The printing of knitted fabrics has advanced significantly over the past decade, yet challenges remain due to their limited dimensional stability. This makes it particularly difficult to achieve consistent results in circular fabric printing, especially with cotton blends containing spandex. These materials are highly elastic, and their drape weight and tension during feeding can be influenced by speed variations, making pattern control complex. Maintaining geometric designs is even more challenging, and some patterns are hard to reproduce effectively. Therefore, precise control throughout the entire process is essential.
1. **Printing Process for Spandex Knitted Fabrics**
The process typically involves: weaving open film rolling → loose cloth → steaming → pre-shaping → pre-treatment → dyeing → printing → steaming → washing → dehydration → drying → setting. Each step must be carefully managed to ensure quality and consistency.
2. **Color Separation Design**
Computerized color separation is critical. It begins with understanding the fabric type, elasticity, pattern complexity, number of colors, and printing method (direct, discharge, or reverse). The design must be scaled appropriately to match production requirements. For example, when spandex content is between 3% and 5%, the warp direction should be stretched by 13–18%, while the weft direction is reduced by about 3%. When spandex is 6–15%, the warp expansion increases to 20–26%, with a 5% reduction in the weft. These adjustments depend on printing speed, which is usually kept between 30–40 m/min for better control.
3. **Complex Color Positioning**
Knitted fabrics have a looser structure than woven ones, allowing for better ink penetration. In direct printing, 8–16 filaments are typically used to maintain precision. During stack printing, reducing the number of complex colors helps prevent paste buildup. For anti-printing, careful attention is given to edge clarity and coverage. The size of the draft must align with the color layout, ensuring accurate reproduction of intricate patterns.
4. **Draft Size and Net Making**
The size of the draft is also affected by the net-making technique. Inkjet and wax-jet nets are commonly used, but they may lack the accuracy of traditional film-based nets. Proper temperature and humidity in the netting chamber are crucial for maintaining accuracy, especially when working with fine edges or complex patterns.
5. **Nickel Mesh Selection**
The choice of nickel mesh depends on the pattern fineness and fabric permeability. Common mesh sizes include 40, 60, 80, 105, 125, and 155. Thicker meshes are used for larger patterns, while finer meshes are ideal for detailed work. For plush or velvet fabrics, the mesh size must be adjusted to balance detail and print quality.
6. **Pre-Treatment for Spandex Fabrics**
Pre-shaping is essential to eliminate internal stress and prevent permanent wrinkles. This involves steaming and heat setting at temperatures between 85–190°C. The fabric is often set wider than the finished product to allow for recovery. Proper pretreatment also removes impurities and improves dye uptake, ensuring better printing results.
7. **Rotary Screen Printing Techniques**
Controlling the feeding speed and tension is vital. At lower speeds (below 20 m/min), the fabric stretches less, while higher speeds cause more elongation. The pressure and scraper selection vary depending on the pattern type—thin stems require smaller scrapers, while large blocks need larger ones. The printing speed must be fixed during the rotary screen production phase to avoid pattern distortion.
8. **Printing Process Formulation**
The formulation of the printing paste depends on the fabric type and dye. For reactive dyes, ingredients like sodium alginate, alkali agents, and urea are used. Urea levels vary based on fiber type, with higher amounts needed for synthetic fibers. K-type dyes work well on cotton, while P-type or PX-type dyes are preferred for modal and bamboo fibers.
9. **Discharge Printing Process**
Discharge printing requires careful preparation of the paste, including starch ether, sodium alginate, and whitening agents. The whiteness level is adjusted according to the desired result, and the mixture is thoroughly stirred before use. Proper drying and steaming are essential to ensure color development and uniformity.
10. **Post-Printing Steps**
After printing, the fabric is dried at controlled temperatures to avoid discoloration. Steaming is then performed to fix the dye, with timing and temperature varying based on the dye type. Washing is done carefully to remove excess chemicals and prevent color bleeding. Finally, the fabric is set to stabilize its shape and improve its appearance.
11. **Conclusion**
Circular screen printing of spandex-containing knitted fabrics, especially discharge printing, was once considered a challenge. However, through careful process control and technical improvements, it is now possible to achieve high-quality prints. Ensuring no wrinkles, proper tension, and precise sizing from the start is key to success. With the right techniques, spandex knits can now be printed efficiently and consistently.
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