GB2159464A

GB2159464A - Laser printer

Info

Publication number: GB2159464A
Application number: GB08413990A
Authority: GB
Inventors: Marshall Greenblatt; Thomas G Kandel; J Bradford Merry
Original assignee: DIRECT LASER PRINTING CORP
Current assignee: DIRECT LASER PRINTING CORP
Priority date: 1983-06-01
Filing date: 1984-06-01
Publication date: 1985-12-04
Also published as: GB8413990D0; DE3419852A1

Abstract

A method and apparatus for directly printing on a substrate 2. The substrate is uniformly coated with a radiation-curable ink at 4, a radiation beam 6 is scanned across the substrate while modulating it on-off in accordance with stored digitized information 20 to be printed to tack the radiation-curable ink to said substrate at a plurality of discrete areas in correspondence with the stored information, and the untacked ink is removed from the substrate at 26, whereby the stored information is printed on the substrate, with a final fixing by the application of pressure or heat at 30. <IMAGE>

Description

SPECIFICATION Method and apparatus of direct, high speed printing The present invention is directed to a method and apparatus for high speed, direct printing and decorating.

In recent years, substantial efforts have been directed toward providing faster and more economical methods and devices for printing. Thus, information is outputted in electrical form from computers, word processors, plotters and other electronic machines at high speeds, and it is desirable for printers to be able to print such information as rapidly as it is provided, or as near to this speed as possible.

Additionally, since presently available optical character recognition technology permits visibly recorded information in either alphanumeric or pictorial form to be rapidly digitized and stored, high speed printers of digital input may become a replacement for printing presses, decorators and photocopiers. Thus; it is desirable to provide high speed printers which are as fast as or faster than photocopiers, but which may be more economical to manufacture and use and have better print quality.

In the development of high speed printer technology, many different types of devices have evolved, including impact printers, thermal printers, ink jet printers, and laser printers. Among the fastest of these is the electrophotographic laser printer, as it is possible to modulate and scan a beam of electromagnetic energy more rapidly than mechanical printing elements can move.

Basically, the laser printer utilizes a scanning laser front end, and a rear end which is similar to that used in a conventional photocopying machine, and which employes an image tranfer drum, belt or plate between the light beam and the substrate to be prnted on. Thus, in the front end of a laser printer, a beam of coherent, collimated light is scanned across a substrate or drum while being modulated on-off in accordance with stored, digitized information to be printed. At the rear end, the substrate or drum is electrically charged and has a photoconductive substance disposed thereon, which changes charge in accordance with the laser light incident thereon, forming a charge pattern (latent image) whch corresponds to the information to be printed.Toner substance is attracted to the latent image of the photoconductor in accordance with the charge pattern, and is ultimately transferred from the intermediate photoconductive surface to the paper being printed on, where it is fixed by heating, pressure or other means.

It should be appreciated that printing by the laser printer is not directly by the light beam on the paper, but rather is dependent on an intermediate image transfer substrate or drum. The necessity for such intermediate substrate is a factor which has made laser printers complex, expensive, and difficult to make and service, and at least in part for these reasons, their use to date has not been widespread.

It is the purpose of the present invention to provide a method and apparatus for high speed printing and decorating which achieves speeds comparable to those attained by laser printers, but which does not require an intermediate substrate or drum, and consequently which is more compact, faster to operate, inexpensive and more economical to manufacture and maintain since there are fewer parts in the original equipment and fewer parts which may require replacment.

A further technology which is of interest by way of background to the present invention, is ultraviolet curing and printing. A large number of photopolymerizable coatings and inks have been developed which chemically cure and harden when exposed to suitable ultraviolet radiation. Typically, such coatings are used as an exterior finish for metallic items such as beer and soda cans, plastic items such as laminates, and wood surfaces and veneers. Additionally, photopolymerizable inks are used for printing applications, wherein characters in such ink are written or otherwise applied to paper or other substrate, and are subsequently cured by exposure to ultraviolet radiation of the entire surface of the substrate, only some of which was inked.Of course, such prnting is relatively slow because it is limited by the speed in which the ink can be applied to the surface in alphanumeric or other format of the information to be printed.

