type is generally carried out by using a cooking pot or in an overflow dyeing machine; the continuous type generally adopts a "D" type box or a crawler box for flat steaming. The bleaching of semi-finished products for knitting printing is often carried out by chlorine bleaching, oxygen bleaching, chlorine bleaching, and double oxygen bleaching.

Knitted printed semi-finished products require uniform scouring, uniform whiteness, good hair effect, small latitude, and weight. Improper pre-treatment, resulting in quality problems, the same as the processing of woven fabrics, such as uneven scouring, uneven bleaching, lack of whiteness, etc., often caused by the difference in color and color, color difference or shade Dim and so on, but also some of its unique quality problems, should be paid attention to during processing.
2.2 square meters of weight problem The finished product of knitted fabrics is one of the important indicators of customer quality requirements, and must be guaranteed; if it exceeds too much, it will increase the production cost, so under the premise of ensuring the weight of finished products It is very important to control the weight of the rough cloth. In the pre-treatment and printing process, there are factors that reduce the gram weight, such as the de-mixing during pre-treatment, the elongation of the fabric after a certain tension during the printing process; there are also factors that increase the gram weight, such as the fabric before pretreatment. The retraction, the amount of paint and additives added to the fabric during printing. However, due to the difference in the tension of the processing equipment, the gram weight varies with the different printing area, and the weight of the finished product fluctuates up and down on the basis of the weight of the blank. In order to avoid the problem that the weight of the finished product does not meet the requirements after printing, unless there are many production experience, the pioneer sample is generally required. After the same process route as the production of large goods, the result of the change in the weight of the knitted fabric after processing the equipment, To develop and correct the blank fabric weight index when producing large quantities of goods.
2.3 Gross effect The gross effect is not good, resulting in uneven coloration of the printed block surface. In severe cases, there will be “exposed bottom” phenomenon, or the depth cannot be improved. Pay special attention when making dark large block surface pattern. The reason for this is that, in addition to the usual "sufficient scouring", the "soft treatment" process unique to knitted fabric pretreatment is also an important cause of poor handling of knitted fabrics. Due to improper selection of softener or excessive dosage, it will cause water repellency of the finished fabric. Therefore, the pre-knitting treatment for printing should also pay attention to the “soft treatment”. It is not possible to simply apply the finishing process of bleaching and dyeing knitted fabrics. Select softener, reasonably set its dosage, otherwise it will affect the fastness after printing. While improving the feel of the fabric, it also takes into account the wool effect of the knitted semi-finished products and ensures the smooth processing of the printing.
2.4 Weft problem The pre-treatment of knitted fabrics is mostly rope-like processing, and the structure is looser. The problem of weft is more serious than that of shuttle fabrics. If it is a floral pattern, the problem of the weft can be appropriately relaxed. If it is a lattice type, the weft requirement is higher. Measures to control and improve the weft: check the direction of the weft before the splitting joint, make it consistent, and correct it when cutting the head; the pre-press setting machine should be equipped with automatic wefting device; Set the width of the door to reduce the amplitude of the post-press tension to reduce the curvature of the curved flower produced by the post-press setting. The quality requirements of knitted fabrics, such as door width, shrinkage rate, weight per unit area (square meter weight), strength, coil length, density, etc., should meet the standards of national standards or customer requirements. The intrinsic quality of the grey cloth has a great relationship with the width of the door, the square meter weight and the shrinkage rate, and must be taken seriously.
3 Circular mesh printing of knitted fabrics <br> The circular knitting process of knitted fabrics is generally:
Full coating: semi-products → splitting → seaming → shaping → printing → baking → setting → roll coating / activity: semi-product → sectioning → seaming → shaping → printing → baking → washing → drying → setting → Finished product

3.1 The choice of new paint printing auxiliaries The new paint printing auxiliaries consist of the following four pillars:
(1) Excellent paint dispersion system (Dystar paint, etc.);
(2) Excellent self-crosslinking adhesive system (German, American company and homemade varieties);
(3) Excellent printing paste system (German, American company and homemade varieties);
(4) Excellent hand fastness adjustment aid system.
When it comes to paint printing, it is up to these additives to achieve satisfactory results. The quality of traditional paint printing products is poor. Usually, chemical knitted fabrics that do not meet the standards of new printing technology are different from ordinary woven fabrics. There are many practical problems different from woven fabrics when printing. The key issues are as follows: Narrative:
Additives and unreasonable printing process:
(1) After the research of the coating dispersion system, the conclusion is that the pigment is ground by a high-power shaped simple sand mill to a particle size distribution range of 0.25-1.5 μm, which is caused by 0.25 μm. Decrease in light blocking and color strength.
In actual printing, the choice of coatings focuses on price, color strength, vividness and fastness characteristics. After years of research, we believe that Dystar's 1mperon system is the first choice from an environmentally friendly or irreplaceable point of view.
(2) Adhesive system The new self-crosslinking adhesives are mostly PA series, and the small amount is PU series. Regardless of the type of adhesive, the first consideration in the synthesis and selection is how to solve the contradictory unity problem of hand and fastness. Followed by the transparency of the film, it directly relates to the vividness of the printed product. The components that make up the binder and the types of different monomers, and their effect on the membrane, are described in a number of ways and are not described here.
After research, we believe that the important condition that the adhesive must meet is to achieve a good balance between adhesion and cohesion. The adhesive film has the same adhesion to the fabric and the good cohesive force of the adhesive. Should be improved as much as possible. If the adhesion is weak, or it is weaker than the cohesive force, the rubbing fastness of the printed fabric surface will be reduced to cause fading marks. If the cohesive force is weak, the particles of the paint will be easily rubbed off.
The paint printing adhesive should be subjected to an acidic condition (pH 4) during the heating reaction. Dry hot air is the best fixing medium. Wet steam or superheated steam is not suitable as a fixing medium. Because it can only obtain about 60-80% of the fixing effect (the high temperature steam contains 100% water vapor as a result of hydrolysis), resulting in a decrease in various color fastness indicators. Generally, the thickness of the coating printing adhesive after film formation is about 5-10 microns, which is difficult for the naked eye to observe. The coating is cohesively deposited in the film, so the adhesion of the adhesive affects the adhesion of the film to the fiber. This process is mainly for physical adsorption and is strengthened by hydrogen bonding. Some kinds of binders can form true chemical bonds under specific conditions and even for fibers, which is due to the formation of polymer film in the film. Formed by special reactive groups. When synthesizing or selecting adhesives, we pay special attention to its important parameters such as glass transition temperature, minimum film forming temperature, particle size fineness and its distribution range, surface tension and electrolyte resistance. In a sense, it is more strictly defined whether it is an excellent adhesive suitable for knitted fabric printing. The choice of PA or PU adhesive should be determined according to different fabric requirements.


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August 31, 2019