Abstract:
A casting roller having a variable temperature surfaces comprises a rotatable cylindrical shell ( 12 ). Axially aligned heating electric elements ( 14 ) are equally spaced and internal to an outer surface of the rotatable cylindrical shell. A brush assembly ( 16 ) is in electrical contact with the heating elements during a portion of the rotatable cylindrical shell rotation about an axis. A stationary core ( 26 ) is internal to the rotatable cylindrical shell. An annular space is between the stationary core and the rotatable cylindrical shell. A cooling fluid fills ( 22 ) at least a portion of the annular space.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     Reference is made to commonly-assigned copending U.S. patent application Ser. No. 10/795,010, filed Mar. 5, 2004, entitled COMPLIANT PRESSURE ROLLER WITH UNIFORM NIP PRESSURE, by Bomba et al.; AND U.S. patent application Ser. No. 10/889,561, filed Jul. 12, 2004, entitled AXIALLY COMPLIANT PRESSURE ROLLER UTILIZING NON-NEWTONIAN FLUID, by Richard D. Bomba; the disclosures of which are incorporated herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates in general to casting rollers and in particular to a casting roller having a radial variable surface temperature.  
       BACKGROUND OF THE INVENTION  
       [0003]     In extrusion and embossing operations the surface temperature of the rollers contacting the molten material or web material is an important process parameter. Typical roller designs provide single roller bulk temperatures as established with internal circulation of a fluid media. The maximum temperature is normally limited to a value in which the material can be easily stripped from the roller surface.  
         [0004]     One attempt to solve this problem can be found in expired prior art, U.S. Pat. No. 2,526,318, which vaguely describes a configuration providing two regions of variable temperature but provides no detail of design criteria or performance capabilities. Subsequent prior art, U.S. Pat. Nos. 5,945,042; 6,260,887; and 6,568,931 similarly describe a method to provide more than one temperature region around the circumference of a roller but provide no detail of design criteria or performance capability. Prior art U.S. Pat. No. 6,554,755 describes a roller design with the ability to provide a localized region of temperature difference, but the sole embodiment of this method is to create a means of compensating for shell deflection.  
         [0005]     At the contact point of either the molten material or web a region of higher temperature is desirable to improve contact between the materials and the roller surface and to improve replication of the roller surface but this temperature is normally too high to allow stripping the material from the roller surface. Therefore, a lower surface temperature is required at the stripping point, which limits the wetting of the roller surface and pattern replication.  
         [0006]     The purpose of this invention is to provide at least two regions around the periphery of a roller with sufficient temperature difference to provide improved wetting and replication in one region and allow for uniform stripping of the material from the second region.  
       SUMMARY OF THE INVENTION  
       [0007]     Briefly, according to one aspect of the present invention a casting roller having a variable temperature surfaces comprises a rotatable cylindrical shell. Axially aligned heating electric elements are equally spaced and internal to an outer surface of the rotatable cylindrical shell. A brush assembly is in electrical contact with the heating elements during a portion of the rotatable cylindrical shell rotation about an axis. A stationary core is internal to the rotatable cylindrical shell. An annular space is between the stationary core and the rotatable cylindrical shell. A cooling fluid fills at least a portion of the annular space.  
         [0008]     The present invention provides detailed design criteria and expected performance capabilities based on finite element analysis to investigate the effect of roller diameter, shell thickness and materials of construction  
         [0009]     The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a cross-sectional view of a casting roller according to the present invention.  
         [0011]      FIG. 2  is a perspective view, partially in section, of another embodiment of a casting roller according to the present invention.  
         [0012]      FIG. 3A  is a cross-sectional view of yet another embodiment of a casting roller according to the present invention.  
         [0013]      FIG. 3B  is a perspective view, partially in section, of the embodiment shown in  FIG. 3A .  
         [0014]      FIG. 3C  is an enlarged cross section view of the shell and support shoe of the embodiment shown if  FIG. 3A .  
         [0015]      FIG. 4  is a chart showing the calculated cooling efficiencies of various roller design configurations according to the present invention.  
         [0016]      FIG. 5  is a chart showing the calculated reheating efficiencies of various roller design configurations according to the present invention.  
         [0017]      FIG. 6  is a cross-sectional view of a pattern on the surface of a roller.  
         [0018]      FIG. 7  is a chart of temperature versus pressure showing pattern replication.  
         [0019]      FIG. 8  is a chart showing calculated shell deformation at maximum temperature.  
