Abstract:
A casting component comprising essentially no macro porosity by having the casting material fill the mold cavity with a laminar flow is disclosed. In one embodiment, the gas within the mold cavity will not be entrained when the flow is laminar. Control of this flow behavior is directly related to the microstructure and thixotropy of the alloy. The SLC slurry have a thixotropic nature and a non-turbulent way of flowing into a casting die, allowing the process to be capable of producing cast parts having thin sections, geometric complexity and close dimensional tolerances without entrapped gas porosity. The thixotropic aluminum slurry may be characterized by a uniform primary aluminum particle size in the range of 50 to 80 microns. A uniform distribution of this microstructure throughout the injected aluminum volume encourages laminar flow of the aluminum into the die cavity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/058,972 (THT 0003 MA), filed Oct. 2, 2014, and U.S. Provisional Application No. 62/058,982 (THT 0005 MA), filed Oct. 2, 2014. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to vertical die casting operations. Generally, in a vertical die casting press, a frame supports one or more vertical shot sleeves, and each sleeve receives a shot piston mounted on a shot piston rod connected to a hydraulic cylinder. The shot sleeve receives a molten die casting metal which is forced upwardly by the shot piston into a die cavity defined between a vertically moveable upper die member and a lower die member. The lower die member defines an opening through which the metal within the shot sleeve is forced upwardly into the die cavity to form a die cast part. After the molten metal has cooled within the die cavity, the upper die member is disengaged from the lower die member, and the lower die member can be shifted to a station where the part can be removed. The remaining solidified metal or biscuit within the shot sleeve can be removed by elevating the shot piston and pressing the biscuit laterally from the shot piston. When multiple shot sleeves are used in a press, the shot sleeves can be indexed between a metal receiving station and a metal injection or transfer station. Further details regarding the design and operation of vertical die casting presses may be gleaned from a variety of readily available sources including, for example, U.S. Pat. Nos. 3,866,666, 4,799,534, 5,332,026, 5,660,223, 6,913,062. 
         [0003]    The present disclosure also relates to semi-solid molding (SSM) of metal alloys and the equipment and methods used for SSM, including vertical die casting presses, details of which may be readily gleaned from a variety of readily available sources including, for example, U.S. Pat. Nos. 6,901,991, 3,954,455, 4,434,837, 5,161,601 and 6,165,411. SSM is also discussed in a book entitled Science and Technology of Semi-Solid Metal Processing, published by North American Die Casting Association in October, 2001. 
       BRIEF SUMMARY 
       [0004]    The ability of a casting to be leak free when pressurized by helium gas is primarily related to the casting being free of porosity. Porosity can exist on a macro scale from gas entrapment due to a turbulent metal flow during the casting process. Porosity can also exist on a micro scale due to solidification shrinkage during the casting process. In either case, a flow path can exist within a casting wall that allows the helium to leak through the casting wall. 
         [0005]    In accordance with the teachings of the present disclosure, the presence of porosity can be essentially eliminated by proper control of the casting microstructure. More specifically, macro porosity can be eliminated by having the casting material, e.g., aluminum alloy A356, fill the mold cavity with a laminar flow. Gas within the mold cavity will not be entrained when the flow is laminar. Control of this flow behavior is directly related to the microstructure and thixotropy of the alloy. Because of the thixotropic nature of the SLC slurry and the non-turbulent way that it flows into a casting die, the process is capable of producing cast parts having thin sections, geometric complexity and close dimensional tolerances without entrapped gas porosity. Preferably, the thixotropic aluminum slurry will be characterized by a uniform primary aluminum particle size in the range of 50 to 80 microns. A uniform distribution of this microstructure throughout the injected aluminum volume encourages laminar flow of the aluminum into the die cavity. 
