How Thermal Fax Machines Work

Thermal fax machines work by implementing heat to print a fax.
Clive Streeter/Dorling Kindersley Collection/Getty Images

Despite the fact that we're standing on the cusp of a "paperless age," fax machines still are used to send important documents quickly when an email doesn't cut it. Business owners would argue that owning a reliable fax machine is essential. But most of us don't really know how these machines work or which type we should purchase. In this article, we'll take a look at the inner workings of the thermal fax machine, an old-fashioned, but useful and inexpensive business tool.

Thermal fax machines send and receive information the same way other fax machines do. The scanning mechanism uses photo sensors to scan the document and read thousands of tiny dot areas to determine whether each dot area is black or white. The machine then encodes and compresses this information and sends it over a telephone line to another fax machine. See "How Fax Machines Work" to get a closer look at this process.


What sets thermal fax machines apart from other fax machines is the method they use to print out that information accurately on the receiving end and make it readable on a piece of paper. Thermal fax machines don't even need an ink cartridge to do this. Instead, they utilize the powers of thermal technology to print a fax, or facsimile, of the original document.

Two kinds of thermal fax machines can do this, and they both use fairly simple methods. One kind stores the necessary "ink" in the paper itself, while the other kind extracts it through a process of melting down special ribbon against the paper.

How is heat used to create a print-out? Read the next few pages to learn just how these machines work their thermal magic.


Direct Thermal Technology Fax Machines

Thermal paper commonly comes in a roll that feeds into the machine.

Certain kinds of thermal fax machines use thermal paper to print out the faxes they receive, using what's called the direct thermal method. This kind of paper became so synonymous with fax machines that people came to refer to it simply as "fax paper." However, these days, thermal paper is also a popular choice for printing machines that print receipts and barcode labels for retail products.

What makes the paper special is the coating of chemicals on it. If you've ever felt thermal paper, you probably immediately noticed that it feels different from plain paper. It also often has an odd chemical smell. The chemical coatings give it these characteristic textures and smells. They also lend it a slippery surface that makes it difficult to write on.


The chemical coating allows the paper to react to heat. The particular chemicals used vary depending on the machine and the manufacturer, but can consist of dyes, antioxidants, inert pigments, and waxes, among other things [source: Petrovic]. As the machine applies heat in the right places and those areas of the paper rise to the appropriate threshold temperature, the chemicals react and actually cause the paper to darken in those spots.

So, to take advantage of the chemical's ability to darken paper, the fax machine incorporates a printhead that applies heat in the right places. This printhead uses a heat resistor to convert electricity into usable heat. The principle behind this conversion is Joule's first law, which explains that a heating effect comes from the flow of currents and resistance. The heating element receives the encoded information, which tells the printhead where to apply heat and where not to apply it. In the end, the page comes out with readable information on it.

Thermal paper usually consists of at least three layers:

  1. Substrate layer: This is simply the paper layer.
  2. Base layer: This includes a binding element, which can consist of such things as starches, gelatin and certain alkali salts. Another component of this layer is what's called a porosity improver, which helps heat travel through the paper.
  3. Active layer: This layer contains a colorless chemical formula that reacts to heat.

In addition to these three layers, the paper might also include a protective layer to prevent heat from fading or even ruining the document after printing. The paper can feed into the machine in rolls, and an automatic paper cutter separates pages.

This kind of fax machine is said to use the "direct thermal" printing method to distinguish it from the machines that use the "transfer method," which we'll talk about next.


Meltdown: Thermal Transfer Fax Machine

Thermal transfer ribbon is the middle-man between the print head and the paper in the transfer process.
NCR Corporation

Though thermal paper is still useful for thermal printers of receipts and bar codes, a different kind of thermal method has taken over for most modern thermal fax machines [source: Sems]. Whereas thermal paper allows for direct application of heat to the paper, the thermal transfer method uses a third agent between the heating element printhead and the paper. This middleman is a ribbon that contains the necessary "ink." However, it's still considered a thermal fax machine because heat is necessary to transfer the ribbon's ink onto the paper.

Unlike with the direct method, you don't have to buy special paper for thermal transfer machines; instead this kind of machine can use plain paper. However, the printhead uses roughly the same basic technology as the one that prints on thermal paper. The printhead provides heat to the thermal transfer ribbon in the appropriate places and melts these areas down. This melting process makes the ribbon release the ink it contains onto the paper fed through the machine.


The ribbon itself can consist of a base film made of a material such as polyester, and attached to that is the substance that makes up the ink [source: Watanabe]. Another layer of protective coating keeps it from sticking to the printhead. The ink layer is typically made of one of three materials:

  1. Wax: Thermal ribbons consisting of wax are often the cheaper option. However, these ribbons don't produce the highest quality print-outs. Documents printed using wax thermal ribbons can easily smudge after being exposed to heat or scratched.
  2. Resin: Resin is a material secreted naturally from certain plants or artificially manufactured. Though it originates in a viscous (gooey and sticky) state, it later hardens. Resins are extremely useful in the manufacturing of such things as paints and plastic. Using a resin material, thermal ribbons print a higher quality document that doesn't easily smudge. However, it's more expensive than the wax kind.
  3. Wax/Resin Composite: This is a popular middle-of-the-road choice material for thermal ribbons because it's not as expensive as resin and not as low quality as wax.

