Abstract:
A dual pressure preload system preloads a doctor blade assembly in intimate contact with an anilox roll. The doctor blade assembly is attached to at least one end wall of an ink chamber. The ink chamber directly applies ink to the circumferential surface of the roll. A pressure chamber is attached to the ink chamber on the side opposite the roll and maintains the doctor blade assembly adjacent the roll. A differential pressure is produced by the pressure chamber providing a consistent preload force applied to the anilox roll by the doctor blade assembly. 
     In one embodiment, a relatively compressible fluid, such as a gas, is compressed, which, in turn, acts on a relatively incompressible fluid, such as a liquid, the relative incompressible liquid supplying the pressure in the pressure chamber. In another embodiment, a clamp provides a biasing force, independent of the pressure chamber, for independently preloading the doctor blade assembly in intimate contact with the anilox roll, in the event of a failure of the pressure chamber.

Description:
FIELD OF THE INVENTION 
     This invention is directed to a dual pressure preload system for controlling the force applied to a member, and more specifically a preload system for maintaining a doctor blade assembly in intimate contact with an anilox roll by preloading the blade assembly with a preselected force. 
     BACKGROUND OF THE INVENTION 
     Flexography, also referred to as flexographic printing, is a rotary letterpress printing process, traditionally using flexible elastomeric printed rolls and fast-drying inks. Flexographic printing is used for printing flexible packaging materials, including paper and plastic films, multiwall bags, corrugated containers, envelopes and paper-back books. The printing apparatus usually encompasses a reverse-angle doctor blade assembly adjacent an anilox metering roll. The anilox roll typically has approximately 100,000 cells per square inch of surface area dispersed on its outer surface. The ink distribution system operates by flooding the anilox roll with ink, thus flooding the cells on the roll&#39;s surface. As the anilox roll rotates, the reverse-angle doctor blade shaves the surplus ink flush with the surface of the cells. The result is a uniformly metered ink film applied by the anilox roll to the surface of the printing roll. 
     A pressure preload system for a flexographic printer usually places a doctor blade assembly in intimate contact with the outer surface of an anilox roll to control the amount of wiping action applied to the roll&#39;s surface. The doctor blade assembly is usually attached to the end wall of an ink chamber located adjacent the circumferential surface of the anilox roll. 
     A common problem associated with prior art preload systems is the difficulty in applying and maintaining a sufficient force to the doctor blade assembly so that the doctor blade is maintained in intimate contact with the outer surface of the anilox roll. It is crucial that the proper amount of wiping action be applied to the surface of the roll to prevent ink leakage and to achieve the desired printing quality resulting from a uniform application of ink to the printing roll, without causing excessive doctor blade or roll surface wear from unnecessarily large loading forces. 
     One type of known preload system employs a series of mechanical hand screws which are manually adjusted by an operator to apply a desired force to a doctor blade assembly. The screws are located on the assembly supporting the blade and apply a fixed force to the assembly. A major drawback of this system is that the screws must constantly be manually readjusted by a human operator during operation of the printing press. If the operator incorrectly sets the pressure or there is a sudden change in the conditions of the press, there is a likelihood of premature blade failure or inconsistent wiping action on the roll. There is also an obvious danger of injury to an operator who must adjust the screws while the press is operating. 
     A second type of known preload device applies a preload force to the doctor blade assembly by using conventional inflatable air pressure tubes or cylinders which must be manually preset by an operator to apply the desired preload force to the doctor blade assembly. The air pressure is normally supplied to the cylinders by the printing plant&#39;s in-house air pressure system. Typical air pressure systems are regulated to 50 psi or more. However, only a relatively low pressure is required to provide a preload force sufficient to maintain the doctor blade assembly in intimate contact with the roll. Low pressure air systems, however, are not practical. Inherent in low pressure air supply systems are the difficulties involved in supplying and maintaining a low pressure. Moreover, because air is easily compressible and does not readily transmit compressive forces, there is a risk that the air pressures to be applied to the blade loading system will result in widely variable preload forces on the doctor blade assembly, leading to inconsistent wiping action, excessive wear or ink leakage. Furthermore, if there is a momentary loss or reduction of the plant air pressure, there is a major risk of ink leakage or, in the case of air pressure failure, the ink chamber moving away from the roll resulting in ink flooding. Inconsistencies in the wiping action and blade failures can result in poor quality printing and lower graphic capabilities. 
