A thermal printer prints by heat. A line of tiny resistors — the print head — is pressed against a paper coated with a dye that turns dark at around 60 to 80 °C, and the pattern of heated dots becomes the characters. There is no ink, no ribbon and no cartridge: the colour is already in the paper, and the printer only decides where to release it.
That is why a receipt printer can be the size of a small box and still print a receipt in under a second. It is also why a receipt fades, why the paper matters as much as the machine, and why the single most common service call on a receipt printer is not a broken head but the wrong paper.
The three layers that make the paper work
Thermal paper is not ordinary paper with a coating sprayed on at the end. It is a base sheet with a heat-sensitive layer 20 to 30 µm thick, and a top coat over that which protects the head from the dye and the dye from the environment.
The heat-sensitive layer holds a leuco dye and a developer that are solid and separate at room temperature. Heat melts them together, the dye donates an electron to the developer, and the pair turns dark. Cool it down and the colour stays — the reaction is irreversible, which is why the print cannot be erased and why a receipt left on a car dashboard goes dark all over rather than fading evenly.
203 dpi is eight dots per millimetre, and that sets everything else
Receipt printers are almost all 203 dpi. Dots per inch divided by 25.4 gives dots per millimetre, and 203 ÷ 25.4 is 8. So the head has one heating element every 0.125 mm, and the print width in dots is simply the print width in millimetres multiplied by eight.
| Paper width | Print width | Dots per line | Font A characters per line |
|---|---|---|---|
| 80 mm | 72 mm | 576 | 48 |
| 76 mm | 64 mm | 512 | 42 |
| 58 mm | 48 mm | 384 | 32 |
| 44 mm | 36 mm | 288 | 24 |
The last column is what a shopkeeper actually notices. Font A is 12 dots wide and 24 dots high, which on a 203 dpi head is 1.5 mm by 3.0 mm. Forty-eight characters is enough for an item name, a quantity and a price on one line. Thirty-two is not, which is why 58 mm receipts wrap — and why a receipt layout designed for 80 mm looks broken on a 58 mm printer even though both print correctly.
The 512 versus 576 question
If a datasheet quotes 512 dots per line against a 72 mm print width, the two numbers disagree: 512 dots at 8 dots/mm is 64 mm. Either the print width is 64 mm or the head is not 203 dpi. Ask which, because the answer changes how the receipt layout has to be designed.
Speed is paid for in darkness
A print head has a fixed amount of time per line to deliver energy into the paper. The pixel row pitch is 0.125 mm, so at 220 mm/s each row gets about 0.57 ms and at 260 mm/s about 0.48 ms. The controller compensates by driving more current, which is why the power adapter is part of the printer specification and not an accessory.
| Model | Print width | Speed | Head life | Cutter | Adapter |
|---|---|---|---|---|---|
| POS-8250 (80 mm) | 72 mm | 260 mm/s | 100 km | Full or partial | 24 V / 2.5 A |
| POS-8370 (80 mm) | 72 mm | 220 mm/s | 100 km | 1.5 M cuts | 24 V |
| POS-5890U (58 mm) | 48 mm | 70 mm/s | 50 km | None | 8.5 V / 2 A |
Two things follow. A 58 mm portable printer runs at 70 mm/s not because it is cheap but because it runs on an 8.5 V adapter and has half the head life of a counter unit — a smaller power budget buys fewer dots per second. And when an 80 mm printer suddenly prints grey at full speed, the first thing to check is the adapter: a 12 V supply on a 24 V printer will print, slowly and faintly, until the head cools enough to print nothing.
What the print head is, and what ends it
The head is a ceramic substrate with a line of resistors and their drivers, pressed against the paper by a rubber platen roller. It has no wearing parts in the ordinary sense — nothing rubs except paper — and yet it is the component with a published life, quoted in kilometres of paper: 50 km on a small portable, 100 km on a counter printer.
