Thermal printers, Part 3: The thermal transfer approach

The thermal printer is an alternative to inkjet and laser printers and has a significant place in application niches for which it is especially well suited.

Thermal transfer printers start with the thermal print head but, in contrast to direct printers, use the print head to transfer a solid ink from a “ribbon” onto a surface to produce a permanent print (there is another approach, discussed later).

Instead of printing directly onto the surface of the printing material, the print head presses the wax-coated ink ribbon (or ribbons) onto the surface of the printing material. This causes the wax to melt, which exposes the ink, which is then transferred to the printing material and dries there (Figure 1).

Clearly, this is a more complicated scheme than direct thermal printing (Figure 2).

The surface being printed on – the printed media – can be almost any kind of paper stock as well as vinyl, polyester, nylon, or other thicker materials. The “inking” ribbon is usually made from wax, resin, or a combination of the two and bonds to the label supply surface with a combination of heat and pressure to create a long-lasting, high-quality image. This technique provides excellent printing quality and durability.

There is an advantage to this type of thermal printer as compared to a direct thermal printer. The prints are long-lasting and much less likely to fade over time, and the printer can even be designed to print in full color by using multiple color ribbons in parallel. Of course, the ribbon or ink source will need to be replaced, so there are long-term maintenance costs as well as user issues (especially in non-office situations such as retail or point-of-sale settings. Also, there are more moving parts, so these printers may be slightly more prone to operational issues (jamming, overall reliable), but a well-designed unit is durable and dependable.

Thermal transfer printing is a good fit for long-term persistence, where the printed media will be used for more than six months or will be exposed to harsh conditions such as chemicals, light, high temperatures, or water. Typical applications include labels for asset tracking, marking laboratory specimens and blood bags, outdoor applications, permanent identification, and cold and freezer storage.

Another major benefit is that thermal transfer supports a large range and variety of printing materials, each with a different cost and longevity; polyester is the most durable with the longest lifetime. The label material itself can be any color which will improve label appearance and attractiveness in some cases and even allow for color-coded labels.

Full-size printers: Available direct thermal printers are usually small to fit the application space and needs. However, larger-size, full-color, high-volume thermal transfer printers do not use a wax-coated ribbon for their colors. Instead, they use crayon-like wax – essentially solid-ink sticks – for the four primary printing colors: black, cyan, magenta, and yellow (not the same as the red, green, and blue primary colors for light displays mixing).

One of the earliest and best-known of these was the Phaser series with a choice of 800 or 1200 dpi resolution made by test-and-measurement vendor Tektronix (Figure 3); they sold this printer business to Xerox about 20 years ago.

The Phaser unit melted the primary-color sticks as needed to produce lush, richly colored (technically known as fully saturated) images and text on almost any sort of stock. The hues of the image were deep and rich, and the printed “ink” had a raised surface that closely resembles high-end engraved printing. This unit’s high-accuracy color rendition was needed for color-printing tests and photo-quality color prints and could produce up to 14 pages per minute (ppm) in full color. It directly supports many basic and advanced data formats and data protocols as a high-end unit.

(A few years later, Xerox replaced the Phaser 850 with their VersaLink C500 unit, which appears to use the four primary colors but as conventional powdered toner rather than solid-ink sticks. However, there is still an active market for used models of the Xerox/Tektronix units and even the original Tektronix ones, and replacement ink sticks for these models are still made and readily available.)

The next part of this article looks at the design specifics of a complete thermal printer, emphasizing the direct thermal units.

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References

• Global Market Insights Inc., “Thermal Printing Market Size”
• Future Market Insights, “Thermal Printing Market”
• Graphics Products, “Thermal Printers vs. Inkjet Printers: Which Printer Should You Choose?”
• Wikipedia, “Thermal printing”
• S. Patent 3,496,333, “Thermal Printer”
• CDW, “How Does a Thermal Printer Work?”
• How-To Geek, “Printing Without Ink or Toner? How Thermal Printers Work”
• VentureBeat, “Best Thermal printer 2021 – 7 Thermal printers Reviews”
• Brother Mobile Solutions, “Direct Thermal vs. Thermal Transfer – Which Print Technology Works Best?”
• Peak Technologies, “What is Thermal Transfer Printing?”
• Science Direct, “Thermal Printers”
• Xerox, “VersaLink® C500”
• Xerox, “PHASER 850 Color Printer”
• Texas Instruments, “POS Printers”
• Cypress Semiconductor Corp., “Developing highly-integrated thermal printer-based applications”
• Circuit Digest (India), “Thermal Printer interfacing with PIC16F877A”
• Visuality (Poland), “Thermal Printer Protocols for Image and Text”