         [0020]      FIG. 9  is a chart showing calculated shell deformation for shell subject to a nip pressure and a supported by an internal pressure gradient.  
         [0021]      FIG. 10  is a cross section of an axially compliant pressure roller forming a nip with at second roller. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     The present invention will be directed in particular to elements forming part of, or in cooperation more directly with the apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
       Embodiment 1  
     Electrically Heated Shell With Fluid Media Cooling  
       [0023]     The outer shell  12  of the roller is machined to accommodate a series of electrical heating elements  14  closely spaced, near the outer surface of the shell. A brush assembly  16 , mounted to the roller, is utilized to provide electrical power only to heaters in the desired heating zone  18 . The internal surface  20  of the outer shell  12  would be exposed to a fluid media  22  to remove the heat added by the heating elements and to continue to remove heat from the process material. The fluid media inlet  32  is attached to the roller by a commercially available rotating joint. The diameter of the shell is based on desired heating dwell time, final cooling temperature, line speed, and materials of construction. The cooled region  24  removes heat from the outer shell  12  and the cast material  86 , shown in  FIG. 1  and  FIG. 10 . The fluid media  22  is circulated in the annular region beneath the internal surface  20 .  
       Embodiment 2  
     Roller With Two Surface Temperature Zones Created With Thermal Fluid Media At Different Temperatures  
       [0024]     Referring to  FIG. 2 , the outer shell  46  rotates about a fixed inner shell (stator)  40  on carbon bearings  54 . A carbon seal  52  is mounted adjacent to the bearings to prevent fluid leakage. The stator is machined with at least two separate flow passages  42 . A close fitting baffle  44  between the stator  40  and the outer shell  46  creates a boundary between the fluid streams. Higher temperature fluid  47  is circulated through one part of the stator  40 . Contact with the outer shell  46  as it passes over this region heats the shell to the desired process temperature. The second region is maintained at a lower temperature by circulating the low temperature fluid  49 . The stator  40  is designed to minimize contact points between the higher and lower temperature zones.  
       Embodiment 3  
     Thin Shelled Roller With Internal Support And Two Different Temperature Regions  
       [0025]     This embodiment is similar to the embodiment above in that the inner shell is fixed, outer shell rotates about inner shell and two temperature zones are created by thermal medium circulation. This embodiment utilizes a thin outer shell  70  supported internally by a series of pivoting loading shoes  72 . The loading shoes  72  are independently adjustable to change the force exerted on the outer shell  70  and to compensate for deformation of the outer shell due to thermal and mechanical loads. The thermal medium circulates within the region containing the shoes and is baffled at two points by a first axial baffle  74  and a second axial baffle  76  to create a higher temperature region and a cooling region. This heat transfer medium may also serve as a hydrodynamic lubricant between the shoe and the inner surface of the shell.  
         [0026]     The thin outer shell  70  is an advantaged in this application for it provides a more thermally responsive system that can heat and cool more quickly. This can also be realized as a smaller diameter for equivalent line speed. The smaller diameter translates into less work to create patterned surface and a higher contact stress in the nip for a given load.  
         [0027]     Referring to  FIGS. 3A, 3B , and  10 , the roller consists of at least three main regions. In this representation, the outer shell  70  rotates in a counter clockwise direction with respect to these regions. The cooling region  41  creates a series of flow passages  42  exposed to the inner surface of the shell  25 . High temperature fluid  2  supply  60  impinges on the inner surface of the shell to remove heat. High temperature fluid  2  return collects the fluid from the flow passages  42  and directs the flow out of the roller. A close fitting baffle  44  is positioned at the entrance to the cooling region  41  to provide a separation of fluids between the cooling region and the load shoe region  45 . A first axial baffle  74  is located at the exit of the cooling region  41  to separate fluids between the cooling region  41  and the reheating region  43 . Similar to the cooling sector  41 , the reheating sector  43  is a flow distribution sector in which high temperature fluid  1  supply  56  impinges fluid on the inner surface of the shell with fluid removal through the high temperature fluid  1  return  58 . A second axial baffle  76  is mounted at the exit of this region to separate this region from the loading shoe region  45 . The purpose of this sector is to increase the shell temperature prior to entering the nip point  35 .  