         [0006]    The primary aluminum particles of cast products produced according to the methodology of the present disclosure, with the aforementioned particle size distribution, are free of encapsulated eutectic at the micron scale. Accordingly, micro porosity can be essentially eliminated in castings produced using the methodology of the present disclosure because micro porosity does not readily occur within the primary aluminum particle. This is presumed to be caused by a temperature gradient surrounding the particle that exceeds the local liquidous temperature of the alloy. The primary aluminum particle solidifies directionally to form a solidification shrink free particle. This temperature gradient is what promotes the globular geometry of the primary aluminum particle. In addition, there is a lower shrink potential in the eutectic surrounding the primary aluminum particle due to the lower bulk energy (temperature) of the alloy. Another factor in promoting a shrink free microstructure is the proximity of neighboring primary aluminum particles. The solute boundary layer as well as the solid/liquid interface of neighboring particles interact with one another as solidification occurs. The physical proximity of the neighboring particles along with the thermal characteristics of temperature gradient surrounding the particles provides a low solidification shrinkage environment in which the eutectic solidifies. 
         [0007]    The present disclosure recognizes that contemplated embodiments of the present disclosure are particularly well-suited for casting components for use in constructing hermetically sealed housings, including, for example, hermetically sealed housings for digital data storage. More particularly, the present applicant has recognized that contemplated embodiments of the present disclosure are particularly well-suited for casting components for use in constructing hermetically-sealed, helium-filled hard drives. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
           [0009]      FIG. 1  is a front elevation cross-sectional view of a vertical die casting press according to one or more embodiments of the present disclosure; 
           [0010]      FIGS. 2A and 2B  are simplified cross-sectional views of a portion of a gate plate and lower mold plate with a molten material temperature gradient profile according to alternative embodiments of the present disclosure; 
           [0011]      FIG. 3  is a front elevation cross-sectional view of a shot sleeve and shot piston with a representative molten material temperature gradient profile; 
           [0012]      FIGS. 4A-4D  schematically depict shot sleeve footprints according to selected alternative embodiments of the present disclosure; 
           [0013]      FIG. 5  illustrates a molded part according to one or more embodiments shown and described herein; 
           [0014]      FIG. 6  is a front elevation cross-sectional view of a vertical die casting press according to one or more embodiments of the present disclosure; and 
           [0015]      FIG. 7  is a front elevation cross-sectional view of a vertical die casting press according to one or more embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    One embodiment of a vertical die casting press  100  according to the present disclosure is shown in  FIG. 1 , and comprises a shot piston  10 , a shot piston rod  12 , a thermally directed press subassembly  20 , and an upper mold ejection subassembly  30 . The thermally directed press subassembly  20  comprises a shot sleeve  210 , a gate plate  220 , a lower mold plate  230 , and an upper mold plate  240 . The shot piston  10  is positioned within the shot sleeve  210 . A hydraulic cylinder can be mechanically coupled to the shot piston rod  12 , which is configured to advance the shot piston  10  mechanically through the shot sleeve  210 . 
         [0017]    The upper mold plate  240  and the lower mold plate  230  define a die cavity  250  there between. The upper mold ejection subassembly  30  engages the thermally directed press subassembly  20  along the upper mold plate  240  of the thermally directed press subassembly  20 . The shot piston  10  may be provided with a water cavity  16  to enhance thermal control in the die casting press  100 . It is contemplated that a variety of assemblies may be employed as alternatives to a hydraulic cylinder, shot piston  10 , and shot piston rod  12  to provide pressurized molten material  40  to the thermally directed press subassembly  20 . In operation, the thermally directed die casting press subassembly  20  generally receives molten material  40 , transfers the molten material  40  through the gate plate  220  and the lower mold plate  230  to the die cavity  250  between the upper mold plate  240  and the lower mold plate  230 . 
         [0018]    The lower mold plate  230  comprises a gate port  232 , a die port  234 , and an injection nozzle  236  extending from the gate port  232  to the die port  234  across a thickness dimension of the lower mold plate  230 . As used herein, a “plate” is not limited to a structure having a uniform planar surface. A plate may have variation, non-planar portions, abutments, or other additional features. In the illustrated embodiment, a major portion of the injection nozzle  236  comprises a contracting nozzle taper  237  along a laminar injection path  238  (see  FIG. 2 ) extending towards the die port  234 . It is contemplated that the injection nozzle  236  may additionally comprise an expanding a nozzle taper  239  along the laminar injection path  238  extending towards the die port  234 , i.e., downstream of the contracting nozzle taper  237 . As illustrated in the embodiment of  FIG. 1 , it is contemplated that the injection nozzle  236  can be configured such that a minor portion of the nozzle  236  comprises the expanding nozzle taper  239 . In many embodiments, it is contemplated that the contracting nozzle taper  237  will be positioned between the gate port  232  and the expanding nozzle taper  239 , and the expanding nozzle taper  239  will be positioned between the contracting nozzle taper  237  and the die port  234 . 
