Patent Application
20110014573
This invention relates to an optical imaging head, a printing plate construction, and methods for direct engraving of flexographic printing plates.
Flexography is a method of printing whereby a flexible plate with a relief image is wrapped around a cylinder, the relief image is inked, and the ink is then transferred to a suitable printable medium. The process is used in the packaging industry wherein the plates must be sufficiently flexible and the contact sufficiently gentle to print on uneven substrates such as corrugated cardboard as well as flexible materials such as polypropylene film. The quality of the printing in this manner is inferior to processes such as lithography and gravure, but nevertheless it is useful in certain markets. In order to accommodate the various types of printing media, the flexographic plates should have a rubbery or elastomeric nature whose precise properties can be adjusted for each particular printable medium.
In addition, when the flexographic printing plates are formed and/or imaged in a flat form, they should be flexible for bending around a cylinder for rotary printing. This can present more of a problem than with offset lithographic plates because the thickness of flexographic printing plates is generally several millimeters instead of fractions of a millimeter. Materials that are flexible, such as one or two μm films, can be rigid and inflexible at one or more mm.
It has long been recognized that the simplest way of making a flexographic printing plate would be by direct engraving using laser beam ablation, thereby eliminating the need for complex post plate image processing such as multiple types of exposures, washing with solvents and long drying of the plate.
Despite the limitations of carbon dioxide lasers, they are now being used commercially in flexographic engraving machines. They are known for slow and expensive imaging with limited resolution. However, the advantages of direct engraving are sufficient to ensure their commercial use in instances where fast imaging and high print quality are not required. It would be preferable to use infrared diodes that produce radiation in the near infrared and infrared (approximately 700 to 1200 nm) and have the advantages of high resolution and relatively low laser cost so that they can be used in large arrays. Until now, although the use of such lasers is described in many publications, they are not In industrial use because even when combined with the most sensitive imageable elements available, satisfactory engraving has not been achieved.
Engraving with an infrared diode laser (or ablative imaging) differs from engraving with a carbon dioxide laser in that a compound absorbing suitable radiation (that is, IR radiation) is usually incorporated into the imaged coating. The recent availability of high power (for example, 8 watts) IR-laser diodes opens up opportunity for the use of relatively low cost laser diode arrays capable of engraving flexographic blanks as described in WO 2005/84959 (Figov).
Relief depth in the resulting image is an issue with laser engraving because the deeper the required relief, either more power is required or it takes longer to engrave or image the plate, for a specific material. Use of material which ablates more easily is another approach adapted to achieve a deeper relief in the same engraving time. Direct engraving of a flexography plate requires carving three-dimensional (3-D) areas, on plate material, with a laser system. This is remarkably different from two-dimensional (2-D) imaging techniques that require post processing steps to produce the 3-D features.
The requirements, mentioned above, introduce several challenges for the laser imaging system and the related media:
The layers in the plate should be optimized in such a way that both printing performance and imaging performance are optimized so that printing layers are most suitable for high resolution imaging by one laser source and for printing high resolution dot, low dot gain and excellent ink transfer. The other imaging layers, which will not be used for printing, are optimized for fast imaging with a second laser source to achieve high throughput, without comprising good printing characteristics. U.S. Pat. No. 7,419,766 (Kimelblat et al.) shows an example of a multi-layer flexographic plate wherein the top layer is an ablatable layer designed to be ablated by a laser source, and the second layer is not ablatable.
Briefly, according to one aspect of the present invention a system for engraving flexographic printing plates includes a flexographic printing plate comprising from at least two ablation layers, a first ablation layer and a second ablation layer wherein the first ablation layer is a printing layer and the second ablation layer is a non-printing layer; a first group of one or more radiation sources each emitting radiation having substantially the same intensity; a first set of one or more optical elements coupled to the first group of one or more radiation sources for imaging radiation emitted from the first group of one or more radiation sources on the first ablation layer; a second group of one or more radiation sources each emitting radiation having substantially the same intensity; a second set of one or more optical elements coupled to the second group of one or more radiation sources for imaging radiation emitted from the second group of one or more radiation sources on the second ablation layer; wherein the intensity and spot size of said first group of one or more radiation sources is different from the intensity and spot size of the second group of one or more radiation sources; and wherein the first and second groups of radiation sources operate simultaneously.
