Technically the simplest thermal printer are things like wax seal or a cattle brand. Signet rings are pressed into the soft wax sealing a letter - creating a simple message.
Heat can transform material in several ways -
| cutting, engraving, melting, gassifying, oxidising or carbonising a material | |
| removing particles by thermal shock. | |
| speeding up chemical reactions such as dyeing | |
| releasing chemical reactions by melting wax like materials | |
| adhering material to a base - the mechanism used in thermal transfer printers - and indeed in laser printer fusers. |
Direct Thermal Print
The most common thermal printing uses a dye and a mild acid coated on the paper as a wax. Where the wax melts the acid changes the dye from light to dark. This method of print has been widely used in fax machines because it was low cost to implement, simple for users and fairly quick in operation. Problems are that:
-the output never looks all that good, the paper surface is slightly glossy with the acid wax
- paper decays with time, yellowing particualrly where it is exposed to light, whilst the writing tends to fade.
- the thin roll-fed paper looks rather nasty and often comes out irregular sizes or with torn corners.- faxes use rather large pixels so the printing looks crude.
The first two problems are difficult to fix, they are in the nature of the material. Problems with appearance could easily be addressed by having a thicker pre-cut paper and higher resolution printheads.
Fax paper also got a reputation for being expensive although this isn't necessarily the case compared with laser and inkjet print. Seeming expensive and looking cheap were a fatal combination so direct thermal printing has been pretty much confined to low cost fax machines, point of sale printers and ATMs and labelling.
A few attempts were made in the early 1980s to popularise direct themal printers as general purpose computer output devices but they never caught on in that role.
Thermal Transfer
Thermal transfer printing is quite widely used for labels, barcodes and even photographs. The paper can be any material - it is commonly oll fed labels but photographic printers are often sheet fed. Colours take the form of a wax or resin coated onto a ribbon.
There are several techniques used with termal transfer ribbons
Thermal printheads are basically a row of tiny resistors embedded in a ceramic bar. This kind of device is not particularly expensive to make in moderate quantity, hence their use in low cost fax machines. Printheads can be made narrow - a few dozen pixels, or the width of an A4 page with several thousand pixels as they are in fax machines.
Thermal transfer ribbons are a very thin plastic backing of something like polyester coated in coloured wax or resin. Wax is cheaper and gives a strong print but not so robust. Resins can often melt into and amalgamate with plastics like vinyl giving a very tough waterproof image. Where printing is done the colorant melts and transfers, leving blank backing meterial behind. This is normally wound onto a take-up spool and disposed although it may go for recycling.
Printheads and ribbons can be narrow or wide.
Narrow ribbons take the same approach as most inkjet or dot-matrix printers and scan a relatively small printhead typically producing a line of text back and forth across the page. Because of the aster scan action this technique isn't particularly quick but it is flexible - the page can be any shape that will fit the machine. There are colour plotters made this way, particularly by Roland.
Page-width ribbons use a wide printhead and a wide ribbon to transfer the whole image in one pass down the page. This is a quick process - potentially just taking seconds for a monochrome page.
To get a colour page use the normal cyan, magenta yellow and black colourants. The Roland plotters have four different ribbons, one for each colour, and these are automatically loaded onto the printhead. Photographic printer ribbons come with a succession of coloured page-shaped panes, one for each colour, and the printer passes the page through three or four times.
Colourant that isn't used in one pass can't really be re-used, there would be gaps in the ribbon. This may not be very important because thermal transfer is often used for photographs and labelling where page cover may be 20 - 50% - far higher than expected with inkjets and laser printers. It does suggest that recycling the ingredients would be a good idea. Controlled disposal of themal transfer ribbons might be a good idea anyway because the takeup spool contains a negative image of everything the printer did - so if anything in the least confidential was done the spool shouldn't be disposed of in ordinary waste.Dye Sublimation printers might count as a sub-species of thermal transfer devices. The idea of sublimation is that the colourant vapourises and then migrates into the substance of the paper or plastic being printed.
Thermal Printers
Thermal printers can often deal with thermal direct of thermal transfer images, the head is essentially the same, its just a matter of higher temperatures or longer contact at the print position. Label printers can often do either, its just a matter of changing a temperature setting to match the material.
Other Thermal PrintersThe earliest writings were engravings on clay tablets, but even if baked clay does count as thermal print that technique isn't much used today. Information that is to have a long life is still often engraved as it is with monumental sculpture. Writing can't outlive it's medium and whilst paper can be surprisingly long-lived some plastics and metals will presumably last longer.
Laser Cutters
Laser engravers and cutters are a form of thermal plotter.
Machines like the GCC Laser-Pro focus a beam onto a small area exciting specific atoms - carbon in the case of a CO2 laser. Laser engravers cut the pattern into the material - gassifying it. Laser engraver construction is roughly like a pen plotter. One type of high speed laser printer uses scanners and mirrors but simply burns the message into the paper. These are specialist devices but they do illustrate that there are lots of ways to print with heat. Laser -based thermal printers that can burn their message into a medium but most thermal printers are less aggressive - they use a chemical reaction.
Sinclair printers
Sinclair sold an unusual thermal printer with some of their low cost computers. The Sinclair approach was to use a metal foil coated paper. The printhead made a firm electrical contact with the foil on one side then used point contacts to burn parts of the paper away in a dot matrix patern.
Perhaps predicatbly there were one or two problems with this approach.
The paper was a 40 column roll - acceptable perhaps as a till roll but not for much else. There was only one source of paper supply and it was quite expensive.
Printers weren't very reliable.