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.
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.
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.
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.
Under ideal circumstances, when the plastisol ink is applied to the garment, pressure and heat combine to force the ink into the fabric and when the ink cools the transfer adheres to the fabric. However, in the real world of garment decorating, the circumstances are rarely ideal and transfer adhesion can be a problem. When applying transfers to fabrics where adhesion may be questionable, such as some nylons and polyesters, or when only light pressure can be applied to the transfer, such as when applying puff transfers, or for transfers containing fine details, or in any other case where transfer adhesion is problematical, you may want to add a hot-melt adhesive powder to the transfer.
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. There are several methods for adding them to transfers, depending on the type of transfer and the type of powder. Finely ground adhesive powders can be mixed with the ink before printing, generally 10-15% powder by weight. This is the usual method with puff transfers and for transfers which will be foiled.
More often 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.
Heat Transfer Paper
Not just any paper can be used for plastisol transfers and the paper you select will have a major effect on how easily your transfers print, how they apply to the garment, and how the transfer looks after it is applied.
Paper and Transfer Type: The first thing to consider when you are selecting a transfer paper is what kind of transfer you are printing. Hot-split transfers require a different type of paper than cold-peel transfers, although some types of paper can be used for both. The type of paper used can also have a significant effect on the opacity of hot-split transfers, as the finish of the paper will effect how much of the ink adheres to the paper as it is peeled off the T-shirt. Obviously, the more ink that adheres to the paper, the less remains on the shirt, and the less opacity the transfer has. Transfers incorporating glitter inks, or transfers intended for athletic uniforms where glossy finishes are desired will require a paper with an extremely smooth finish.
Dimensional Stability: If you're printing multi-color transfers, you need a paper with great dimensional stability because in the course of printing a multi-color transfer the paper will tend to shrink as it is repeatedly heated. This, of course, will make accurate registration difficult or impossible. There are ways of managing this that I will explain later in the article, but for now, just remember that if you are printing multi-color transfers, you must select a paper that has as much dimensional stability as possible.
Curling: The transfer paper must lay flat during printing, curing, and transfer application so another important characteristic of transfer paper is its resistance to curling. Some types of paper tend to curl when heated or exposed to changes in humidity, making them completely unsuitable for heat transfers. Curled paper is extremely difficult to handle, print, and apply in a heat transfer press. Generally, the thicker a paper is, the more curl resistant, however, thicker paper costs more and increases transfer application time, so simply printing transfers on thicker paper is not the solution. A compromise has to be made between paper thickness, cost, and application time.
Paper and Transfer Shelf Life: Transfer papers have special coatings to increase dimensional stability, curl resistance, the thickness of the ink layer adhering to the garment in hot-split transfers, and the smoothness of the transfer finish in cold-peel transfers. Another important characteristic of the coatings used in heat transfers is their ability to improve ink hold out. The ink in a plastisol transfer is a mixture of powdered resins and liquid plasticizers. Because the ink is gelled but not cured, it is possible for the plasticizers, over time, to seep from the ink into the paper. A sure symptom of this is a greasy halo around the ink. This is plasticizer that has soaked into the paper. Once the plasticizer has left the ink film, the transfer can no longer be used because the ink will be brittle and adhere to the fabric poorly. Most transfer papers have a coating applied that forms a barrier between the ink and paper, preventing the plasticizer in the ink from soaking into the paper and prolonging the shelf life of the transfer. Well made transfers will have a shelf life of at least a year if maintained in an environment free from extremes of temperature and humidity.
Paper Cost: The final consideration in selecting paper is the cost. Although it is tempting to economize when selecting paper, the consequences of producing bad transfers, transfers that damage your customers garments, is severe. More expensive papers will be easier to print, produce better looking transfers, and have a longer shelf life. Save the inexpensive transfer paper for situations where you are producing simple transfers that will be applied under your supervision without too long a delay between printing and application.
The Screen Printing Press
Transfers can be printed on almost any type of screen printing presses. For short runs of simple designs, transfers can be printed on T-shirt presses in a manner similar to the way that T-shirts are printed. However, most transfers are printed on either commercial or home-made graphics presses. The typical clam-shell graphics press, with a vacuum bed hold-down and mechanical screen lift is ideal for printing transfers. If it also features a power-driven floodbar and squeegee, so much the better. For high-volume production, nothing can beat a cylinder press but you have to sell a lot of transfers to pay for one. Since plastisol ink is considerably thicker than most graphic inks you may have to add weight to the print heads or squeegee holders on some smaller presses so they will be able to push the ink through the screen. Any press that can be used to print signs or posters can be used to print transfers.
The Curing Unit
While transfers can be printed very successfully on home-made presses, home-made curing equipment is not such a good idea. The ink used in plastisol heat transfers has to be heated in a very controlled manner until it is just gelled. Although some people have been successful making transfers by gelling the ink with flash-cure units, home-made conveyer dryers, heat transfer presses, or other makeshift devices, there is simply no substitute for a large, well-made, commercial dryer.