It is therefore an object of the present invention to provide an improved high speed printing or decorating method and apparatus.

It is a further oblect of the invention to provide a printing method and apparatus which utilizes a high speed scanning energy beam, but which does not require intermediate image transfer substrates.

It is still a further object of the invention to provide a method and apparatus for printing, which includes relatively few parts, and which may be relatively inexpensive to manufacture and use.

It is still a further object of the invention to provide a printing method and apparatus which may attain throughput speeds comparable to or greater than those attained by photocopying, and may be used as a substitute therefor.

In accordance with the invention, the above objects are accomplished by coating a substrate to be printed on with a photopolymerizable radiation-curable substance, scanning a beam of radiation across the substrate while modulating it on-off in accordance with stored, digitized information to be printed or copied to tack the radiation curable substance to the substrate at a plurality of discrete areas corresponding directly to the stored, digitized information, and removing the untacked coating substance from the substrate, thus developing a visible image.

In accordance with the invention, the printing is effected directly on the substrate by radiation which impinges thereon and tacks coating substance thereto, and there is no requirement for intermediate or transfer surfaces, or electrical charging of a surface or the substrate, as in laser printing or conventional photocopying. Addition ally, no specially treated paper or substance is required as in some types of printing and copying, and the radiation-curable inks and coatings used have been developed. In order to keep ink and coating application and substance costs to a minimum, the entire substrate is initially coated, and after exposure to the printing beam the excess coating which has not been tacked is removed and is recirculated, to be used again for subsequent inking and coating applications.

Additionally, it may be desirable or necessary to fix or fuse the tacked substance to more permanently attach it to the substrate, in which case, after the excess coating substance has been removed, the substrate is exposed to radiation emitted by a quartz lamp, or to a pressure-applying means, or to other suitable relatively inexpensive and efficient fusing means.

The invention will be better understood by referring to the accompanying figure, which illustrates an embodiment thereof.

Referring to the figure, substrate or web 2 is paper or other material to be printed on or decorated, and is conveyed past an optional corona 33 to charge the substrate and then past ink- containing trough 4 and radiation scanning beam 6 by rollers 8 and 10, or other suitable mechanical conveying means. Trough 4 contains radiation curable ink or coating in either powder or liquid form, and is arranged to evenly distribute the ink on web 2 as it passes under the trough. For example, in the illustrative embodiment trough 4 is a container having a trap door 12, which is movable between a closed position in which no ink is allowed to escape from the trough and an open position in which gravity causes ink to escape and to be applied uniformly to the web as it passes thereunder.In the alternative, trough 4 may have a trap door which is moveable to a plurality of positions to deposit more or less ink depending on the speed of movement of web 2, on the material of which the web is comprised, or on other factors, or may be another type of coating or ink applicator. Additionally, in an operating embodiment, means would be provided for automatically and continuously replenishing the ink supply in trough 4, for example by pumping ink from a large reservoir directly or indirectly into the trough or from recirculation means 28.

As mentioned above, the ink can be in either powder or liquid form and can be one of many radiation curable inks which are available. For example, the powder can be an ethylenically unsaturated epoxy polyester polymer as descrbed in U.S. Patent No. 4,163,810, which is incorporated herein by reference.

Turning to the optical system utilized in the invention, laser 14 is a device which emits an intense, coherent beam 15 of ultraviolet radiation or other wavelengths. The beam is shaped by beam shaping optics 16 and is incident on acousto-optic or electro-optic modulator 18. The acousto-optic modulator is made of glass or crystal, and being compressible, can support waves of alternating compression and rarefaction, which are created by an acoustic transducer connected to said glass or crystal block, when excited by electrical signals.

Such acousto-optic modulators are well known, and may for example be made of lead molybdate or germanium.

