GENERAL INFORMATION ABOUT THE HEAT TRANSFER PROCESS
The heat transfer process has become increasingly popular among screen printers and T-shirt manufacturers. There are a number of advantages to the process, even though it does add another step into the process of screen printing. The transfer process uses a specially formulated paper as a medium for the ink. The image is printed on the paper rather than on a T-shirt, or other product, and the paper, with the use of heat, then transfers a completed image to the final product as another step.
One of the most prominent advantages is that the printing process is shifted into two locations. There is less likelihood of the products being damaged by ink spills or general messiness of the printing shop. Irregular, rather than flat, surfaces are easier to handle with a transfer than by rigging a screen to form around these contours. If there are problems that emerge in the printing process, they can be corrected without losing any of the shirts, or products, that are to be printed. T-shirt printers can work in small studios with transferrable images brought from another source.Manufacturing a heat transfer requires the use of:
- Appropriate ink (plastisol for silkscreen; lithography ink can also be used in connection with plastisol for adherence)
- Release paper on which the image is printed
- A carefully controlled process
Transfer paper is a smooth-finish paper. Using a smooth paper permits the application of a finer and more exacting image. Since the paper is smooth, the image will copy without the imperfections caused by a textured surface. Also, the printing process itself requires less ink, since there is no need to compensate for the roughness of the surface. A thinner surface of ink is possible. Blank transfer papers can be bought from a variety of sources.
Since the paper is smooth and nonporous, less application pressure is needed to apply the ink. The ink will rest on the surface rather than need to be pushed down into the crevices of a surface. Squeegee operation can be more gentle, and there is less wear on the screen itself.
When using the squeegee, or setting blanket pressure, review the prints during the process. If the image is breaking up, more pressure is needed. If the image is smudging as it’s printed, less pressure is required. A smooth ink surface is the goal. The pressure of the squeegee varies depending upon the size and intricacy of the image layer being printed.
While printing on transfer paper, one concern is the avoidance of a layer picking up a previous layer while printing. The paper is designed to release the ink onto another surface and sometimes in printing subsequent layers, the screen will pick up bits of the previous layer. If this happens, reduce the pressure on the surface of the screen in subsequent passes, and use slightly less ink.
• • • TEMPERATURE • • •
350°F is the industry standard. Too much heat will cause the papers to become brittle and tear. Too little heat will cause poor adhesion and poor washability.
• • • TIME • • •
The amount of time necessary to complete the transfer is generally about 10 to 30 seconds. The time varies with different thicknesses and different transfer presses. Essentially, enough time is needed for the plastisol inks to heat up, soften, and work their way into the fabric. If the original printing process over-cured the transfer or printed an ink layer that is too thin, the ink will not be able to melt and sink into the weave of the fabric. The image will merely stick to the surface of the garment or the object without actually creating a bond. It will easily crack off or peel.
• • • PRESSURE • • •
Enough pressure is needed to drive the softened plastisol ink into the garment’s weave. Pressure should be monitored carefully. Too little pressure will result in poor adhesion and a short life for the printed image on the garment. It is safer to apply too much pressure than to apply too little.
Transfer papers are used with other applications besides silkscreen. Lithography inks canbe printed on the sheets, and backed up with a white plastisol layer to provide the cured adhesion to a garment.
The actual manufacturing process for these inks is very similar to the process used in screen printing colors. Color layer after color layer is applied. As a final layer, a white back-up of plastisol ink is printed over the entire image.
This layer becomes the bond. It is like a covering of glue that holds both the fabric and the lithography inks and provides a secure hold between them.
Sublimation or inkjet printers can also produce a transfer. When a single copy, or a very limited number of copies, of an image is needed, this is the most efficient method. Specially treated transfer paper is run through an inkjet and the image is printed onto the paper. This image is then adhered to the garment in much the same way as the heat transfer images are.