Four things shorten that life faster than paper does:
- Abrasive paper. Cheap thermal paper uses a coarse clay filler. It is the same paper that prints faintly, so the saving is paid twice.
- Dust and paper fibres. They bake onto the resistors and create a permanent pale stripe. This is the line that runs down every receipt and gets blamed on the software.
- Cleaning with the wrong solvent. Isopropyl alcohol on a cold head, with a lint-free swab, is the standard. Acetone and abrasive erasers remove the protective glaze along with the dirt.
- Static and a bad ground. In a dry room, a head that is not grounded properly accumulates charge and punches through its own driver ICs. The symptom is a block of dead dots, not a gradual fade.
Thermal transfer is a different machine with a similar name
Everything above describes direct thermal: heat acts on the paper. Thermal transfer is the other branch. There, the head heats a ribbon coated with wax or resin, and the ribbon deposits material onto ordinary label stock. The paper does not need a coating, the print survives heat and sunlight far better, and the machine needs a ribbon, a take-up spindle and a tension mechanism.
The practical split: receipts, tickets and queue numbers are direct thermal, because the paper is cheap and the print only has to survive a week. Shelf labels, asset tags and anything that must survive a year in daylight are thermal transfer. A direct thermal label on a sunlit shelf is legible in March and blank by August.
Label machines also read their media. The 4-inch class in our own range uses dual transmissive and reflective sensors, accepts stock from 0.06 to 0.30 mm thick, and prints 800 dots across a 100 mm width — the same 203 dpi arithmetic, applied to a wider head.
Why the receipt fades, and what to do about it
The dye reaction is reversible under the wrong conditions. Three of them matter in a shop:
- Heat above about 60 °C turns the whole sheet dark — a receipt left on a dashboard or beside a griddle.
- Plasticisers in PVC folders, and the oils on skin, dissolve the top coat and lift the print.
- UV light fades the print from the top down, which is why the first line of a window-displayed receipt is always the faintest.
The fix is storage, not a different printer: a paper bag in a drawer, not a PVC sleeve on a sunny counter. For anything that must be legible for years — a warranty receipt, a signed delivery note — print two copies and file one, or use thermal transfer.
Where thermal printers are the wrong choice
A thermal printer cannot produce a durable document, cannot print colour, and cannot print on plain paper. If a workflow needs an A4 invoice, a two-colour label or a document that survives a year on a shelf, the answer is a laser or an inkjet, and the receipt printer stays for the receipt. The mistake is trying to make one device do both: it produces a receipt-sized invoice on paper that fades.
FAQ
Does a thermal printer need ink or a ribbon?
No. Direct thermal printers need only paper; the colour is in the coating. Thermal transfer printers need a ribbon, and those are a different machine with a take-up spindle and a tension path. If there is no ribbon in the box and the paper is shiny on one side, it is direct thermal.
Can I put thermal paper in a laser printer?
No. A laser fuser runs at roughly 200 °C, well above the 60–80 °C the coating activates at, so the sheet will darken, smear and very likely jam inside the fuser. Thermal paper is for thermal printers only.
How can I tell which side of the paper is coated?
Scratch it with a fingernail. The coated side turns dark grey where the pressure and friction generate heat; the other side stays white. The coated side must face the print head, which is why a correctly loaded roll prints and a reversed one comes out blank.
What is ESC/POS?
It is the command set receipt printers use. The application sends bytes — text, line feeds, cut commands, cash drawer pulses — and the printer executes them. It is not a page description language like PostScript: there is no font embedding and no layout engine, which is why the receipt layout lives in the application.
Why is only half my receipt printing?
Three causes, in order of frequency: the paper is narrower than the print width in the app’s layout, the head has a dead block from static damage, or the roll is slightly misaligned and the mechanism is printing off the edge. A dead block produces a consistent vertical white stripe; misalignment produces a ragged edge.
Where to go next
Print width, dots per line and head life are the three numbers that decide what a receipt printer can do — the rest of the datasheet is reassurance.
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