     There is a need for a simple but effective system for applying a preload force to a doctor blade assembly that avoids these problems. The present invention fills that need. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a dual pressure preload system for applying a preload force to a member. The preload system is preferably used to preload a doctor blade assembly against an anilox roll, but is not strictly limited to this particular application. The preload system can be used in other printing apparatus or any apparatus necessitating the preloading of a member. 
     In its broadest aspect, the invention is directed to an apparatus for applying to a member a preload force having a selected magnitude and direction. A first force is generated having a component in a preselected direction. A second force is generated having a component in a direction opposite to that of the first force. The magnitudes of the first and second force components are greater than the magnitude of the preload force, and the first force component is greater than the magnitude of the second force component. The preload force is generated as the net vector sum of the first and second force components. 
     In a preferred embodiment, the invention is directed to a preload system for applying a preload force to a doctor blade assembly to maintain the blade in intimate contact with an anilox roll. An ink chamber having an opening on one side is positioned adjacent a portion of the circumferential surface of an anilox roll along the roll&#39;s longitudinal axis. The opening of the ink chamber faces the roll and contains an ink supply. A doctor blade assembly comprising at least one doctor blade is mounted on at least one end wall of the opening of the ink chamber. The free edge of each doctor blade is in intimate contact with the tangential surface of the roll. A pressure chamber attaches to the ink chamber on the side opposite the anilox roll. A reciprocally-movable piston, having a front and back surface, divides the pressure chamber into two smaller compartments. One end of a rod is attached to the back surface of the piston. The first compartment is pressurized to a first preselected pressure, and the second compartment is pressurized to a second preselected pressure. The second pressure is greater than the first pressure. The first and second pressures generate a pressure differential between the two compartments. A net preload force in the direction of the roll is provided by the pressure differential, and is transmitted to the doctor blade assembly to maintain it in intimate contact with the anilox roll. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
     FIG. 1 is a top plan view of a preferred embodiment of an ink metering system employing the invention as described herein. 
     FIG. 2 is a partial sectional view of the preferred embodiment taken along line 2--2 of FIG. 1, and including a schematic representation of certain features of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 and 2 illustrate an apparatus 10 for applying a preload force to a doctor blade assembly. The apparatus 10 comprises an anilox roll 12 mounted for rotation about its central axis, such as by an axle 14. An ink chamber 16, as described below, is positioned adjacent a portion of the roll&#39;s circumferential surface and preferably extends substantially parallel to the longitudinal axis of the roll 12. The ink chamber preferably, but not necessarily, extends along the entire length of the roll 12. Two end seals 18 are positioned at either end of the chamber 16 and preferably are in intimate contact with a portion of the end surfaces of the roll 12 to prevent ink leaking from the ink chamber 16 at the ends of the roll 12. 
     As seen more clearly in FIG. 2, one side of the ink chamber 16 has an opening which faces the anilox roll 12 and contains an ink supply 17. A doctor blade assembly 22, which extends preferably substantially the length of the ink chamber 16, is attached to at least one end wall 20 of the ink chamber 16 by any suitable means, such as, but not limited to, being welded or bolted on. The doctor blade assembly 22 preferably, but not necessarily, comprises a pair of doctor blades. The doctor blades 22 are angled toward one another with respect to the tangential surface of the roll 12 so that one edge of each blade is in intimate contact with the roll 12. 
     A cross tie 24 parallel to the circumferential surface of the roll 12 supports a series of pressurized chambers 26. Three pairs of pressurized chambers 26 are evenly spaced along the length of the cross tie 24. The pressurized chambers 26 apply a preload force as described below, along the length of the ink chamber 16 to maintain the doctor blade assembly 22 in intimate contact with the roll surface to provide proper wiping action. Each pressurized chamber 26 is coupled to the ink chamber 16 by a clamp 28. The clamp is preferably, but not necessarily, a conventional scissor clamp. The clamp 28 is pressurized by a pressure source which is preferably independent of the air source to the pressurized chambers 26. The pressure source (not shown) is preferably an air source, but can be any suitable pneumatic or hydraulic source. The clamp 28 also serves to insure that once preloaded against anilox roll 12, the doctor blade assembly 22 will maintain intimate contact with the roll in the event that a plant air pressure failure occurs. 
