Abstract:
Apparatus and a method for selectively vapor-coating an extrusion die to extrude honeycombs with desired distributions of web thicknesses, wherein at least one perforated impedance or preferential coating plate with a desired pattern of holes is positioned adjacent to, but spaced apart from, a face of the extrusion die to be coated, whereby a coating gas passed through the coating plate and into the die imparts a desired coating distribution to the extrusion die slots that is effective to closely control the web thickness distribution in honeycombs extruded therefrom.

Description:
BACKGROUND OF THE INVENTION 
     Extrusion dies are useful in forming cellular or honeycomb ceramic substrates for use in catalytic converters, which are utilized in exhaust systems of internal combustion engines. In order to reduce back pressure within the exhaust system, it is necessary that the cell walls or webs have a substantially thin cross-sectional dimension so as to provide a substantially large open frontal area. However, the thin walled structure must be protected so as to withstand normal automotive impact requirements. One way to protect the inner cells is to increase the thickness of the cell walls in peripheral portions of the extruded ceramic substrate, thus providing additional strength to withstand external loads. 
     European Patent Specification EP 0 674 018 B1 teaches the use of blocking plates during the recoating of dies to obtain uniform slot width. The blocking plates are used to limit the amount of coating applied to worn dies that need to be recoated and be within constant web thickness. U.S. Pat. No. 5,952,079 uses a mask in the form of a Teflon tape to block off outer portions of a die during liquid nickel plating, so as to inhibit the occurrence of edge chipping of a honeycomb structure. Problems can arise in connection with the latter method, however, in that it is difficult to control plating gradients and to achieve uniform results under varying conditions of liquid flow and tape alignment. 
     The present invention overcomes problems encountered in the prior art by setting forth a controlled approach for obtaining desired web thicknesses of a honeycomb structure, by controlling the flow of gases within slots of a honeycomb extrusion die during the chemical vapor deposition (CVD) coating of the die. 
     SUMMARY OF THE INVENTION 
     The present invention relates to the manufacture of extruded honeycomb structures and more particularly to improved methods and apparatus for manufacturing honeycomb structures of modified web geometries exhibiting improved strength and enhanced flow properties. The invention provides a desired distribution of web thickness in an extruded honeycomb substrate. 
     The desired web distribution is obtained by providing desired distributions of slot widths in honeycomb extrusion dies during chemical vapor deposition (CVD) coating of the dies. Such desired coating distributions may be variable to provide variable slot widths across the die face, or constant to provide a constant slot width. Also if desired, a portion of the coating may be variable and an adjacent portion may be constant. When the flow of CVD gas is reduced in the peripheral extrusion area of the die, less coating thickness will result in such region as compared to other regions where gas flows are not reduced. As a result, the extruded webs in such peripheral region of the honeycomb substrate will be thicker and stronger than the other region. 
     The present invention thus provides a way to better control the distribution of the thickness of extruded honeycomb webs by controlling the CVD coating thickness distribution in a honeycomb extrusion die. In a specific embodiment, the invention provides means for controlling the distribution of a CVD coating to a die in such a manner so as to provide thicker webs in a peripheral region of a honeycomb substrate resulting in additional strength to withstand external loads. Coating hardware to produce a gradual increase in the web thickness over a desired region of a honeycomb substrate is also provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates one embodiment of the invention. 
         FIG. 2  illustrates an other embodiment of the invention. 
         FIG. 3  Illustrates a further embodiment of the invention. 
         FIG. 4  is a plan view of an impedance plate or preferential coating plate of the present invention. 
         FIG. 5  is an enlarged portion of the plate of  FIG. 4 . 
         FIG. 6  is a further embodiment of an impedance or preferential coating plate of the present invention. 
         FIG. 7  illustrates a preferred embodiment of the invention. 
         FIG. 8  illustrates an other preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an arrangement for coating a die with a reduced amount of coating in a peripheral region of the slots. An extrusion die  10  for forming honeycomb structures is shown having an upstream slot portion  12  and a downstream hole portion  14 , bounded by a solid outer rim portion  16 . A spacer ring or shim  18  is positioned on the downstream or hole side  14  of the die  10 . The shim  18  has an inner dimension substantially equal to the outer dimension of the hole portion  14 , forming an open inner portion  20  bounded by a ring portion  22 . A disk member  24 , also having an open inner portion  26  bounded by an outer ring portion  28 , is positioned on the spacer ring  18 . The ring portion  28  of the disk  24  extends over that portion of the die  10  where it is desired to limit the thickness of the coating. If desired, the disk member  24  may have an angled surface facing the die  10  which will affect thickness variation of an applied coating. The thickness of the spacer  18  also controls in part the amount of coating obtained on the peripheral region of the die. 
