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| Paperless heat transfer printing machine |
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Q: What printing processes are commonly used for fabrics?
A: There are currently two major printing processes for fabrics: silk-screen printing and heat transfer printing.
Silk-screen printing consists of lithographic printing and circular-screen printing. Both require the use of a stencil to obtain the desired patterns. In order to fix the ink, the fabric needs to go through the processes of steaming, fixation, washing and rinsing. Such printing technique is well-developed - a conventional machine can print up to 60m/min, but it produces a large amount of wastewater, as every 100m of fabric could use up to 5m3 of water.
As far as water consumption is concerned, heat transfer printing, developed since the 1960s, offers an alternative and has found extensive applications in various industrial sectors. Dyestuff is used to color the patterns printed on the heat transfer paper, which is then pasted on the fabric surface to fix the patterns under certain temperature. Such method saves the finishing process and eliminates the problem of wastewater. The downside, however, is an increased consumption of paper, and the method can only apply on polyester fabric.
Q: What is paperless heat transfer printing?
A: Paperless heat transfer printing shares similar principles as those of the conventional paper transfer method. But instead of using special heat transfer paper, paperless transfer printing requires a special kind of substrate. Dyestuff is used to dye the pattern printed on the substrate, which is reusable. The substrate then transfers the pattern onto the fabric inside the heating zone of a specially designed paperless printing machine.
A photo printed on fabric using paperless heat transfer method |
Q: What are the advantages of paperless heat transfer printing over the conventional methods?
A: Compared with the conventional methods, paperless heat transfer printing does not require any post-treatments such as steaming and washing, and the substrate can be reused to produce new patterns. So the technique helps to save cost and reduce the consumption of water and energy. It is estimated that energy cost makes up 50% of the total cost of a printing factory.
With paperless heat transfer technique, it is easier to print complicated patterns or photos on fabric, and with lower cost, while the printed images are better defined than when employing conventional methods. These advantages lead to more flexibility in textile design.
Q: Why bridging agent is sometimes used in paperless heat transfer printing?
A: Disperse dyes used in heat transfer printing is hydrophobic by nature, while natural fiber such as cotton is hydrophilic. This is why heat transfer printing cannot be directly applied on fabrics made of natural fibers.
To solve the problem, a bridging agent can be used, linking the natural fiber on the one hand and disperse dye on the other. This way, heat transfer printing can be realized on natural fiber.
Currently, there are bridging agents for use in pure cotton, polyester-cotton blend and pure silk.
Answers were provided by Wang Hongfeng, an associate professor at the Department of Material Engineering, Beijing Institute of Clothing Technology.
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