Example embodiments of the present invention relate to a method of electro-mechanical engraving a molding pattern consisting of small, individual cavities, in a rigid surface for the specific purpose of micro replicating the inverse of said three-dimensional pattern into a continuous web of material.
A light redirecting film may be used in a variety of applications. For example, a light redirecting film may be used as part of a liquid crystal display (LCD) to increase the power efficiency of the LCD. Increasing the power efficiency of a LCD (or other similar display) may be significant. Liquid crystal displays are often included in mobile devices (e.g. cellular telephones, laptop computers, digital cameras, etc.) which run on batteries. It is desirable for these mobile devices to maximize the operating time of their batteries. Although battery technology is improving, one way to increase the battery life of a mobile device is to reduce power consumption of the device without degrading quality. By making liquid crystal displays more efficient, the battery life of a mobile device can be extended, which is of great benefit to the user.
The optics of light redirecting films are very specific and detailed. Most light redirecting films comprise elongated prisms, in some cases with varying height or horizontal position along the length of the prisms. For example, PCT Published Application WO 00/48037 teaches the use of a diamond turning machine with a fast tool servo cutting head to produce rollers with a molding pattern to micro replicate a light redirecting film with elongated prisms that vary in height or horizontal position along their length.
For some applications, discrete optical elements have considerable advantages. Discrete optical elements are short in both length and width compared to the dimensions of the optical substrate on which they lie. Discrete optical elements may have advantages such as reducing wet-out of films, obscuring cosmetic defects, and reducing Moiré effects when the film is assembled into an LCD. As taught in U.S. patent application Ser. No. 11/388,582, incorporated herein by reference, discrete optical elements also have the distinct advantage of being able to vary in shape, position, density, pitch, length, and other attributes in a two-dimensional pattern on a light extracting film.
There is a need for methods to fabricate molds with large numbers of discrete cavities with well-defined and varying shapes and distributions in a short amount of time.
Example embodiments of the present invention relate to a method including electro-mechanical engraving of a molding pattern in a rigid surface (e.g. a pattern roller). In some embodiments the cavities engraved in the molding pattern have complex cross sections. Other example embodiments of the present invention relate to a method including micro replicating optical elements on a light extracting film. Other embodiments of the present invention relate to an apparatus including a rigid surface. A molding pattern is formed in the rigid surface by electro-mechanical engraving (e.g. Gravure electro-mechanical engraving). In some embodiments the molding pattern is formed using multiple diamond cutters, where the relative positions of the cavities formed by the multiple cutters are accurately positioned to within very close tolerances.
A molding pattern is a three-dimensional surface shape formed on a rigid surface, such that the negative of the three-dimensional surface shape is accurately imparted to the surface of an object molded from the rigid surface. The molding pattern in the present invention comprises many well-formed cavities cut in the rigid surface, and the shape of the cavities is substantially the negative of the shape of the protrusions on an object molded from the rigid surface.
In accordance with example embodiments of the present invention, the manufacturing process is able to produce a molding pattern for optical films that can be used in a variety of applications. For example, by using the manufacturing process in accordance with example embodiments of the invention, a mold may be produced having discrete cavities with complex cross sections. A single mold may contain cavities with multiple cross sectional shapes. The cavities of multiple shapes may be interspersed with each other, and may be located relative to each other with high precision. The cavities may be cut at high speeds, allowing fabrication of a complete mold with many cavities within a reasonable amount of time. An optical film produced from the molding pattern comprises discrete optical elements and may have advantages in display quality, manufacturing cost, and optical efficiency.
FIGS. 8A-C show illustrations of various types of neighboring, overlapping cavities.
There are numerous uses for cylinders with patterns of small cavities on their surfaces. Typically they are used to replicate the pattern of cavities in the cylinder surface into or onto another material in a continuous manufacturing process. For example, in Gravure printing, a series of cavities formed in a desired pattern on a cylinder surface are used to collect and then transfer ink or a coating material onto the surface of a continuous web.
