Direct thermal is typically used to print tags, tickets, and labels that have a relatively short usable life.
Bulk food labels
POS sale receipts
Direct thermal is well suited for these applications because it is typically less expensive than thermal transfer, and the long life durability attributes of thermal transfer are not needed.
Now; why isn’t direct thermal well suited for longer life applications?
Direct thermal papers have a coating on them that contains leuco dyes. When this coating that contains the dyes is exposed to heat from a thermal printhead, a reaction takes place and the paper “turns black in the area exposed to the thermal printhead. This same reaction can take place unintentionally when the direct thermal paper is exposed to heat (~85C and higher) or light (particularly UV light) over a period of time.
You may recall the early fax machines that used direct thermal technology. If you kept your original fax that was printed on direct thermal paper for a period of time, it would eventually become discolored and fade.
Testing recently completed showed the following:
Direct thermal images have very good scratch and smear resistance when tested using conventional evaluation methods (crockmeter). When tested this way, they exhibit scratch and smear properties at least as good as an image printed with a premium (wax/resin) thermal transfer ribbon.
However, a direct thermal image can easily be marred by striking the unprinted area of the paper with a sharp edge like a fingernail or stylus. If you strike a direct thermal paper with your fingernail or a stylus, a dark line will result. This does not happen with thermal transfer.
Direct thermal images have very good resistance to water (as thermal transfer images do).
Direct thermal images have poor resistance to chemicals – even what we would consider the “harmless” chemicals like IPA or Formula 409 cleaner. Thermal transfer has excellent chemical resistance (particularly premium wax/resin and super premium resin type ribbons).
As stated above, direct thermal has very poor temperature resistance. 7 out of the 9 direct thermal papers we tested started to show “graying” at temperatures as low as 85C (185F) after only 5 hours. 9 out of the 9 direct thermal papers showed graying after exposure to 85C for 24 hours. This exposure had no effect on the thermal transfer images (even those printed with wax ribbons).
9 out of the 9 direct thermal papers showed graying after only 24 hours of exposure to UV light. This exposure had no effect on the thermal transfer images.
Direct thermal images are not as dark/black as thermal transfer images. As measured with a Macbeth densitometer, thermal transfer images ranged from 1.9 to 2.1, whereas direct thermal images ranged from 1.2 to 1.5. (A densitometer measurement of 1.8 or higher is desirable.) Not only does a darker/blacker image look better to the eye, but, a darker/blacker bar code will result in a higher scan grade.
The direct thermal images also tended to exhibit more voiding in large text or all black areas.
In short, like everything else, direct thermal has its advantages and disadvantages…
Advantages of direct thermal:
Usually less costly as compared to thermal transfer (but not always)
More user-friendly in the sense that the user has only one media type to worry about
Very good scratch and smear resistance
Very good water resistance
Although less black than thermal transfer, the print quality of direct thermal is still very good.
Disadvantages of direct thermal:
Very little temperature resistance
Very little UV light resistance
Image can easily be marred by a sharp object
Image is relatively brown/light
Poor chemical resistance
Direct thermal paper is very abrasive to a thermal printhead, whereas, the backcoat of a thermal transfer ribbon is more “printhead friendly”.
Direct thermal requires significantly higher print energy than thermal transfer. When a printer is in “direct thermal mode”, the energy level is much higher than “thermal transfer mode” on a printer.