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The screen printed way

Still Afloat in a Sea of Digital Bits & Bytes: Working With Heat Transfers

Wayne Potter is vice president of marketing development for Air Waves Inc., Lewis Center, Ohio. He has 30-plus years’ experience in the heat transfer and digital imaging market.

In this brave new digital world, are there still places for old standby analog processes? Yes, the production of heat-applied graphics is alive and well! Heat transfers, as we call them, are still a viable way to decorate both garments and other textile products.

So why would anyone in his or her right their right mind want to decorate with a legacy process such as iron-on heat transfers when they can complete an entire garment using an inkjet printer in mere minutes? The fact that we’re dealing with that buttonless, collarless, round neck, pull-over garment commonly called the “basic T” offers much of the answer to this question:

  • Traditional transfers are easy to produce or acquire.
  • They’re affordable when compared to the capitol investment in direct-to-garment digital printing.
  • They allow embellishers to capitalize on special effects such as puff inks, glossy lithographic images with metal-flake, and post foiling the puff surface for fashion appeal.
  • They avoid the pitfalls of uncertainty and deliver printed garments just-in-time for purchase.
  • They easily personalize garments and ad-specialty items.

As embellishers, we all recognize the most common form of T-shirt decoration is still direct screen printing with plastisol or water-based inks. More recently, other methods have worked their way into T-shirt decoration. For example:

  • laser technology on specialty printed release papers;
  • inkjet technology on specialty printed release papers; and
  • direct-to-garment inkjet printing.

But by and large, more shirts are still embellished via direct screen printing and analog heat transfers.

Why buy?

Now that we have explored some of the more salient reasons for using heat-transfer decoration, let’s look at the how-to part of the equation. Most individuals and commercial decorators opt to purchase heat transfers rather than produce their own. Why might this be? Typical reasons cited are:

  • the high cost and/or limited availability of fashion-quality artwork needed to produce their own heat transfers;
  • the cost of the specialized equipment required for producing heat transfers;
  • a lack of technology and understanding about the processes involved.

There are many good manufacturers of high-quality stock and custom heat transfers. Most have catalogs or on-line offerings and I encourage you to take a look. What surprises me more, though, is that more established screen-printing shops don’t print their own. After all, a screen printer already has nearly all the equipment and expertise needed to produce basic heat transfers. If you are going after fashion retail, you will probably want to consider purchasing stock heat transfers for this category, but for basic personalization of 20-60 shirts, you can produce the iron-on heat transfers yourself.

Transfer specifics

Simply stated, you print the decoration directly to a special release paper rather than directly to the garment. The freshly printed paper and ink is then passed through a conveyor dryer where the semi-liquid plastisol ink is heated to its gel-point—where you would stop with a flash-curing procedure. It is important to note here that the plastisol ink is not fully cured. The full cure will be done at a subsequent point during heat-transfer application to the garment.

These printed heat transfers can be stored at moderate room temperatures and transported to satellite application locations. When you ultimately want to perform the application, you simply place the heat transfer ink-side down on the garment, then apply heat and pressure using a commercial heat-transfer machine. This machine application will force the transfer ink onto the garment and complete the final ink cure in the process.

For the most part, heat transfers fall into two categories—hot-split and cold-peel transfers—the primary differences being the way the heat transfers are applied to the fabric substrate and the finished appearance of the print. When applying a hot-split transfers the paper is removed immediately after the garment is removed from the heat source. The transfer-ink film actually splits in the process, standing up the fabric nap. While most of the ink remains on the shirt, some is left on the release paper. Hot-split transfers have a very soft hand and, if properly prepared and applied, are indistinguishable from a direct-print decoration.

Cold-peel heat transfers, on the other hand, are allowed to cool before stripping the release carrier paper—thus the stiffer feel and, in some cases, glossy look. The cold-peel transfer typically offers good opacity on dark fabrics.

In addition to the basic types, heat transfers can be produced using inks with a blowing agent that expand under heat giving the resultant image considerable relief. When combined with halftone process images, puff becomes a highly saleable fashion effect.