Pattern cylinders may also be used in micro replication processes. In Ultraviolet (UV) curing replication, a material is applied to the cylindrical surface in a way to fill the cavities, the UV cured material cured by UV exposure and then separated from the cylinder. Other examples of micro replication processes include hot embossing and extrusion roll molding. In hot embossing, a pattern cylinder is pressed under high pressure and temperature against a preformed polymer web to create a homogenous product with a micro structured surface. In extrusion roll molding, a thin layer of molten polymer is pressed against a pattern cylinder to create a homogeneous product with a micro structured surface. The inherent appeal of a pattern cylinder, regardless of the process in which it is utilized, is its ability to enable manufacture of a product in a continuous manner. Continuous processes may provide lower manufacturing costs than batch processes.
Given the appeal of continuous manufacturing processes, one can efficiently create a cylinder containing the desired pattern of small cavities. Not only can one be able to create the exact desired pattern of small cavities, but also the time required to create the desired pattern over a large surface area must be reasonable. Surface areas to be patterned with small cavities typically range from 50 square inches to 3400 square inches, most ranging from 500 to 1500 square inches. It is desirable that the process required to create the pattern of small cavities could be accomplished in less than three days.
Many techniques are known in the art for creating pattern cylinders, but the list of techniques becomes shorter when the desired pattern has some optical utility. There are numerous uses for replicated patterns of small cavities that have optical utility. These applications include, but are not limited to, holograms used in security and packaging applications, micro lens arrays for telecommunications and imaging systems applications and light redirecting films used in backlit display systems. Examples of light redirecting films include diffusers, light collimating films and polarization recovery and recycling films.
Diamond tooling can be utilized to achieve the surface quality and roughness characteristics required for the creation of optical features. The precisely shaped and polished diamond tooling can be engaged with the cylindrical surface using any one of a variety of techniques, to either remove, shape or form the material on the cylindrical surface. While diamond tooling is expensive, it is preferred to more common and less expensive alternative tool materials. Alternative tool materials, such as carbide or high-speed steel, are not capable of producing optical quality features due to the relatively large grain size of the materials. This large grain size results in microchips along the cutting edges that will produce a surface with unacceptable roughness properties, thereby not meeting the requirements for a surface having optical utility. In order to produce optical quality features directly in a cylinder, the features can be cut, formed, scribed, ruled or otherwise created using a diamond tool or stylus.
Alternatives exist for indirectly creating a cylinder with the desired pattern of small cavities. For example one may create the desired pattern into a flat work piece, and through subsequent operations produce a flexible copy of the original that can be wrapped around a cylindrical surface. Utilizing such a technique will produce an easily detectable seam or line in the finished product, whose frequency is equal to the circumference of the cylinder. In some applications this may be acceptable, but in many applications it is not. This is particularly true when the finished product produced by replication or material transfer is of a length greater than the circumference of the cylinder.
It is well known that the time required to create a cylinder with the desired pattern of small cavities is a significant limitation of the various alternative methods for patterning such cylinders. In particular, the time required to create a cylinder with the desired pattern of small cavities can make the creation of such cylinders prohibitively expensive. All alternative processes have a capability to produce between one and ten features per second. At this rate, the time required to pattern an average cylinder used in a continuous manufacturing process would be measured in months. For example, if the desired pattern consists of 25,000 cavities per square inch and the surface area to be patterned is 1,500 square inches, at a rate of ten features per second, the time required to pattern a cylinder is 43 days. It is preferred to be able to create more than 100 cavities per second, and more preferred to be able to create more than 500 cavities per second.
Known techniques exist in the art for producing continuous optical grooves on a cylinder using diamond turning. In this process, a non-rotating diamond tool is engaged with a rotating cylinder. Typically a symmetric tool is engaged normal to the surface of the cylinder to produce a symmetric groove, consisting of two or more angled surfaces. While it is engaged with the cylinder it also may be moved further into the surface or retracted from the surface of the cylinder to produce variations in feature depth. If this is done in a continuous, non-linear motion, the resulting groove will consist of two or more curved surfaces. U.S. Pat. No. 6,581,286 (Campbell et al.) describes such a process for forming simple, continuous features by thread cutting. The grooves produced by this process are symmetrical, continuous and consist of two curved surfaces.
