US 3280731 A
Description (OCR text may contain errors)
Oct. 25, 1966 w. PROSKAUER 3,280,731
STENCIL SCREEN FRAME ATTACHMENT AND TENSIONING MEANS Filed Jan. 28, 1964 INVENTQR. 1 g. 4 WALTE/P PROKAb/H? /FWJ m United States Patent 3,280 731 STENCIL SCREEN FRAIViE ATTACHMENT AND TENSIONING MEANS Walter Proskauer, San Francisco, Calif., assignor to Electrostatic Printing Corporation of America, San
Filed Jan. 28, 1964, Ser. No. 340,700 3 Claims. (Cl. 101127.1)
This invention relates to electrostatic printing systems and, more particularly, to an improved stencil screen structure in an electrostatic printing system.
In electrostatic printing systems such as is described in Patent No. 3,081,698 by Clyde Childress et al., an image establishing electrode may be formed by a conductive stencil wherein the printing or image forming areas comprise fine mesh open screen and the nonprinting areas are suitably masked. Opposite the screen there is placed paper or other material to be printed and this is backed by a conductive plate. The screen and plate are connected across a source of potential to establish an electric field therebetween. A finely divided printing powder, having a particle size capable of passing through the open areas of the stencil, and having electroscopic properties, is applied to the screen to pass through the openings in the printing areas into the electric field. The electric field carries the powder particles across an air gap and toward the oppositely charged plate. However, the material being printed upon which is placed in the air gap intercepts the powder, and the powder forms the image thereon of the open areas of the printing stencil. The image can then be fixed in any suitable manner such as by heat, solvent, vapor or by any other suitable means.
In this system, the stability of the image to be printed depends upon the stability of the screen which is used to support the stencil. Under normal operating conditions and at average temperatures, the screen can be tightly prestretched over its frame and will remain dimensionally quite stable so that there is no problem in maintaining the stability of the image. However, when the screen is used for a stencil in printing on objects, such as glass bottles at elevated temperatures, the uneven heating of the screen causes the individual wires of the mesh to ex pand unevenly. This uneven expansion causes the screen and, therefore, the screen image region, to warp and buckle with subsequent inability of the stencil to print an undistorted image. The screen cools somewhat when the hot object is removed, as it must be, to permit the next hot object to take its place. As a result, the screen undergoes successive expansion and contraction as a result of which distorted images may be printed.
An object of this invention is to provide in an electrostatic printing system an improved screen support for maintaining screen dimensional stability.
Another object of this invention is to provide in an electrostatic printing system a screen which remains tightly stretched over its frame even at elevated temperatures.
Still another object of this invention is to provide a novel and useful support for a stencil screen in an electrostatic printing system.
These and other objects of the invention are achieved by an arrangement in which the frame for holding the screen is equipped with an elastic flange consisting of a strip of rubber or similar elesatic material. The screen is stretched over the flange and fastened to the sides of the frame. Means are provided for raising the elastic flange so that it may be brought firmly to bear against the screen. The elasticity of the flange causes it to maintain a tension against the separate wires of the screen so that when a hot object is brought near this screen and causes the screen wires to expand, the pressure within 3,280,731 Patented Oct. 25, 1966 "ice the elastic flange acts individually on each wire of the screen, allowing elongation of the wires to take place, yet maintaining essentially equal and suflicient tension, in the individual wires so that the screen does not tend to warp or deform.
The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:
FIGURE 1 is an isometric view of an embodiment of the invention;
FIGURE 2 shows a cross-section taken along the line 2-2 of FIGURE 1.
FIGURES 3 and 4 are also sections taken along the line 3--3 of FIGURE 1. They respectively show the characteristics of the elastic flange strip under different conditions of strain deformation.
Reference is now made to FIGURE 1 where there may be seen an isometric view of an embodiment of the invention. It consists of a frame 10 over which the stencil printing screen 12 is stretched. The screen is securely fastened to the sides of the frame 10 by a bonding strip 14. An elastic flange strip 16 is held in a channel which is grooved on the face of the frame so that it maintains tension against the screen 12.
