An iron-on decal is an image printed on special paper that allows it to be transferred to fabric by applying heat and pressure. Iron-ons are one of the four primary types of decals; the other three types are slide off, varnish, and pressure sensitive. The term decal is from the French decalquer, meaning to trace or to copy. Decals are traced, or more accurately, stenciled, onto a fabric screen. Ink is forced through the stencil and onto a printing substrate in a process is known as screen printing. Iron-on decals are printed on a temporary substrate that is designed to release the image when exposed to pressure and heat. The act of ironing the decal causes the image to move from its paper backing to the fabric, hence the name "iron on."
The concept of using stencils to print images has been employed throughout recorded history. There is even evidence to suggest that the first stencils were made from large leaves and were used by prehistoric people to create cave paintings. Over time, stencil printing was used for decorating such items as furniture, walls, and fabrics. A drawback of this technique was that holes in the stencils are connected by "bridges," which show up as interrupting gaps or "islands" in the final printed image. This problem was solved in the early 1900s by Samuel Simons, who devised a way to print through a silk fabric screen. The silk allowed the ink to slip around the fine fibers and give a smoother looking image. This modified method of stencil printing is known today as "silk screening." Simon's invention was first officially discussed in his 1907 British patent, but unfortunately it was slow to catch on commercially. Not until 1938 did a group of practicing artists under the federal Works Projects Administration began to seriously explore the technique. By the end of World War II, the silk screening process was recognized as a valuable printing method, and today it is one of the most widely used of all printing technologies. In the last 40 years, printers have developed techniques to screen print different types of ink on a wide variety of substrates. Heat-transferred graphic decals are made using these advanced techniques.
Production of iron-on decals requires stencil-making materials, inks, a porous printing screen, and a printing substrate to receive the final image.
Stencils are made from nonporous paper or plastic coated with lacquer, gelatin, or a combination of glue and tusche, a heavy ink-like substance. These materials are either oil soluble or water soluble, depending on the type of printing. The stencil blocks portions of the screen during the printing process, so that ink only touches the paper in designated spots.
The inks commonly used for iron-on decals fall into two categories, plastisol type and sublimation type. Both types use pigments created from a variety of metals, clays, plants, and synthetic chemicals to provide color. These pigments are suspended or dissolved in a liquid solvent such as mineral spirits, alcohol, or water. Plastisol ink is a lacquer-based ink that is specially designed for use on fabrics. It is dried by heating it to 300°F (149°C) for several minutes. This type of ink is also quite thick and requires some special handling to produce a good image. Sublimation ink is not really an ink at all but rather a dye-like pigment. When exposed to heat, the pigment vaporizes and moves from its temporary support to the fabric, where it becomes permanently bonded to the fibers. Synthetic fibers such as polyester and nylon are particularly effective at bonding with sublimation inks. Both of these ink types are available in a wide array of colors, so that almost any image can be reproduced.
The screens used in this process are typically finely woven fabrics, like silk, nylon, and dacron, or stainless steel meshes that are stretched tightly over a rigid frame. Small scale printing screens may be wood or plastic. Large commercial screens are typically made of metal.
The printing substrate used for heat transfers depends on the type of ink employed. Plastisol inks can only be printed on a specially coated paper stock that is designed to absorb the inks. When the paper is placed face down on fabric, and heat and pressure are applied, the coating melts and allows the image to transfer to the fabric. The coating produces a rubbery feel on the finished garment. On the other hand, sublimation dyes are printed on uncoated paper stock and are transferred as a result of the chemical change from solid to gas.
The process of creating iron-on decals involves three key steps: preparation of the stencil to be used to print the image; the screen printing process itself; and transfer of the image to the fabric substrate.
- 1 The first step is to create a stencil of the image. As described above, stencils are made from paper or plastic sheets, which are coated with a thin layer of lacquer or gelatin. An outline of the image to be printed is cut into this thin layer and the lacquer or gelatin is peeled away. A solvent is then used to adhere the stencil to the underside of the fabric screen. Once the stencil has dried on the screen, the paper or plastic backing sheet is removed, leaving only the film layer. The portions of the stencil that were cut away expose a section of the screen through which ink can be forced. Such stencils may be produced from photographic images by a similar process, but instead of cutting away the portions of the image, a light-sensitive material is used to mark an image on the screen. Alternatively, the stencil may be painted directly on the screen using tusche.
