WO2006026103A2 - Method and apparatus for emulating brush strokes on mass produced art - Google Patents

Abstract

A screen printing method and apparatus is provided for emulating textured brushstrokes on prints that have been previously printed. The screen printing system and process include steps of forming the screen by providing a mesh (14) with a thread count between 86 and 230 threads per inch, applying emulsion (12) having a thickness between 50 and 200 microns to the mesh (14), positioning an image film (18) adjacent the emulsion, exposing the emulsion to a light source (22), removing non-hardened areas of emulsion from the mesh to form image openings (24a). The method also involves selecting appropriate ink formulations, preparing the squeegee/floodbar, and various press mechanics.

Description

TITLE OF INVENTION: METHOD AND APPARATUS FOR EMULATING BRUSH STROKES ON MASS PRODUCED ART CROSSREFERENCE TO RELATED APPLICATION This application claims priority of co-pending U.S. Provisional Patent Application No. 60/600,192, filed 10 August 2004, which is incorporated by reference in its entirety herein. BACKGROUND OF THE INVENTION This invention is directed to a method and apparatus for mass producing art prints (hereinafter "print(s)") substantially having the texture of an original hand-painted work of art. Mass-produced prints are typically produced by printing processes such as lithography, letterpress and offset processes that result in a substantially flat, one- dimensional print. Hand-painted originals, on the other hand, are frequently characterized by the appearance of numerous textured brush strokes and pallet knife techniques that add depth and character. In the past, apprentices and others have been hired by known art publishing houses to add hand-painted texturing techniques to prints in order to give the print the look of original artwork. The process is time-consuming and adds significant cost to the final product. Tn addition, no two final products are exactly alike, rendering it difficult for the original artist to give approvals based on a specimen. SUMMARY OF THE INVENTION The invention herein is directed to a method and apparatus for adding emulated textured brushstrokes and/or pallet knife techniques to prints with a height and density that blurs the line between the original artwork and the print, while maintaining a production economy that yields the desired price point for the finished product, preferably a price point suitable for the mass market. The invention herein comprises a printing system and process that creates an enhanced reproduction of a work of art by applying noticeable deposits of ink on a substrate bearing the reproduction so as to substantially replicate hand-applied texturing techniques. A novel screen printing system and process is used to deposit the ink in a manner that supplements a print that has been previously printed on paper, canvass or other substrate. The invention is conveniently disclosed in two parts: (1) ink deposit, and (2) control of ink deposit. 1. Ink Deposit. While conventional screen printing relies on ink shear and capillary action to transfer ink to a substrate, the instant invention utilizes an extrusion process to emulate the visual and tactile effect of hand-applied texture. Relatively heavy deposits of ink are controllably transferred to the substrate utilizing a stencil and mesh system that provides a plurality of relatively large ink-holding chambers juxtaposed over the areas of substrate where the deposits are to be made, with preferred stencils having a thickness normally in excess of 100 microns and the polyester mesh having a preferred thread count of 86-230 threads per inch. High viscosity, low thixotropic ink is then forced through the stencil by means of a squeegee, which preferably has a rounded print edge rather than the traditional sharp print edge. The interaction of mesh count, stencil thickness, ink rheology and squeegee contact is the platform to achieve large ink deposits on the substrate. 2. Control of Ink Deposit. The high wall thickness of the stencil, a correctly chosen mesh count, the use of correctly applied force to push the ink through the stencil and the use of a relatively high viscosity, low thixotropic ink with moderate flow-out characteristics yields an esthetic edge definition to the deposited ink that substantially emulates hand-applied texture. BRIEF DESCRIPTION OF THE DRAWINGS The drawings are in schematic form and not necessarily to scale. Fig. 1 is a plan view of a prior art substrate bearing a printed work of art; Fig. 2 is a cross-sectional side view of a conventional screen-printing mesh and stencil system; Fig. 2a is a cross-sectional side view of a screen-printing mesh and stencil system after development and creation of image-transferring openings; Fig. 3 is a cross-sectional view of a synthetic thread construction; Fig. 4a is perspective view of a squeegee; Figs. 4b and 4c are end elevation views of squeegees of different variations; Figs. 5a, 5b and 5c are cross-sectional views of screen-printing mesh and substrate in association with ink of various viscosities and thixotropies; Fig. 6 is a top plan view of a pallet and the locations of exemplary brackets; Fig. 6a is a top plan view of an adjustment bracket; Fig. 7 is a fragmentary elevation view of an adjustment bracket attached to a pallet; Fig. 8 is an illustration of an example of an assembly line with round registration points; Fig. 9 is an illustration of an assembly line with line contact registration.