In other applications the diamond tool is moved parallel, perpendicular or at an angle to the rotational axis of the cylinder or work piece while engaged. A combination of any or all of these motions can occur simultaneously and the choice of motion selected is a function of the desired shape of the cavity being produced. Example applications of this method include the fabrication of the individual segments of a large mirror surface assembly.
Example embodiments of the present invention create a molding pattern for micro replication including a metal cylinder that is patterned with small, individual, three dimensional cavities by electro-mechanical engraving for the specific purpose of replicating the inverse of said three dimensional pattern into a continuous web of material.
Example
The reflective liquid crystal display D shown in example
The transflective liquid crystal display D shown in example
Light redirecting film 2 may include a thin transparent film or substrate 8 having a pattern of discrete individual optical elements 5 of well defined shape on the light exit surface 6 of the film for refracting the incident light distribution such that the distribution of the light exiting the film is in a direction more normal to the surface of the film.
Each of the individual optical elements 5 may have a width and length many times smaller than the width and length of the film, and may be formed by depressions in or projections on the exit surface of the film. These individual optical elements 5 may include at least one sloping surface for refracting the incident light toward the direction normal to the light exit surface. Optical elements 5 may have an aspect ratio greater than 0.5. Optical elements 5 may have a depth greater than 15 micrometers. These optical elements may take many different shapes. U.S. Patent Application Publication No. U.S. 2001/0053075 A1 titled “Light Redirecting Films and Film Systems” is hereby incorporated by reference in entirety. This application illustrates many variations of individual optical elements. However, one of ordinary skill in the art would appreciate other variations of optical elements of light redirecting systems that are covered by embodiments of the present invention.
As illustrated in example
The individual optical elements 5 may have multiple shapes and sizes on a light redirecting film 2. The individual optical elements 5 may also be placed on the surface in irregular patterns, where the spacing between neighboring elements varies. For example, random or pseudo-random placement of individual optical elements 5 on the light redirecting film 2 may be useful to avoid Moiré patterns or other optical effects when the light redirecting film is placed in an assembly with other optical components.
Irregular patterns comprise cavities that are placed in such a way that they do not follow a regular pattern such as a matrix, grid, or linear arrangement. Random patterns comprise cavities with locations chosen by a random process.
The backlight BL may be substantially flat or curved. The backlight BL may be a single layer or multi-layers and may have different thicknesses and shapes. The backlight BL may be flexible or rigid and be made of a variety of compounds. Further, the backlight may be hollow, filled with liquid, air, or be solid, and may have holes, ridges, or other optical deformities.
The light source 26 may be of any suitable type (e.g. an arc lamp, an incandescent bulb which may also be colored, filtered or painted, a lens end bulb, a line light, a halogen lamp, a light emitting diode (LED), a chip from a LED, a neon bulb, a cold cathode fluorescent lamp, a fiber optic light pipe transmitting from a remote source, a laser or laser diode, or any other suitable light source). Additionally, the light source 26 may be a multiple colored LED, or a combination of multiple colored radiation sources in order to provide a desired colored or white light output distribution. For example, a plurality of colored lights such as LEDs of different colors (e.g., red, blue, green) or a single LED with multiple color chips may be employed to create white light or any other colored light output distribution by varying the intensities of each individual colored light.
A back reflector 40 may be attached or positioned against one side of the backlight BL as schematically shown in example
Thermoplastic films with textured surfaces have applications ranging from packaging to optical films. The texture may be produced in a casting nip that consists of a pressure roller and a pattern roller. Depending on the pattern being transferred to the thermoplastic film, it can be difficult to obtain a uniform degree of replication across the width of the film. It can also be difficult to obtain this uniform degree of replication and have a smooth backside to the film.