FIGURE 2 is a cross-section taken along the line 2-2 of FIGURE 1. It illustrates further details of the construction of the device shown in FIGURE 1. The elastic flange strip 16 is set in a channel 18 which extends around the perimeter of the frame facing the screen. Elastic strip 16 is bottomed against a series of individual metal strips 20 which are located at the bottom of channel 18. These individual metal strips 20 may be pushed upward by turning set screws 22. Access to the individual set screws is gained through access holes 24 located on the underside of frame 10. As each set screw 22 is turned, each metal strip 20 pushes a section of the elastic strip 16 upward against the screen 12. This elastic strip is deformed so that it exerts a constant tension on metal screen 12.
FIGURE 3 shows a fragmentary cross-section of the device taken through the elastic flange along the line 33 of FIGURE 1, before the set screws have been tightened and FIGURE 4 shows the same cross-section after the set screws have been tightened.
After the metallic screen 12 has been securely bonded to the frame 10 by bonding strip 14, the metallic strips 16 lying in the channel underneath the rubber strip are each raised by each set screw to thereby bring the rubber firmly to bear against the metallic screen all around the frame. Rubber has the property of being essentially non-compressible, but as it is brought to bear against the tightly-stretched screen, it deforms in the manner shown in FIGURE 4. This deformation causes the rubber strip to be under considerable pressure throughout its volume. Subsequently, when a hot object is brought near this screen and thereby causes the screen wires to expand due to the positive thermal-coefiicient of expansion of the metal screen, the pressure within the rubber acts individually on each wire of the screen, allowing elongation of the wires to take place, yet maintaining essentially equal and sufiicient tension on the individual wires so that the screen does not tend to warp or deform.
The set screws are adjusted so that the initial deformation of the rubber is suflicient to compensate for the total elongation the wire will undergo when in proximity to the hot object on which electrostatic printing is performed. The rubber acts in the manner of a large number of individual compensating devices that maintain contemperature. This is analogous to a system that can tension each wire by individual and separate spring tensioning devices, an operation which is otherwise clearly not possible since a screen may have on the order of 200 wires per inch, for example.
' There has accordingly been described and shown herein an improved screen structure for use in an electrostatic printing system in which deformation of the screen does "not take place at high temperature because constant'tension is maintained on .the individual wires of the screen. This invention makes it possible to use such an electrostatic printing process to print upon objects at elevated I temperatures.
" Although in the description of the embodiment of the invention rubber is described as being the elastic material, it is to be understood that other elastic materials with suitable characteristics may be considered for maintaining tension on the individual Wires of the screen without departing from the scope and spirit of this invention.
1. An electrostatic printing screen assembly comprising frame means, a groove cut in the perimeter of said frame means, a rubberstrip extending out of said groove, an image forming screen, means holding said screen stretched over said rubber strip and fastened to the sides of said frame means, and a plurality of means in the bottom of said groove for adjusting the pressure of incremental lengths of said rubber strip against said screen, said incremental lengths being adjacent to one another-and extending around the length of said elastic flange means. 7
2. An electrostatic printing screen assembly as recited in claim 1 wherein each of said plurality of means for adjusting the pressure of incremental lengths of said rubber strip comprises a rigid strip having a predetermined 7 length located in said groove underneath said rubber strip,
and an adjustment screw threaded through the portion of said frame adjacent said groove for varying the pressure of said strip against said rubber strip.
3. A printing screen assembly comprising a frame, a groove cut in the perimeter of said frame, an image forming screen, means located at the sides of said frame for holding said screen stretched over said frame, and tensioning means for maintaining said screen stretched undistorted despite differential tensioning stresses due to variations in temperature comprising a plurality of separate rigid strips extending along the bottom of said groove, set screw. means for each strip threadably mounted in said frame for individually raising and lowering each of said strips, and a flange made of a material having rubber like properties and supported by said plurality of separate rigid strips and extending out of said groove to contact said screen for maintaining said screen stretched despite variations in temperature thereof.
References Cited by the Examiner UNITED STATES PATENTS 1,277,248 7/1918 Newgass. 1,892,268 12/1932 Flockhart 101-127.1X 2,8o2,419 7/1957 Gramatzky 101 41s.1 3,081,698 3/1963 Childress et al. 10l--l14X 3,176,843 4/1965 Hoskins et a1 378X FOREIGN PATENTS 634,954 1/1962 Canada.
ROBERT E. PULFREY, Primary Examiner.
H. P. EWELL, Assistant Examiner.