The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.
The combination of radiation sources with high power broad spots and low power fine spots, referred to as a hybrid optical head system (HOHS), is well suited for 3-D processing of direct engraving flexography applications. The HOHS is described in detail in the U.S. Patent Publication No. 2008/0153038 (Siman-Tov et al.).
The HOHS may be configured with at least two groups of radiation sources, the groups comprising at least one radiation source, wherein the radiation sources within the groups emit radiation having the same intensity and spot size, different from the intensity and spot size of radiation sources in other groups. The groups of radiation sources operate simultaneously. Radiation sources include, but are not limited to, lasers, laser diodes, multi-emitter laser diodes laser bars, laser stacks, fiber lasers, and the like. For example, a lower power fine laser source may assist in processing solid areas; however, a high power broad laser source may only operate in areas that are greater than or equal to its spot size. The laser sources, fine and broad, may be integrated into a single optical head, or separated into their own separate mounted heads. In each configuration, the laser sources are controlled and driven independently of each other.
A fine laser source, or a multiplicity of fine laser sources, may comprise diode lasers having a single emitter, such as, for example, both fine and broad source lasers are available in a fiber-coupled and non-fiber-coupled configurations. In the fiber-coupled configuration, the laser is coupled to a fiber using a separate focusing lens or a lens defined by processing the fiber end to a surface capable of refracting the light into the fiber. The size of the aperture emerging from the fiber is determined by the radial dimension of the fiber. The light that is output from the aperture diverges and needs to be imaged by using a lens, or system of lenses, to result in the desired spot size.
In operation, a flexographic plate 200 is attached to rotating drum 124 and then spun. While spinning, control device 104 directs broad laser source 116 to ablate certain large areas on imaging layer 204 that are greater than or equal to the spot size of the broad laser source 116; while fine laser source 108 is directed to ablate certain small areas on imaging layer 204, areas requiring fine detail and large areas where fine laser source 108 is directed to operate. Laser sources 108 and 116 are moved on their respective carriages 112 and 120, so as to locate the laser sources 108 and 116 in the area where they need to operate.
The imaging process described above is not new, it can be accomplished by deploying an imaging head presented in the U.S. Patent Publication No. 2008/0153038, imaging a flexographic plate 200 (described in U.S. Pat. No. 7,419,766).
Printing layer 404 is constructed from a combination of materials such as thermosetting acrylates, polyurethanes, vulcanized rubbers, synthetic rubbers and other thermosetting elastomers. Those materials, by their design or in addition include in the matrix materials such as fillers, making printing layer 404, imageable by infra red (IR) based laser and possessing certain mechanical and chemical properties, and therefore is most suitable for high quality printing. Some of the main characteristics of such printing layer 404 are: good mechanical properties; good resistance to heat, mechanical and chemical attack; good affinity to different inks; and ability to be imaged by laser sources to produce high resolution dots, and being able to hold small dots. Due to these characteristics, printing layer 404 is well suited to serve as a printing layer. Non-printing layer 408 is constructed from materials such as thermosetting acrylates, polyurethane, vulcanized rubbers, synthetic rubbers, and other thermosetting elastomers. Those materials, by their design or in addition include in the matrix materials such as exothermic oxidizing groups and fillers with high tendency to decompose with heat and ablate, or having low density or entrapped air within them, or having weak bonds which can ablate easily. Non-printing layer 408 may be softer and less durable than printing layer 404, and therefore will easily ablate, exhibiting high imaging throughput.
Fine laser source 108 is designed to image printing layer 404 and broad laser source 116 is designed to ablate the non-printable layer 408. The typical thickness of printing layer 404 is in the range of 30-350 microns and of non-printing layer 408 is in the range of 100-1000 microns.
In operation as is depicted in
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.
Reference is made to commonly-assigned U.S. patent application Ser. No. 11/615,025 (U.S. Patent Publication No. 2008/0153038), filed Dec. 22, 2006, entitled HYBRID OPTICAL HEAD FOR DIRECT ENGRAVING OF FLEXOGRAPHIC PRINTING PLATES, by Siman-Tov et al., the disclosure of which is incorporated herein.