         [0019]      FIGS. 2A and 2B  are simplified cross-sectional views of a portion of the gate plate  220  and lower mold plate  230  with a molten material temperature gradient profile  225  extracted from  FIG. 3 , which shows the molten material  40  contained within the shot sleeve  210  above the shot piston  10 . The present inventors have recognized that the molten material  40  contained within the shot sleeve exhibits a significant temperature drop moving from a central region  218  of the shot sleeve  210  towards a periphery  219  of the shot sleeve  210 . It is noted that the illustrated temperature gradient profile  225  is an estimation of the thermal properties of the molten material  40  contained within the shot sleeve  210  and it is contemplated that the temperature gradient of the molten material within the shot sleeve  210  may vary from that which is illustrated in  FIG. 3 . Accordingly, it is noted that the size of the central region  218  is presented in  FIGS. 2A ,  2 B and  3  merely as an approximation and, as such, it is contemplated that its precise size in relation to the remainder of the shot sleeve  210  will depend upon the particular thermal characteristics of the shot sleeve  210  and the molten material  40  contained therein. For the purposes of practicing the concepts of the present disclosure it is noted that a limited amount of experimentation may be needed to determine the suitable size of the central region  218  because, as is explained in detail below, the design of the gate plate  220  should account for the size of the central region  218 . More specifically, the gate plate  220  should be designed to sample molten material  40  from relatively low and relatively high temperature portions of the shot sleeve temperature gradient profile  225 . 
         [0020]    Referring to  FIGS. 1 ,  2 , and  6 , the gate plate  220  comprises a thermally directed funnel gate  260  extending from the shot sleeve  210  to the gate port  232  of the lower mold plate  230  across a thickness dimension of the gate plate  220 . The gate port  232  to which the thermally directed funnel gate  260  extends may be advantageously offset relative to a central region  218  of the shot sleeve  210  (See  FIG. 1 ). Alternatively, the gate port  232  to which the thermally directed funnel gate  260  extends may be substantially centered with the central region of the shot sleeve  210  (See  FIG. 6 ). The thermally directed funnel gate  260  comprises a contracting funnel taper  267  along a turbulence-inducing injection path  268  extending towards the gate port  232  of the lower mold plate  230 . As is shown in the embodiment of  FIG. 1 , when read in light of  FIGS. 4A-4D , which are described in detail below, the thermally directed funnel gate  260  may comprise a bi-laterally truncated funnel, which is generally distinguishable from a circular funnel by the presence of flattened side portions in the otherwise circular cross section of a circular funnel. Examples of these flattened side portions are illustrated in  FIGS. 4A-4D . It is contemplated that the funnel gate  260  may be of any shape that spans the relatively low T and relatively high T portions of the shot sleeve temperature gradient profile  225 . 
         [0021]    As shown in  FIGS. 2A ,  2 B, and  3 , the shot sleeve  210  defines a temperature gradient profile  225  rising from relatively low T portions at a periphery  219  of the shot sleeve  210  to relatively high T portions at a central region  218  of the shot sleeve. In some contemplated embodiments, the contracting funnel taper  267  of the thermally directed funnel gate  260  may be skewed to sample molten material from an off-center portion of the shot sleeve temperature gradient profile  225 . More specifically, referring specifically to  FIGS. 2A and 3 , in some embodiments, it is contemplated that the thermally directed funnel gate  260  can be skewed such that molten material  40  sampled from relatively high T portions of the shot sleeve temperature gradient profile  225  moves towards the gate port  232  of the lower mold plate  230  at a higher velocity than molten material sampled from relatively low T portions of the shot sleeve temperature gradient profile  225 . Referring to  FIGS. 2B and 3 , in still further contemplated embodiments, the thermally directed funnel gate  260  can be skewed such that molten material  40  sampled from relatively low T portions of the shot sleeve temperature gradient profile  225  moves towards the gate port  232  of the lower mold plate  230  at a higher velocity than molten material  40  sampled from relatively high T portions of the shot sleeve temperature gradient profile  225 . 
