The Four Most Common Pre-processing Methods For Digital Printing

- Aug 11, 2018-

The four most common pre-processing methods for digital printing

Abstract: Compared with the traditional printing process, inkjet printing has greatly improved and improved in the production process, cost and precision. It has achieved multi-color, multi-variety and small-lot printing in cotton textiles, wool textiles and silk. Widely used in natural fiber textiles and chemical fiber textiles, and gradually become themainstream of the printing market.

Compared with the traditional printing process, inkjet printing has greatly improved and improved in the production process, cost and precision, achieving multi-color, multi-variety and small-lot printing, in cotton textiles, wool textiles, silk, etc. Widely used in natural fiber textiles and chemical fiber textiles, it has gradually become the mainstream

of the printing market.

In order to improve the final printing quality and meet the requirements of the subsequent printing process, the textiles must be pre-processed before inkjet printing. This paper mainly discusses the pre-treatment measures in the inkjet printing process from several common textile materials.

Pretreatment of cotton fabric

Cotton textiles have the advantages of soft gloss, soft texture, strong durability, good coloring, etc. They are used in the traditional printing process, and the process flow is also the most perfect and mature.

The inkjet printing process mainly includes the processes of pre-treatment, drying, printing, drying, steaming (120 ° C, 8 min, fixing the reactive dye), washing and drying. Therefore, cotton textiles only need to ensure that their printability meets the special requirements brought by the difference between inkjet printing and traditional printing process.

For example, in the inkjet printing process, the transfer mode of the ink to the surface of the cotton textile is in the form of sprayed by the nozzle, so it is necessary to additionally add the thickener sodium alginate when pre-rolling the auxiliary agent (sizing) on the surface of the cotton textile in the early stage of printing. In this way, the ink spread on the cotton fabric is suppressed, and the printing precision of the cotton textile is improved. At the same time, it is necessary to ensure that the alkaline environment of the textile surface fiber and ink and the temperature and humidity of the printing workshop are within a certain range (generally, the temperature is maintained at 18 to 25 ° C and the relative humidity is greater than 50%). 

 As for the other, as long as the cotton textiles can conform to the traditional printability.

Pretreatment of silk textiles

For inkjet printing of silk textiles, the quality of the printing is mainly measured in terms of the definition of the contour of the flower and the color strength of the printed color. Similar to cotton textiles, we want to prevent the bleeding of the printing ink after it is transferred to the surface of silk textiles; at the same time, we must ensure the fixing effect of the printing ink on the surface of silk textiles. In view of the characteristics of the silk fiber, we have higher requirements for the paste in the slurry component and the proportion of each additive component in the sizing process. 

In the inkjet printing of silk textiles, the pastes we commonly use are P3 paste and DGT-7 paste. The proportion of the paste should also consider the thickness of the textile. The thickness of the silk textile can be divided into thin textiles such as cotton spinning and medium-thick textiles such as plain satin.

For thin textiles such as cotton spinning, because the moisture absorption is small and the fabric is thin, the ink is easily permeated. Therefore, when the paste is selected, a material with good water holding property should be selected, and at the same time, a part of the synthetic thickener should be included in the paste. It is used to improve the water holding capacity of the paste, improve the coloring effect of the textile, and avoid the phenomenon of seepage. Moreover, it is also required that the selected paste has good wettability and permeability to the textile in the early stage treatment, which will eventually affect the ease of film formation and thickness of the paste on the textile surface: too thin will affect the textile during sizing The uniformity of the paste on the surface; too thick will affect the color fastness of the subsequent printing. In general, the haze rate of thin textiles is controlled at about 3.5%.

For medium-thick textiles such as plain satin, P3 paste also has a good coloring effect, but the thickness of the textile affects the penetration of the paste, so it is necessary to appropriately reduce the amount of paste of the P3 paste, and add a certain amount of The penetrant or the padding method is changed to improve the penetration effect of the P3 paste. In general, the paste formation rate of the medium-thick textile is controlled at about 3%. 

In the slurry component, urea acts as a moisture absorbent, and as the component increases, the amount of ink is increased, but the fineness of the printing is also affected. For example, when the urea component exceeds 5%, the black of the thin textile The penetration is very obvious. According to the actual production experience, the comprehensive coloring and product fineness factors, the urea component of the thin textile is determined to be 3% to 5%, and the urea component of the thick textile is determined to be 5% to 8%.

In order to ensure the subsequent reaction of the printing ink with the textile surface fiber to improve the color fastness, we must try to control the pH value of the slurry, avoid the hydrolysis of the ink before the steaming, and promote the dye in the ink and the surface fiber of the textile. The reaction takes place during the steaming process to improve the fixing effect.

Pretreatment of chemical fiber textiles

The chemical fiber inkjet printing process has the same pre-treatment process as natural fibers, but most of the chemical fiber structures lack hydrophilic groups or fewer hydrophilic groups, and because they are artificially treated, their surface structure is relative to natural fibers. It is relatively smooth, so the hydrophilic effect of most chemical fiber textiles is poor, which is not conducive to the subsequent printing process of the inkjet printing process. Therefore, it is necessary to improve the hydrophilicity of chemical fibers, including chemistry, in the pre-treatment. Two methods and physical methods:

 

Among them, there are usually three chemical modification methods: the first one, introducing a large number of hydrophilic groups in the macromolecular structure of the fiber by chemical reaction such as polymerization, copolymerization, etc., thereby improving the hydrophilicity of the fiber molecule; The fiber molecule can be graft copolymerized with the hydrophilic substance; the third type can also be hydrophilically treated on the surface of the fiber: a hydrophilic compound is added on the surface of the chemical fiber textile, that is, a hydrophilic finishing agent, which is currently commonly used on the market. There are two kinds of hydrophilic finishing agents: one is an acrylic monomer, and the other is a surfactant having a hydrophilic portion and a fixing portion in the structure.

The use of the first two methods will reduce some of the excellent properties of the fiber raw materials, such as the deterioration of the color fastness and the hardening of the product. Therefore, the commonly used chemical modification method is mainly the third one, that is, the use of a hydrophilic finishing agent, which is relatively simple to handle, low in cost, and can improve the moisture absorption performance of the fiber under the premise of protecting the original properties of the fiber, but the disadvantage is that after the treatment The fiber has poor hydrophilic durability and poor washing resistance, so it is also necessary to carry out special treatment in the subsequent gas-vapor curing. 

Physical modification methods include blended textiles or composite textiles blended or composited with hydrophilic materials. The mixing of materials is not part of the discussion herein and will not be described here; for single composite fibers, it can also pass through fibers. Structure microporation, fiber cross-section profile, fiber surface roughening to change the fiber's morphological structure, cross-sectional shape and surface contact angle, etc., in order to improve the hydrophilicity of the fiber, to achieve the inkjet printing process Fitness. 

In addition, there are many kinds of chemical fibers, and the properties of various textiles are also different. It also needs to be different depending on the materials in the printing process. For example, acrylic fiber is a thermoplastic fiber, which is easily deformed during steaming and under stress. It is easy to deform and needs to be processed accordingly.

Conclusion: blended, interwoven, composite textiles

With the emergence of new materials and the advancement of textile technology, the structure and composition of textiles will also change with each passing day, such as blending, interlacing or composite textiles of various fiber materials. How to ensure that these new materials can meet the adaptability of inkjet printing process, which needs to be based on the surface properties of textiles, through theoretical analysis, combined with actual production to explore a reasonable prepress processing method, in order to improve the application range of inkjet printing process.