DESCRIPTION OF THE PREFERRED EMBODIMENT As described below, a process of ink extrusion to a substrate is used within a production printing process having a well known sequence of events: image capture, color correction, properly executed offset-litho print to canvas or similar structure, a hand- embellished piece on the litho printed sheet as a guide for screen printed separations, color separations generated for the screen-printing embellishments, creation of the stencil/mesh system balanced to the separations, preparation of squeegees and floodbars that are balanced to the desired results in terms of durometer, radius and rigidity, preparation of ink colors balanced to the image and production parameters, set-up of press mechanics balanced to the desired result, balance of UV curing to the mixed ink set, print production and post-production procedures as appropriate to the client. Referring initially to Figure 1, a substrate 10 bearing a printed work of art 1 1 is to be enhanced in accordance with the invention. The work 1 1 is conveniently a lithograph of an original work of art; although it will be apparent to those of ordinary skill in the art that a work reproduced by substantially any printing process can be enhanced in the manner described below. A lithograph has been chosen because it is a cost-effective way of producing a substantial number of reproductions at a relatively inexpensive price-point and therefore likely to be the most commonly used printing method for creating the pre- enhanced print. The pre-enhanced print is then subjected to the instant invention's preferred manufacturing process, which may conveniently be broken down into five parts: (1) screen preparation and exposure, (2) creation of embellishment separations, (3) ink formulation, (4) squeegee and floodbar preparation, and (5) press mechanics. 1 . Screen Preparation. As illustrated in Figure 3, a mesh 1 14, of synthetic thread construction, having a thread count of approximately 86 to approximately 230 threads per inch is glued or otherwise affixed very tightly to a metal frame (not shown). The tension of this mesh may be numerically characterized in Newtons/cm², the optimal value being in the range of approximately 18 to approximately 25 Newtons/cm². Those of ordinary skill in the art also recognize that the mesh must be sufficiently stretched across the frame to move quickly away from the substrate during the printing process after making momentary contact to initiate ink transfer. Proper tensioning of the mesh creates a "spring action" of the mesh away from the substrate which is highly preferred in conventional screen printing to avoid smearing and maintaining correct acutance of the desired image. The screen frame must be sufficiently rigid to support the tension levels without distortion of the frame bar alignment. Excessive tension will inhibit proper squeegee interaction by inducing a bowing effect in the squeegee attack angle. Lastly, the mesh material is preferably died yellow or amber to minimize light exposure undercut. Once the proper mesh tension has been achieved, a novel stencil/emulsion is applied to the mesh. Figure 2 is a highly magnified, cross-sectional side view in schematic of a conventional screen-printing mesh and stencil system, shown with frame and image film. The stencil/emulsion system comprises a photosensitive substance 12 attached to a porous fabric or stainless steel mesh 14. The mesh is attached to the frame 15. The image film 18 is pressed intimately against the stencil / emulsion during the exposure process. The stencil/emulsion thickness used in conventional screen printing is typically less than 15-20 microns because a greater thickness interferes with the capillary action that pulls the ink from the mesh to the substrate. The emulsion is applied to the mesh in accordance with manufacturer's instructions. In contrast to conventional stencil/emulsion systems, the novel stencil/emulsion system herein comprises a thick layer of photo-chemically active emulsion that is preferably approximately 50 microns to approximately 200 microns thick. As is known in the art, the emulsion in the emulsion/stencil system becomes impervious to water where impinged by light of a suitable wavelength, typically ultraviolet light within a specified narrow band of wavelengths. The emulsion remains water-soluble where it is not impinged by the light, and the degree of hardening is a function of its total exposure to the light source. Exposure, in turn, is a function of time and distance, with a known intensity of the light source. Here, stencil systems capable of achieving emulsion-over-mesh deposits of 50 to 200 microns are required. Direct/indirect systems such as Murakami MS Thick Film have been found to be suitable. A direct/indirect system is preferred and should be applied with the mesh in firm and uniform contact to the emulsion film then firmly coated with appropriate direct emulsion such as Murakami One Pot SoI on the well side of the screen. Two to four coats are generally sufficient to adhere the indirect film. Upon proper application the drying is best facilitated at approximately 78 to approximately 85° F, with relative humidity of about 45% and substantial air flow across the mesh surface. When correctly dried, the clear carrier base film of such a product such as Murakami MS film will be easily removed prior to exposure. 