The importance of being able to accurately control the amount of heat applied to the dryer cannot be overstated. For efficient and profitable transfer production you should be able to maintain the heat the transfer reaches as it goes through the dryer to with in ten degrees or less. Moreover, the dryer must be designed and built so that it can maintain the set temperature consistently throughout the entire production run and from job to job day after day. Both over-cured and under-cured transfers can be a disaster for the screen printer. Undercured transfers will offset, smudge, and smear when stacked, and within a short period of time, be unusable, either because of offsetting, or because the plasticizer in the ink has migrated into the paper. Overcured transfers will stack and store well, but be will difficult or impossible to apply to the garment because the over-gelled ink is no longer thermoplastic enough to adhere well to the garment.
Exacerbating the pitfalls of improperly cured transfers is the fact that the bad transfers may not be discovered to be defective until months after they have been printed and sold to the end user. Also, improperly cured transfers end up costing someone not just the price of the transfer, but also the value of the garments they were unsuccessfully applied to.
The Transfer Press
Although it is possible to apply plastisol transfers to garments with a hand iron and an ironing board, this old-fashioned technique is impractical because It takes too long and will result in too many spoiled shirts. For productive and profitable transfer applications you will need a well made heat transfer press. When buying a heat transfer press, you get what you pay for. Inexpensive presses will be smaller, slower, harder to use, and more likely to apply transfers improperly, resulting in spoiled shirts. In theUS, transfer presses are available at prices starting at $500 and going up to several thousand. When selecting a press, keep in mind the three keys for correct heat transfer application, time, temperature, and pressure. 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, although solid state controls will respond faster to a cooling platen and keep the temperature more stable. To determine which of several models of heat transfer presses will maintain a stable high heat, divide the watt rating of the press by the number of square inches in the heated platen. The higher that number is (the more watts per square inches) the faster the platen will heat up, both at the beginning of a production run and between transfer applications. The platen should be several inches larger all around than the largest transfer you will apply.
Time: Dwell time is the length of time that the heat transfer press must apply heat and pressure to the transfer to attach it to a T-shirt. Inexpensive presses are equipped with a bell timer that must be reset at the start of each application cycle and ding when the time has elapsed. They are not particularly accurate but since most transfers have an application latitude of several seconds, generally they are adequate for the task. More expensive presses are equipped with automatic timers that reset with each application cycle and measure the dwell time very accurately. The most expensive presses feature an electrical or pneumatic device that automatically closes and opens the press. For high speed, high volume production or for extremely accurate applications these machines are worth the extra cost.
Pressure: The amount of pressure the transfer press applies to the transfer is as important as the time and temperature variables. Unfortunately, on inexpensive models it is the least easily calibrated and the one usually ignored by most press operators. The most common type of pressure adjustment is a screw, knob or similar device that can be turned one way or the other to increase or decrease the pressure applied to the transfer. Although the press operator can adjust the pressure, he has no way of know what the pressure actually is, and can determine the correct pressure only by expensive and time-consuming trial and error. The more expensive machines, with electrical or pneumatic opening and closing, will also have easy pressure adjustments and may have a gauge that indicates the pressure.
Although it is tempting to purchase an inexpensive heat transfer press, the more expensive machines are easier to use and will apply transfers better and faster. If you purchase a model with automatic opening, one operator can operate two or more presses at once, substantially increasing production per employee.
Single-color transfers: Single-color transfers are extremely easy to print. Remember to reverse the image in the design and select the correct paper for your transfer type (hot-split, cold-peel, glitter, etc.) Print the design on the paper just as if you were printing a sign or poster. Depending on the transfer type and the fabric the transfer will be applied to, you may want to apply a layer of hot-melt adhesive powder. Next, run the transfer through your dryer. Obtaining the correct degree of gelling of the ink layer is crucial. Generally, you want to heat the ink to 180-250º F (82-121º C).
Although the temperature the ink reaches as it goes through the dryer directly affects the amount of gelling and thus determines how well the transfer will adhere to a garment, it's the gelling that's important, not the temperature. Fortunately it's easy to measure the amount of gelling directly. Here's how. Print a set of test transfers, increasing the belt speed a small amount with each transfer you print. You will soon find a transfer that leaves the dryer with the ink liquid enough to offset when the transfers are stacked. Obviously, this transfer has not been heated enough. Slow the dryer belt down a small amount (so that the next transfer printed will be in the dryer a longer time) and print another transfer, run it through the dryer, and check for ink offset. Keep slowing the dryer belt until the transfers are heated just enough that the ink does not offset.