         [0022]    As shown in  FIG. 6 , in another embodiment, the contracting funnel taper  267  of the thermally directed funnel gate  260  may be centered on the shot sleeve  210  and extend towards the periphery  219  to sample molten material  40  from the relatively low T portions of the shot sleeve temperature gradient profile  225  so to move toward the gate port  232  at a higher velocity than the molten material  40  sampled at the relatively high T portions of the shot sleeve temperature gradient profile  225 . 
         [0023]    It is contemplated that the characteristics of the thermally directed funnel gate  260  may alternatively be quantified with reference to the “high temperature” and “low temperature” shot-to-port path lengths defined by the gate  260 . More specifically, referring to  FIGS. 2A and 3 , the thermally directed funnel gate  260  is shaped such that a high temperature shot-to-port path length for molten material  40  sampled from relatively high T portions of the shot sleeve temperature gradient profile  225  is longer than a low temperature shot-to-port path length for molten material  40  sampled from relatively low T portions of the shot sleeve temperature gradient profile  225 . In  FIGS. 2B ,  3  and  6 , the thermally directed funnel gate  260  is shaped such that a low temperature shot-to-port path length for molten material  40  sampled from relatively low T portions of the shot sleeve temperature gradient profile  225  is longer than a high temperature shot-to-port path length for molten material  40  sampled from relatively high T portions of the shot sleeve temperature gradient profile  225 . As shown in  FIG. 6 , the thermally directed funnel gate  260  may be shaped to have a second low temperature shot-to-port path length for molten material  40  sampled from relatively low T portions of the shot sleeve temperature gradient profile  225 . 
         [0024]    As shown in the embodiment of  FIG. 7 , the gate plate may comprise two thermally directed funnel gates, with a central funnel gate and a peripheral funnel gate. Each thermally directed funnel gate would extend from the shot sleeve to a respective gate port. Each thermally directed funnel gate would comprise a funnel. The funnel of the central funnel gate generally samples molten material from the relatively high T portion of the shot sleeve temperature gradient profile from the central region of the shot sleeve. The funnel of the peripheral funnel gate generally samples molten material from the relatively low T portion of the shot sleeve temperature gradient profile from the periphery of the shot sleeve. 
         [0025]      FIGS. 4A ,  4 B,  4 C, and  4 D schematically depict shot sleeve footprints  212  according to selected alternative embodiments of the present disclosure. Although the illustrated shot sleeve footprints  212  are substantially circular, it is noted that the concepts of the present disclosure need not be limited to circular shot sleeve footprints  212 . Rather, it is contemplated that the shot sleeve footprint  212  may take any shape allowing for a temperature gradient profile with the aforementioned relatively high and low temperature portions. For convenience, the shot sleeve footprints  212  have been presented with indicia calling out a shot sleeve center  213 , a shot sleeve interior edge  215 , and shot sleeve footprint radii  214 . Further, as is described in detail below, the thermally directed funnel gate  260  defines a sampling footprint  211   a ,  211   b ,  211   c ,  211   d  that extends over the shot sleeve  210  along a projection that spans the relatively low T and relatively high T portions of the shot sleeve temperature gradient profile  225 . 
         [0026]    As shown in  FIGS. 4A and 4B , the shot sleeve  210  may define radial sampling footprints  211   a  and  211   b . The sampling footprints  211   a  and  211   b  have a first and second semi-circular end, each with a radius r. As shown in  FIGS. 4A and 4B , the sampling footprints  211   a  and  211   b  are defined as “radial” because the major radial portion of the footprint extends along a single shot sleeve footprint radius  214 . For example, it is contemplated that a sampling footprint comprising one or more minor, non-linear radial portion or other types or irregularities would still be defined as radial because it would comprise a major radial portion similar to that shown in  FIGS. 4A and 4B . As shown in  FIG. 4A , the radial sampling footprint  211   a  covers the center  213  of the central region  218  of the shot sleeve  210 . Alternatively, the radial sampling footprint  211  may start with its first end on the shot sleeve footprint center  213  and extend along a shot sleeve footprint radius  214 . In the alternative embodiment of  FIG. 4B , the radial sampling footprint  211   b  is displaced from the center  213  of the central region  218  of the shot sleeve  210 . 
