How do thermal printers work?
The short answer
Thermal printers, as the name suggests, use heat to print. One type of thermal printer uses heat to cause a chemical reaction in special paper, and the other type uses heat to melt solid ink, which is then pressed on to the print material.
The long answer
When you get a receipt at the grocery store (or an outrageously long receipt from a CVS pharmacy), it's printed using a thermal printer.
What is a thermal printer?
The name is somewhat self-explanatory: it is a printer that relies primarily on heat to produce the design of whatever you need to print.
How does a thermal printer work?
Thermal printers use a heated printhead (the component of a printer that transfers the data you need to print) to activate special paper to release ink, or transfer and press solid ink on to the print material.
There are two types of thermal printers.
Type 1: Direct thermal printers
Direct thermal printers work by applying heat from the printhead on to chemically treated paper. Where and when the paper heats up, it releases ink.
Here you can see how applying heat with a lighter releases the ink.
The magic of direct thermal printing lies with the specially treated paper, also known as thermopaper. Thermopaper has multiple layers: top coat, thermal coat, pre-coat, base paper, and back coat.
Diagram of multi-layered thermopaper
Put simply, the top, pre, and back coats are there to protect the dyes released in the thermal coat and enhance print quality, and the base paper is there as a foundation. But the fun stuff is happening in the thermal coat.
There are three key components that make up the thermal coat: heat-sensitive dyes, dye stabilizers, and a solvent surrounding these parts. When heat is applied, here's what happens:
The heat-sensitive dye becomes visible.
The solvent melts, allowing the dye and dye stabilizer to form a bond.
The dye stabilizer ensures the dye remains visible, even after heat is removed.
Left: The dye (rectangle) and dye stabilizer (polygon) are separated by a solvent.
Center: Heat is applied, making the dye visible and melting the solvent.
Right: The dye stabilizer forms a bond with the dye and maintains the visibility even after heat is gone.
These printers are very low maintenance, don't require ink, and can print super fast. The only downsides are that you can only print in one color, you need specific paper to print, and the print fades over time in part because of its sensitivity to light, heat, and abrasions.
Interestingly, I learned that bisphenol A (BPA) is commonly used for the dye stabilizer in thermopaper. This is somewhat troubling as BPA has been known to be an endocrine disruptor and cause hormonal disturbances.
However, a recent study out of the United Arab Emirates found that the estimated daily intake of BPA from paper-to-skin transfer was not concerning, even among cashiers more frequently exposed to handling receipts.
Type 2: Thermal transfer printers
Thermal transfer work by the printhead applying heat to a ribbon made up of wax or resin which is then transferred on to the printing material.
"Thermal Transfer Printing" by Mikhail Ryazanov is licensed under CC BY-SA 3.0 DEED.
In the diagram above, you can see how the thermal element (printhead) heats up the ink ribbon which then gets transferred on to the label.
Like direct thermal printers, heat is still the key reason the printing is able to occur. But it is more versatile than a direct thermal printer because you can use multiple ribbons to elicit multiple colors, you don't need special paper, and the prints can last a longer time.
BONUS: If you want to see some serious enthusiasm for printers, I highly suggest this energizing video put out by Brother walking through how to choose between direct thermal or thermal transfer.
Check out some other curious questions:
Sources
Brother UK. (2023, March 2). How Do Thermal Label Printers Work?. Brother UK. https://www.brother.co.uk/support/answers/how-do-thermal-label-printers-work
CDW. (2022, March 14). What is a Thermal Printer and How Does it Work?. CDW. https://www.cdw.com/content/cdw/en/articles/hardware/what-is-a-thermal-printer-how-does-it-work.html
ChemistryViews. (2020, May 5). How Does Thermal Paper Work?. YouTube. https://www.youtube.com/watch?v=N821Toa9tOU
Davis, J. (2023, April 11). The Ultimate Guide to Thermal Printing. Gordon Flesch Company. https://www.gflesch.com/blog/thermal-printing-ultimate-guide
Koester, V. (2017, May 7). How Does Thermal Paper Work?. ChemistryViews. https://www.chemistryviews.org/details/ezine/11146606/How_Does_Thermal_Paper_Work/
Semerjian, L., Alawadhi, N., & Nazer, K. (2023, March 28). Detection of bisphenol A in thermal paper receipts and assessment of human exposure: A case study from Sharjah, United Arab Emirates. U.S. National Library of Medicine. https://pubmed.ncbi.nlm.nih.gov/36976811/