         [0028]     The loading shoe region  45  consists of an axial member machined to form pockets  29  along the length to accommodate each of the loading shoes  72 . The force applied to each loading shoe is independently adjustable by means of the shoe loading mechanism  78 . A manually controlled worm screw adjusting mechanism has been shown but is not limited to this implementation. High temperature fluid  3  supply  48  is located at the inlet of the loading shoe  72  to shell interface to maintain continuous supply of fluid for the hydrodynamic lubrication at this point. High temperature fluid  3  return  50  collects excess fluid for removal from the roller. Each of these regions is arranged around the periphery of the fixed inner shell  40  and constrained at a central point to accommodate differential thermal expansion. The fixed inner shell  40  is rigidly attached the machine structure through a support bracket  80  on each end. The outer shell  70  is constrained to rotate about the fixed inner shell  40  through bearings  28  mounted on each end. A seal  30  is adjacent to the bearings  28  to prevent leakage of the fluid from the inner flow chambers. A drive sprocket  82  can be used to rotate the shell to overcome nip forces and internal frictional forces.  
         [0029]     Each of these embodiments can be described in terms of dimensionless parameters that are based on the physical properties of the cast material to be processed, the desired process conditions, the physical properties of the roller and the desired manufacturing rates. Heat transfer textbooks describe transient heat transfer using similar methods, but no direct method is taught for a design considering a combination of cast materials and roller design criteria subject to more than one heat transfer process.  
         [0030]     A dimensionless temperature ratio, theta, can be formed based on the following parameters; T infinity , which is described as the bulk temperature of the heat transfer media. T initial , which is defined as the initial temperature of the roller shell or cast material depending on the particular process function and T end , which is defined as the desired temperature at the end of a given process operation. A smaller value of this ratio indicates a higher efficiency in achieving desired process goals.  
           θ   TR     ⁢     :       =         T   end     -     T   infinite           T   initial     -     T   infinite             
 
         [0031]     Another dimensionless parameter can be formed, dimensionless time, commonly referred to as tau in open literature. This parameter normalizes the time dimension by utilizing the thermal diffusivity of the material and the thickness.  
         τ   ⁢     :       =         t   dwell     ·   α         (       t   shell     2     )     2           
 
         [0032]      FIG. 4  shows a chart in which the dimensionless temperature ratio, theta, is plotted on the ordinate and dimensionless time is plotted on the abscissa. The family of curves is of the exponential form as shown in the equation below:  
             Θ   t     ⁡     (     x   ,   k   ,   h   ,   t     )       ⁢     :       =     P   ⁢           ⁢   1   ⁢           ⁢   s   ⁢           ⁢       1   0     ·     ⅇ       -     (     x   -       K   mat     ·     k     h   ·   t           )         1   -       K   mat     ·     k     h   ·   t                         
 The coefficients shown in this equation are fitted to the finite element results of various process simulations and related to physical properties of the cast material, roller geometry and physical properties of the roller. The subscripts of the fitted curves end with the letters cl. The subscripts on the curves al, steel, ag, and cu indicate shell materials of construction; aluminum, steel, silver and copper respectively. In addition, the subscript values of 0625 and 125 indicate shell thickness values of one sixteenth of an inch (0.00158 m) and one eight of an inch (0.0031 m) respectively. The calculations are based on a heat transfer coefficient, h, of 350 Btu/(hr*ft 2 *° F.) (1990 watt/(m 2 *° C.) and cooling a commercially available plastic material with a thermal diffusivity of 0.005 ft 2 /hr (0.00000013 m 2 /s). 
 
         [0033]     The roller design can be determined from these equations for a particular set of requirements. Defining a dimensionless temperature ratio based on the temperature of each zone, heat transfer media temperatures and molten material temperature, the chart of  FIG. 4  can be use to either determine a process efficiency for a given operating condition and roller configuration or for a chosen process efficiency an operating speed can be determined for a given roller diameter, shell thickness and material of construction.  FIG. 5  shows a chart plotting dimensionless temperature ratio, theta, against dimensionless time, tau, for shell reheating resulting from finite element calculations. The subscripts follow the same convention as denoted for  FIG. 4 . The calculations are based on an internal heat transfer coefficient, h, of 350 Btu/(hr*ft 2 *° F.) (1990 watt/(m 2 *° C.) and an external heat transfer coefficient of 3 Btu/(hr*ft 2 *° F.) (17.03 watt/(m 2 *° C.).  
         [0034]     Referring now to  FIG. 10  an axially compliant pressure roller is referred to in general by numeral  10 . Axially compliant pressure roller  10  is comprised in general of a stationary inner core  26  and a plurality of loading shoes  72  which are pivotally mounted to the stationary inner core  26 . A series of non-magnetic dividers create a plurality of annular chambers and each of the loading shoes  72  occupies one of the annular chambers.  
