Patent Application
20060115635
This invention relates to direct laser engraving of a flat substrate for use in a printing process, such as intaglio printing. More particularly, the invention relates to an optical scanning system and substrate material particularly suitable for high-speed and high-resolution engraving.
Many printing processes utilize substrates, platens or forms as printing surfaces to transfer an image to a printable medium. One such process is called intaglio printing. Intaglio printing involves application of printed indicia or images below the surface of a platen or substrate that is utilized as a printing surface. Traditionally, intaglio substrates have been prepared by mechanically engraving or chemically etching a recessed pattern into the printing surface of the substrate, which defines an image. The pattern may comprise an array of dots in the printing surface of the substrate. The recessed pattern, such as the array of dots, define tiny recesses within which ink is held and transferred to the printable medium, such as a sheet or surface. This intaglio process is typically used in die stamping or in engraved processes, sometimes referred to as copper plate printing. It is also used in connection with pad printing, which is typically used to decorate plastic surfaces, as well as in the gravure printing process.
Mechanical engraving and chemical etching techniques are time-consuming processes. Mechanical techniques are typically slow due to limitations of engraving equipment. A mechanical stylus must be used to engrave the image into the substrate, which requires a certain amount of time to penetrate and cut the substrate material. Furthermore, accuracy of the engraving becomes an issue when the stylus becomes worn and dull. On the other hand, chemical etching is time consuming due to the many steps involved. Chemical etching is a multi-step process that first involves producing the image onto a film negative, such as with an imagesetter. Once the film is produced, it becomes a mask that can be laid on top of a copper or steel substrate having a thin film coating of sensitizing material. The substrate and mask combination is exposed to light for subsequent chemical development, which transfers the mask to the copper plate. After development, the substrate is ready for acid etching to complete the process. Accuracy is also an issue with chemical etching, due to the limited controllability of the chemical-etching process.
Another technique involves direct laser etching, which is a single-step process that requires much less time than mechanical engraving and chemical etching techniques. In this technique, a laser is used to directly engrave the substrate material. However, because metals have a high reflectance, the laser/metal interaction is not conducive to producing plates having sharp engravings. With metals and a majority of plastics, direct laser engraving causes the material to melt, which creates the recessed areas, but also creates pooling of melted material around these recessed areas. This pooling of material acts as a ridge surrounding the recessed areas, which adversely affects the accuracy and usefulness of the printing surface of the substrate. Thus, accuracy remains an issue. Furthermore, although the direct laser technique is only a single-step process, the speed of the engraving process still remains an issue at higher resolution levels, which require the laser to engrave a higher number of tightly focused dots to achieve such resolutions. With presently known systems, the engraving process time is increased when the resolution level is increased.
The system and method of the present invention addresses these and other problems associated with direct laser engraving of substrates.
An optical scanning system for laser engraving a plurality of data subrasters into a substrate to form a raster of engraved data defining an image on the substrate. Each subraster has a length dimension and a width dimension. The system includes a transport assembly having an objective lens and a mirror, the mirror capable of reflecting a substantially collimated scanning beam incident thereon in a direction transverse to an axis of the incident beam such that it is directed to the objective lens. The objective lens is capable of focusing the scanning beam on the substrate to engrave a set of data in the width dimension of the subraster and the objective lens and mirror combination is capable of moving along the axis of the incident beam to allow subsequent engraving of other sets of data in the width dimension until a complete subraster is formed along its length dimension. The objective lens and mirror combination is also capable of returning to its starting position to begin engraving of a subsequent subraster of the plurality of subrasters forming the raster of engraved data.
In a particular embodiment, an optical scanning system for laser engraving of a plurality of subrasters of data into a substrate to form a raster of engraved data is provided and includes a scanner capable of deflecting an input laser beam incident thereon from a first beam direction to create a scanning beam. The system also includes a beam expander capable of receiving the scanning beam and expanding it to create an expanded scanning beam. A transport assembly of the system has an objective lens and a mirror, wherein the mirror is capable of reflecting the expanded scanning beam in a second beam direction transverse to the first beam direction such that it is incident on the objective lens. The objective lens and mirror is capable of moving along an axis defined by the first beam direction. The objective lens is capable of focusing the expanded scanning beam on the substrate to engrave a set of data oriented in a width dimension of the subraster and is also capable of moving along the first beam axis to allow subsequent engraving of other sets of data oriented in the width dimension until a complete subraster is formed to define a length dimension of the subraster. The objective lens and mirror combination is further capable of returning to its starting position to initiate engraving of a subsequent subraster.