2. Exposure. Each color component of the reproduction requires a separate Iitho film. The Iitho film 18 accordingly contains a color component of the image to be embellished. The image portion of the Iitho film is dense black (having a D-max sufficient to block required exposure levels of ultraviolet light) while the remaining portion of the Iitho film is transparent to the light. The coated screen and Iitho film are placed in a vacuum frame (normally used in the printing industry) to create intimate contact between stencil system and Iitho film. A light source 22 exposes the emulsion through the Iitho film. The light passes through the transparent (non-image) portions 23 of the Iitho film, impinging on and cross-linking the underlying layer(s) of emulsion. The light is blocked by the non-transparent image areas 24 on the film, thereby leaving an image in the emulsion that remains water soluble and washes away during development. This process creates image-transferring openings 24a (Figure 2a) that, as described below, act as ink chambers within the otherwise cross-linked (hardened) non-transferring areas 23a of emulsion. Each ink chamber terminates in an orifice from which the ink is discharged onto the substrate, as described below. Thus, following the exposure to ultraviolet light and subsequent developing washout, the resulting ink chambers are formed by the areas of stencil/emulsion that wash away. The designed mesh and stencil/emulsion combination have created an ink chamber of greater volume than conventionally found in screen printing processes because of the greater (hardened) emulsion thickness—up to approximately 200 microns as described above—which form the walls of the ink chambers. The purpose of a larger chamber is to hold more ink for transfer to the print, as described below. Each orifice has cylinder-like walls that are formed by the remaining stencil/emulsion. These "cylinders" fill with ink that discharges onto the substrate via an extrusion effect created by interaction with the squeegee and floodbar assembly. This concept will be expanded upon after Section 5, below. 3. Ink Formulation. Turning next to the ink, the preferred ink is a high viscosity, low thixotrophy ink. This is a significant change in the traditional application of ink rheology to screen printing, where low viscosity, low thixotropic (high out-flow) inks or highly thixotropic with medium to high viscosity (low flow-out ink) is the norm. Thixotropy is defined as the property exhibited by certain liquids/gels of becoming fluid when put in motion and returning to a semi-solid state upon rest. The term "thixotropic" is known in the printing industry. When an ink is thixotropic, it acts as a lower viscosity liquid when in motion, but exhibits an apparently high viscosity when at rest. An analogy may be imagined by recalling the characteristics of such commonly encountered home products as Heinz^® ketchup and mayonnaise. A high viscosity, high thixotropic ink will develop a high surface tension that makes it difficult to obtain satisfactory ink flow through the chambers in the stencil. In addition, as illustrated in Figure 5a, the ink reaching the substrate below the stencil will tend to retain the "stepped" edge defined by the chamber's shape, and look unnatural on the enhanced work. If the viscosity and thixotropy are too low the resulting print does not develop sufficient texture to be easily noticed and displays a lack of clarity, as illustrated in Figure 5c. Instead, it is desirable for the ink to have a thixotropy that results in a generally convex deposit on the substrate, as illustrated in Figure 5b. In selecting an ink, shear is not a critical consideration, and pigmentation is preferably in a concentration that does not exceed 18% by weight. The preferred ink has a rheology that exhibits a moderate degree of flow-out, but not so much that the column height of ink collapses beyond a slight rounding of the edges (as illustrated in Figure 5b). In addition, the viscosity cannot be so thick that the ink hangs in the chamber, but cannot be so thin that flow out causes blurring or lack of definition. A preferred ink is TUVN 3000-6-2 by TW Graphics, City of Commerce, California U.S.A. Gloss levels may be modified by use of appropriate flattening agents. 