Abstract:
A doctor blade head for coating a transfer roller includes a semi-cylindrical cavity to enable streamline fluid flow therethrough, and check plates at the inlet and outlet openings for permitting unidirectional flow into the inlet opening from said pump and out of the outlet opening toward the pump. An hydraulic accumulator acts as a fluid pressure and fluid velocity balancing device, and includes a rolling diaphragm piston moving in a cylinder that is connected to the inlet fluid path, with a spring impinging on the piston to absorb pressure surges and compensate pressure dropoffs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable. 
       FEDERALLY SPONSORED RESEARCH 
       [0002]    Not applicable. 
       SEQUENCE LISTING, ETC ON CD 
       [0003]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    This invention relates to doctor blade systems for applying coatings in a printing or coating process, and in particular to a new design for the doctor blade head. 
         [0006]    2. Description of Related Art 
         [0007]    In the application of liquid substances to a moving web or successive sheets of material, it is considered well known in the art to apply the liquid using a rotating transfer roller, and to directly apply the liquid uniformly onto the roller by means of a doctor blade assembly. The doctor blade assembly generally includes a reservoir chamber extending the length of the transfer roller and in contact with the circumferential surface thereof, and a pair of doctor blades extending longitudinally on either side of the chamber. The doctor blades are angled obliquely toward the transfer roller surface, and serve both to seal the reservoir chamber to the roller and to form a uniform film of liquid on the roller transfer surface. The assembly also must include some means to seal the reservoir chamber at the ends of the roller, so that the liquid is not flung from the roller into the surroundings, and so that the liquid may be pumped through the reservoir during the transfer process. Such transfer systems are used in flexographic and gravure printing, adhesive applicators for substrates such as paper or plastic, coating applicators in many different industrial processes, and the like. Exemplary system are described in U.S. Pat. Nos. 4,821,672 and 6,576,059 issued to Nick Bruno. 
         [0008]    It is apparent that the doctor blade head must provide uniform coating of the transfer roller to the utmost extent, so that the printed output is as perfect as possible. Factors that may cause defects in the liquid layer on the transfer roller may include the transfer roller itself, which is furnished with a micro-etched pattern designed to sustain the liquid film that is transferred to the printing or coating roller. The pattern may also carry air into the doctor blade cavity and cause bubbles to form in the coating liquid in the cavity, leading to defects in the coating and printing drop-outs in the final product. This effect is also exacerbated by the rotational velocity of the transfer roller and printing roller, and may limit the production speed of the printing press. 
         [0009]    Indeed, the doctor blade cavity may be viewed as a closed space having fixed side and end wall, except for the rapidly and constantly moving side wall formed by the transfer roller engaged by the doctor blade head. In the prior art the cavity is typically a flattened rectangular chamber, and the fluid flow is end-to-end through the cavity. It is quite possible for turbulence to occur within the flowing liquid, which retards the flow rate and requires higher pumping pressure to maintain the fluid flow through the chamber. Turbulence may be increased by the motion of the transfer roller surface forming one side of the cavity, again limiting the speed of the printing press. 
         [0010]    In addition to the issue of turbulence, the pump that provides the pressurized fluid to the cavity typically creates pulses of pressure, particularly since pneumatically operated piston pumps are easiest to use and maintain in a transfer coating machine. Instability in the fluid pressure may also contribute to turbulence in the fluid and an ultimate degradation in printing quality. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    The present invention generally comprises an improved doctor blade head for coating a transfer roller. The doctor blade head is provided with several salient features that enable a high velocity flow of coating liquid longitudinally through the doctor blade chamber, while requiring a lower overall fluid pressure across the chamber. Higher fluid flow rates through the chamber enables the chamber to be replenished with fresh fluid more often, and less air (foam) is introduced into the fluid from the anilox roller surface that moves across the doctor blade opening. 
         [0012]    In one aspect, the chamber is configured as a quasi-cylindrical cavity that is more similar to a round pipe than prior art designs, thereby allowing fluid flow with less restrictions (resistance) than previous chamber cavity designs. The decreased resistance increases the fluid velocity and decreases the pump pressure required to move fluid through the cavity. 
         [0013]    In a further aspect, the doctor blade head is provided with a check plate mounted in the inlet side of the chamber that allows fluid into the cavity from the cavity inlet that is connected to a pump, but does not let fluid back-flow out of the cavity through the inlet side. There is another check plate mounted in the outlet side of the chamber that allows fluid to flow out of the cavity but prevents fluid flow into the chamber from the outlet side. These check plates enable the system to maintain a very low and unchanging fluid pressure in the cavity of the chamber. They also keep the chamber cavity completely filled at all times of operation, not allowing air into the cavity from outside the chamber system, which can cause large starvation spots (dropouts) on the anilox roller. 