         [0035]     Referring to  FIGS. 3A, 3B , and  3 C loading shoe  72 , which is eccentrically mounted, is shown. A pivot point  15  and shoe adjusting pin  17  are attached to loading shoe  72 . A non-magnetic, metallic material is used in the construction of the loading shoe  72 , but the present invention is not limited to this embodiment. The curved surface  33  of loading shoe  72 , has a curvature that is slightly smaller than the curvature of the inner surface of the thin walled outer shell  70 . This creates a converging cross section at the interface between these components.  
         [0036]     In  FIG. 10 , the axially compliant pressure roller  10  comprises a non-rotating stationary core  26 , which is the main support structure for the axially compliant pressure roller  10 . A non-magnetic, metallic material is used in the construction of the stationary core  26 , but the present invention is not limited to this embodiment. The stationary core  26  has a cylindrical form in which axial holes  27  have been provided. At least one of these holes is used to house the magnetic field generator  37 . In the preferred embodiment one magnetic field generator  37  is associated with each of the plurality of loading shoes  72 . This allows for local adjustments to the thin walled outer shell  70 . In an alternate embodiment a magnetic field generator  37  may be located in each of the plurality of loading shoes  72  as shown in  FIG. 3C .  
         [0037]     Axial holes  27  are used for the circulation of heat transfer media within the core. A series of pockets  29  are created in a radial direction to serve as supports for the loading shoes  72 . Seats on the stationary core  26  enable mounting of bearings  28  and fluid seals  30 .  
         [0038]     In operation, the hydrodynamic effect of a viscous fluid subject to the shear stress created by the relative velocity of the thin walled shell with respect to the loading shoe, develops a pressure profile within the converging region of viscous fluid  11 . This pressure acts on the thin walled shell curved inner surface of shell  25  and the curved surface of the shoe  33 . The pressure acting on the shoe results in a force normal to the curvature at the center of pressure. This force is resisted by the spring preloading force acting on the loading shoe  72 . The pressure acting on the rotating thin walled outer shell  70  creates an internal force on the shell. The net difference in force acting on the shell from the internal hydrodynamic action and the external nip force will result in a localized deformation of the thin walled shell in this region.  
         [0039]     A thin walled shell of small shell diameter is possible with this embodiment because the structural design of the shell is not dictated by beam bending criteria or shell crushing criteria. The wall thickness of the shell can be significantly thinner because the surface of the shell subjected to the external nip force is directly supported internally by the pressure created by the interaction of the magneto-rheological fluid (not shown) and the loading shoe  72 .  
         [0040]     The thin walled outer shell  70  is constrained with bearings  28  to rotate about the stationary core  26 . The rotation of the shell can be imparted by the friction force at the nip point  35 , shown in  FIG. 10 , or with an external drive mechanism as shown by drive sprocket  82 . Along the curved inner surface of shell  25 , for a given convergent interface, relative velocity, and fluid viscosity a uniform pressure is developed. The annular chambers in conjunction with the loading shoes  72 , magneto-rheological fluid, and axially variable magnetic field generator  37  can be subjected to variable hydrodynamic pressure forces by changing the viscosity of the fluid. The ability to exert axially variable pressure along the thin walled shell results in localized deformation changes of small magnitude and at a much higher frequency than possible by other prior art.  
         [0041]      FIG. 8  shows the results of finite element calculations used to model the effect of the variable internal pressure capability of this apparatus on the radial profile of the roller surface in the nip point. The dimensions of the shell can be represented in terms of the following quantities; a flexural rigidity of approximately 1800 lb-in (203 newton*m) and a shell thickness to diameter ratio of 0.025. The flexural rigidity is defined as the quantity of the product of the material elastic modulus and the shell thickness cubed divided by the quantity of the product of a constant value 12 and the quantity of the difference of 1 and Poisson&#39;s ratio squared. An average nip pressure of 250 psi, (1.724 MPa) placed on the thin walled outer shell  24  along a localized region parallel to the axis of rotation, has been used in this calculation. The variable (UX) is the radial displacement in the x-direction, which is also normal to the applied nip pressure region. A greater positive value indicates further deformation toward the center of the roller shell.  