4. Squeegee Preparation. As illustrated in Figure 4a, traditional squeegees used in screen printing processes are usually formed from a 3/8-inch by 2 inch surface block of polyurethane compound having 90° corners. They are available in various durometer variations, but 65 - 85 Shore D is typically used. They are available in single, double or triple durometer variations, Figure 4b-c. Higher durometer creates a more rigid print edge. As illustrated in Figure 4c, the typical sharp edge of the squeegee 40 is replaced with a rounded edge for the preferred squeegee. The rounded edge has a preferred radius of approximately .020 inch to approximately 0.118 inch. Squeegee parameters during actual printing as relates to press mechanics are attack angle, travel speed and downward force. Traditionally, the 90° edge between the 3/8 inch side and the 2-inch side is held against the ink as the squeegee 40 is pulled lightly across the screen to break the surface tension of the ink. The preferred squeegee is applied to the mesh at an angle of attack from about 10° to about 25° from vertical where the term "vertical" denotes the direction substantially orthogonal to the substrate. The squeegee travels across the mesh at a speed that is slower than conventional speed as more dwell time is required to move the greater ink volume. The downward force exerted against the mesh is higher that conventional force in order to convert from capillary to extrusion action. Higher durometer blades are preferable to minimize deflection of the vector force against the mesh. The overall goal is to increase the squeegee surface area in contact with the ink and mesh beyond the norm, generating a greater volume of ink flow through the mesh. This is very contrary to conventional process that endeavors to shear the ink away from the mesh. 5. Press Mechanics. After proper development of the stencil/emulsion system, modifying the squeegee and preparing the ink, the screen is positioned on the press and the image is thereafter transferred to a substrate such as a canvas, clothing or paper sheet by applying a suitable ink to the mesh and firmly dragging the squeegee across the mesh. At this stage many adjustments can be made to the press to fine-tune the resulting print. Off- contact and peel rate adjustments are most critical, as too little or excessive adjustment of either feature may cause smearing, dimensional distortion and lack of clarity of the final image. Both these adjustments interact with squeegee pressure and can affect ultimate goal of controlled ink deposit at high volume. Care must be taken to balance these parameters. An increase of travel speed of either floodbar or squeegee will decrease ink deposit. As the colors are sequentially applied, the topography of the substrate changes significantly and requires adjustment of the above mentioned parameters to achieve the desired results.

Additional Information. In conventional screen printing operations, capillary action traditionally pulls the ink down to the substrate as the shearing force of the squeegee breaks the surface tension at the top of the ink chamber. To enhance capillary action, low viscosity inks have been favored. Once the capillary action between stencil system and substrate begins to pull the ink down to the substrate, the rheology of the ink determines its controllability. Conventionally, screen printing has called for a very sharp squeegee print edge and only a light pressure on the mesh by the squeegee; i.e., just enough to shear the ink and permit capillary action to pull the ink down to the substrate, with the ink flowing in the general shape of the mesh opening. The application of more pressure is avoided because too much pressure by the squeegee distorts the stencil, causing ink to get under the stencil and smear, thereby resulting in a wider ink flow than desired. With the instant invention, the volume of the ink to be deposited is significantly increased making capillary action ineffective. The instant mesh and stencil system have created a much larger ink deposit chamber than presenting conventional systems. The chamber thus created can hold more ink for transfer to the substrate, but the depositing of the ink on the work must be controlled. Capillary action is ineffective because the quantity of ink is too great, surface tension of the mass is high, and only a small proportion of the ink will be pulled down through the mesh and onto the substrate. Accordingly, a novel ink transfer process is utilized that preferably employs non- conventional screen-printing ink transfer mechanisms and techniques. First, the ink to be deposited on the work is preferably extruded through the mesh and chamber rather than pulled through by capillary action. In order to extrude the ink, the modified squeegee is preferably used to apply sufficient force against the stencil to push the ink through the stencil rather than simply break the ink's surface tension. The combination of high wall thickness in the chamber, correct mesh count and proper ink rheology create a chamber or column through which ink can be extruded with the rounded edge of the squeegee. The correct ink rheology permits the ink to flow through the chamber, be forced onto the substrate where it achieves moderate flow-out that yields an esthetic edge definition similar to a painted edge. The preferred printing process also requires control of ink drying in addition to ink deposit. Use of ultraviolet curable inks allow for "instant" drying and a preferable method of control for the previously created ink deposit. The curing of UV inks is dependant on the dose and duration of the UV source, as well as the thickness of the ink deposit and reactivity of the