According to another aspect of the invention, an optical scanning system is provided that is capable of engraving at two different resolutions.
According to another aspect of the invention, a substrate is provided for use with a direct laser engraving process to create an intaglio printing substrate. The substrate consists essentially of a thermoset plastic which substantially vaporizes in response to an impinging laser beam that engraves portions of the substrate, thereby substantially eliminating the formation of slag material adjacent to engraved portions of the substrate.
According to another aspect of the invention, a substrate is provided for use with a direct laser engraving process to create a printing substrate, wherein the substrate comprises a base material and an inorganic ceramic material disposed on the base material.
According to yet another aspect of the invention, methods of manufacturing such a substrate and methods for making such a substrate into a printing substrate in accordance with the principles of the present invention are provided.
According to yet another aspect of the invention, a method of laser engraving a substrate for use in a printing process is provided. The method comprises the steps of: (a) directing a substantially collimated scanning beam having a beam axis to an objective lens that is movable along the beam axis, wherein the scanning beam defines a scan width; (b) focusing the scanning beam through the objective lens and onto the substrate; (c) engraving onto the substrate a set of subraster width data having a width equal to the scan width of the beam; (d) continuously moving the objective lens along the beam axis to subsequent positions relative to the substrate and engraving subsequent sets of subraster width data to form a complete subraster; (e) incrementing the substrate and engraving an additional subraster adjacent to the previously completed subraster; and (f) repeating the steps of incrementing the substrate and engraving an additional subraster until a complete raster made up of a plurality of subrasters is created that defines an engraved image on the substrate.
These and other aspects of the invention will become apparent from the specification, drawings and claims.
To understand the present invention, it will now be described by way of example, with reference to the accompanying drawings in which:
While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail one or more preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated.
The system 10 requires an input beam 30. A laser assembly 31 is provided to produce the input beam 30, which is incident on the scanner 20 from a first beam direction. The laser assembly 31 includes a laser 32 (preferably a DC excited CO2 laser), a beam compressor 34 and a modulator 36 driven by a modulator driver 38. Referring to
The upper limit beam 66 and the lower limit beam 68 expand until they reach a second lens L2, which is spaced a focal length f2 from the focal plane 64 of the first lens L1. In a preferred embodiment, the focal length f2 is 300 mm. Utilizing the preferred pair of lenses L1 (15 mm focal length) and L2 (300 mm focal length), the beam expander 22 acts as a 20× beam expander (300/15 =20). Thus, the beam expander 22 in the preferred embodiment expands the 1 mm input beam to a substantially collimated 20 mm output beam. A fundamental consequence of beam expansion is a complimentary compression of scan angle of the beam (in the preferred embodiment, compression from ±20° to ±1°). This reduction in scan angle imparts a practical field angle to the objective lens 28.
Another consequence of this configuration in the preferred embodiment is the relaying of a 1 mm beam aperture at the scanner 20 to a 20 mm beam aperture at the 300 mm focal distance of lens L2. Thus, as shown in
Referring again to
Referring to
Referring to the right side of
The present invention utilizes a method of engraving the substrate wherein a plurality of data subrasters are engraved to form individual swaths, which, in combination form a complete raster of data representing an image to be engraved. Referring to the schematic diagram of
During engraving, the laser 32 remains on. Instead of turning the laser 32 on and off, the modulator 36 shifts the beam 30 to a position analogous to ON. An unshifted beam position is analogous to OFF. During engraving, this shift happens at a very high rate. In the OFF position, the beam 30 exits the modulator 36, strikes a dump mirror (not shown), and deflects into a beam dump (not shown) to absorb unwanted laser power. When the modulator 36 shifts the beam 30 in an ON position, the shifted beam 30 bypasses the dump mirror and impinges on the scanner 20. Based on these ON and OFF positions, each data point or dot can represent an engraved point (ON) or an unengraved point (OFF) on the surface of the substrate.