The electrical signals are provided by signal source 20, which is a source of digital signals corresponding to the information to be printed on web 2. For example, signal source 20 may be the output of a word processor or electronic typewriter, or stored signals resulting from visibly recorded information which has been captured by an optical character recognition device from an original document or pattern to be printed. When the video signal is off, represented by a digital "zero" output of signal source 20, there is no electrical signal on the acoustic transducer means of modulator 18 and light beam 15 goes straight through the modulator block, and is stopped by optical stop 21, which for example may be a black heat sink.

On the other hand, when the video signal is on, the output of signal source 20 is a digital "one", and an electrical signal is present on the acoustic transducer means of modulator 18, setting up an acoustic wave in the modulator block, which creates an effective diffraction grating in the modulator block and causes the light beam traversing the block to be deflected at the Bragg angle.

As seen in the figure, when the beam is deflected at the Bragg angle it is incident on scanning reflector means 22, which scans the beam 6 across the web. By way of example, scanning means 22 can be an oscillating mirror or galvanometer, a rotating polygon with mirrors on each face, or an acousto-optic or electro- optic deflector. In order to insure that the video signal is printed in correct line correspondence with the stored digitized information, signal source 20 and driver 23 of scanning means 22 are electrically synchronized with each other by electronic synchronization means 24.

The laser beam, which in the illustrative embodiment is produced by a helium cadmium device emitting at 325 nm, is modulated at a rate of several megahertz, and if for example, a resolution of 300 spots per inch on web 2 is desired, 2400 pulses in a line video signal would cover a web eight inches in width. The web may be arranged to move at a speed such that by the time the scanning means has gone through one complete cycle and returned to the start position, the web has advanced the width of a dot in the vertical video or frame direction. In this way, the entire surface of the web can be covered with discrete dots, and by selective turning on and off of the light signal, any desired image can be represented.

It should be appreciated that the details of the optical scanning front end of the present invention are well known as for example, the system is similar to that used in known laser printers.

The light beam emitted by laser 14 is arranged to be of appropriate wavelength, and of sufficiently high power density to effect 'curing' of the ink or coating which it strikes. In the present application, the term "curing" means cross linking of the coating to such an extent within a particular localized region so that the coating adheres to the substrate or is "tacked" to it in that region, and the coating is a solid film over the region.

The chemical reactions involved in curing and tacking may be adhesion of the coating to the substrate, cohension of the coating molecules to neighboring molecules, and temperature change of the coating molecules due to catalytic reaction on and/or in the molecule, or a combination of such reactions.

Since the laser beam is modulated on-off as described above, only discrete areas of the fully coated substrate are irradiated and undergo tacking. After tacking is effected, the untacked coating may pass an optional discharge corona 34 which helps remove coating from the substrate. Removal constitutes the development' of the image. The development occurs by "lifting-off" the image background, which is the opposite of the way that the image is created in conventional graphic or copying processes.

In the illustrative embodiment, the excess coating is removed by gravity, wherein the web is conveyed so that after tacking it is turned upside down, and the non-tacked coating falls off the substrate. Alternatively, the excess coating can be removed by a convection process by blowing or sucking away the coating, and air removal unit 32 is illutratively depicted in the figure. Additionally, the coating may be removed by electrostatic attraction by charging a nearby surface differently than the coating and electrostatically pulling the excess coating off, or by magnetic attraction by placing a magnet of polarity opposite to particles in the coating near to it and magnetically pulling the excess coating off. Additionally the discharge corona 34 may lessen the attraction between untacked ink and substrate.

The excess coating which is removed may be collected and recirculated for further application to the susbstrate. In the figure, collection trough 26 is illustrated into which the excess coating falls, and recirculation means 28, which for example may be a pumping system, is used to recirculate the coating to application trough 4.

