Process Requirements of Heat Transfer Printing Technology
Heat transfer printing is a special printing process that uses heat transfer ink (i.e., thermal sublimation ink) to print any image on color inkjet paper (or thermal sublimation special inkjet printing paper) or ordinary ink on rubber mold heat transfer paper. After that, it is heated to a certain temperature within a few minutes by a heat transfer heating device, and the pattern on the paper is transferred to materials such as ceramics, glass, metal, and cotton textiles, making the colors of the pattern look realistic.
The heat transfer printing process mainly involves mastering three key elements: (1) the heating temperature of the drum; (2) the transfer speed; (3) the pressure and temperature of the rubber roller. Below are explanations of these three points:
When printing drum markings using the heat transfer method, the drum needs to be heated first. During heating, the surface temperature of the drum is generally heated to 80-90°C. This ensures that there is an optimal temperature of 60-70°C during the printing process when moving the drum from the heating area to the heat transfer machine. The heating of the drum can utilize the temperature after baking the paint, that is, the drum is not air-dried after leaving the baking area, and the marking can be directly printed. Alternatively, electric heating can be used for galvanized drums or cooled painted drums. For electric heating, a heater can be self-made. Only the area where the marking needs to be printed needs to be heated, and the entire drum does not need to be heated. Our company's self-made simple heater is shown in the figure below. The drum is placed next to the heater in an upright position, about 80-100mm away from the resistance wire, and it can be placed on the heat transfer machine for printing in approximately 1 minute.
The speed of printing drum markings using heat transfer is controlled by the speed of the pressure roller. Generally, adjusting the linear speed of the roller to 8-9m/min is sufficient. If the speed is too fast, the transfer will be incomplete, with some patterns remaining on the transfer film and causing missing parts of the pattern. If the speed is too slow, the transfer film will stick to the drum, affecting quality. In addition to adjusting the roller speed appropriately, the unwinding speed and winding speed of the transfer film must also be adjusted accordingly (refer to the process diagram).
Pressure and temperature of the rubber roller
When the rubber roller is pressed onto the drum for printing, there must be a stable pressure. Generally, the system pressure is adjusted to 0.2MPa. The actual pressure on the drum also depends on the distance between the rubber roller and the drum after the cylinder retracts (see simplified process diagram). Adjusting the distance between them can adjust the printing pressure within a certain range. The standard for adjustment is to ensure that the printed pattern is clear and complete, while preventing deformation and indentation of the drum body.
In addition to pressure requirements, the rubber roller, like the drum, also needs heating. Its heating is achieved by the heating tube above the rubber roller of the heat transfer machine, and the purpose of heating is to heat the transfer film when it is pressed onto the rubber roller. Generally, the temperature control display of the heat transfer machine is set to 150-170°C. It should be noted that the distance between the sensor probe and the rubber roller must be between 20-30mm at this time, and if the distance is too large or too small, the temperature needs to be reset.
In order to ensure the printing quality of the marking, the length of the marking should ideally be controlled within 500mm and the height within 150mm. This keeps the temperature of the drum body consistent from the beginning to the end of the printing process and ensures quality.
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