- 2 Image transfer is accomplished by forcing various inks, which are placed on top of the screen fabric, through the stencil and onto the printing substrate. A rubber squeegee is used to force ink through openings in the stencil. Ink application may be done by hand or by automated printing processes. The different colors are laid down sequentially, one at a time. The plastisol type must be heated to cure after each application. This curing is done at temperatures of 225-250°F (107-121°C). The ink must be heated to this temperature for about one minute and then cooled before the next color can be added. (The sublimation type requires no heat curing.) It should also be noted that the colors are laid down in reverse order, from last to first. Therefore, the finished decal resembles a multi-layer sandwich. The bottom layer is the release paper, followed by what are called the detail colors. Then the background colors are laid down one at a time.
- 3 Upon completion of the printing process, the image is ready to be transferred to a T-shirt or other garment. Before transferring the image, the fabric must be laid out on a smooth hard surface. For a T-shirt, a piece of heavy cardboard is inserted between its front and back to provide firm support and prevent the transfer from sticking to the back of the garment. Heat and pressure are then applied using an ordinary household iron or a special piece of equipment called a dry mounting press. The latter consists of two flat, electrically heated metal plates that provide even pressure and heat to the fabric. Using the dry press allows for better image transfer with less chance of scorching. For iron-on decals made with plastisol inks, the dry press is set to about 300°F (149°C). This temperature is adequate to melt the lacquer layer holding the inks. Continued heat and pressure for one to three minutes cause the molten ink/lacquer film to migrate to the fabric. When the heat source is removed, the ink/lacquer combination cools and binds to the fabric. After cooling is complete, the paper backing is peeled away. A similar process is used for sublimation inks, except no carrier layer is needed because the pigments vaporize and transfer directly to the fabric as result of heat and pressure. A temperature of 350-375°F (177-191°C) is usually sufficient to affect this change. Sublimation inks transfer more cleanly than the plastisol type since there is no lacquer coating or film to be transferred.
The quality control measures employed for decals are meant to ensure that the image transfer is clean and crisp. The following factors have been determined to be critical to image quality:
- The screen fabric must be properly adhering to the stencil.
- The right type of adhesive must be used to adhere the stencil to the screen.
- Contact time for the adhesive must be limited to avoid softening the stencil.
- The stencil must not be cut with poor or dull tooling.
- The screen must be properly cleaned after each ink application.
With plastisol inks, care must be taken during image transfer to ensure that residual lacquer layer does not stick to the paper and cause smearing of the image.
The decal manufacturing process generates waste in the form of excess materials used in stencil production (lacquer, gelatin, and paper). The very nature of the stencil dictates that some material will be wasted, because unused portions of the image are cut away. Excess inks that are wiped off the screen also contribute waste products, as do solvents used for cleaning equipment. Depending on the chemistry of the specific materials employed, the waste may be flammable and considered hazardous. To a large extent this depends on whether the waste material is water or solvent based.
Methods of producing iron-on decals stand to be improved as advances are made in ink and paper coating chemistry. The development of quicker drying inks with a wider range of colors and better adherence to paper and fabric substrates would substantially increase the efficiency with which these products are made. The current trend toward increased regulations to protect the environment will likely impact the types of inks, solvents, and lacquers that are used in decal production. Development of environmentally safe, or "green," products would be an asset to the industry.
For example, a recent improvement in computer technology has led to a better screen printing process, known as FM screening. This process allows printers to use smaller, more uniform screens, resulting in smoother images. Another interesting advance in decal printing now allows those with a personal computer to make their own iron-ons. Several manufacturers offer specially treated iron-on paper that can be printed with a standard color printer. An image on this paper can then be simply and easily ironed on to shirts or other garments. This process can be used by anyone to create unique and memorable garments, but it cannot replace the commercial printing processes currently used for screen printing iron-on decals.
Where to Learn More
Grattaroti, Rosalie, ed. Great T-shirt Graphics. Rockport Publishers, 1993.
Swerdlow, Robert. The Step by Step Guide to Screen-Process Printing. Prentice-Hall, 1985.
McDougall, Paul. "FM Screening: Big Gains from Tiny Dots?" Folio: the Magazine for Magazine Management, January 15, 1994, p. 25.
Strashun, Joann. "Screening Options Gain Momentum." Graphic Arts Monthly, February 1994, P. 55.
Toth, Debora. "Printers Ponder Screening Options." Graphic Arts Monthly, August 1995, p. 45.