         [0014]    The invention also provides an hydraulic accumulator for stabilizing the pump pressure that feeds the chamber. The hydraulic accumulator acts as a fluid pressure and fluid velocity balancing device, and includes a rolling diaphragm piston moving in a cylinder that is connected to the inlet fluid path, with a spring impinging on the piston. If there is a fluid pressure spike from the chamber supply pump, it enters the cylinder though the inlet manifold, and pushes the rolling diaphragm to move outwardly in the cylinder against the spring, thus storing the energy and fluid from that pressure spike. As the fluid pressure decreases from the pump and in the chamber cavity, in between strokes, the spring pushes the stored fluid into the chamber cavity so that the hydraulic accumulator releases that energy and fluid into the chamber. The result of this that pressure spikes are attenuated and pressure dropoffs are compensated, so that there is continuous fluid flow through the chamber at a very stable fluid pressure. As the supply pump delivers more or less fluid, the hydraulic accumulator keeps the fluid pressure stable, and the chamber cavity completely filled when used in conjunction with the check-plates. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0015]      FIG. 1  is a perspective rear view of the doctor blade head of the invention, shown engaged with a transfer roller. 
           [0016]      FIG. 2  is a partially cross-sectioned perspective view of the hydraulic accumulator of the invention. 
           [0017]      FIG. 3  is a partially cross-sectioned perspective view of the doctor blade chamber outlet assembly of the invention. 
           [0018]      FIGS. 4-6  are cross-sectional elevations of the hydraulic accumulator shown in  FIG. 2 , depicting sequentially the operation of the accumulator. 
           [0019]      FIGS. 7-9  are cross-sectional elevations of the outlet check plate assembly shown in  FIG. 3 , depicting sequentially the operation of the outlet check plate. 
           [0020]      FIG. 10A  is a cross-sectional side elevation of a typical doctor blade head known in the prior art, and  FIG. 10B  is a cross-sectional side elevation of the doctor blade head of the invention. 
           [0021]      FIG. 11  is an exploded view of the hydraulic accumulator of the invention, and  FIG. 12  is an exploded view of the outlet check plate assembly of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The present invention generally comprises an improved doctor blade head for coating a transfer roller that delivers a high velocity flow of coating liquid longitudinally through the doctor blade chamber, while providing a lower, more stable fluid pressure across the chamber. As shown in  FIG. 1 , the doctor blade head  20  generally includes a channel-like structure  21  having a central web portion  22  with a plurality of mounting brackets  23  for securing the doctor blade head to a supporting framework (not shown). The head  20  includes a longitudinally extending cavity  24  ( FIG. 10B ) that has a longitudinally extending opening  26 . A pair of doctor blades  27  are secured in opposed, parallel fashion adjacent to the opening  26 , and are disposed to impinge on a rotating transfer roller  28 , whereby a film of coating fluid is applied to the roller. The roller may comprise an anilox roller or the equivalent known in the prior art. 
         [0023]    As shown in  FIG. 1 , a fluid pump  30  has its output connected through tubing or hose to an inlet assembly  31  disposed at one end of the structure  21 , and the inlet of the pump is connected to an outlet manifold assembly  32  disposed at the other end of the structure  21 , so that fluid from the pump flows the length of the cavity  24  before returning to the pump. 
         [0024]    A salient feature of the invention is an hydraulic accumulator  41  for stabilizing the pressure applied to the fluid in the doctor blade cavity  24 . The hydraulic accumulator  41  is located in the inlet assembly  31 , and is shown in  FIGS. 2 ,  4 - 6 , and  10 . With regard to  FIGS. 2 and 10 , the hydraulic accumulator is comprised of an outer housing plate  42  and an inner housing plate  43  in stacked relationship and secured by bolts to the outer surface of the web  22  of channel-like structure  21 . The housing plates are provided with cylindrical recesses  44  and  47  that are equal in diameter and axially aligned to form a closed cylindrical space. A rolling diaphragm piston  52  is entrained between the opposed faces of the plates  42  and  43 , defining a variable volume fluid chamber  56  at the inner side and an outer chamber  44  that is open to ambient pressure. A spring  45  is seated in an annular groove  46  in the recess  44  to exert a resilient force to bias the piston  52  to extend into the recess  47  of plate  43 . A fluid passage  48  extends through the plate  42  and is connected at its outer end to a standard male connector  49  for a supply tubing extending to the pump outlet. Within the plate  43  a fluid passage  53  is aligned with and joins the passage  48 , the fluid passage extending to fluid chamber  56 . 