photo initiator package. In general, more energy or reactivity is required to affect an adequate cure as the thickness of the ink deposit increases. Most commercially available (non-nitrogen) curing lines have a typical upper limit of 300 Watts/inch of capacity. Given this dose limitation, the next possible adjustment is duration. However, greater exposure duration equates to higher heat levels and distortion as well as longer cycle time or slower production. The solution lies in balancing a custom photo-initiator package to a custom doped lamp. Custom lamp doping is achieved by the introduction of certain ferrous elements to the internal lamp ingredients to create a higher spectral output; i.e., more UV energy in certain ranges such as 360-380 nm. PALLETLEVELINGSYSTEM When the print to be enhanced is placed under the stencil during the production process, it is highly preferred that it be essentially completely level across its entire surface; if it is not, the ink flow through the mesh is adversely affected and the ink's configuration on the substrate is less than desirable. The substrate bearing the image that is to be enhanced is accordingly placed on a substantially flat pallet, and held firmly to the pallet by vacuum heads as is known in the art. The pallets, however, are not essentially flat, and discrepancies in the hundredths of inches or less can affect the visual quality of the enhanced work owing to the less-than-ideal ink transfer that results from such minute distortions. Another feature of the invention accordingly resides in a pallet leveling system that improves the quality of ink transfer by permitting both upward and downward forces to be exerted in different regions of the pallet in order to resolve height differences a small as approximately 0.008 inch or less. Figure 6 is a top plan view of a pallet 50 utilizing the leveling foregoing system. As illustrated in Figure 6, the pallet leveling system preferably comprises three rows of 6 adjustment brackets 70 underlying the pallet. As shown in Figure 7, a fragmentary front elevation view of the arrangement of Figure 6, a fixed bracket 72 (see also Figure 6a) is affixed to the master frame 73, and a translatable bracket 74 is affixed to the bottom of an aluminum pallet 50. A threaded member 78 extends upward from the fixed bracket through an accommodating hole in the translatable bracket. A nut 80 is turned clockwise or counterclockwise about the threaded member to respectively lower or raise the overlying portion of the pallet. As the nut is advanced or lowered, it pushes up or pulls down on the affected region of the pallet. Adjustments are made by monitoring dial indicators as the print head is moved back to front along the pallet. A system for registration of the image is important for practicing the invention. Registration techniques well known to those having ordinary skill in the art may be utilized. The particular registration technique used will be dependent upon several factors including the rigidity of the substrate. As the flexibility of the substrate increases, the difficulty of maintaining accurate registration also increases. In some techniques flexible substrates tend to ripple or collapse when touching a narrow positioning pin. Accordingly, the particular registration technique used will depend in part on the flexibility of the substrate. By way of example, and not by way of limitation, illustrated in Figure 8 is a schematic illustration of an example of an assembly line constructed in accordance with the invention for enhancing printed artwork. It will be understood by those having ordinary skill in the art that the illustrated assembly line is but an example and may or may not be utilized based upon numerous factors including, but not limited to, the physical characteristics of the substrate, the type of printing equipment utilized and other factors. The assembly line depicted in Figure 8 comprises a number of pallets positioned at a respective number of stations 52, 54, 56, through which the pallets 50 pass. In practice, an assembly line comprising fourteen such stations has been utilized. A substrate 10 of printed artwork is placed on a pallet 50 at a load station 52, and clamped to the pallet by vacuum heads in the pallet in a known manner. The pallet is then transferred under microprocessor control to a first printing station 54 where the enhancement process commences with the first of several ink-transfer operations described above. While the substrate is being enhanced at the first printing station 54, a second pallet 50 is being loaded with a second substrate at the load station 52. When the ink transfer operation is completed at station 54, the pallets are advanced to the next respective station, and so forth so that all stations are eventually performing essentially simultaneously under microprocessor control, with the fully enhanced substrate being off- loaded at the last station. As the pallets and their respective substrates are transferred from station to station, a means must be provided for ensuring that the substrates are perfectly registered at each station so that