     The flow of gas in  FIG. 1  is into the upstream slot side  12  of the die  10 . With the disk member  24  overlying a peripheral portion of the die holes  14 , a reduced amount of coating material flows into the peripheral region of the slots, resulting in such slots having a greater width than those that were not covered. Thus when material is extruded through the die  10 , the outer webs or cell walls will be thicker and thereby stronger to withstand external loads. 
       FIG. 2  illustrates a different set-up to obtain the same results as that obtained with the arrangement shown in  FIG. 1 , that is to obtain less CVD coating in the peripheral area of a honeycomb extrusion die. An extrusion die  30  for forming honeycomb structures is shown with an upstream slot side portion  32  and a downstream hole side portion  34  bounded by a rim portion  36 . A shim or spacer ring  38  having an inner dimension equal to the outer dimension of the hole portion  34  is positioned on the slot side  32  of the die, and a disk member  40  is positioned on the shim  38 . The disk member  40  has an inner dimension selected to overlie that peripheral portion of the extrusion die  30  where a reduced amount of coating is desired. 
     An optional additional disk member  42 , similar to member  40 , may be positioned on the downstream hole side  34  of the die  40 , with or without the use of a shim. The use of such a downstream disk member will change the coating thickness distribution in the peripheral region beyond that which would be obtained if no downstream disk were used. Like the results obtained with the device of  FIG. 1 , the hardware of  FIG. 2  produces a reduced gas flow about the peripheral region of the die and thus results in wider slots in such region. 
       FIG. 3  shows another approach of producing a die with a desired flow distribution of CVD coating material in selected slots. The hardware illustrated in  FIG. 3  will provide a reduction in coating thickness about the peripheral region of the die, similar to that produced by the embodiments shown in  FIGS. 1 and 2 . Die  44  is shown having its hole side  46  upstream and slot side  48  downstream bounded by a solid rim  50 . An impedance plate  52  is shown having a drilled or perforated central portion  54  of constant thickness, a drilled or perforated variable thickness area  56  surrounding the central portion  54 , and a solid rim portion  58  corresponding to rim portion  50  of die  44 . The through-holes in the impedance plate  52  are of uniform diameter and preferably aligned with the holes in side  46  of die  44 . If desired, a spacer plate, not shown, may be positioned between the die  44  and the impedance plate  52 . 
     With this particular arrangement, the flow of coating material produces a variable reduced coating in the slots about the periphery of the die and a constant slot wall coating thickness centrally of the die. However, it should be understood that almost any desired coating distribution in the slots across the die could be obtained by different thickness profiles being produced on the impedance plate  52 . That is, the thicker the impedance plate, the thinner the corresponding die slot coating. Further, non-uniform diameter holes could be formed in plate  52 , if desired. 
       FIGS. 4 and 5  show a preferred form of an impedance plate or preferential coating plate  60  for use with a round extrusion die, such as die  80  of  FIG. 7 . The preferential coating plate  60  is of uniform thickness, but is provided with plurality of through-holes  62  in concentric patterns or rings  64 , bounded by an outer rim  66 . Although various patterns  64  of holes  62  may be utilized to obtain different desired distributions of slot wall thickness, the plate  60  is designed to gradually reduce the coating distribution in the slots of a round die, in radially outwardly concentric patterns from the center of the die. 