Preferred polymers for the formation of the surface structures include polyolefins, polyesters, polyamides, polycarbonates, cellulosic esters, polystyrene, polyvinyl resins, polysulfonamides, polyethers, polyimides, polyvinylidene fluoride, polyurethanes, polyphenylenesulfides, polytetrafluoroethylene, polyacetals, polysulfonates, polyester ionomers, and polyolefin ionomers. Copolymers and/or mixtures of these polymers to improve mechanical or optical properties can be used. Preferred polyamides for the optical elements include nylon 6, nylon 66, and mixtures thereof. Copolymers of polyamides are also suitable continuous phase polymers. An example of a useful polycarbonate is bisphenol-A polycarbonate. Cellulosic esters suitable for use as the continuous phase polymer of optical elements include cellulose nitrate, cellulose triacetate, cellulose diacetate, cellulose acetate propionate, cellulose acetate butyrate, and mixtures or copolymers thereof. Preferably, polyvinyl resins include polyvinyl chloride, poly(vinyl acetal), and mixtures thereof. Copolymers of vinyl resins can also be utilized. Preferred polyesters of the invention include those produced from aromatic, aliphatic or cycloaliphatic dicarboxylic acids of 4-20 carbon atoms and aliphatic or alicyclic glycols having from 2-24 carbon atoms. Examples of suitable dicarboxylic acids include terephthalic, isophthalic, phthalic, naphthalene dicarboxylic acid, succinic, glutaric, adipic, azelaic, sebacic, fumaric, maleic, itaconic, 1,4-cyclohexanedicarboxylic, sodiosulfoisophthalic and mixtures thereof. Examples of suitable glycols include ethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, 1,4-cyclohexanedimethanol, diethylene glycol, other polyethylene glycols and mixtures thereof.
A typical extrusion roll molding system comprises an extruder that extrudes molten polymeric material into a nip. The nip is formed between a pattern roller and a pressure roller. The molten polymer is forced into the pattern roller pattern by the pressure roller and cools. The polymer exits the nip in a semi-solid to solid state. Rubber pressure rollers may be used to provide a relatively uniform pressure across the casting nip, since their coverings can deform to accommodate any thickness non-uniformities in a melt curtain. These thickness non-uniformities may be due to the presence of thick edges from neck-in or from other causes of non-uniform flow from the extrusion die. However, the rubber coverings may not have a surface with low enough roughness to produce a glossy (e.g. smooth) backside surface.
In alternative embodiments, a pattern roller may be micro replicated into an optical film using other molding processes known in the art, including but not limited to UV casting, hot embossing, and solvent casting.
Example embodiments of the present invention relate to a method of electro-mechanical engraving a molding pattern in a rigid surface. The molding pattern may be for micro-replicating optical elements during manufacturing of light redirecting films. The electro-mechanical engraving may be Gravure electro-mechanical engraving. Gravure electro-mechanical engraving processes have been used to produce printing rollers in the printing industry. However, example embodiments of the present application use electro-mechanical engraving for the entirely different purpose of making a molding pattern.
In typical electromechanical engraving, the cavities cut on a cylinder are placed in a regular pattern.
Example embodiments of the present invention significantly modify the electromechanical engraving process to provide irregular positioning of cavities for replicating three-dimensional micro-features. The electromechanical engraver may be modified to allow varying offset between columns of cavities, to allow arbitrary, irregular, or random cavity positions. The arbitrary, irregular, or random cavity positions may also cause the cavities to intersect and interlock in arbitrary, irregular, or random ways.
The varying offset between columns of cavities can also be allowed to be zero, thereby causing the electromechanical engraver to engrave multiple columns of cavities in the same X location. This capability can be useful in several ways for making tools for micro replication. The edges of neighboring cavities that are engraved sequentially by an electromechanical engraver cannot have sharp edges between them, due to momentum of the engraving head and stylus.
