Solid Ink Printers

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Solid Ink Printers - Some DetailsDetail about Solid-Ink Printers
The basic idea of the solid ink or "phase change" printer is an inkjet with meltable ink - something like a wax.

Ink that undergoes a phase-change in this way should have several advantages:


 
A wax like substance not unlike a child's crayon will give nice bright colours and a strong bond to the paper. A solid ink can carry plenty of pigment based colourant, with few of the "transport" issues that pose problems in liquid inks or laser printer toners.
Solid Ink need not be very sensitive to page materials in the way liquid inks are. Anything the ink will bond to should be printable - paper, cardboard, transparencies and plastics.
The solid ink will re-freeze on contact with the page. There is no wait for a liquid ink to dry out.  The print should remain stable when the paper is handled - even in a damp environment.
A cartridge of liquid ink needs wrapping in a in a plastic container. Toner powder normally needs wrapping to protect the user. Solid ink can simply be supplied in litte bricks ready to slot in when convenient. Solid ink shouldn't evaporate, leak, crush or spill in the way other materials do.

Or to summarise the phase change printer gives the bright, tough look expected from a laser printer but doesn't need the complicated electrophotography processes of a laser using toner powder.

Various companies have experimented with phase change inkjet printers over the years. Tektronix developed marketable models in the mid 1990s but in the year 2000 sold the printer division to Xerox.

Xerox confuses things somewhat by using the "Phaser" name for both solid ink and rather more ordinary laser printers presumably on the grounds that what customers want is a "value proposition" or some other bit of marketing speak.

The Xerox / Tektronix "Phasers" have been quite succesful, particularly in photograph and prototyping work. Xerox are the major producers of this class of machine.

Problems

Tektronix and Xerox overcame several problems to make this kind of printing possible. In some ways the materials and techniques are well established. Basic material is rather similar to the wax / resin used in thermal transfer and inkjet printing is familiar.

Thermal transfer printers use a simple mechanism to move wax or resin onto a page. The problem is that they rely on a foil coated in material. This approach tends to be both:

  • wasteful because only a small part of the colourant is used and the foil is discarded
  • slow because the printer has to make three or four passes over the page for colour pictures.
Phase-change inkjet designs initially took the approach used by a conventional inkjet, using a carriage or a shuttle printer with a pagewidth head producing one line of pixels per pass. Tektronix produced their first phase-change inkjet working this way in 1991. However the printhead with it's heaters was heavy so:
  • the carriage was heavy and time was wasted in accelerating and decelerating it
  • printers suffered margin drift spoiling its graphic abilities
  • vibration was an issue. The big heavy printhead was compared to a pile driver in the vibration it made.
Tektronix had a rethink of the process in 1995 and produced a new class of printer, the "phaser"

Phasers

The key innovation in the phaser is to use an intermediate drum to build an image before transfering it to the paper. The drum is anodised aluminium, large enough to hold the image to be transfered to the printed page and heated to the point where the colours set but will still transfer to paper.
 
Alongside the drum is a near pagewidth inkjet printhead, made in stainless steel and heated to 135 centigrade. The printhead has columns of piezoelectric elements for each of the four colours so as the drum turns the four successive nozzles each contribute their colour in a pixel-position. The temperature of the printhead is scalding in human terms but well below the point at which semiconductors fail. Click for larger pic

Ink is replenished from the solid sticks by being melted into the printheads collectors and distributed within the head assembly by pipes.
 
There is not a printhead for every column on the page. The drum rotates a turn then the printhead steps right a column. The image is built up as a succession of stripes from each column of printheads. More columns of printhead elements means faster print. Xerox heads have more than a thousand printhead elements across the pagewidth so there are hundreds of multi-colour stripes building up simultaneously ound the drum. 

The drum itself is heated to about 65 centigrade so that the solid colour cools, sticks but remains malleable.

When the image is complete after several turns the transfer roller moves in and the whole thing is fused to the paper. To do this the paper is warmed and passed across the drum with a pressure roller forcing it down on the ink, which adheres and transfers.

The drum is thinly coated with silicone oil after each transfer - sufficient to ensure the colour doesn't bake on. This is done by a maintenance kit which is the only consumeable beyond the little bricks of ink.

In the phaser the printhead doesn't have to be a shuttle flying back and forth. The process is graceful, the drum turns and the printhead makes one steady pass. One new column of pixels is made for each turn of the drum. There are usually enough printhead elements so that the drum makes about 8 turns to build a page, then a ninth to transfer the image. However the number of printhead elements can be varied to make mono printers, cheaper printers or faster printers. Its a very flexible technology.
 

Benefits
 
The resulting pages look very good. Bright colours give a wide gamut
Because the printhead only deals with the drum it keeps a constant and optimum spacing giving very consistent appearance.
Transfer to the paper is at just 65 centigrade, far cooler than any laser printer fuser.

 
Horizontal resolution is controlable if the quantity from the inkjet is small enough. The printhead can make large or small steps across the drum to give fast print or high resolution.
A very wide variety of paper, card, transparency or other material can be handled. The printer can print on more or less anything that can pass through the paper path. There is very little drying time.
Xerox have managed to incorporate 1236 injets into an A4 head with a speed of 24,000 drops per second per jet. Phaser printers have speeds to 35 pages per minute with full-cover A4 pages at less than 7p each.

Issues

The need to pre-heat the drum, printhead and the ink sticks means startup times are quite long - as much as 15 minutes. Also when the printhead re-starts it purges itself of ink that had hardened. Xerox phasers overcome this by having the printer enter a standby state where ink in the head is held just above it's freezing point.

Solid ink isn't just any old wax. It should be liquid with a low viscosity at a temperature that is not very high. It should be a robust solid at oom temperature. It needs to adhere very well to ordinary paper and of course it needs exactly the right colour characteristics.

The only mass -market solid inkjet at present is the Xerox Phaser. People tend to be wary of minority technologies- but there is no need. Laser printers are a more standard technology and do a pretty similar job so they are direct competition. Solid inkjets survive by competing on price - as low as 1p per page for mono A3 and 7p per covered page for full colour A4.

Overall

Solid ink technology is good for office and workgroup printing particularly in work like trade catalogues. Solid ink printers can be fast, produces eally good looking pages and can even be quite economical for mono print of text pages.

One limiting factor is that solid ink is unlikely to be much use in home printing because there just isn't enough throughput. Solid ink products struggle in the small office market because purchase costs are too high. To some extent printer economics depend on mass production and the solid inkjet hasn't yet achieved that.


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© Graham Huskinson 2010