What is thermal paper?
Thermal paper carries a chemical coating, typically a leuco dye and a developer, that reacts and turns dark when heated. A direct thermal printer has no ink, toner or ribbon. Its printhead is a row of tiny heating elements across the width of the paper, and each element darkens a dot of the coating as the paper moves past. Thermal paper is used for receipts at point-of-sale (POS) terminals, shipping and barcode labels, ticket and kiosk printers, and older fax machines.
Direct thermal vs. thermal transfer
| Direct thermal | Thermal transfer | |
|---|---|---|
| Media | Special thermal paper or labels | Plain paper, label stock or synthetic material |
| Ribbon | None | Wax, wax-resin or resin ribbon melted onto the media |
| Durability | Fades with heat, light and age | Long-lasting, resists heat and abrasion |
| Typical use | Receipts, short-term shipping labels | Asset tags, product labels, long-term labels |
If a thermal printer question mentions a ribbon, it is thermal transfer, and the media does not need to be heat-sensitive. Impact printers also use ribbons, but they strike the ribbon with pins instead of heating it.
Heat sensitivity and handling
Because the image is a chemical reaction, thermal paper keeps reacting to its surroundings. Heat from a hot car dashboard or a nearby heater can turn the whole sheet gray or black. Sunlight, friction, and contact with some plastics, solvents and oils fade the print over time. Store rolls in a cool, dark, dry place, and do not rely on thermal receipts for records that must last; scan or copy them instead.
Only one side is coated. If a new roll is loaded backward, the printer feeds paper but prints nothing. A quick check is to scratch the paper with a fingernail: the coated side shows a dark mark.
Feed assembly and maintenance
The feed assembly holds the roll and uses a rubber platen roller to pull the paper across the printhead while keeping it pressed against the heating elements. Objective 3.8 lists three thermal maintenance tasks:
- Replace paper: use the roll width and core size the printer specifies (80 mm and 58 mm are common receipt widths), load it with the coated side toward the printhead, and leave a short tail out before closing the cover.
- Clean the heating element: power off, let the head cool, and wipe the element with isopropyl alcohol on a swab or a printhead cleaning pen. Some manufacturers recommend doing this at every roll change.
- Remove debris: clear paper dust, label adhesive and torn paper from the paper path and the platen roller.
Troubleshooting
- Blank output: roll loaded upside down, or plain paper loaded instead of thermal paper.
- Faded or light print: darkness (heat) setting too low, a dirty heating element, or old or heat-damaged paper.
- White vertical lines or gaps: debris on the element, so clean it. If the line stays in the same place, one or more heating dots have failed and the printhead needs replacing.
- Gray background: paper exposed to heat before printing.
Thermal paper on the CompTIA A+ exam
Under thermal printers, Core 1 objective 3.8 lists the feed assembly, special thermal paper, and three maintenance tasks: replace paper, clean heating element and remove debris. Know that a direct thermal printer prints only on thermal paper, that the paper is heat-sensitive and fades, and that a printer feeding blank paper usually has the roll loaded backward. Do not confuse thermal printers with thermal inkjet printheads, which use heat to fire liquid ink.