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Dot Matrix Ink 
The most common way for a dot-matrix printer to ink the page is using a conventional sticky-ink in multistike fabric ribbon approach.
One alternative used in the 1980s but now largely abandoned is to use a carbon coated single strike ribbon.
Print-ribbon might seem rather uninteresting, but it is actually the point at which printing occurs. Using the wrong ribbon will at the least educe print quality, but it could wreck the print-head.
The ribbon fabric is one of the limits on dot-matrix performance. The more close-weave the fabric the smaller the minimum dot size. If the fabric is too open then printhead pins will stick through the cloth and become bent.
Ink is held in pores in the cloth mostly by its viscosity and partly through capillary action. The ink is usually a greasy consistency - like that in a ball-point rather than that in a fountain pen. Handling a new ribbon lightly leaves just a trace on the fingers. When the printhead pins hit the ribbon hard it transfers sufficient to the paper to make a clear mark, and more ink gradually fills the dry area of fabric so that it can be used again.
To prevent ink smearing the paper where it is not wanted the printhead is usually surrounded by a ribbon shield. The shield is usually made of mylar or thin metal and it's job is to leave a window for printing just where the print-needles meet ribbon and paper and nowhere else. Sometimes the ribbon -shield is part of a ribbon cartridge, often it is built into the printer. The shield is continually in contact with the paper and ibbon so it will wear away and it is really a sort of long-life consumable.
Manufacturers striving for high print qualities also produced single-strike carbon ribbons. The ribbon is a thin plastic sheet with a coat of carbon-black. This kind of ribbon was more common on daisy-wheel and golf-ball printers, but was used on some 24 and 48 pin dot matrix designs. Anyone wanting high quality print is now likely to use an inkjet or laser printer, so this kind of ribbon is obsolete.
The consistency of the ink is set by several sometimes conflicting needs:
- it needs to be retained in the ribbon in a transferably wet state over a wide temperature range and for some time when the printer is not in use
- it needs to transfer with a very short impact from a small pin
- it should not transfer to fingers during handling
- the ribbon should have a long life, so the ink from unused areas should transfer into used areas.
- ink should dry immediately on the paper - except in the case of coloured inks where some mixing of inks over 3 - 5 seconds may be desirable.
- coloured inks should not flow out of their stripes in a ribbon over periods of years.
- the ink has a role in print-jewel lubrication.
Ribbon on Spools
Ribbons were held on spools and wound backwards and forwards by motors in old printer designs, an idea they inherited from typewriters. Ribbon would be supplied on a spool and the user put the spool on it's winder, pulled the ribbon across the printer, through the printhead shield and onto the take-up reel. A couple of turns around the take-up reel would secure it in place. Users could not keep their hands clean doing this. On the other hand a ribbon on a spool is just about the cheapest and most effective way to deliver ink other than in a bottle. Furthermore it is easy for anyone with an ink-ribbon manufacturing plant to make ribbons that will fit their competitors machines, so prices for ribbon spools are rarely extortionate.
Some of OKI's early dot matrix printers had the selling point that they could use ordinary typewriter ribbons.
The spools might need two motors with gear chains and switches to detect the ribbon end - simple and effective, but expensive and usually found on more expensive printers. Manufacturers designed various ratchet mechanisms driven by the carriage motor to control the spools.
The ribbon is constantly being exhausted and worn away by the action of the print-head. To increase ribbon life they are often mounted at an angle across the print path - when the printhead is carriage left it uses the lower part of a thick ribbon, then uses the top part when it is at carriage right. Of course this principle doesn't work properly if most of the printing is on narrow label stationery just 3 inches wide in a 14 inch carriage - and it won't work for ribbons with a colour stripe.
Ribbon in Cartridges
As already suggested users can't keep their hands clean using ibbons wound on a spool. Manufacturers can help with this by putting the spools in a cartridge.
Circular ribbon spools make poor use of space inside the printer, an oblong cartridge fits much better. The ribbon is a continual "mobius" strip folded in the cartridge. This design makes better use of space, it is also potentially simpler because now the ribbon is driven in one direction all the time. The design problem shifts to finding a way to stop the ribbon getting jammed. Ribbon on spools does not usually jam because it can be kept under tension all the time. Ribbon in a cartridge is rather more difficult to control. Ribbon jams in brand new cartridges are not unknown, and with cartridges costing anything up to £50 this can really irritate the users. A damaged ribbon is a liability because the point of damage may bend a printhead needle - and that really will prove expensive.
Costs And Benefits
The cost of ink ribbon cartridges for dot matrix printers varies widely. Smaller printers tend to have tiny cartridges costing about £5- £7 each. Big printer cartridges are more commonly in the £25 to £50 range. The cost of putting ink on paper with a dot-matrix printer can be as little as 0.1p for an A4 page using a big cartridge.
It is relatively easy to re-ink ribbons if they are not too badly worn, there a companies that sell little winding rigs to do the job, together with the right consistency of ink. There are also specialist re-inking services. As demand for dot matrix printers declines these facilities and advantages will decline.
Users and corporate IT department are often a bit hostile to dot-matrix printers compared with inkjets and laser printers. Installing a ribbon cartridge is often not as easy as installing a toner cartridge, people get impatient and don't read the instructions, then the print-head jams and wrecks a brand-new ribbon.
Problems with loading cartridges aren't the main reason for the dot matrix printer losing popularity. Serial dot matrix printers are inherently rather slow because the pixel pattern on the page is being produced by just a few pins and they are only capable of moving a few thousand times a second so a page containing millions of pixels can take a minute to print. (Paralllel "line printers" overcome this by having dozens of pins but they are expensive to make)
The pins on a matrix head also make an irritating noise. About the best that can be said for the graphics is that after an immense time something vaguely recognisable might be produced. And dot matrix printers aren't cheap to buy because they have been undercut by the economies of scale inkjet printers can achieve.
Dot Matrix Resurgent
The surprise is that manufacturers still producing dot-matrix machines don't revert to the big power in their armoury, drop the idea of expensive cartridges and revert to using cheap ribbon reels. Those of us who want print at the lowest cost don't mind having to wash our hands from time to time.
© Graham Huskinson 2010