     Inside each pressurized chamber 26 is a double acting piston and rod assembly 29. The rod 32 is preferably attached to one side of a piston 30 having a front and back surface, such that the rod 32 is perpendicular to the piston&#39;s back surface. The rod 32 extends from the piston surface through the clamp 28 until it is flush with the outer wall of the ink chamber 16. The piston 30 is reciprocally movable and divides pressurized chamber 26 into two separate compartments 34, 36. As the piston 30 moves, it changes the area of each compartment 34, 36. Each compartment 34, 36 is pressurized by its own pressure source produced by first and second pressure cylinders 38, 40 respectively. The pressure cylinders 38, 40 are preferably air-oil cylinders, but could be virtually any type of pressure cylinder, including, but not limited to, a purely pneumatic or hydraulic cylinder. The oil is located in the lower portion of the cylinders 38, 40 and flows into both compartments 34, 36 of the pressurized chamber 26 through inlets 39, 41. The use of a hydraulic fluid such as oil provides a relatively incompressible fluid, which in turn provides an immediate and uniform force in the direction of the doctor blades 22. A hydraulic system is preferred over a purely pneumatic assembly which has a great deal of compliance and could provide only a delayed and variable force, due to a pneumatic fluid&#39;s compressible nature. 
     An air supply 46 supplies air to each pressure cylinder 38, 40. The air supply 46 is preferably the printing plant&#39;s air pressure system, but pressurized air can be supplied from any suitable source. Located between the air supply 46 and each pressure cylinder 38, 40 are conventional pressure regulators 42, 44. The air sent to the regulators is monitored by pressure gauges 48, 50. The pressure regulators 42, 44 are arranged in parallel so that a desired air pressure is applied to both pressure cylinders 38, 40. Because both cylinders 38, 40 are supplied by a single air source 46, the pressure applied to each cylinder 38, 40, will be approximately the same. If there is a change in the pressure being supplied to the regulators 42, 44 there will be a corresponding proportional drop in the pressure sent to each cylinder 38, 40. The regulators are adjusted so that there will be a small pressure differential between the pressure received by the first cylinder 38 and the second cylinder 40. Because of this relationship between the first and second cylinders 38, 40, a constant pressure differential value can be maintained between the first and second compartments. For example, a pressure differential of 5 psi could be maintained between the pressure cylinders regardless of the actual individual air pressures received by each cylinder 38, 40. 
     The pressure differential is necessary to produce a net preload force against the anilox roll. Since the rod 32 is attached to one side of the piston, the areas of the piston surfaces are not equal. Defining the piston surface facing the first compartment 34 as the front surface, and the piston surface facing the second compartment 26 as the back surface, the presence of the rod 32 results in the piston having a smaller back surface area relative to the front surface area. In order to produce a net preload force in the direction of the anilox roll, different pressures must be applied to each piston surface. As is well known, the force applied to the piston&#39;s surface is proportional to the pressure operating on each piston surface multiplied by the piston surface area. A greater pressure must be applied to the piston&#39;s back surface relative to the piston&#39;s front surface to produce the desired pressure differential resulting in a net preload force acting against the anilox roll. 
     The resulting net preload force produced in the direction of the roll 12 causes the doctor blades 22 to be in intimate contact with the roll 12. The air-oil cylinders 38, 40 provide a substantially incompressible system which is capable of transmitting an evenly applied force from a relatively small net pressure. In this case, the net force is the result of a pressure differential produced in the pressure chamber 26 which is sufficient to maintain the doctor blades 22 in intimate contact with the roll 12. 
     The use of a pressure differential to preload the doctor blades 22 allows a high air pressure to be used to achieve a low net force by reverse pressurizing one side of the cylinder. Adjustment to the system can be factory preset so that no operator air adjustments will be required. 
     Because the pressurized chambers 26 are uniformly spaced along the outer wall 21 of the ink chamber, a constant force is applied to the doctor blade assembly 22. 
     The force to be applied to the doctor blade assembly 22 can be determined from the general relationship 
     
