Circumferentially variable surface temperature roller

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

FIELD OF THE INVENTION

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

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.

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.

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.

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

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.

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

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

FIG. 1 is a cross-sectional view of a casting roller according to the present invention.

FIG. 2 is a perspective view, partially in section, of another embodiment of a casting roller according to the present invention.

FIG. 3A is a cross-sectional view of yet another embodiment of a casting roller according to the present invention.

FIG. 3B is a perspective view, partially in section, of the embodiment shown in FIG. 3A.

FIG. 3C is an enlarged cross section view of the shell and support shoe of the embodiment shown if FIG. 3A.

FIG. 4 is a chart showing the calculated cooling efficiencies of various roller design configurations according to the present invention.

FIG. 5 is a chart showing the calculated reheating efficiencies of various roller design configurations according to the present invention.

FIG. 6 is a cross-sectional view of a pattern on the surface of a roller.

FIG. 7 is a chart of temperature versus pressure showing pattern replication.

FIG. 8 is a chart showing calculated shell deformation at maximum temperature.

FIG. 9 is a chart showing calculated shell deformation for shell subject to a nip pressure and a supported by an internal pressure gradient.

FIG. 10 is a cross section of an axially compliant pressure roller forming a nip with at second roller.

DETAILED DESCRIPTION OF THE INVENTION

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

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

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

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.

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.

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.

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.

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.

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. ${\theta }_{\mathrm{TR}}\text{:}=\frac{T_{\mathrm{end}}-T_{\mathrm{infinite}}}{T_{\mathrm{initial}}-T_{\mathrm{infinite}}}$

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. $\tau \text{:}=\frac{t_{\mathrm{dwell}}·\alpha }{{\left(\frac{t_{\mathrm{shell}}}{2}\right)}^2}$

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: ${\Theta }_t\left(x,k,h,t\right)\text{:}=P1s{1}_0·e^{\frac{-\left(x-K_{\mathrm{mat}}·\frac{k}{h·t}\right)}{1-K_{\mathrm{mat}}·\frac{k}{h·t}}}$ 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*ft²*° F.) (1990 watt/(m²*° C.) and cooling a commercially available plastic material with a thermal diffusivity of 0.005 ft²/hr (0.00000013 m²/s).

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*ft²*° F.) (1990 watt/(m²*° C.) and an external heat transfer coefficient of 3 Btu/(hr*ft²*° F.) (17.03 watt/(m²*° C.).

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.

Referring to FIGS. 3A, 3B, and 3C 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.

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.

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.

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.

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.

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.

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'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.

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.

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.

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.

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.

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.

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

• 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

Claims

1. A casting roller having a variable temperature surface comprising: a rotatable cylindrical shell; axially aligned heating electric elements equally spaced and internal to an outer surface of said rotatable cylindrical shell; a brush assembly in electrical contact with said heating elements during a portion of said rotatable cylindrical shell rotation about an axis; a stationary core internal to said rotatable cylindrical shell; an annular space between said stationary core and said rotatable cylindrical shell; and a cooling fluid filling at least a portion of said annular space.

2. A casting roller as in claim 1 wherein a baffle confines said cooling fluid to a portion of said annular space.

3. A casting roller as in claim 1 wherein said cooling fluid is selected from a group comprising organic synthetic media that has an operating temperature range of 60 C to 350 C (140 F-662 F).

4. A casting roller as in claim 3 wherein the viscosity of the fluid varies from 8.2 centipoise to 0.26 centipoise.

5. A casting roller as in claim 3 wherein specific heat of said media varies from 1.62 kJ/kg*K at 60 C to 2.82 kJ/kg*K at 360 C.

6. A casting roller as in claim 3 wherein thermal conductivity varies from 0.125 W/m*K at 60 C to 0.086 W/m*K at 360 C.

7. A casting roller as in claim 3 wherein density of said media varies from 1016 kg/m3 at 60 C to 801 kg/m3 at 360 C.

8. A casting roller as in claim 1 wherein said heating elements are cartridge heaters.

9. A casting roller as in claim 8 wherein said heaters are 0.25 inch diameter with a heat flux output of approximately 60 W/in2.

10. A casting roller having a variable surface temperature comprising: a rotatable cylindrical shell; a stationary core internal to said rotatable cylindrical shell; an annular space between said stationary core and said rotatable cylindrical shell; baffles creating a first and second region in said annular space; a first fluid at a first temperature circulating in said first annular space; and a second fluid at a second temperature circulating in said second annular space.

11. A casting roller as in claim 10 wherein said first fluid is a cooling fluid and is selected from a group comprising organic synthetic fluid has an operating temperature range of 60 C to 350 C (140 F-662 F).

12. A casting roller as in claim 11 wherein a viscosity of the fluid varies from 8.2 centipoise to 0.26 centipoise.

13. A casting roller as in claim 11 wherein a specific heat of said fluid varies from 1.62 kJ/kg*K at 60 C to 2.82 kJ/kg*K at 360 C.

14. A casting roller as in claim 11 wherein thermal conductivity of said fluid varies from 0.125 W/m*K at 60 C to 0.086 W/m*K at 360 C.

15. A casting roller as in claim 11 wherein a density of said fluid varies from 1016 kg/m3 at 60 C to of 801 kg/m3 at 360 C.

16. A casting roller as in claim 10 wherein said first annular space covers one third of the circumference of the shell at a time.

17. A casting roller as in claim 10 wherein said second annular space covers two thirds of said cylindrical shell at a time.

18. A casting roller as in claim 10 wherein: said rotatable cylindrical shell is thin; and a shoe supports said rotatable cylindrical shell at a nip.

19. A method of casting a web of material comprising: contacting said web within a heating zone of said roller; raising said web to a casting temperature; forming an impression on said web; maintaining said web in contact with said roller through at least a portion of a cooling zone on said roller; and stripping said web from said roller after said web has cooled.

20. A method as in claim 19 wherein said roller surface temperature is increased 30 degrees centigrade to 60 degrees centigrade greater than a web glass transition temperature.

21. A method as in claim 19 wherein said roller surface temperature is decreased to 3 degrees centigrade to 5 degrees centigrade less than a web glass transition temperature.

22. A casting roller having a variable surface temperature comprising: a rotatable cylindrical shell; a stationary core internal to said rotatable cylindrical shell; an annular space between said stationary core and said rotatable cylindrical shell; baffles creating a first and second region in said annular space; a first fluid at a first temperature circulating in said first annular space; a second fluid at a second temperature circulating in said second annular space; wherein said rotatable cylindrical shell is thin; and a shoe which supports said rotatable cylindrical shell at a nip.

23. A casting roller as in claim 22 wherein: a third region is created by said buffer surrounding said shoe; and a third fluid at a third temperature is in said third region.

24. A casting roller as in claim 23 wherein said third fluid lubricates said shoe.