So, in the thermal transfer method, the printhead receives the encoded information about when and where to heat up, just as in the direct thermal method. However, this time, the ribbon runs along between the paper and the printhead. As the printhead heats the ribbon's ink material, it releases the ink, which proceeds to melt onto the paper.

Knowing the thermal methods of these kinds of fax machines allows us to understand their advantages, which we'll discuss on the next page.


Advantages of Thermal Fax Machines

Thermal fax machines have fewer components and take up less space than other fax machines. 
Todd Warnock/Stone Collection/Getty Images

Most agree that the biggest advantage of thermal fax machines is the cost. As a tried and true machine with basic components, thermal fax machines are cheaper to build, and thus cheaper to buy. The cost benefits can prove better in the long run as well, especially compared to ink-jet fax machines, which require expensive ink cartridge refills. For those machines that use the direct thermal method, the only refill they need is the paper itself.

Another major advantage of thermal fax machines is their durability, which is due to their relative simplicity. They have fewer moving parts than their newer, more sophisticated competition, ink-jet and laser fax machines. In fact, in direct thermal fax machines, the only moving part is the system that feeds the paper through the machine past the printhead.


Although transfer method machines are a bit more sophisticated than direct method machines, they aren't as complex as ink-jet machines. Because neither kind of thermal fax machine is very complex, they don't break as frequently, and more often than not, when they do break, they are easier to fix. As a result, these machines often last longer.

Retail sellers of thermal fax machines often tout that they tend to be more compact than other kinds. So, if space efficiency is a factor in your workplace, thermal fax machines are a viable option.

Despite these advantages, thermal fax machines have become unpopular in many modern offices for reasons we'll discuss on the next page.


Disadvantages of Thermal Fax Machines

Direct thermal fax machines can feed out faxes that immediately curl up, making them frustrating to work with.
John Turner/Stone Collection/Getty Images

Although the advent of thermal fax machines allowed for faster and easier business communication for decades, the machines have some weaknesses, especially compared to newer technologies. Because of the distinctly chemical smell and feel of thermal paper, many people dislike using it. And because it's originally fed from a roll, thermal paper faxes have a tendency to curl up. The fact that it's difficult to write on can make it particularly frustrating as well.

But perhaps the biggest problem associated with thermal paper comes from the very quality that allows it to work. Because it's made to darken with heat, the documents might darken after printing if the paper is stored in a warm place. Leaving your important fax in a hot car, for instance, can easily ruin it. And, because laminating is a heat-related process, laminating thermal paper doesn't work very well [source: Leech Printing].


Even if you protect the thermal paper from extreme temperatures, it doesn't have a very long shelf life. Gradual fading can render it useless after too long, despite the fact that it often comes with a protective coating. Different sources say that we can only expect documents on thermal paper to be useful for about three to five years. Even in optimal conditions, which are at temperatures lower than 77 degrees Fahrenheit (25 degrees Celsius), thermal paper probably won't last much longer than that [source: Sandt Products].

Many organizations in charge of record-keeping make sure to avoid keeping documents printed on thermal paper. Instead, they recommend converting any important records to plain paper by using a photocopier. Office workers often do this by habit when they receive faxes on thermal paper. However, by using up both thermal and plain paper this way, the cost benefits of the thermal fax machine can go to waste and even make it more expensive than other fax machines [source: Passaic County].

Though it used to dominate the faxing world, thermal paper retreated to the back burner in the year 2000, when the use of plain paper officially over took "fax paper" itself in fax machines [source: Kenmochi]. Since then, it has continued to fall in popularity for fax purposes.

Even though thermal transfer fax machines don't use thermal paper, they have some disadvantages as well. For instance, especially when the thermal ribbon consists of mostly wax, the print tends to easily smudge. And unlike with the direct method where the only refillable need is the paper, replenishing the thermal ribbon periodically is necessary for the transfer method machines.

Both direct and transfer methods tend to only print documents of average quality at best, especially compared to ink-jet fax printers. Both kinds of thermal fax machines have environmental disadvantages as well. Thermal paper is not recyclable like plain paper is, and the ribbon used in the transfer method cannot be recycled either.

To learn more about fax machines and other emerging technologies, browse the links on the next page.


Lots More Information

Related HowStuffWorks Articles


  • "Doc's column." Rice Lake Weighing Systems. (April 30, 2008)
  • "Frequently asked questions: How do I manage records on thermal paper?" National Archives of Australia. (April 30, 2008)
  • Leech Printing. "Frequently Asked Questions." Leech Printing, Ltd. (April 30, 2008)
  • Passaic County. "Fax vs. Mail vs. Modem." Passaic County, N.J. Office of Natural Resource Programs. (May 1, 2008)
  • "Thermal Fax Supplies: Frequently Asked Questions" Fullmark, Pte Ltd. (May 1, 2008)
  • Petrovic, Ivan, et al. "Themal Paper." International Patent Application WO 2006/060589 A2. (May 1, 2008)
  • "Ribbon Selection." IntelliTech, Intl. (April 30, 2008)
  • Sandt Products. "Bond, Machine & Thermal Paper Roll." Sandt Products: Manufacturers of Paper Roll & Folded Specialties. (April 30, 2008)
  • Sems, Marty. "Fax It To Me." Harware. Aug. 2000, Vol. 4, Issue 3, Pages 28-51. (April 30, 2008)
  • Watanabe, Shinya, et al. "Biaxially oriented polyester film for thermal transfer ribbon." United States Patent 6537657. (May 1, 2008)