It may be necessary or desirable to fix or fuse the tacked area of coating to the substrate after development has been effected. This can be done by exposure to a radiation device which is much less expensive than a laser, (for example a quartz heat lamp, calrod), gas oven, or induction heater, and quartz lamp 30 is illustratively depicted in the figure. Alternatively, such fusing may be effected by pressure means, for example a laminating roller; solvent vapor; induction excitation of the coating; and/or a combination of these or other suitable methods.

It should be noted that to keep the cost and power level of the laser as low as possible, the degree of tacking performed should be sufficient only to resist the excess removal process. Additionally, the dominant lines in the output spectrum of the laser should coincide with a peak or relative high in the absorption spectrum of the particular coating used. While the invention is illustrated primarily in connection with the use of coatings and inks which are curable by radiation in the ultra- violet part of the spectrum, it should be noted that it may be possible to utilize radiation in other portions of the electro- magnetic spectrum with the appropriate coatings and inks.

There thus has been described a method and apparatus for directly printing on substrates at high speeds and without requiring the use of an intermediate image transfer substrate. The method and apparatus disclosed can be used either as a printer or as a replacement for conventional photocopying. Additionally, use without the corona units as disclosed is possible, and by simultaneously scanning across both sides of the web or substrate, both sides can be simultaneously printed.

By way of example, it appears that printing on an 8 2 by 11 inch sheet of paper at a speed of one sheet per second with a resolution of 300 spots per inch in both horizontal and vertical directions is possible, and would require a laser modulation frequency of 4.2 megahertz.

In the following claims the term "printing" is used generically and includes decorating, or printing of non-alphanumeric patterns or indicia.

Claims

1. A method of high speed direct printing of stored, digitized information on a substrate, comprising the steps of, coating said substrate with a radiation-curable substance, providing a radiation source which emits a beam of radiation at wavelengths which are effective to cure said substance, scanning said beam across said substrate while modulating it on- off in accordance with said stored digitized information to tack said radiationcurable substance to said substrate at a plurality of discrete areas in correspondence with said stored digitized information, and removing the untacked coating substance from said substrate, whereby said stored information is printed on said substrate.

2. The method of claim 1 wherein the steps of coating comprises coating the entire substrate with said radiation-curable substance.

3. The method of claim 1 wherein said radiation source is a laser which emits ultraviolet light.

4. The method of claim 1 wherein said scanning is affected in a raster pattern.

5. The method of claim 1 further including the step of fixing the tacked coating substance to said substrate after the untacked substance is removed.

6 The method of claim 1 wherein said radiation-curable substance is a radiation-curable powder.

7. The method of claim 1 wherein said removed untacked coating is recirculated to be used again in the recited coating step.

8. An apparatus for high speed direct printing of stored, digitized information on a substrate, comprising means for coating the substrate with a radiationcurable substance, radiation source means for emitting a beam of radiation at wavelengths which are effective to cure said substance, means for scanning said beam across said substrate while modulating in on-off in accordance with said stored digitized information to tack said radiation-curable substance to said substrate at a plurality of discrete areas in correspondence with said stored digitized information, and, means for removing the untacked coating substance from said substrate, whereby said stored information is printed on said substrate.

9. The apparatus of claim 8 wherein said means for coating comprises means for coating the entire substrate with said radiation-curable substance.

10. The apparatus of claim 8 wherein said radiation source means comprises a laser which emits ultraviolet light.

11. The apparatus of claim 8 wherein said means for scanning comprises means for scanning said beam in a raster pattern.

12. The apparatus of claim 8 further including means for fixing the tacked coating substance to said substrate after the untacked substance is removed.

13. The apparatus of claim 8 wherein said radiation-curable substance is a radiation-curable powder.

14. The apparatus of claim 8 wherein said means for scanning includes acousto-optic modulator means.

15. The apparatus of claim 12 wherein said means for fixing comprises lamp means for providing infra-red radiation.

16. A method of high speed direct printing of stored, digitized information on a substrate, substantially as described herein.

17. Apparatus for high speed direct printing of stored digitized information on a substate, substantially as described with reference to the accompanying drawing.