As already mentioned, a particular advantage of the system 10 is its ability to provide two different resolutions with the same optical system (1200 dpi and 2400 dpi in a preferred embodiment). This is accomplished by narrowing the modulation pulse width at 2400 dpi to half of the modulation pulse width at 1200 dpi, while doubling its repetition rate, which doubles the dot count along the subraster width from 40 to 80. Correspondingly, the motor speed of the scanner is doubled from 10,000 rpm to 20,000 rpm, which provides a full double-resolution dot array (data set) across the width dimension of the subraster. The modulation pulse width is narrowed by reducing the intensity of the beam. Since the dots are formed by a Gaussian focused beam contour, reducing the pulse duration by one half reduces the dot exposure (intensity) on the substrate, which, in turn, sufficiently reduces the dot width to facilitate the double-resolution engraving. The repetition rate of the modulation pulse width is changed via software control. Since the exposure (intensity) is reduced at the 2400 dpi resolution, the total energy remains the same as that at 1200 dpi resolution. Since the total energy is the same, the total engraving duration is the same. Thus, the system is capable of doubling its engraving resolution without increasing engraving time.
From the foregoing description, it is apparent that changing the resolution of the system is rapidly accomplished without the need for critical mechanical changes, such as changing the objective lens to focus to a smaller dot size, which can be very costly. Furthermore, two different resolutions can be engraved with the same optical system, which creates the same subraster format to cover the same total area during the same total time, and the same laser providing the same optical power.
As yet another aspect of the present invention, it has been found that the use of a thermoset plastic material as the substrate substantially eliminates unwanted slag formation around the engraved points of the substrate. The thermoset plastic material substantially vaporizes in response to the impinging laser beam that engraves points of the substrate, thereby substantially eliminating the formation of the slag material. Desirable results have been achieved by including a mineral filler with the thermoset material. Preferably, the mineral filler has a grain size smaller than a smallest feature of the engraved portions of the substrate. Preferably, the grain size is in the range of about 3 to 5 microns. However, the grain size can be varied to match a particular resolution. The filler adds strength to the substrate material, thereby maintaining the accuracy and detail of the engraved portions of the substrate. In a preferred embodiment, silica is utilized as a filler for the thermoset material. Additionally, a flame retardant can be included to minimize flame and smoke formation from the impinging laser beam.
Thermoset plastics provide for more accurate laser engraving due, in part, to their strength and resistance to flow. The polymer component consists of molecules with permanent cross-links between linear chains that form a rigid three-dimensional network structure which cannot flow. The tightly cross-linked structure of thermosetting polymers immobilizes the molecules, providing hardness, strength at relatively high temperature, insolubility, good heat and chemical resistance, and resistance to creep. The use of a thermoset plastic material for a substrate has a significant impact on the cost of printing processes that utilize such substrates. Thermoset plastic substrates are much less expensive than copper or steel substrates and they do not sacrifice engraving accuracy, and hence, printing accuracy.
It is contemplated that a variety of thermoset plastic materials can be utilized in accordance with the principles of the present invention. Such materials include epoxies, unsaturated polyesters, phenolics, amino resins (such as urea- and melamine-formaldehyde), alkyds, allyl family (such as diallyphthalate), silicone molding compounds, and polyimides (such as bimaleimides).
In cases where durability is a priority, it has further been found that a substrate having an inorganic ceramic material is suitable for a printing process, and particularly suitable for use in the system and process described herein. In an embodiment as shown in
An inorganic ceramic material component, layer or coating of a printing substrate provides excellent resistance to abrasion encountered during the printing process. Among other things, ink ingredients used in printing processes can cause wear to the substrate. The ceramic material of the substrate is more resistant to abrasion than the thermoset substrates, while offering excellent engraving accuracy. The substrate incorporating the inorganic ceramic material provides an excellent alternative to the thermoset substrates. It has been found that an inorganic ceramic material that is vitreous in nature offers particularly excellent resistance to abrasion and is particularly suitable for use in a printing substrate.
