While doing some research on creating plastisol heat transfers, I came across this article written by the fine folks at union ink company. It is lengthy, but pretty much tells you everything you could ever want to know about making transfers.
“The basic process for producing heat-applied plastisol transfers is uncomplicated. You print a design with plastisol ink, but instead of printing it directly on the garment, you print the design on special paper. The paper is then passed through a conveyer dryer where the ink is heated until it has gelled just enough to be dry to the touch. It’s important not to cure the ink too much.
The resulting print, called a transfer, can be stored until needed. When you want to apply the transfer to a T-shirt, place the garment in a heat transfer press, put the transfer on top of the garment, ink side down, and close the press. The heat and pressure applied by the press will force the ink into the garment and finish curing it. When the press is opened and the paper is peeled off the shirt, the ink remains behind. When done correctly, a heat-applied plastisol transfer will be as permanent as a direct print and under some circumstances nearly indistinguishable.
Why Use Transfers?
Why would you want to print transfers that require extra materials (the paper), extra labor (the transfer application process), and more equipment (a heat transfer press) when you can print directly on the garment? There are several situations where plastisol transfers are actually more efficient, economical, and profitable than direct printing.
For example, suppose you have a contract to provide decorated T-shirts for a once-a-year event but you have no way of knowing in advance how many shirts will be sold. If you prints too few shirts, you will unnecessarily limit your sales. If you prints too many shirts you’ll take an expensive loss on the unsold shirts. However, if you print the design on transfers, you can take the transfers, a transfer press, and a stock of blank shirts to the event and decorate the shirts to order. At the end of the day you have some unsold blank shirts, which you can put back in stock to sell another day, and some surplus transfers, which only cost you a few cents each so you can afford to throw them away. Sales are increased and waste is reduced.
Plastisol heat transfers may also be the most profitable decorating method when you have to reprint a design frequently, but in small quantities. Let’s assume that you have a design that you print four or five times a year, but each order is for a small quantity of shirts. The labor involved in setting up the press each time will significantly increase the cost of the job. If you print an entire years supply of transfers in one press run, you can store them, then quickly and inexpensively apply them to blank garments as each order comes in. Job costs are considerably reduced and the shirts can be decorated in minutes.
Plastisol heat transfers are also a popular method of decorating baseball caps. Baseball caps are difficult to print well because of the complications involved in printing on a curved surface. Transfers for baseball caps can be printed very quickly because the design is so small that you can print several on one sheet of paper. Applying the designs is also quick and easy with a special cap transfer press that automatically wraps the transfer around the curve of the cap.
Another factor in favor of plastisol transfers is that, for beginners at least, only simple, inexpensive equipment is necessary. Although large transfer companies that produce millions of transfers a year use complicated and expensive production equipment, for printing small orders or to get started in heat transfer production, a simple press, easily manufactured by the screen printer, a conveyer dryer, and a heat transfer press is all that is required.
Types of Transfers
The two most common types of plastisol heat transfers are Hot-Split and Cold-Peel. The main difference between the two is the way they are applied. When applying hot-split transfers, the transfer paper is removed immediately after the heat transfer press is opened. Because the plastisol ink layer is still hot and relatively fluid, it splits. Most of the ink remains on the T-shirt, but some adheres to the paper. Hot split transfers have a very soft hand and when properly applied, are almost indistinguishable from a direct print. Because hot split transfers leave a thinner layer of ink on the T-shirt, you may have problems with opacity, especially on dark-colored garments.
When applying cold-peel transfers, the paper is not removed until the ink and garment have cooled. The entire ink layer adheres to the T-shirt. Cold-peel transfers are quite stiff and have a characteristic smooth or glossy look. They have excellent opacity and are popular on athletic uniforms.
Other types of transfers include hot-peel, where the paper is peeled while the transfer is still hot but the ink layer does not split, puff transfers, where the applied transfer has the three-dimensional look of a direct-print puff design, and process color transfers, where a half-tone process color print is applied to the garment.
The Design
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:
1. 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.
2. 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″ (1.6 mm) and halftone dots should be no finer than 13-15 mils (.33-.38 mm). 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 power. We will cover that later in the article.
Large areas of solid colors may cause problems because they requires 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.
3. 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. This rule is not infallible. You will find situations where you have to experiment to determine the correct color sequence. Generally the more colors and the more complicated the design is, the more often you will have to print test transfers and apply them to determine the correct color sequence. Color sequences for hot-split transfers may be different than for cold-peel transfers.
4. 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.
5. 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 (6.35 mm) or more between one color and the next. There are methods for minimizing this that we will cover later in the article, but 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.
The Screen
One of the crucial variables in plastisol transfer production is ink film thickness. For this, the correct mesh is the key. Ideally, the ink film thickness should be between 3 mils (.0762 mm) for transfers where opacity is not required, to about 6 mils (.1524 mm) in high opacity and glitter transfers. Remember that ink film thickness is more important in hot-split transfers because only part of the ink layer adheres to the T-shirt. The rest is stripped off with the paper. Since wet film thickness gauges and micrometers are readily available and not too expensive, you may want to conduct tests to determine exactly which mesh counts will give you the best results in your plant. You can use the following numbers as guidelines to get started. (In all cases, the specifications are for monofiliment polyester meshes with the smallest thread diameter available in the mesh count specified.)
For detailed images, try a 125 threads/inch mesh (49 threads/cm). For general transfers, use 86 to 110 threads/inch mesh (34-43 threads/cm). For transfers where opacity is required, use 60 to 75 threads/inch mesh (24-30 threads/cm). For glitter transfers, use 25 to 35 threads/inch mesh (10-13 threads/cm). Carefully tensioned mesh on retensionable frames is an advantage, but not essential.
Transfer printing is one situation where direct film emulsions definitely offer an advantage over direct liquid emulsions. The smooth substrate side of capillary film will print a crisp, sharp image on the transfer paper. Use the thickest capillary film available, in order to print a thick, even layer of ink. If capillary films are not available, coat the mesh with direct liquid emulsion and after the first application has dried, apply another layer on the substrate side of the mesh to create a thicker, smoother stencil.
The Ink
Although many plastisol inks can be used in transfer printing, you will get the best results using a plastisol that has been specifically manufactured for transfers. Most ink manufacturers have developed a variety of specialized transfer inks. A good transfer plastisol should be reasonably opaque for use in hot-split transfers and low tack for printing multi-color transfers. It should have a wide temperature range in which it will gel but not cure after printing and also a wide temperature range for final application to the garment. It should have a viscosity suitable for printing on paper, not stringy or sticky.
Specialized transfer inks include high-opacity inks for transfers intended for printing on dark garments, puff transfer inks for printing puff transfers, and glitter inks for printing glitter transfers. You may also find a use for clear transfer inks as a backing layer for designs with fine details and process color designs”