Review of nonimpact print technologies

by MIKE WILLIS

The alternatives to laser printing

Abstract: New technologies are set to replace currem impact techniques throughout the whole computer market, with the recently launched LED printer i~halleng~n~ the laser.

Keywords: data processing, printers, nonimpact printing, LEDs.

Mike Willis is a senior engineer in the Non Impact Printing Group at Cambridge Consult- ants Limited.

N onimpact printing is becoming such an all embracing title de- scribing many diverse techno-

logies and applications that it needs

some kind of definition. It is often used as an alternative to ‘laser print- ers’ - which use electrophotography to form images, but really should include other methods such as ink jet, thermal, magnetic and electrographic recording. Any process which prints without heavy impact of needles or

type from digital input is a non- impact printer (NIP). Although they are mostly to be found in the clean environments normally reserved for

computers, the technology is also applicable to some of the printing industries problems of handling vari-

able information - an area in which we at Cambridge Consultants special- ize. In this article I shall review the technologies likely to become the leaders in each of the computer print- er segments, and round off by looking at the LED arrays which show pro- mise and are competition for lasers in low speed laser printers. The most important message is that there is a lot of technology in research and deve- lopment labs ready for this major growth area of information techno-

logy.

Personal computer printers

At the slowest, low-cost end of the printer market, the personal computer user probably has an impact dot matrix printer capable of printing very readable text, graphics and, in a

few cases, in colour as well. Two NIP technologies are beginning to move into this segment - thermal transfer and ink jet. Brother appears to be leading the move to thermal transfer, which is related to the old direct thermal methods, which printed on coated paper, but now uses thick or thin-film thermal printheads backing a transfer ribbon in contact with plain paper (see Figure 1). The advantage

over impact methods, apart from silent printing, is a potential increase in print quality due to higher resolu- tion. The resistors which form the thermal elements can be packed at densities exceeding 161mm - higher than most current laser printers pro-

duce.

, //q, Pressure rail

i

/ Tronsfer ribbon

-._Thermal print head

Figure 1. The thermal transfer process.

Ink jet printers have already been introduced by many companies in- cluding Siemens, Canon and Hewlett Packard. Traditionally, the drops have been ejected from tiny nozzles by piezoelectric elements - which is the technology Siemens and others use. But the recently introduced H-P Think Jet uses the bubblejet prin-

VOI 27 no 3 apt%11985 0011~84X/85~03003~4$03.00 0 1985 Butterworth & Co ~P~blishers~ Ltd. 33

ciple. Behind each nozzle is a small resistive element. When this is heated a vapour bubble grows rapidly in the ink behind the nozzle, forcing out an ink drop. The embodiment of this invention is a disposable printhead complete with ink supply sufficient for 500 pages. Throwing the head away solves a lot of the reliability and life problems a designer faces! Colour is relatively easy to implement with ink jet, but the greatest drawback for rapid growth may be the current dependence of print quality on the paper used.

Work processing

Work processing demands high print quality above all - provided readily by the daisy wheel. The major limita- tions, however, are noise, slow speed and the lack of a graphics capability with the fully-formed characters. Al- though two out of three of these are satisfied by the latest needle printers, nonimpact printers using thermal transfer and ink jet are on the side- lines, poised to enter this market. IBM recently launched its resistive ribbon thermal technology in the USA. A thermal printhead is replaced by an array of electrodes which con- tact a resistive layer on the back of the transfer ink ribbon, so that the ribbon itself becomes the heated element. Diablo has demonstrated a thermal

transfer page printer, which uses a page wide printhead and delivers eight pages a minute - the through- put of small laser printers.

On the ink jet side, Epson and Exxon have high-quality serial print- ers available producing letter, or near letter quality text. How acceptable they will become is difficult to say. As previously mentioned, there is often some dependence of quality on the paper used. This will be reduced as better inks are developed.

Page printers

Distributed word processing demands higher throughputs, and here the low- speed page printers using electro- photography will predominate. The technology employed is similar to that used in photocopiers. A photoreceptor is charged up in the dark, and then light is used to discharge it selectively to form a latent charged image. This is developed by using oppositely charged toner particles, and the toned image can then be transferred onto plain paper and fused using heat and pressure (see Figure 2).

As you can see, the multistage process is relatively complex. To keep development costs down most small printers are based on photocopier ‘print engines’. Lasers used to be the most popular light source, a focussed beam being scanned across the drum

Erase Charge lamp corotron

\ \

Photoreceptor

I

drum

Developer unit

; (-)

Transfer corotron

Figure 2. The electrophotographic process.

34

by a rotating mirror drum. Recently photoreceptors sensitive to the near infrared have been developed allow- ing the use of soiid state laser diodes. These are much smaller and cheaper than gas lasers, allowing printers such as the Canon CX to have the footprint of a typewriter and a price tag of $3 500.

But lasers are not the only imaging systems available. A lot of intensive research worldwide has produced a variety of solid state systems. These have the advantage of taking up less space and require no scanning mechanisms and hence no mechanical parts, meaning improved reliability. Arrays of LEDs are likely to become the most popular, with the Agfa P400 with a resolution of 16 points/mm leading the high resolution machines. Other methods are arrays of liquid crystal shutters - Epson and Casio are developing this system - and magneto optic light valves, of which the Philips LiSA system is an ex- ample. All these methods use page wide arrays with one LED or light valve per print position across the page. LED arrays are cheaper to make than light valves, but the red LEDs require a red sensitive photo- conductor, and may dissipate 10 or 20 watts of heat. Light valve arrays are more expensive, but can switch any colour light, allowing a better match between the light source and the photoreceptors spectral sensitivity.

