Printing Plastisol Transfers
By Mark Goodridge
Are you a textile screen printer who's looking for a new product to sell or a way to increase your profits in some of the jobs you're printing now? Heat-applied plastisol transfers may be exactly what you're searching for. They can increase sales and profits and they're cheap and easy to make. Although plastisol heat transfers are not for the complete novice, they're simple enough that any textile screen printer who regularly prints with plastisol should be able to make them.
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?
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
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.
1. Reverse the design
2. Avoid both fine details and large areas of solid color
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
4. Avoid trapping colors
5. Allow for paper shrinkage
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.
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.
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.
Union Ink recommends the following inks for transfer production. (Each ink name is
linked to that ink's technical data sheet.)
For transfer printing with Union's high-opacity Maxopake and Athletic Gloss inks, add 5-10% Hot Split Additive (PLUS-9040) by weight. For producing transfers with Mix-Opake inks, add 15% MIXO-TRANS.
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
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
The Curing Unit
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
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.
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.
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