         F.sub.C =(P.sub.1 A.sub.1 -P.sub.2 A.sub.2)N               (1) 
    
     in which 
     P 1  =first pressure 
     A 1  =area of cylinder piston 
     P 2  =second pressure 
     A 2  =area of effective piston-rod end 
     N=number of cylinders 
     F c  =total pre-load force. 
     The total preload force can also be determined by the following equation: 
     
         F.sub.c =2(l)F.sub.B sin θ                           (2) 
    
     wherein 
     l=blade length 
     F B  =axial blade load/inches of length 
     N=number of cylinders 
     Θ=the angle between the y component of the blade force and the resultant component of the blade force as seen in FIG. 2. 
     By substituting equation (2) into equation (1), the second pressure P 2  can be calculated, given the known desired preload force, first pressure and Θ=13° as follows: 
     
         P.sub.2 =P.sub.1 A.sub.1 /A.sub.2 -2(l)F.sub.B sin13°/ NA.sub.2 (3) 
    
     The above described arrangement provides a constant pressure differential between the first and second pressure compartments 34, 36 regardless of the pressure transmitted by the air supply 46. This results in a constant force being applied to the doctor blade assembly 22 which provides consistent wiping action to the roll 12 and allows for better overall printing quality and higher graphic capabilities. 
     The ink metering system operates in the following manner. As an illustrative example, values have been given to various parameters such as the pressure and the radii of the piston and rod. However, these values are not intended to limit the scope of the invention. A common air supply 46 provides an air pressure to first and second pressure regulators 42, 44 placed in series relative to one another. Each pressure regulator 42, 44 is connected to first and second pressure cylinders 38, 40. The pressure regulators 42, 44 are adjusted so that a greater pressure is transmitted to the second pressure cylinder 40 relative to the first pressure cylinder 38. The first and second pressure cylinders 38, 40 are connected to a pressure chamber containing a piston and rod assembly 29. The slidable piston 30 divides the pressure chamber 26 into two smaller compartments 34, 36 with the rod end being connected to one of the piston surfaces and contained within the second compartment 36. 
     The second compartment 36 is pressurized by the second cylinder 40 at a pressure of 54.7 psi, for example, and the first compartment 34 is pressurized by the first cylinder 38 to a pressure of 50 psi. Due to the presence of the rod 32 on one side of the piston surface, the area of the piston surface in the second compartment 36 is smaller than the area of the piston surface in the first compartment 34 by the surface area of the rod end. When the pressure chamber 26 receives the pressures produced by the first and second pressure cylinders 38, 40 a pressure differential is created inside the pressure chamber. 
     If the radius of the piston surface was 1 in for example, then the area of the front surface area would be the following: ##EQU1## wherein r 1  =piston radius 
     Likewise, if the radius of the rod is 0.3125 in. then the area of the expose back surface area of the piston is as follows: ##EQU2## wherein r 2  =piston radius 
     r rod  =rod radius 
     The resulting forces acting on each piston surface are as follows: ##EQU3## 
     By substituting equations (6) and (7) into equation (1) the resulting preload force is as follows: 
     
         F.sub.C =(F.sub.1 -F.sub.2)N 
    
     If N=6, as shown in the drawings, 
     
         F.sub.C =(157.08-155.05)×6                           (8) 
    
     
         F.sub.C =12.18 lb 
    
     As can be seen from the example the resulting preload force is much smaller than the forces in either compartment. 
     As described above, the net preload force is applied by rod 32, which is attached to the outer wall 21 of the ink chamber. A series of pressure chambers 26 are linearly distributed across the chamber&#39;s outer wall 16. Attached to each end wall of the ink chamber 16 is a doctor blade 22. The ink chamber 16 is adjacent an anilox roll 12 with the edge of the doctor blades in intimate contact with the roll&#39;s surface. The force produced by the pressure chamber 26 is applied to the outer wall of the ink chamber 16 which places a constant preload force on the doctor blades to maintain the blades in intimate contact with the roll. 
     In the event of plant pressure failure, safety features are placed in series with the second pressure cylinder 40. A check valve 52 prevents the back flow of hydraulic fluid in the event of plant pressure failure and during normal printing operations. A restrictor valve 54 retards the flow of hydraulic fluid in the event of a plant failure to delay the inevitable loss in pressure which occurs as a result of the failure. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.