         [0042]     The curve with diamond shaped markers represents the expected shell deformation under nip load but without internal support. The curve with triangular shaped markers represents the effect of applying a localized pressure on an area equivalent to the curved surface of the loading shoe  72  acting at the center of the shell with an average pressure of 50 psi. (0.344 MPa) The curve with rectangular shaped markers represents the positive effect on the radial deformation obtained by applying a gradient pressure profile along the inner surface of the shell ranging from 15 psi to 20 psi (0.103 MPa to 0.137 MPa). Utilizing basic fluid dynamic principles it has been calculated that a pressure of approximately 30 psi (0.206 MPa) can be developed in this region given a fluid of viscosity of approximately 10 Pa-s sheared between the outer shell and the curved surface of the shoe with an average shear rate of 250 l/s.  
         [0043]      FIG. 10  shows a cross sectional view of a typical two roller nip utilized in the extrusion cast web formation. An axially compliant pressure roller  10  is loaded radially into the interface of the molten resin  86  and a second roller  84 . Utilizing a non-contacting deformation detector  88  such as a laser triangulation gage or an eddy current device, the resulting shell surface deformation can be measured. This measurement data can be utilized to control internal loading conditions along the axis of the roller by sending a deformation signal  90  to microprocessor  92 , which alters the strength of one or more of the magnetic field generators  37 .  
         [0044]     In addition to the magneto-rheological fluid described previously, this apparatus can accommodate other fluids without magneto-rheological properties but which exhibit non-Newtonian characteristics (viscosity of fluid is dependent on shear rate imposed). Localized pressure variations can be created through adjustment of the gap between the outer shell and the curved surface of the shoe. The average shear rate in this gap is proportional to the surface velocity of the shell divided by the gap height. Non-Newtonian fluids exhibit a logarithmic relationship between viscosity and shear rate. External manipulation of the gap combined with a fluid with desirable shear sensitive properties provides an additional means of creating localized pressure differences within each chamber.  
         [0045]     A key advantage of this invention is the ability to replicate a pattern with lower nip pressure due to the increased surface temperature at the point of contact of the molten material with the patterned roller surface. One example of this has been modeled with computational fluid dynamics software, Polyflow, in which a resin material, polycarbonate was subject to a pressure boundary condition and the flow of the material into a fine patterned geometry was studied.  FIG. 6  shows the two dimensional representation of the resin material and the patterned geometry.  FIG. 7  shows the improvement in pattern replication as mold surface temperature increases. An equivalent level of replication can be obtained for a given temperature as a significantly lower applied pressure. An increase in patterned surface temperature of 10% can result in a decrease in applied pressure of 67.5% for an equivalent replication efficiency.  
         [0046]     In one example, a finite element analysis has been performed on the shell of six inch diameter by 20″ face with a wall thickness of 0.125 inch, constructed of aluminum to determine the effect of circumferentially variable heating on the mechanical stresses and thermal deformation.  FIG. 9  shows a plot of the calculated shell deformation. The lower curve shows the resultant effect of uniformly distributed pressure on the outer shell surface at the point of maximum temperature. The upper curve shows the resultant effect of the application of an internal compensation pressure adjusted to minimize surface deformation. A greater positive value indicates a greater deformation away from the center of the shell.  
         [0047]     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 scope of the invention.  
       PARTS LIST  
       [0000]    
       
           10  axially compliant pressure roller  
           11  converging region of viscous fluid  
           12  outer shell  
           14  heating elements  
           15  pivot  
           16  brush assembly  
           17  adjusting pin  
           18  heating zone  
           20  internal surface  
           22  fluid media  
           24  cooled region  
           25  inner surface of shell  
           26  stationary core  
           27  axial holes  
           28  bearings  
           29  pockets  
           30  seal  
           32  inlet for fluid media  
           33  curved surface of shoe  
           35  nip point  
           37  magnetic field generator  
           40  fixed inner shell (stator)  
           41  cooling region  
           42  flow passages  
           43  reheating region  
           44  close fitting baffle  
           45  loading shoe region  
           46  outer shell  
           47  higher temperature fluid  
           48  high temperature fluid  3  supply  
           49  lower temperature fluid  
           50  high temperature fluid  3  return  
           52  carbon seal  
           54  carbon bearings  
           56  high temperature fluid  1  supply  
           58  high temperature fluid  1  return  
           60  high temperature fluid  2  supply  
           62  high temperature fluid  2  return  
           70  outer shell  
           72  loading shoes  
           74  first axial baffle  
           76  second axial baffle  
           78  shoe loading mechanism  
           80  support bracket  
           82  drive sprocket  
           84  second roller  
           86  material entering nip  
           88  deformation detector  
           90  deformation signal  
           92  microprocessor