Process overview

If you are going to be successful producing heat transfer designs it all starts with your design and artwork. When designing for heat transfer production, always keep in mind the process, as detailed below:

  1. Remember that your transfer image must be reverse of the final printed transfer image on the fabric. In other words, the film positive you produce as an intermediate step must be wrong-reading emulsion side up.
  2. Fine detail is not only difficult to reproduce in most heat transfers process, but can lead to unacceptable results, in many cases. Thin lines close together and small type closes up. Your design-line weight should be no thinner than about 1/16” and halftone screens no finer than about .35mm. If your design is comprised of a lot of fine detail, consider backing it up with a solid color to hold it together. This will avoid poor application and premature wash failure.
  3. Consider the color printing rotation and color trapping. With most direct-printed designs, the black outline (if any) is printed last. When producing a screen-printed heat transfer it is just the opposite. The color that is printed first on the release paper ends up on the top of the applied transfer image. Keep this in mind when you design your heat transfer. The color rotation for hot-split transfers can be different than for a cold-peel. The major manufacturers of screen-printing inks have excellent technical service and are more than glad to help you through the printing learning curve.
  4. Remember that the release papers used to produce heat transfers dimensionally change when exposed to process dryer heat. A large sheet of paper can shirk up to a quarter-inch during the production process. Such shrinkage can be like trying to hit a moving target if you do not account for it in your design and production of multi-colored heat transfers.
  5. Ink-film thickness should be between three and five mils. If the ink film varies widely your resultant image will not look good after transferring. As for screen emulsion, most folks are using direct film emulsions for transfer printing. The smooth surface of a capillary film gives you much sharper images on the commercial release papers available for transfer printing.
  6. You will get the best results using plastisol inks specifically designed for heat-transfer applications. The major ink suppliers have developed specialized ink sets for production of heat transfers. The ideal heat-transfer plastisol should be fairly opaque, and low-tack for printing multi-colored images. Heat-transfer inks should have a wide enough temperature range at which they will gel and not cure after printing. Additionally, consider the final application temperature range of the fabric to which you intend to apply the finished heat transfer. Some of the new synthetic fabrics will melt or distort at temperatures that cotton blends will easily withstand.
  7. Clearly, under normal application conditions, most plastisol heat transfers will become liquid and, with the pressure of application, intermix with the fibers of the fabric, making a good mechanical attachment. This mechanical attachment is directly related to wash resistance. Where some fabrics (nylon, polyester) warrant more foolproof application, you should consider the addition of a hot-melt adhesive powder. The application of the adhesive powder is commonly performed after the last color is printed using a drop-spreading device and vacuuming off the excess powder. I have even seen individuals using a large metallic saltshaker to disperse the powder adhesive on to the last image before passing it through the dryer. Remember, you must remove all the excess powder from the paper or it will spoil your transferred image.
  8. Curling release paper presents a continuing challenge in the production of heat transfers. Some release papers designed for heat transfer production have a greater propensity to curl when exposed to heat. Make sure you experiment with the most popular papers and decide for yourself which meets your specifications.

Adding up the bill

The last thing that you might want to consider is the cost of it all. If you don’t already have it, at the very least you will need the following to print your own transfers:

  • Equipment to stretch, coat, expose, dry and reclaim the screens required for printing.
  • A screen-printing press. This can be as simple as a one-color bench printer, although you should consider multi-color equipment if you are going to control your registration and production times.
  • Although, in theory, you can dry a transfer using a flash unit, a process conveyor dryer is more suitable for transfer production.
  • A transfer press.

Depending on what workhorses are currently in your stable, your start-up cost can range from $7,500 to $10,000 for a minimal set-up, although I think it is more likely you’re looking at $15,000 to $20,000, especially if you are starting form scratch. There are many high-quality manufacturers that advertise in this magazine where you may begin your search for the equipment needed. Most of these companies have long histories of producing quality equipment and have talented professionals to help get your started.

If you find the cost and learning curve of your own production a bit daunting, you should consider the possibility of outsourcing your production to one of the many heat-transfer houses that also advertise here.

Clearly the scope of this article is not to provide an intimate step-by-step instruction, but rather to give you an overview and framework for further investigation. As Theodore Paul Wright, the well-known aeronautical engineer and educator noted, given an industrial process, costs are always higher at the beginning and decline in the future. This is true of producing airplanes or heat transfers. Even though the learning curve is steeper for some than others, armed with the information in this article and your basic instinct I trust your reward from producing heat transfers will be sooner than later!