HEAT TRANSFER PRINTING (Thermal Transfer Printing)
The principle of heat-transfer printing is somewhat similar to that of familiar decal transfers. In this method, the design is first printed on paper with printing inks containing disperse dyes. The printed paper (called transfer paper) is then stored until ready for use by the textile producer.
Iron-on (or press on) prints used as emblems and decorations on garments (or other areas of textile fibers) are usually vinyl plastic or pigments mixed either into a paraffin wax or a thermoplastic base. The heat of the iron melts the substance, which then becomes bound to the fabric when cooled. These iron-on emblems stiffen the fabric in the area of print. When subjected to machine wash and tumble drying, they gradually loose colour and become considerably lighter and faded. They are however, not adversely affected by dry cleaning.
SUBLIMATE – the ability of substance, upon application of heat, it turns from solid to gas without going through the stage of liquidity and when cooled solidifies again – disperse dyes have this property.
When fabric is tobe printed, it is passed through a heat transfer printing machine which brings the printed transfer paper and the unprinted fabric together, face-to-face and passes them through the machine at about 210degree centigrade. Under this high temperature, the dye on the printed paper sublimates and is transferred onto the fabric completing printing. No further treatment is required. The process is relatively simple and does not require the expertise necessary for producing roller or rotary screen printing.
Only Disperse dyes sublimate and the only ones that respond in a way that permits heat-transfer printing. The process is therefore limited to fabrics that are made with fibres that have affinity to disperse dyes. Some of the fibres that have affinity to Disperse dyes are acetate, acrylics, polyamides and polyesters.
In Heat-transfer printing, the textile printer purchases printed paper from a source of highly specialised papers. The transfer paper is printed in accordance with the designer’s or customer’s art work. Stock patterns (ready for use transfer designs) are usually available with heat transfer paper supplier.
Heat transfer printing may be transferred to garment parts (logos, emblems, motifs, borders). Special designed heat presses are used to apply heat transfer design on garment parts.
Main advantage of heat transfer printing is space and cost saving due to the elimination of equipment like dryers, steamers, washers and tenter frames.