Tools for micro replication may need to have cavities that are deeper than can be cut in a single cut, or they may need to be made out of harder materials than are typically electromechanically engraved, such as nickel or nickel-phosphorous alloys. Engraving deep features in these harder materials might stress the diamond stylus to fracture, or optical-quality surfaces may no longer be achieved. Engraving multiple columns of cavities in the same X location can address these issues by cutting a cavity to increasing depths in each column. For example, as illustrated in
The cylinder may be hollow or solid, but in either case the cylindrical surface will have some associated thickness of the specific material required to achieve the desired results. In particular, if the desired pattern of cavities is to have optical utility, the cylindrical surface may be a non-ferrous material or alloy, so that it may be machined successfully using a diamond tool. In addition the material preferably has a very small grain structure or is amorphous, and is free of inclusions, pits, voids and other defects that will affect the optical utility of the desired pattern. Examples of such materials include, but are not limited to: copper and copper-nickel alloys, nickel and nickel-phosphorus alloys and high purity aluminums.
The cylindrical surface into which the machining will occur can either be wrought or plated. In wrought form the material may comprise the entire cylindrical surface or it may be pressed or otherwise sleeved over an existing cylindrical surface made from a less expensive material. In plating, an electro-chemical process or alike is used to transfer the desired material uniformly and in a thin layer onto the outside of the cylinder surface. This plating process can also be done over a mandrel to create a thin sleeve of the preferred material. This sleeve can then be patterned with the desired cavities or grooves and then transferred to the preferred cylinder for use in replication or material transfer. This sleeve can also be removed from the mandrel and transferred to the preferred cylinder prior to forming of the desired cavities.
After forming of the desired pattern of cavities, the sleeve can be removed and used as a belt in a replication or material transfer process. The sleeve can also be cut to size and used in the flat state for a replication process such as injection molding or thermoforming.
In another embodiment, the roller may comprise a polymer surface. Polymer surfaces may be diamond turned or electro-mechanically engraved and have high replication fidelity. Polymer rollers are useful as a replication master for electro-plating processes and also can be used for casting of aqueous and solvent dispersions of polymers. Suitable polymers include PTFE, Delrin, and Capton.
Electromechanical engravers and diamond turning machines with fast tool servos are both examples of cutting machines for cylinders, but they have significant differences. Diamond turning machines must be built to extremely fine tolerances to achieve high positioning accuracy in the direction of the cylinder's axis as well as radially (normal to the surface of the cylinder). Furthermore, if the fast tool servo attachment moves large distances at high frequency, it may induce vibrations in the radial direction, reducing depth accuracy. Achieving high accuracy in depth also requires either finishing the roller surface on the same diamond turning machine before cutting cavities, or providing a very precise roller surface and mounting the roller with extremely low runout. An example diamond turning machine with fast-tool servo capability is the Vertical Drum Lathe available from Moore Tool Company of Bridgeport, Conn.
Electromechanical engravers are made with mature technology that addresses some of these issues in a cost-effective and reliable way. A diamond stylus is mounted on an arm attached to a rod in an engraving head that also has a shoe mounted in it. Typically the stylus has a simple cross section consisting of two straight cutting edges that meet each other in an apex; sometimes the apex is chamfered. A force is applied to the engraving head, causing the shoe to rest against the roller surface. As the cylinder turns, twisting motions are induced in the rod by electromagnets, causing the stylus to move in and out of the cylinder surface. The motion of the stylus is synchronized with the rotational position of the cylinder using an encoder attached to the rotational axis. Electromechanical engravers are designed to achieve high speed, for example up to 8,000 Hz, for fast production of rollers for printing and material-transfer applications. They are also capable of large cutting depths, for example up to 100 micrometers or more. The HelioKlischograph K500 available from Hell Gravure Systems of Kiel, Germany is an example electromechanical engraver. Electromechanical engravers can also be configured to engrave on flat surfaces using similar methods, either rotating the workpiece or by a relative translational motion. A short description is provided here, but is not meant to limit the definition of electromechanical engraving. Those skilled in the art will know additional variations not described. U.S. Pat. No. 6,515,772, incorporated here by reference, describes control and setup methods for an electromechanical engraving system.