the ink-transferred image at each printing station is not only exactly aligned with the ink-transferred images from the other printing stations but also exactly aligned with the printed image that was on the substrate that was loaded onto the pallet at the loading station 52. Moreover, the colors imprinted at the various printing stations must not only align with the other printed colors, but must also align with the design on the substrate that is being enhanced. Otherwise, the ink transfers may be perfectly aligned with each other but be unaligned with the un-enhanced image, thereby rendering the final product unacceptable. Accordingly, the position of the substrate on the pallet must be registered with great precision, and the pallet itself must be precisely registered at each station. There are numerous well-known techniques for registering the pallets at the stations, and mechanisms for doing so are included within multi-station print production systems. These techniques are commonly known to one with ordinary skill in the art. Three locating points shown in Figure 8 as an "X" are utilized. Two points are on the same longer edge of the substrate and the third point is at an adjacent corner of an opposing (usually shorter) edge. As a guideline however, experience has shown as a substrate becomes less rigid, e.g. art canvas, a broader surface area of contact (approximately 3 inches to 4 inches of line contact) at each of the three locations is advantageous (See Figure 9). With more rigid substrates (e.g. high quality art paper of cover stock weight), a smaller surface area of contact (ideally a circle) at each of the three locating points is preferred. Clearly what has been shown is a new, novel and no obvious method of printing which is of great commercial and aesthetic value. It is not the intention to limit the invention to the particular embodiment disclosed but the invention will be understood to be limited only by the claims.

Claims

CLAIMS WHAT IS CLAIMED IS: 1. A method for producing prints comprising the steps of: a) Positioning a mesh having a first surface and a second surface and having a thread count between approximately 86 and approximately 230 threads per inch in a substantially rigid frame; b) Applying a photo-chemically active emulsion to the first surface of the mesh to a thickness of between approximately 50 microns and approximately 200 microns; c) Positioning the first side of the mesh relative to a top side of a photosensitive substrate; d) Positioning an image film against the bottom side of the photosensitive substrate; e) Exposing the photo-chemically active emulsion through the image film to a light source; f) Removing non-hardened photo-chemically active emulsion from the mesh; g) Positioning the first surface of the mesh above a top surface of a print substrate; h) Applying ink to the second surface of the mesh so that ink travels through the mesh and the areas in which the photo-chemically active emulsion has been removed and onto the top surface of the print substrate. 2. The method of claim 1 wherein the tension of the mesh in the frame is between approximately 18 Newtons per square centimeter and approximately 25 Newtons per square centimeter. 3. The method of Claim 1 , including the step of Drying the photo-chemically active emulsion at a temperature between approximately 78 degrees F and approximately 85 degrees F. 4. The method of Claim 1 wherein the ink has a high viscosity and a low thixotrophy. 5. The method of Claim 1 wherein the ink is applied to the second surface of the mesh by using a squeegee. 6. The mothod of Claim 1 including the step of: Adjusting the distance between the top surface of the print substrate and the first surface of the mesh to cause predetermined movement of different portions of the print substrate when the ink flows onto the top surface of the print substrate to vary the thickness of the ink on the substrate. 7. The method of Claim 1, including the step of repeating steps a through h of Claim 1 for a plurality of different meshes. 8. The method of Claim 7 wherein only one print substrate is utilized. 9. The method of Claim 8 including the step of: Registering each of the meshes relative to the print substrate so that the ink received by the top portion of the print substrate through each of the meshes is in a predetermined location. 10. An apparatus for producing prints comprising: a) A substantially rigid frame; b) Amesh having a first surface and a second surface and having a thread count between approximately 86 and approximately 230 threads per inch affixed to the frame; c) Hardened photo-chemically active emulsion affixed to the first surface of the mesh to a thickness of between approximately 50 microns and approximately 200 microns; d) A print substrate having a top surface and a bottom surface and positioned so • that the top surface of the print substrate is proximate the first surface of the mesh; e) ink extending from the second surface of the mesh through the mesh and the areas in which the hardened photo-chemically active emulsion is present and onto the top surface of the print substrate.