         [0027]    Additional alternative radial sampling footprints  211   c  and  211   d  are shown in  FIGS. 4C and 4D . Referring to  FIG. 4C , the contracting funnel taper  267  of the thermally directed funnel gate  260  defines a substantially diametrical sampling footprint  211   c  that overlies the circular shot sleeve footprint and is asymmetric with respect to the center  213  of the circular shot sleeve footprint  212 . The sampling footprint  211   c  in  FIG. 4C  is defined as diametrical because it overlies the shot sleeve footprint center  213  and extends along a first shot sleeve footprint radius  214   a  a distance a and along a second footprint radius  214   b  a distance b, both of which are co-linear. For example, it is contemplated that a sampling footprint comprising one or more minor, non-linear radial portion or other types or irregularities would still be defined as diametrical because it would overlie the shot sleeve footprint center  213  and extend along two shot sleeve footprint radii  214  as shown in  FIG. 4C . To ensure asymmetry, the distances a and b are not the same length. In other embodiments, the shot sleeve footprint radii  214  may be non-linear. 
         [0028]    Referring to  FIG. 4D , the contracting funnel taper  267  of the thermally directed funnel gate  460  defines a non-radial sampling footprint  211  that is displaced from a center  213  of the central region  218  of the shot sleeve  210  and extends along a projection that spans the relatively low T and relatively high T portions of the shot sleeve temperature gradient profile. 
         [0029]    Referring collectively to  FIGS. 4A-4D , it is noted that it may be advantageous to ensure that the sampling footprint  211   a ,  211   b ,  211   c ,  211   d  of the thermally directed funnel gate  260  extends to the outer boundary of a prime slurry portion of the shot sleeve footprint  212 , which is defined herein as the portion of the shot sleeve footprint  212  where the temperature gradient profile  225  (see  FIGS. 2A ,  2 B, and  3 ) is at least greater than the point at which primary aluminum solidification in the slurry begins, e.g., at approximately 610° C. for aluminum alloy A356. It may also be advantageous to exclude the sampling footprint  211  from extending beyond the prime slurry portion of the shot sleeve footprint  212 . Typically, the sampling footprint  211  of the thermally directed funnel gate  260  will not extend to the interior edge  215  of the shot sleeve  210  because the slurry temperature there will be well below the point at which primary aluminum solidification in the slurry begins. 
         [0030]    In some embodiments, the contracting funnel taper  267  of the thermally directed funnel gate  260  defines a sampling footprint  211  having a cross-sectional area that is a function of the die volume. For example, the cross-sectional area of the sampling footprint  211  can be selected such that the volume of the funnel gate  260  is at least approximately 40% of the volume of the die. 
         [0031]    The present inventors have recognized that the concepts of the present disclosure can be used to produce parts that are well-suited for constructing hermetically sealed enclosures because the cast parts are typically substantially free of porosity that would otherwise require sealant impregnation or other means of addressing the typical porosity of cast metals. In addition, by limiting the porosity of the parts, a vertical die casting press according to the present disclosure can be used to create parts with relatively thin sections, geometric complexity, and close dimensional tolerances. For example, as shown in  FIG. 5 , the vertical die casting press and associated methodology disclosed herein may be utilized to produce a housing component or other molded part  50  that is suitable for use in a hermetically-sealed, helium-filled data storage drive, e.g., a hard disc drive. While the molded part  50  may be any die cast part, and may be made from a variety of materials, it is contemplated that vertical die press assemblies according to the present disclosure will be particularly advantageous in semi-solid molding (SSM) of metal alloys, as discussed above. More particularly, it is contemplated that the housing component  50  can be manufactured by injecting an aluminum alloy slurry from the shot sleeve  210  through the thermally directed funnel gate  260  and the injection nozzle  236  into the die cavity  250 . 