     As a specific illustration, plate  60  is shown having concentric patterns a, b, c, d, and e, each being radially outwardly from the center of the plate and radially disposed from each other. As noted in the illustration, the diameter of the holes  62  in each pattern decreased from 0.046″ in the central hole pattern a to 0.033″ in outer hole pattern e. See also  FIG. 5  for a blowup of the outer three patterns c, d and e, where all of the holes  62  in pattern c have a diameter of 0.039, all those in pattern d have a diameter of 0.035″, and all those in pattern e have a diameter of 0.033″. It thus can be seen, that in each particular pattern, the diameter of all the holes in that pattern is the same. When positioned on a die, the outer extent of pattern e would complement the outer extent of the feed holes or the periphery of the slot face of the die. Since the hole diameters in each pattern decreases for the central pattern a to the outer most pattern e, as CVD coating gas is passed through preferential coating plate  60  into an extrusion die upon which it is positioned, the distribution of coating on the slot walls will be gradually reduced from a central portion of the die toward the periphery. That is, the larger the diameter of the holes in the impedance plate, the thicker the corresponding die slot coating, and the smaller the hole diameters, the thinner the coating on the slot walls of the die. However, the distribution of the pattern of holes in a impedance plate does not have to be a gradual decrease or increase in diameter from a center or a periphery of a given die, but such distribution in the plate may be such so as to produce any variation of a desired coating thickness across the face of the die. 
       FIG. 6  shows a further preferred embodiment of an impedance plate or preferred coating plate for use with an oval extrusion die. Like plate  60 , the preferential coating plate  70  is of uniform thickness with an outer rim  76 . However, plate  70  is provided with an oval-like pattern  74  of flow-through holes  72  which may extend from a central portion of an oval extrusion die to the periphery of the die slot face. The preferential coating plate  70  is provided with an array of oval-like patterns, u, v, w, x, y, and z, extending from center pattern u to outer pattern z, having an outer periphery corresponding to the outer periphery of the die slot face upon which the plate is to be positioned. The pattern y is of a normal oval configuration, and the remaining patterns are actually percentages of the normal, but are substantially concentric with each other. 
     If it were desired to extrude a honeycomb substrate with gradually increasing web thicknesses from a center portion toward a peripheral portion, holes  62  in the various patterns u through z of plate  70  would have gradually decreasing diameters. Again, however, size of the holes selected for the various patterns and even the particular pattern arrangement chosen, may be varied to produce the desired distribution of slot widths in a honeycomb extrusion die. Accordingly, if some special or unique extruded honeycomb substrate were desired for a particular application, the pattern of holes would not necessarily have to be substantially concentric, and even the diameter of all of the holes in a particular pattern would not have to be the same. Although probably not necessary, it is conceivable that an impedance plate could incorporate both patterns of different diameter holes and a variable plate thickness to produce some interesting flow distributions. 
       FIG. 7  illustrates a honeycomb extrusion die  80  having an impedance or preferential coating plate  78 , such as plates  60  or  70  positioned on an upstream face of the die. The extrusion die  80  has a slot face  82  and a hole face  84 . A ring-like spacer member  86  positions the plate  78  in spaced-apart relationship from the slot face  82  of the die  80 . The spacer ring  86  of  FIG. 7  has a thickness of ¼″, but may be of any desired uniform thickness depending on the desired coating distribution. The preferential coating plate  78  is provided with an array of through-holes (not shown) with a range of hole diameters decreasing toward the plate periphery as illustrated in  FIGS. 4 and 5  of the drawings. In this embodiment the plate has a uniform thickness of ⅛″, but again the uniform thickness may vary depending on the number of plates utilized and the spacing from the extrusion die. As shown by the arrow F, the flow of CVD coating gas is through the preferential coating plate  78  and into the slot face  82  of the die  80 . 
     Finally,  FIG. 8  illustrates honeycomb extrusion die  80  having a plurality of impedance or preferential coating plates  88  mounted on the upstream slot face  82 , but spaced-apart from the die by a spacer ring such as  86 , and from each other by a plurality of spacer ring members  90 . The multiple impedance or coating plates  88  are typically thinner than the single plate  78 , and may be on the order of about 0.015″ thick. Each impedance plate will incorporate an array of through-holes (not shown) of decreasing diameter toward the plate periphery, as illustrated in  FIGS. 4 and 5  of the drawings. Also, when utilizing multiple impedance plates  88 , the spacer rings  90  are also relatively thin, such as on the order of about 0.005″. The use of multiple impedance or preferential coating plates  88 , has the advantage of controlling the degree of coating and the desired coating distribution by changing the number of and/or the hole distribution in the preferential coating plates. Again, as shown by the arrow F in  FIG. 8 , the flow of the coating gas is through the multiple plates  88  and into the slot face  82  of die  80  to produce a desired coating distribution on the slot walls of the die. 
     Although the now preferred die coating embodiments of the invention have been disclosed, it will be apparent to those skilled in the art that various changes and modifications may be made thereto without departing from the spirit and scope thereof as defined in the appending claims.