Ceramic Industry

SPECIAL SECTION/DECORATING: Creating Indelible Images

April 1, 2007
Short-run custom imprinting can be achieved on a variety of ceramic and glass substrates through ink jet heat transfer sublimation.



The dye sublimation transfer process begins with a computer image that, for the purposes of the ceramic and related industries, is printed to an ink jet printer using special inks and paper. The image can then be transferred onto an array of hard and soft substrates through a heat transfer. Aided by pressure, this type of heat transfer converts ink into a gas, which allows it to migrate into the substrate. As the gas solidifies, it creates an indelible impression within the final product.

Compared to decorating methods such as pad and screen printing, which can require high volumes and offer a more limited color range, sublimation transfer requires no volume minimums or elaborate setups, and millions of full digital colors can be used. In addition, while the image is located on the top of the substrate with other technologies, sublimation places the image underneath a polymer coating, which yields a highly durable decorated surface.

Ceramic tile applications include wall surfacing and inserts for items such as wood boxes, serving trays, coasters and trivets. A recent boon has surfaced with the introduction of glass tile. In addition to being compatible with all uses of ceramic tile, glass is fully walkable and submersible in water.

Preparing the Substrate

Appropriate substrates include ceramics, porcelain, glass and stone, as well as items ranging from T-shirts, baseball caps and towels to jewelry, aluminum and wood. Hard substrates are surfaced with a special polymer coating, while soft substrates should be constructed of synthetic fibers (primarily 100% polyester to be compatible with sublimation heat transfer).

Hard substrates (other than stone surfaces) are graded to be as flat as possible and free of surface anomalies, such as pimples (raised defects), dimples (recessed defects) or eclusions. Once the substrates are graded and cleaned so they are free of debris, the proprietary coatings, which feature a clear polymer base, are robotically sprayed on the surface, and then heat cured. The coating enables the substrate to be receptive to sublimation. Without the coating, the dyes would solidify on the surface in no particular order and simply rub or wash off.

While raw, uncoated substrates are cheap and readily available, it is often impractical to use common spray- or paint-on polymers for the necessary coatings. The result is sometimes acceptable, but generally not. It is very difficult for generic polymers to provide the appropriate color, consistency, and UV or scratch resistance achieved with proprietary coatings.

Sublimation places the image underneath a polymer coating, which yields a highly durable decorated surface.

Printing Requirements

The ink used in the sublimation transfer process is a special mixture of water and heat-reactive dyes. These dyes are available in an array of four to eight colors, including cyan, magenta, yellow and black. Depending on the type of ink jet printer used, special light-colored dyes, such as light black, light cyan and light magenta, are also available.

The familiar "bubble jet" technology makes up the bulk of ink jet printers currently available. These printers use heat and pressure as the propellant for their ink. However, sublimation inks become activated under a heat press, and, as such, do not like to be heated twice (once onto the paper and again on the way into the substrate).

Thus, proprietary inks call for specialized ink jet printers. At the time of this writing, sublimation inks are available exclusively for printers with micropiezo print heads. Printers built to these specifications fire their ink droplets via pressure under ambient temperature conditions. Epson manufactures several compatible ink jet printers, including models C8X, 1280, R800/1800, 4000/4800, 7600/7800 and 9600/9800. Sublimation solutions for larger, wide-format printers include the Roland Hi-Fi Jet Pro and Mimaki's JV3 and JV4.

Laser- or light emitting diode (LED)-based printers have been on the market for some time. However, these types of printers are not as scalable or readily available as ink jet. Another barrier for market penetration of toner-based printers is their increasingly vendor-proprietary toner formulations. In an attempt to thwart the efforts of "driller/fillers," printer manufacturers are developing toners that cannot be readily reverse engineered. Unless printer manufacturers slow the tweaking of their toners, it is not likely that widespread sublimation alternatives to ink jet will be developed in the near term.

Special inks and printers also require special papers, and the best of these papers are coated specifically for sublimation. There are two basic types of paper on the market today: general purpose for most products and high release for soft products. As the description implies, general purpose paper yields an acceptable image across a broad spectrum of items. If more color saturation is required, however, high release paper is preferred.

Many types of sublimation-compatible papers are available. Some end users prefer only one or two brands of paper, while others keep and use products from different vendors. The variety allows end users to experiment with different papers and substrates in order to achieve the desired results.

Transferring the Image

Heat pressing closes the loop in the life cycle of a sublimation transfer, using substrate- specific temperatures (365-450°F), pressures (30+ psi) and dwell times (6-10 minutes for a single item; dwell times increase along with the number of items). When the ink is heat activated, it evaporates and then migrates into the polymer coating. As the vapor cools, it solidifies and reconstitutes itself into a high-resolution, (virtual) continuous tone version of the original ~200+ dpi dot pattern. For example, once sublimated, a 300-dpi print can be converted into an image meeting or exceeding 2400 dpi.

Heat presses can be curved (for substrates such as mugs) or flat, and their capabilities range from those suitable for hobbyists to high-volume commercial applications. On a manual press, the user turns a large nut to set the pressure, then presses down and pulls up on a handle once the pressing is complete. Alternatively, air-operated presses feature automatic settings for pressure and timing. The most prominent heat presses are those from the George Knight Co., Inc. (Brockton, Mass.) and Hix Corp. (Pittsburg, Kan.).

After pressing, the imaging area of the substrate must not be touched while the item cools. The coating can be abraided or removed if the item is mishandled before cooling is complete.

General Application Notes

Many variables need to be taken into account in order to achieve the desired results with sublimation imprinting. Substrates should be receptive to polymer coatings and versatile enough to be finished with a glossy, matte or satin appearance. The substrate's surface should be clean (to avoid dimples, pimples, debris and inconsistent application) and as smooth as possible to ensure consistent coverage.

In addition, the color of the substrate should be light or even white, since blank spaces in the image will enable the substrate to show through. If the decorated item will be used in contact with food or drink, U.S. Food and Drug Administration (FDA) approval is also required. Additional variables include:
  • Substrate thickness-the thicker the substrate, the more time and pressure required to imprint the image.
  • Coating hardness-the coating should be hard enough to prevent scratching and ultraviolet fading.
  • Heat absorption-faster heat absorption leads to increased productivity. In general, harder substrates can take longer to sublimate.


Flexible Imaging

Today's digital imprinting offers a platform for ceramic, glass and related items to be commissioned, enjoyed and preserved. The dye sublimation transfer process lends itself well to the customization of these items on demand without extensive advanced planning for the typical job. It is an economical and cost-effective solution for projects that require just-in-time turnaround with a high degree of volume flexibility.

For more information regarding ink jet heat transfer sublimation, contact Condé‚ Systems, Inc., 5600 Commerce Blvd. East, Mobile, AL 33619-9214; (800) 826-6332; fax (251) 633-3876; e-mail info@conde.com; or visit www.conde.com.

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