Patent Document

CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part of U.S. patent application Ser. No. 10/062,975 filed Jan. 31, 2002 by Jeffrey B. Hardesty, now abandoned and wholly incorporated herein by reference. 

   BACKGROUND 
   The present disclosure relates generally to exhaust emission control devices such as automotive catalytic converters. More particularly, the disclosure relates to a method of manufacturing emission control devices having fragile substrates. 
   Exhaust emission control devices comprise catalytic converters, evaporative emissions devices, scrubbing devices (e.g., hydrocarbon, sulfur, and the like), particulate filters/traps, adsorbers/absorbers, as well as combinations comprising at least one of the foregoing devices. This disclosure relates to such emission control devices having a ceramic substrate disposed within a metal housing with a matting material disposed between the substrate and housing. Although the substrate can have any size or geometry, the size and geometry are preferably chosen to optimize surface area in the given converter design parameters. Typically, the substrate has a honeycomb geometry, with the combs through-channel having any multi-sided or rounded shape, with substantially square, triangular, pentagonal, hexagonal, heptagonal, or octagonal or similar geometries preferred due to case of manufacturing and increased surface area. Although converter housings are most often cylindrical, they can be other shapes as well, such as having elliptical, oval, polygonal, and other cross sections. 
   In cases where a chemical washcoat is applied to the substrate, for example, a catalyst material and/or an adsorption material washcoat layer, the materials are wash coated, imbibed, impregnated, physisorbed, chemisorbed, precipitated, or otherwise applied to the catalyst substrate. In addition, a binder and/or support material such as aluminum oxide can be mixed with the chemical washcoat according to the specification of the particular application. Once the washcoat is applied to the substrate, the substrate is fired at a high temperature to sinter the washcoat thereby forming a hard phase of the chemical coating. It has previously been known to fire the washcoat prior to assembling the substrate into the emission control device housing. 
   Newer “thin-wall” substrates used in exhaust emission control devices offer significant advantages over traditional substrates, including a greater geometric surface area per unit volume and faster catalyst light-off due to the lower thermal mass of the substrate. As is generally understood in the art, faster light-off translates to higher conversion efficiency since catalytic converters are not effective during a cold engine start until they reach operation, or light-off temperature. However, thin wall substrates are significantly more fragile and are subject to fracture during stressful manufacturing operations that including stuffing, sizing, and burnoff operations. 
   One method currently employed to manufacture an emission control devices include wrapping the substrate in a retention material, or mat, and stuffing the substrate and mat into a metal housing through the use of a stuffing cone, the cone serving to compress the matting so that it can slide into the housing (see  FIG. 2 ). The retention material enhances the structural integrity of the substrate by applying compressive radial forces about it, reducing its axial movement and retaining it in place. Typically, the retention material is concentrically disposed around the substrate to form a retention material/substrate subassembly. 
   Another method involves disposing particulate retention material between the substrate and the housing. While the retention material can be in the form of a mat, particulates, or the like, it will be referred to herein simply as “mat” and it will be understood that the term “mat” as used herein refers to the retention material regardless of what form it is in. 
   Depending on the type of mat, whether it is intumescent or non-intumescent, the necessary stuffing pressure varies. Intumescent mats are called such because they swell under high temperature. This swelling is a property of a component of the mat, typically vermiculite. Non-intumescent mats do not contain vermiculite. These materials can comprise ceramic materials and other materials such as organic binders and the like, or combinations comprising at least one of the foregoing materials. Non-intumescent materials include materials such as those sold under the trademarks “NEXTEL” and “SAFFIL” by the “3M” Company, Minneapolis, Minn., or those sold under the trademark, “FIBERFRAX” and “CC-MAX” by the Unifrax Co., Niagara Falls, N.Y., and the like. Intumescent materials include materials sold under the trademark “INTERAM” by the “3M” Company, Minneapolis, Minn., as well as those intumescents which are also sold under the aforementioned “FIBERFRAX” trademark, as well as combinations thereof and others. 
