Transfer printing refers to a printing system in which dye is first printed on a paper substrate and then transferred from paper to cloth. Most of the dyes used in transfer printing belong to disperse dyes and a few are solvent dyes. Therefore, heat transfer printing is most commonly used in nylon and polyester. It is also used in acrylic, triacetate, and polyester-cotton blended fabrics with a high content of polyester. Although there is a lot of research on heat transfer printing for other fibers, there is no feasible process for actual production.
When the transfer printing paper which is in close contact with the fabric is heated, the temperature of the paper, dye, and fabric is quickly close to that of the heating surface. Due to the increase in temperature, the dye vapor pressure increases, and the dye vapor flows in and out between the dye solid on the paper and the dye vapor on the paper, and a dynamic equilibrium is established. The sublimation process of the dye continues until the dye vapor on the paper reaches a supersaturated state, and then the dye vapor is absorbed and concentrated on the fiber surface, releasing the supersaturated vapor. By forming a dye concentration gradient on the surface of the fiber, the diffusion of dye molecules into the fiber begins. The conversion mechanism of dyes, including sublimation, supersaturation, concentration, and diffusion, is carried out in dynamic equilibrium until the partial pressure of dye vapor between the printing paper and the dyed fiber reaches the equilibrium value.
Therefore, the transfer printing process has many names, such as vapor printing, dry heat printing and sublimation, and so on.
Dyes suitable for transfer printing should have excellent color value, excellent color fastness, and excellent printing ink adaptability. These dyes can be transferred to polyester fabric within 20 seconds between 190 ℃ and 210 ℃.
Transfer printing paper production method
Intaglio engraving printing: It uses a steel or copper roller with an engraved pattern on it and is the most versatile method that produces consistent results in rapid processing.
Lithography: It can produce the most minute reproduction of a picture on paper. But this method has a restriction, it is not enough ink thickness on the paper.
Rubber plate printing: use the raised pattern area of rubber plate printing. Can produce satisfactory transfer printing paper, and can give a high amount of ink. But the amount of ink transferred to the printing paper is more difficult to control, resulting in changes in the shade of the color.
Screen printing method: It is only used to prepare the transfer printing paper which has been cut into sheets. The paper is used to print special designs on shorter pieces of cloth.
Classification of transfer printing machine
It is divided into two types, that is, hot plate type and roller type transfer printing machine.
- Flat plate hot pressing transfer printing machine is mainly used for clothing printing. In a typical plate transfer printing machine, the printing area consists of an upper and a lower plate. The two plates are heated directly by electricity, and the temperature of the whole working surface is controlled within the error of ±5℃. It is pneumatically pressurized by two cylinders.
The printing process is divided into three steps, clothing feeding, printing, and clothing code pile. Prepare the garment for printing in the feeding area and send it to the printing area under the electric heating head. When the conveyor belt stops, the heating head is pressed down and held until the time set by the dial, while another garment is ready and waiting in the feeding area. At the end of the scheduled heating time, the heating head is raised, the conveyor belt starts and sends the printed garment to the unloading area, while another garment is ready to arrive at the printing area.
2. Continuous rotary printing machine is suitable for fabric printing. A continuous rotary printing machine is mainly composed of a hot metal cylinder and a continuous conveyor belt. The printing paper, the fabric to be printed, and the backing paper are placed separately on a scroll at the entrance of the machine. The printed paper, the fabric to be printed, and the backing paper are fed down to a pair of feeding rollers where they are stacked together and fed into the printing machine. As this overlap passes through the machine, it is held together by the pressure of the conveyor belt towards the large roller.
Large rollers are usually heated by their internal thermal oil, heating temperature can reach 180℃. Under the conditions of temperature, pressure and time, the dye is transformed into a gas, which is transferred to the fabric and diffused into the fiber. After leaving the large roller, the dye is firmly enough attached to the fabric for most purposes without the need for further color fixing or wash processing.
Advantages
Transfer printing achieves accurate and clear pattern outline that other printing methods cannot achieve. This method is very suitable for producing patterns such as plaid, which have many problems with other commonly used printing methods. All colors are printed at the same time, so operation is simplified and processing costs are low. Production can be adjusted quickly by simply changing the printing paper. Short production process and quick delivery.
It has been proved that transfer printing is quite successful in producing knitted fabric printing products. Knitted fabrics have less dimensional stability than woven fabrics. There are many difficulties in producing multi – color knitted prints with common screen printing or roller printing because of the need for accurate floral matching. In transfer printing, all parts of the pattern are printed at once, eliminating problems that drag the fabric as it moves from one printing area to another. Equipment investment is low, small production site. Although it is necessary to control the temperature and feed speed accurately, it does not require a very skilled printer. Low rate of nonconforming products. As the printed fabric no longer requires wet processing, there is no water pollution.
On the other hand, transfer printing has lower yield than roller printing and circular printing. The printed fabric only has color fastness, especially the fastness to brilliance is low. There are certain restrictions when producing dark products. Ordinary printing is easy to change the color, but transfer printing paper, once printed cannot be changed. Compared with the ordinary calico process, the pattern design and engraving of transfer printing is not time-consuming. The resulting glass feel of the fabric is not accepted by the market. In order to improve the feel, further wet processing may be required, thus increasing the production cost.