Other high quality printers using rival technologies should appear very shortly. Diconix, a US company ac- quired last year by Kodak, has demon- strated an ink jet page printer capable of 18 pages/mm and with a resolution of 12 points/mm. The technology is similar to the Digit system developed by previous owners Mead, which is used in a very high-speed web addres- sing and mailing printer. IBM’s at- tempt to exploit ‘continuous’ jet - as distinct from drop on demand ink jet - was far from successful, and so the Diconix machine is watched with interest.

data processing

systems

Paper

J *

what becomes the final print paper. 16 points/mm resolution is now avail- able, and from Versatec a colour machine. Previously only ink jet plot-

Figure 3. The ~u~eto~~~~ic print mechanism.

Magnetography is the print mech- anism in the Ferix 800. A magnetic recording head array scans and mag- netizes an image on a drum coated with chromium dioxide. Dry toner with a small content of ferite particles then develop up the image, which is transferred electrostatically on to plain paper and then fused (see Figure 3). The process has the advantage over electrophotography in that the image does not decay, allowing a ‘duplicator’ function - the same magnetic image can be repeatedly deveioped up until the required num- ber of prints have been made.

All of these page printers can be- come electronic copiers with the addi- tion of a scanner and the appropriate software. As the original document is digitized it can also be transmitted to a remote machine giving a facsimile function as well. The power of the computer allows electronic enlarge- ment or reduction, changes in aspect ratio and automatic straightening of documents.

Computers plotters

Electrographic technology has been used by Benson and Versatec for many years. Charged images are cre- ated directly on dieletric coated paper by an array of high voltage electrodes. Liquid toner forms a visible image on

ters, notably from Applicon, were capable of producing colour prints for this market. Applications include computer graphics dumps for geo- physical data and electronic CAD.

High-speed computer output

lasers expose photoconductor drums at resolutions of 12 points/mm giving very high quality print. The industry

The line printer replacement of the future may not be the laser printer.

of direct mail would not have grown

Medium and high-speed electropho- tographic printers have been around since the late 1970s and can offer very high throughputs - up to 180 pages/ mm (equivalent to 9000 linesimin). Five years ago the paperless office was predicted, but these machines are now seen to increase the volume of printed pages at a rate in excess of 40% per year. High powered gas

Paper out f <Paper m

Power supply

Figure 4. The Delphax 2460 printer uses ionographic processes.

~0127 no 3 april1985 35

so quickly without them, yet they are being challenged by simpler and more robust technologies.

The upstarts are magnetography - already described, and exploited in the high speed market by the Cii Honeywell-Bull MP6090 - and iono- graphy. Delphax, a company set up in Canada in 1980, has developed a print engine used by Anser and Check Technology in high-speed page print- ers. An ion projection head generates a charged image on a hard dielectric drum by getting ion flows to form spots of charge. Dry toner is attracted to this image making it visible. This image is transferred and fixed to the print paper in one stage, using a high pressure roller (see Figure 4).

Print quality is not currently as high as the laser printers, but it offers more in terms of simplicity and reli- ability. Current applications are pre-

dominantly cheque personalization, where the competition will come from the new McCorquodale system which incorporates LED printers developed by CCL.

All of these problems are overcome by using a linear array of LEDs. A printhead may consist of a line of chips, each with between 32 to 128 emitters. These are butted together to

LED arrays - the successors to laser scanning?

This leads us back to LED arrays and the question of how to maintain the quality of the laser printer while reducing the overall cost and improv- ing reliability. Laser scanning systems involve rotating polygon mirrors, or the recently introduced hologons, at high speed on precision bearings. To correct scanning errors, complex op- tical systems are required to maintain a constant size spot of laser light across the photoconductor drum. The imaging system is often bulky and requires factory prealignment .

Figure 6. The LED chip. The circles are the actual emitting areas. The sparkled strips are the anode metallization. The black areas are the gallium arsenide strip.

36

Modules

substrate

Figure 5. An LED array.

form a continuous line of perhaps 3000 switchable light sources on a ceramic substrate (see Figures 5 and 6). For low-speed applications where relatively low light output levels are sufficient, the drive electronics can be integrated on the head. To achieve the McCorquodale specification of 70 pages/min, CCL separated the serial data into parallel drive channels. This allowed the advantages of LEDs - a compact design with no moving parts - to be used at print speeds previous- ly the province of laser prints.

Several manufacturers are offering integrated LED printheads that OEMS can incorporate onto their printers. Currently Oki and Sanyo of Japan, Agfa and McCorquodale of Europe, and Kentek of the USA have announced LED printers. LED arrays can use the mass fabrication techniques of silicon devices, so auto- matic assembly methods should re- duce the cost of imaging, particularly for the low-speed market. The result should be a greater choice of printers in the speed range of 7-70 pageslmin, offering improved performance at lower cost. 17

NIP Group, Cambridge Consultants Ltd, Sci- ence Park, Milton Rd, Cambridge CB4 4DW.

data processing