         [0025]    The inner end of housing plate  43  is provided with a port  54  that communicates with the fluid chamber  56 . The port  54  also provides an annular seat  55  for an inlet check plate  51 , a flexible tongue that is shaped to occlude the port  54 . An inlet opening  57  is formed in the web  22  of doctor blade channel  21  in communication with the cavity  24 , the opening  57  providing a large area through which the fluid may pass so that locally generated turbulence is avoided. The opening  57  also provides space for the check plate  54  to deflect inwardly in a resilient fashion ( FIGS. 5 and 6 ) to allow fluid to enter the cavity  24  from the fluid chamber  56 . However, any retrograde flow from the cavity  24  toward the chamber  56  is blocked by the plate  54  urged to impinge on the seat  55  by the retro-flow as well as its own resilient restoring force. Thus if the input fluid pressure should falter for whatever reason, the check plate  54  prevents backflow out of the cavity  24 , an event that could, for example, potentially draw air into the system and cause starvation spots on the transfer roller. 
         [0026]    Note that bolts are used to join the housing plates to the channel web  22 , along with appropriate seals to contain the fluid, but they are not enumerated herein. 
         [0027]    The hydraulic accumulator  41  functions as shown in the sequence depicted in  FIGS. 4-6 . When fluid from the pump enters the accumulator  41  from fitting  49  and passages  48  and  53 , the fluid flows into fluid chamber  56 , as shown in  FIG. 4 . If there is a pressure spike in the fluid, it will overcome the force of spring  45  and cause the piston  52  to deflect ( FIG. 5 ) and enlarge the fluid chamber  56 , thus absorbing the pressure surge before it is transmitted to the cavity  24 . Note that the hydraulic accumulator does not interrupt the fluid flow to the cavity  24 , which continues as the check plate  51  is opened by the fluid flow advancing through port  54  and opening  57  into the cavity  24 . As the pressure spike passes, the piston  52  is urged by spring  45  to return inwardly, driving excess fluid from chamber  56  into the cavity  24 . The net result is that pressure spikes are attenuated, pressure dropoffs between pump strokes are compensated, and fluid pressure applied to the doctor blade cavity is stabilize to a high degree. 
         [0028]    A further aspect of the invention, shown in  FIGS. 3 ,  7 - 9 , and  12 , is the provision of an outlet check plate in the outlet manifold assembly  32 . The web  22  is provided with an outlet opening  67  at the end that is longitudinally opposed to the inlet assembly, the outlet opening having sufficient area and smooth surface transitions to enable fluid flow therethrough without creating backpressure or turbulence in the cavity  24 . A rectangular housing  61  is secured to the web  22 , and the housing is provided with a chamber  64  extending therethrough. At the inner end of the housing  61  the chamber  64  is aligned in flow communications with outlet opening  67 . A check plate  61  is secured within the opening  67 , the check plate comprising a flexible tongue that is shaped to occlude the opening  67 . An annular seat  65  surrounds the opening  67  and is disposed to engage the check plate  61  in a manner similar to the seat  55  and check plate  51 , except that fluid flow is blocked if retrograde into the cavity  24  but free-flowing out of opening  67 . as shown in  FIGS. 8 and 0 . At the outer end of the housing  61  a transparent window is secured and sealed at the opening of chamber  64 , providing a watch glass for visual inspection of the fluid outflow from the doctor blade chamber. A tapered end  68  protrudes from a lower side of the housing  61 , and a male tubing connector  69  extends therefrom to form a flow path from the outlet  67  past the check plate  61  and through the chamber  64 , thence out of the connector  69  to return to the pump  30 . 
         [0029]    Note that the two check plates  51  and  61  act together to maintain the cavity  24  completely filled with fluid at all times, and enable the system to run at a very low fluid pressure in the cavity, while the hydraulic accumulator regulates and stabilizes the fluid pressure in the cavity. 
         [0030]    Another important aspect of the invention is the shape of the cavity  24  of the doctor blade head  20 . With reference to  FIG. 10A , a typical chambered doctor blade head known in the prior art is provided with a cavity  71  that is generally shaped as a flattened rectangle, with inlet and outlet connections  72  and  73  that open to the cavity in directions that are essentially transverse to the fluid flow along the longitudinal length of the cavity (parallel to the transfer roller axis). As a result turbulence may occur, requiring higher pump pressure and a wider opening between the doctor blades  77  in order to assure complete coating of the transfer roller surface. 
         [0031]    In contrast with the prior art, the doctor blade head  20  of the invention ( FIG. 10B ) provides a cavity  24  that is configured as a quasi-cylindrical cavity that is more similar to a round pipe, thereby allowing a more streamline fluid flow with less restrictions (resistance) than previous chamber cavity designs. Note also that the opening  26  between the doctor blades  27  is substantially narrower than prior art devices; i.e., subtending an angle about the transfer roller axis that is as little as half the angle subtended by prior art devices ( FIG. 10A ). This reduction in contact area, made possible by the hydraulic accumulator  41  and check plates  51  and  61 , reduces vibration between the head and the roller and facilitates the application of a uniform coating. 
         [0032]    The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching without deviating from the spirit and the scope of the invention. The embodiment described is selected to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as suited to the particular purpose contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.