         [0032]    Referring further to  FIG. 5 , an aluminum alloy housing component  50  of a hermetically sealed disc drive will typically comprise one or more thin-walled portions  52  that occupy respective majorities of many of the major faces of the housing component. The concepts of the present disclosure allow a thickness dimension of the thin-walled portions  52  to be less than approximately 2.5 mm (0.1 inches), between approximately 1.5 mm and approximately 1.75 mm (0.06 inches to 0.07 inches), or as low as 0.75 mm (0.03 inches) while preserving the above-noted hermetic properties. It is contemplated, however, that it will typically be preferable to ensure that the gating pad portion  55  of the housing component  50 , which is illustrated schematically on the back side of the housing component of  FIG. 5 , in the area of the storage disc hub, is at least approximately 50% thicker than the aforementioned thin-walled portions  52  of the housing component  50 . 
         [0033]    It is contemplated that the vertical die casting press and associated methodology of the present disclosure limit porosity by forcing molten material to undergo turbulent and then laminar flow, while promoting-non dendritic growth via forced convection. More specifically, referring to  FIGS. 1 and 3 , during the injection of molten material  40  into the die cavity  250 , the molten material  40  is forced through the thermally directed funnel gate  260 . The gate geometry causes forced convection of hotter molten material  40  within the central region  218  with cooler molten material  40  at the periphery  219 . This forced convection disperses the nuclei in the injected volume and promotes non-dendritic growth of primary molten material particles. Temperature gradients within the injected volume result in the non-dendritic or globular morphology of the primary particles. The shot piston  10  moves vertically upward to raise the surface of the molten material  40  within the thermally directed funnel gate  260  to create a turbulent flow of the molten material  40  within the thermally directed funnel gate  260 . This turbulence encourages a thixotropic slurry to develop. The molten material  40  then flows through the injection nozzle  236  with a contracting nozzle taper  237 . The purpose of this geometry is to promote a laminar flow of molten material  40  into the die cavity  250 . This geometry reduces the turbulence in the molten material  40  that might otherwise occur within the thermally directed funnel gate  260 . In this manner, it is contemplated that thin walled portions of housing components manufacture according to the teachings of the present disclosure will be characterized by helium leak rates in the ambient of less than approximately 10 −8  cm 3 /s with the helium at atmospheric pressure or above, i.e., up to approximately twice the atmospheric pressure. In many cases, it is contemplated that the aforementioned leak rate may be less than approximately 10 −9  cm 3 /s. 
         [0034]    For the purposes of describing and defining the present invention, it is noted that “between” does not require uninterrupted succession from one component to another. For example, where the contracting nozzle taper  237  is described or recited as being positioned “between” the gate port  232  and the expanding nozzle taper  239 , it is contemplated that additional tapered or non-tapered nozzle portions may be positioned between the contracting nozzle taper  237  and the gate port  232  or between the contracting nozzle taper  237  and the expanding nozzle taper  239 . 
         [0035]    It is noted that terms like “generally” and “advantageously,” when utilized herein, are not utilized to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to identify particular aspects of an embodiment of the present disclosure or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment of the present disclosure. 
         [0036]    For the purposes of describing and defining the present invention it is noted that the terms “substantially,” and “approximately” are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. For example, a “substantially linear” body may refer to a body with some variation from one end to the second, with some abutments or apertures along the body, or some other minor non-linear features. The terms “substantially” and “approximately” are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
         [0037]    Having described the subject matter of the present disclosure in detail and by reference to specific embodiments thereof, it is noted that the various details disclosed herein should not be taken to imply that these details relate to elements that are essential components of the various embodiments described herein, even in cases where a particular element is illustrated in each of the drawings that accompany the present description. Further, it will be apparent that modifications and variations are possible without departing from the scope of the present disclosure, including, but not limited to, embodiments defined in the appended claims. More specifically, although some aspects of the present disclosure are identified herein as preferred or particularly advantageous, it is contemplated that the present disclosure is not necessarily limited to these aspects. 
         [0038]    It is noted that one or more of the following claims utilize the term “wherein” as a transitional phrase. For the purposes of defining the present invention, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.”