By way of background and example, a particular inorganic ceramic material for use in a substrate for a printing process will now be described with the understanding that the particular example is but one embodiment of many as understood by one of ordinary skill in the art that can be utilized in accordance with the principles of the present invention. In this particular example, the inorganic ceramic material is in the form of a porcelain enamel disposed on a metallic substrate, such as a steel or iron substrate. Porcelain enamels for steel and iron substrates are typically classified as either ground-coat or cover-coat enamels. Ground-coat enamels contain metallic oxides, such as cobalt oxide and/or nickel oxide, that promote adherence of the glass/enamel to the metal substrate. Cover-coat enamels are applied over fired ground coats to improve the properties of the coating. In addition, cover-coat enamels may be applied over unfired ground coats, with both coats being fired at the same time. This is referred to as a two-coat/one-fire system. Cover coats may also be applied directly to properly prepared decarburized steel substrates. Porcelain enamels for aluminum substrates are typically one-coat systems that are applied by spraying. However, two-coat systems can also be utilized. It should be understood, however, that any number of application systems, alone or in combination, can be utilized on various substrate materials to achieve a porcelain-coated substrate.
The basic material of the porcelain enamel coating is called frit, which is a smelted complex borosilicate glass. Frits are produced by quenching a molten glassy mixture that is compounded from numerous components, sometimes more than 20 different components, depending upon the application. Thus, the composition of the frit can be customized and optimized to exhibit certain desired properties for a particular application. As already mentioned, in cases where increased adherence to a steel substrate is desired, for example, cobalt oxide and/or nickel oxide may be included in the frit. In accordance with the principles of the present invention, the frit can be optimized for properties conducive to use as a printing substrate, including, but not limited to, hardness, abrasion resistance, strength at relatively high temperature, heat and chemical resistance, etc. Examples of oxide components that may be utilized include, but are not limited to, SiO2, B2O3, Na2O, K2O‘, Li2O, CaO, ZnO, Al2O3, ZrO2, TiO2, CuO, MnO2, NiO, Co3O4, P2O5, MgO, PbO, Sb2O3, Sb2O5, ZrO2, BaO and F2. Additives may be added to the frit to further influence various properties of the enamel, such as clay, bentonite, electrolytes, fluxes, and coloring oxides.
In a preferred embodiment, the porcelain enamel has a low glass content, i.e., less than 50% by weight, preferably in the range of 35-40% by weight. However, other percentages can also be utilized, as long as the glass content is not too low, which weakens the enamel. Typical porcelain enamels have a glass content between 50% and 60% by weight.
The porcelain enamel may be applied to the substrate by either a wet process or a dry process. Wet process methods include manual spraying, electrostatic spraying, dipping, flow coating, and electrodeposition (electrophoresis). Dry process methods include electrostatic dry powder spraying and sprinkling onto a heated substrate. In a preferred embodiment, the porcelain enamel is applied electrostatically.
Since subsurface abrasion resistance varies with processing variables that affect the bubble structure of the enamel (i.e., gas bubbles that get trapped in the enamel during cooling), it is preferable that gas bubbles are minimized. Enamel compositions may contain crystalline particles (from mill additions or devitrification heat treatment) that can increase abrasion resistance by as much as 50%. In a preferred embodiment, calcium carbonate is included as a filler additive, which increases abrasion resistance.
It should be understood that any inorganic ceramic material can be utilized in accordance with the principles of the present invention. However, it is preferable to utilize a porcelain enamel, particularly one having low glass content and exhibiting excellent abrasion resistance.
While some substrate materials—such as thermoset plastics—can accommodate and take advantage of the faster engraving speeds and fast trolley return speeds associated with the system 10 depicted in
Referring to
Software that facilitates the system to skip over non-engraved areas of the substrate at higher speeds can be incorporated into controls of both systems, the system having a moveable objective lens and the system having a fixed objective lens, to compensate for the slower engraving speeds dictated to these embodiments by the materials they are engraving.
It is understood that, given the above description of the embodiments of the invention, various modifications may be made by one skilled in the art. Such modifications are intended to be encompassed by the claims below.
This application is a continuation-in-part of U.S. patent application Ser. No. 10/159,492, filed May 31, 2002, upon which a claim of priority is based.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10842680 | May 2004 | US |
| Child | 11334135 | Jan 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10159492 | May 2002 | US |
| Child | 10842680 | May 2004 | US |