With modifications described herein, electromechanical engraving machines have several distinct advantages for patterning micro-scale cavities on rollers or molds for optical applications. The shoe resting against the cylinder surface provides a radial positional reference to the surface, allowing highly repeatable depth positioning relative to the cylinder surface. This can save considerable time and expense on the machine to finish the roller or fixture it accurately. Furthermore, the engraver can be made to lower tolerances as well, because it need not provide accurate positioning in the radial direction. In yet another advantage, the shoe riding on the surface provides a damping function against vibration or inaccuracy due to fast and/or large motions of the cutting head and diamond stylus.
In embodiments, the present invention may be used to fabricate light extracting features as described in U.S. patent application Ser. No. 11/388,582, incorporated herein by reference. In this application, individual cavities of finite length and width are cut into a roller, and the roller is used to mold a light extracting film having light extracting features that are substantially the negative shape of the cavities on the roller. A cross section of an example light extracting feature 110 is shown in
As used herein, the cross section of a cavity is its cross section in the plane perpendicular to its longest dimension. A complex cross section of a cavity is a cross section that consists of more than three line segments or that contains curves that are parabolic, elliptical, circular arcs, or other well-defined curved shapes such as those defined by an aspheric equation. A complex cross section stands in contrast to the simple cross section of a diamond stylus typically used with electromechanical engraving machines, which has two line segment cutting edges that meet at an apex, where the apex may have an optional flat tip. Because the shape of the cavity in the mold is substantially the negative of the shape of the diamond cutter that cuts it, and the shape of the cavity in the mold is substantially the negative of the shape of the optical element formed from it, we will speak of complex cross sections as applying equally to cavities, optical elements, and diamond cutters.
Another example of a complex cross section is shown in
In some cases the molding process may cause light extracting features 110 to have a somewhat different shape than the negative of cavities 130. For example, a UV-cured polymer may shrink anisotropically when it cures, perhaps because some dimensions are constrained by the substrate 111. This may cause angles 118a, 118b to be different than angles 138a, 138b, and it may cause the length of segments 116b, 117b to be longer or shorter than the lengths of segments 136b, 137b. Other cases of predictable shape changing during molding will be known to those skilled in the art. In such cases, the cavity 130 cross section may be fabricated in a modified way to correct for such molding distortions. For example, if light extracting feature angle 118a is consistently 1 degree smaller than cavity cross section angle 138a, then angle 138a may be cut 1 degree larger to achieve the desired angle 118a.
In yet another embodiment of the present invention, multiple cavity cross sections may be cut on a single surface.
A mold for a light extracting film with multiple light extracting feature cross sections may be produced by cutting cavities with multiple cavity cross sections, using a diamond cutter for each cross section.
In one embodiment, a first set of cavities is cut using cutter 150, and then cutter 151 is loaded into the electromechanical engraving head, and the second set of cavities is cut. The two sets of cavities are independently determined, so that there need not be a one-to-one correspondence between cavities in the first set and cavities in the second. Independently determined sets of cavities may have different placements, cavity lengths and cross sectional shapes, densities across the pattern, and other differences that will be known to those skilled in the art. In addition, the cavities cut by a single diamond cutter may vary in depth, length, and other attributes.
The two sets of cavities may be accurately positioned relative to each other by the following method. In a test region of the cylinder, preferably near one edge, cavities are cut by both cutters in an interspersed pattern, placing cavities of the first set between cavities of the second set, and vice versa.
In experiments, relative positioning accuracies of less than 5 micrometers may be obtained consistently with the above method, and relative positioning accuracies of approximately 1 micrometer may be obtained by measuring and averaging relative positions of multiple cavity pairs. Some additional relative error between the first and second sets of cavities is induced by inaccuracies in the positioning of the electromechanical engraver, temperature-induced positioning changes, and other sources known to those skilled in the art. The first and second sets of cavities may be kept in relative position to accuracies of less than 20 micrometers with relative ease. Accuracies of 10 micrometers and 5 micrometers have been obtained by application of additional methods known to those skilled in the art, such as careful attention to temperature control, accurate electromechanical engraver positioning components, and averaging relative positions measured on multiple cavity pairs as described herein.
