Successful production of plastisol heat transfers starts with the design. Not all designs will make good transfers and direct print designs that are used to print transfers will need some adjustments. When you are creating designs for transfers you must always keep in mind that the design you are creating will go through an intermediate stage (the paper transfer) before it appears on a T-shirt. This affects nearly every aspect of the design, from the sequence of colors printed to the fineness of detail that you can print. When you are creating a design for heat transfer printing, remember these rules:
Reverse the design
The design that you image on the screen and print on the transfer paper must be the mirror image of the way you want the design to appear on the T-shirt. Remember that after the design is printed on the paper and cured, it is flipped over to apply to the T-shirt. That means that the left side of the design on the screen and the paper becomes the right side of the design when it is applied to the T-shirt. Film positives used to image screens for transfer printing should be wrong reading, emulsion side up.
Avoid both fine details and large areas of solid color
In a transfer design, fine details are bothersome because it's hard to print enough ink and control the temperature of the ink closely enough to insure that fine details will adhere to the T-shirt. Thin lines and fine type are especially difficult. Generally speaking, lines should be no thinner than 1/16" and halftone dots should be no finer than 13-15 mils. It is possible to print and transfer finer details, however this requires considerable experience in transfer production and expensive equipment. If you do have to print fine details, consider adding a backing layer, a layer of plastisol, generally white, black, or clear, that is printed over the entire transfer as the last color down. The backing layer adheres to the garment and the other colors in the transfer, including the fine details, adhere to the backing layer. Another method to ensure the correct adhesion of fine details is by using an adhesive powder.
Large areas of solid colors may cause problems because they require a very even layer of ink, very evenly gelled. Any imperfections in the thickness of the ink layer or degree of gelling of the transfer may result in an uneven thickness of ink transferred to the T-shirt and an obviously defective design.
Generally, reverse the color printing sequence
In most direct printed designs, the dominant color or the black outline is printed last. In transfer production, this is reversed. The color that is printed first on the transfer (and in a multi-color design, winds up on the bottom) appears on the top of the transfer when it is applied to a T-shirt. Keep this in mind when creating a transfer design and print the dominant color either first, or second after a black outline.
Avoid trapping colors
Trapping colors (the practice of printing one layer of ink over another) should be avoided whenever possible. Because it creates differing thicknesses of plastisol on the transfer it complicates the gelling process, and with hot-split transfers, may result in the wrong layer of ink being split during application. It's far better to butt register the colors whenever possible. If butt registration is not possible, make the trap width as narrow as possible.
Allow for paper shrinkage
Transfer prints, unlike direct prints, cannot be printed wet-on-wet. Each ink color must be gelled before the next color is printed. This means that for multi-color designs, the paper is constantly being heated and cooled during the transfer printing process. A large sheet of transfer paper can shrink up to ¼ inch or more between one color and the next. Some shrinkage is unavoidable. Keep this in mind when you are creating transfer designs. A design that is difficult to register when direct printing will be doubly so on transfer paper.
None of these design rules are iron-clad except for the first. However, when learning to print transfers you should start with simple single-color designs then progress to simple multi-color designs before you attempt to produce complicated multi-color designs. Usually direct print designs will need adjustments, sometimes minor, sometimes major, before they can be used in transfer printing.
Hot-melt adhesive powders act as a very adhesive glue when heated and can allow you to apply transfers in situations where otherwise they would not adhere. The adhesive powder is applied to the surface of the transfer after printing. There are several ways for doing this. The most common is to fill a box large enough to hold a transfer with adhesive powder an inch or so deep. After printing the transfer, pass the paper through the box, scooping it under the powder in such a manner that the entire printed surface of the transfer is covered with the adhesive powder. Shake the excess powder back into the box as you lift the transfer out, then gel the transfer. You can also fill a large salt shaker with adhesive powder and shake it onto the transfer, coating the ink thoroughly. Remember to shake the excess powder off the transfer back into a box for re-use before passing the transfer through the dryer.
When making multi-colored transfers, you can apply the adhesive powder to all the colors as they are printed or only to the last. The wet ink will hold a good layer of powder, but some will stick even to the gelled ink.
The Transfer Press
For productive and profitable transfer applications you will need a well made heat transfer press. Applying a transfer correctly requires applying enough heat, and enough pressure, for a long enough time to fuse the ink onto the surface of the garment.
Temperature: There are two types of temperature controls available on heat transfer presses, thermostatic and solid-state. Both will do the job.