You also should test the transfer to insure that the ink is not over-gelled. There are two simple tests for checking this. If the ink is properly gelled, not too much, not too little, you should be able to peel part of the ink layer off the transfer paper, but once peeled off the paper, the ink layer should break very quickly when stretched. The ink layer should be elastic enough to peel from the paper, but not elastic enough to stretch very much. Another test is the ball test. Peel some of the ink off the paper and roll it into a little ball. When you lay the ball of ink down, it should not unroll. If it does, the ink may be over-cured.
These tests will give you some good indications about how well you are gelling the ink, however the only absolutely reliable test is to apply a transfer to a garment. If the transfer applies correctly and looks good, that's the most reliable test. You should regularly pull transfers at random from production runs and apply them to garments to check that they are being properly gelled.
Multi-Color Transfers: Be sure that you have thoroughly mastered single color transfers before you contract to print your first multi-color transfers because they can be much more difficult. The extra difficulties in multi-color transfers stem from the fact that transfers cannot be printed wet-on-wet. After each color is printed, the ink must be gelled before the next color can be printed. These repeated heating cycles will shrink the paper and the first colors printed may be over-gelled by the heating required by subsequent colors.
The first problem to get under control is paper shrinkage. First, select a paper that had good dimensional stability. Next, pre-shrink the paper by running it through the dryer at the same speed and temperature as the printed transfers will be exposed to. If you are printing a short press run, a hundred sheets or so, and you can print the transfers immediately, take the pre-shrunk paper back to the press and start printing. That's all you need to do.
For long press runs and for paper that will not be printed immediately after pre-shrinking, you need to take some extra precautions. For small amounts of paper, especially paper that will be printed fairly soon, it's enough to wrap the paper in plastic and then tape the seams shut. If you keep the paper from being exposed to atmospheric humidity, it will not expand. For large amounts of paper, or for paper that is going to be printed over several days, or for situations where the registration requirements of the design are very precise so the dimensional stability of the paper must be as great as possible, you will need to store the paper in a heated cabinet or closet called a hot-box. The hot box need not be anything elaborate, just an area big enough to store the paper you will need for immediate production. It should be reasonably air-tight to keep out drafts and humidity, and insulated to hold the heat. The hot-box should then be heated to 110-120º F (43-49º C). Pre-shrink the paper and then store it in the hot-box until needed.
After pre-shrinking, print the first color in a multi-color transfer exactly as a single color design, but stack the paper carefully and store it so that as far as possible, it maintains its temperature and doesn't absorb humidity. You may need to wrap the sheets in plastic, or return the paper to a hot-box in lifts as you print out the entire run. It's a good practice to print the entire press run in a multi-color design as quickly as possible. This will minimize dimensional changes in the paper.
Another problem with multi-color transfers is that the first colors printed may be over-gelled by the heating required by subsequent colors. This can be avoided in several ways. The ink you select should have as wide a range of gelling temperature as possible. Gel the first colors at the bottom edge of the temperature range, without under gelling. A dryer that can be set at and will hold a temperature with great accuracy and consistency will be a great help with this problem. Other solutions include applying adhesion powder or printing an overall background layer of ink as the last color down. The first colors will adhere to the background ink, and the background ink, perhaps with an assist from adhesion powder, will adhere to the garment.
A final word of caution, while single color transfers will almost always work, for success with multi-color transfers, especially those designs with fine details, tight registration requirements, and many colors, there is no substitute for experience.
All the work involved in creating plastisol transfers will be of no avail unless they are applied properly. Even the best transfer will ruin a shirt if you apply it wrong. I explained the three variables, time, temperature, and pressure in the section on heat transfer presses. For specific numbers, you will have to do a little experimentation. Depending on the paper thickness and transfer type (hot-split, cold-peel, or puff transfer) the temperatures can range from 350-400º F (177-204º C). Application pressure can range from 35-50 lbf/in2 (2.5-3.5 kgf/cm2). Application time can range from 5-20 seconds. The higher the temperature and pressure, the shorter the application time required. Most transfers will apply quite well at 10 seconds, 380º F (193º C), and 35 lbf/in2 (2.5 kgf/cm2). Hot-split transfers can run at a little higher temperature, shorter time, and lower pressure. Cold-peel transfers should be applied at a slightly lower temperature, higher pressure, and longer time.
Pre-heat the press thoroughly before applying the transfers. When applying hot-split transfers, remove the paper from the shirt immediately after you open the transfer press, for cold-peel transfers, let the transfer and shirt cool down a little. The transfer does not have to be completely cooled, but you should wait at least 30 seconds.
That's about it. There's much more to learn about transfers than I can cover in one article. I didn't explain much about puff transfers and didn't explain process color and foil transfers at all. However, if you follow these instructions carefully, you should have no problem in printing and applying one-color and simple multi-color hot-split and cold-peel transfers. With practice you should be successful with complex multi-color transfers. Remember the most important point about transfers; they offer you an opportunity to print and store a design quickly and inexpensively, without tying up your money in expensive T-shirts. Then apply the design to the shirt at a time that is convenient and profitable for you.
By Mark Goodridge