In an alternative embodiment, multiple engraving heads are mounted on a single engraving machine, for example on opposite sides of the cylinder, and the two sets of cavities are cut simultaneously. In this case, a first portion of the interspersed cavities 160, 161 in the registration pattern may be cut in a test region of the cylinder by both cutters 150, 151. Then the first portion of the cavities 160, 161 may be measured, the relative position of the two cutters is adjusted as above, and then the process is repeated. When the relative position of the first set and second set of cavities is measured to be in correct position, the engraving machine may be allowed to cut the rest of the two sets of cavities simultaneously. This embodiment may achieve higher overall cutting speed at the cost of a second engraving head and machine and control complexity.
When more than two cutters are to be used, the first cutter may cut a first set of cavities to be used as a reference, including one or more registration patterns. Each successive cutter places additional cavities interspersed with the first set of cavities in registration patterns, and the position of the additional cavities relative to the first set of cavities is measured and adjusted as described above. If desired, cavities cut by the third cutter may be measured relative to cavities cut by both the first and second cutters, or they may be measured against only one of the previous sets of cavities.
In another embodiment, multiple cutters may be used to cut sets of cavities that have the same cross section. This may be of use when cutter wear is an issue. For example, in some cases a pattern roller must be fabricated of a hard substance such as a nickel-phosphorous alloy to withstand stresses or wear induced by the molding process. In this case, diamond cutters may wear too much to allow cutting an entire pattern of cavities on the roller using a single diamond cutter. As the cutter wears, cut finish quality may degrade, visual artifacts may emerge, or the cross sectional shape of the cavities may change. Multiple cutters may be used to cut the cavities on the roller to achieve the pattern size desired while keeping wear on each cutter within required bounds. To avoid visible lines in the pattern due to slightly different cutting from the multiple cutters, it is preferred to intersperse cavities from the multiple cutters across the cylinder.
In another embodiment, a first cutter may cut a registration pattern after it has cut a first set of cavities in the cylinder while traveling in a first axial direction. The cavities cut by a second cutter may be registered into correct relative position in the registration pattern, after which the second cutter cuts a second set of cavities while traversing the cylinder in the opposite axial direction from the first cutter. This method may distribute cutter wear more evenly across the cylinder, because the first cutter is worn at the end of its cut where the second cutter has experienced little wear, and vice versa. However, this method may induce additional inaccuracies in machine positioning due to the different axial movement directions.
In another embodiment, a single cutter may be kept mounted in the engraver and used to cut multiple sets of cavities that are described above as being cut by different cutters with the same shape. This embodiment will not reduce overall wear on the cutter, but it will spread the wear across the cavities in a more even distribution. For example, if a cutter cuts a single set of cavities across the cylinder, then the cutter will be sharp at one end of the cylinder and have wear at the other end. If instead the single set of cavities is divided into 2 or more interspersed sets of cavities cut in succession by a single cutter, then the wear on the cutter, and any corresponding changes in surface finish or shape, will be more evenly distributed across the cylinder.
In preferred embodiments, the cylinder is kept mounted on the electromechanical engraving machine from before the first set of cavities is cut until after the last set of cavities is cut. If the cylinder is removed from the engraver, then any inaccuracies in remounting it to the same position will be reflected in additional inaccuracy in relative positioning of any subsequent cavities that are cut. In some cases it is advantageous to remove the cylinder, rotate it 180 degrees to swap its ends in the engraver, and then cut additional cavities that may have an alternate orientation.
In another embodiment, cavities with complex cross sections may be formed by multiple overlapping cuts of a cutter.
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 spirit and scope of the invention.
This application is a Continuation-in-Part of U.S. Ser. No. 10/859,652 filed Jun. 3, 2004, the contents of which are incorporated herein by reference.
Number | Date | Country | |
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Parent | 10859652 | Jun 2004 | US |
Child | 11567770 | Dec 2006 | US |