Heat transfer printing technology was developed by European printing technicians in the late 1960s. This process utilizes the sublimation principle of disperse dyes or inks, employing paper as a carrier. Through temperature-induced sublimation, the printed patterns on the paper are transferred and replicated onto fabric..
Heat transfer printing technology began production in some printing factories in the early 1970s. However, during this stage, most of it involved intermittently pressing the pattern from the transfer printing paper onto garment pieces, ready-made garments, bags, or cut pieces of synthetic fabrics. In the mid-1970s, transfer printing technology began to be used in continuous production, just like traditional printing. Due to the improvements of printing equipment, the quality of the transfer printing base paper improved accordingly, and its machine printing capacity was generally between 2000 and 3000 m³/h.
Classification of Heat Transfer Printing
Heat transfer printing is generally classified into heat-melt transfer printing and sublimation transfer printing. Heat-melt transfer printing is commonly used for cotton products, but its disadvantage is poor hand feel and breathability. Sublimation transfer printing is commonly used for polyester, but its disadvantage is high plate-making costs.
The most commonly used heat transfer printing method is sublimation. The principle is to utilize the sublimation properties of disperse dyes, transferring them to synthetic fibers such as polyester under high temperature conditions and fixing them there.
The specific process involves printing the pattern onto paper using disperse dyes or inks through rollers or flatbed screens (or rotary screen printing machines). Then, the transfer printing paper with the pattern is passed through a transfer printing machine and, under suitable temperature and pressure conditions, the pattern on the paper sublimates instantly onto the fabric surface through physical and chemical reactions, simultaneously diffusing and penetrating into the inner layers of the fibers for fixation. This is the current process for heat transfer printing on polyester fabrics.
Sublimation transfer printing can be further classified according to different printing methods: offset printing, gravure printing, screen printing, and digital printing.
Advantages and Disadvantages of Heat Transfer Printing
Compared to traditional processes, heat transfer printing has the following advantages:
- Smaller footprint and shorter process flow;
- Utilizes the sublimation fixation properties of disperse dyes for complete color development and fixation;
- Eliminates post-treatment processes such as fixing and washing, thus avoiding wastewater issues and making it an environmentally friendly printing method;
- Since transfer printing base paper absorbs significantly less dye than direct printing on fabric, it reduces costs relatively;
- Defects arising from complex color matching and pattern registration can be detected during the printing process and removed before transfer to the fabric, ensuring the highest yield of acceptable finished products;
- Due to the minimal ink penetration of the base paper, heat transfer printed fabrics exhibit clearer patterns, more distinct layers, and more uniform color distribution. The three-dimensional effect is strong, especially noticeable in halftone effects.
Disadvantages:
- Suitable for small-batch, multi-variety products with short lead times;
- Limited fiber application. The transfer process requires high temperature and pressure, so it is mainly used on synthetic fiber fabrics, primarily polyester fibers. A drawback of heat transfer printing is that it is difficult to achieve satisfactory large-scale production on natural fiber fabrics;
- Transfer paper requires high quality and is used in large quantities.
Factors Affecting Heat Transfer Printing
- Transfer Temperature
The temperature depends on factors such as the optimal sublimation temperature of the dye, the heat resistance of the fiber, and the heat transfer time.
For disperse dyes used in heat transfer printing, the sublimation temperature should be lower than the melting point of the fiber macromolecules to avoid damaging the fabric strength. For polyester, a suitable processing temperature is 180–210℃. Within this temperature range, the molecular weights of dyes that sublimate are generally between 230 and 270.
2. Transfer Pressure
Flatbed Printing Machine: The standard transfer pressure is 10 kPa. Insufficient pressure will result in poor adhesion between the transfer paper and the printed fabric, leading to uneven printing and dull colors. Conversely, excessive pressure will alter the feel and style of the printed fabric.
Roller Transfer Printing Machine: To ensure close overlap between the transfer paper and the printed fabric, the blanket must be tightly wrapped around the surface of the heated roller and the appropriate pressure is generally controlled at 12 kPa.
Vacuum Negative Pressure Heat Transfer Machine: Under negative pressure (13.3 kPa), it can achieve good coloring and penetration effects, and the printed fabric has a very good hand feel.
3. Transfer Time
In actual processing, the transfer time is 15~45 seconds. The transfer temperature, fiber type, and the structure and type of woven and knitted fabrics will all affect the transfer time.
4. Colorfastness
The colorfastness of transfer-printed fabrics mainly depends on the following three factors: the fabric being processed; the transfer paper used and the dye printed on the transfer paper; and the transfer printing process conditions.
