How do thermal printers work?

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📓 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.

I'm glad this question came in because I'm a bit of a printer nerd. In college, I was dubbed the "printer whisperer" for my ability to decipher error codes and willingness to give machines more patience than the average disgruntled user.

I am also the proud owner of a Zebra LP 2844 thermal printer, which I used for many years to print shipping labels for my eBay business.

GIF of a thermal printer printing out a label with the Today You Should Know label printed on. — Behold, my pride and joy, my personal 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 work by applying heat from the printhead on to chemically treated paper. Where and when the paper heats up, it releases ink.

GIF of a thermal paper label inks where heat from a lighter is applied — 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. I am typing I am tyypicng and issue weh — 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.

Diagram of the thermal coat of thermopaper. 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 — 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 work by the printhead applying heat to a ribbon made up of wax or resin which is then transferred on to the printing material.

In the diagram above, the thermal element (printhead) heats up the ink ribbon which then gets transferred on to the label. — "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.

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Thanks for reading this week's newsletter! If you have any thoughts, questions, or favorite GIFs, my inbox is always open. Just hit reply to send me a note! :)

All my best,

Caitlin

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📖 Book of the week

Smart Brevity by Mike Allen, Jim Vandehei, and Roy Schwartz

This book is about how to write so people can easily understand what you're trying to say. There is absolutely a time and place for elegant prose and meandering descriptions, but for day-to-day reading (articles, emails, newsletters, etc.), you'll be better served with smart brevity. Unsurprisingly, I found the style and structure of this book to be actionable and memorable. If you write words that other people read, it's worth picking up. And, of course, it's a quick read.