February 8, 2014

thermal dye-sublimation

Kodak PROFESSIONAL 8670 PS thermal printer
photo by Styrous®

I have used the term, thermal dye-sublimation, in a few of my posts. It occured to me that an explaination of what this means is in order. I use a Kodak PROFESSIONAL 8670 PS thermal printer.

A dye-sublimation (or dye-sub) printer is a computer printer which employs a printing process that uses heat to transfer dye onto materials such as a plastic, card, paper, or fabric. These are not to be confused with dye sublimation heat transfer imprinting printers, which use special inks to create transfers designed to be imprinted on textiles, and in which the dyes do indeed sublimate.

This printer uses CMYO (Cyan, Magenta, Yellow, Overcoating) colors, which differs from the traditional CMYK colors in that the black is eliminated in favour of a clear overcoating. This overcoating is also stored on the ribbon and is effectively a thin layer which protects the print from discoloration from UV light and the air, while also rendering the print water-resistant. All four colors are on a strip that lays down the colors.

thermal dye-sublimation CMYO printer ribbon

This extra panel works by thermal transfer printing instead of dye diffusion: a whole layer, instead of just some of the dye in the layer, transfers from the ribbon to the substrate at the pixels defined by the thermal head. This overall process is then sometimes called D2T2 (Dye Diffusion Thermal Transfer).

The process lays one color at a time, the dye being stored on a polyester ribbon that has each color on a separate panel. Each colored panel is the size of the medium that is being printed on; for example, a 6" by 4" dye sub printer would have four 6" by 4" panels. 

 thermal dye-sublimation CMYO color layers

 During the printing cycle, the printer rollers will move the medium and one of the colored panels together under a thermal printing head, which is usually the same width as the shorter dimension of the print medium. Tiny heating elements on the head change temperature rapidly, laying different amounts of dye depending on the amount of heat applied. After being heated into a gas, the dye diffuses onto the printing medium and solidifies.

After the printer finishes covering the medium in one color, it winds the ribbon on to the next color panel and partially ejects the medium from the printer to prepare for the next cycle. The entire process is repeated four or five times in total: the first three lay the colors onto the medium to form a complete image; there may or may not then be a black thermal transfer process; while the last one lays the laminate over top. This layer protects the dyes from degrading or resublimating when handled or exposed to warm conditions.

The advantage of dye-sublimation printing is that it is a continuous-tone technology, where each dot can be any color. In contrast, inkjet printers can vary the location and size of ink droplets, a process called dithering, but each drop of ink is limited to the colors of the inks installed. Consequently, a dye-sublimation printer produces true continuous tones appearing much like a chemical photograph.

The prints are dry and ready to handle as soon as they exit the printer. Since the thermal head doesn't have to sweep back and forth over the print media, there are fewer moving parts that can break down. As the dye never enters a liquid phase, the whole printing cycle is extremely clean; there are no liquid inks to clean up. These factors make dye-sublimation generally a more reliable technology over inkjet printing.

Because the sublimated ink is a gas, it does diffuse a small amount before being absorbed by the paper. Consequently, prints are not razor-sharp. For photographs, this produces very natural prints.

the bummers

Of course, there is a downside to everything. The amount of wasted dye per page is very high; most of the dye in the four panels may be wasted for a typical print. Once a panel has been used, even to just print a single dot, the remaining dye on that panel cannot be reused for another print without leaving a blank spot where the dye was used previously. Due to the single-roll design of most printers, four panels of colored dye must be used for every print, whether or not a panel is needed for the print. Printing in monochrome saves nothing, and the three unused color panels for that page cannot be recycled for a different single-color print. Inkjet printers can also suffer from 'dye wastage' as the ink cartridges are prone to drying up with low usage (without 'heavy use', the cartridge nozzles can become clogged with dried ink). Dye-sublimation media packs, (which contain both ribbon and paper), are rated for an exact number of prints which yields a fixed cost per print. This is in opposition to inkjet printers where inks are purchased by volume.

Also, dye-sublimation papers and ribbons are sensitive to skin oils, which interfere with the dye's ability to sublimate from the ribbon to the paper. They must also be free of dust particles, which can lead to small colored blobs appearing on the prints. Most dye-sublimation printers have filters and/or cleaning rollers to reduce the likelihood of this happening, and a speck of dust can only affect one print as it becomes attached to the print during the printing process. Finally, dye-sublimation printers fall short when producing neutral and toned black-and-white prints with higher density levels and virtually no metamerism or bronzing.

As dye-sublimation printers utilise heat to transfer the dye onto the print media, the printing speed is limited by the speed at which the elements on the thermal head can change temperature. Heating the elements is easy, as a strong electric current can raise the temperature of an element very quickly. However, cooling the elements down, when changing from a darker to a lighter color, is harder and usually involves having a fan/heatsink assembly attached to the print head. The use of multiple heads can also speed up this process, since one head can cool down while the another is printing. Although print times vary among different dye-sublimation printers, a typical cheap home-use dye-sub printer can print a 6" x 4" photo in 45 – 90 seconds. More heavy-duty printers can print much faster; for example, a Shinko CHC-S-2145 dye-sublimation printer can print a 6" x 4" photo in as little as 6.8 seconds. In all cases, the finished print is completely dry once it emerges from the printer.

a bit of history

Alps Electric produced the first quality dye-sub printers for home consumers in the $500–$1,000 price range, bringing dye-sublimation technology within the reach of a wider audience. The company was established in 1948. The Kodak PROFESSIONAL 8670 PS thermal printer was introduced sometime in the 1980's.

I hope this helps to understand the process a little better.

Styrous® ~ February 8, 2014


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