   The swelling property of intumescent mats is useful because it helps to maintain a positive pressure between the substrate and the housing during the thermal cycle imposed on the converter in normal use without having to greatly compress the mat during manufacture. In use, the diameter of the metal housing increases due to thermal expansion to a greater degree than that of the ceramic substrate. Thus, to maintain a positive pressure, a mat must expand to fill the growing gap as the temperature rises. Non-intumescent mats must be stuffed under much greater force to a high level of compression in order to ensure a continued positive pressure between the substrate and housing during use. This high-force stuffing is more time consuming and takes considerable energy, which significantly increases the overall production cost of the converter. 
   Once the substrate and matting material is stuffed into the housing, the housing may be sized and appropriate connections are formed for assembly into an exhaust system. Sizing operations, when necessary, compensate for variations in substrate diameters, and may comprise compressing the housing to produce an overlapped seam, and then welding, or a housing may be reduced by drawing or compressing the housing using a pipe-sizer. 
   Exhaust pipe connections may be formed in or welded onto either end of the housing. The ends of the shell may be sized, e.g., using a spinform method, to form a conical shaped inlet and/or a conical shaped outlet, thus eliminating the need for separate endcone assemblies in at least one embodiment of the exhaust emission control device. In the alternative, one or both ends of the shell can also be sized so that an end cone, an end plate, an exhaust gas manifold assembly, or other exhaust system component, and combinations comprising at least one of the foregoing components, can be attached to provide a gas tight seal. Thus, the connections include portions having varying cross-sections to conform the stream entering the converter to the shape of the substrate, thereby allowing exhaust to flow from the engine into and out of the converter, and through the remaining exhaust system to the tail pipe. 
   After the exhaust pipe connections are formed on the housing, the emission control device is ready to be assembled into an engine. During the converter&#39;s first use, the converter is heated to normal operating temperature, which may be anywhere from 300° C. to more than 500° C. This first use or heating drives off organic binders within the mat and causes the intumescent material within the mat to greatly expand, thus increasing the pressure within the confines between the housing and substrate. Some substrates, particularly the newer, more fragile substrates, can fail under this pressure, rendering the entire converter unusable. 
   For comparision, a graph showing estimated matting pressure as a function of type of operation during the prior art manufacturing method described above is provided in  FIG. 6 . Starting at the left side of the graph, the intumescent matting is stuffed under very low pressure, i.e., less than 10 pounds per square inch (psi). The pressure is greatly increased to about 150 psi during the sizing operation. After sizing, the matting responds by relaxing somewhat, reducing the stress therein and the pressure to about 100 psi. Finally, the converter is heated, e.g., during its first use, which causes swelling of the matting, which increases the pressure by about 80 psi to 180 psi. The pressure may be even higher locally within the matting material due to variations in the matting or the substrate itself. The fragile ceramic substrate is sometimes unable to stand up to these high pressures and fails. 
   It would be desirable to reduce the likelihood of breaking the ceramic substrate during the production or first use of a catalytic converter. 
   SUMMARY 
   Disclosed herein is a method including sizing a housing of an exhaust emission control device over a substrate and intumescent mat subsequently to heating the emission control device, the heating causing the intumescent mat to at least reach a temperature at which the intumescent mat swells. If the substrate has a washcoat layer, it may be left unfired until after it is assembled with the housing, the heat step serving to fire and sinter the washcoat layer. 
   The above-discussed and other features will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a flow chart illustrating a method to produce a catalytic converter; 
       FIG. 2  shows a first step in the construction of a catalytic converter, according to a prior art method; 
       FIG. 3  shows a subsequent step in the construction of a catalytic converter, according to one embodiment of the disclosed method; 
       FIG. 4  shows a subsequent step in the construction of a catalytic converter, according to one embodiment of the disclosed method; 
       FIG. 5  shows a pressure-time diagram illustrating the advantages of the method shown in  FIG. 1 ; and 
       FIG. 6  shows, for comparison, a diagram showing estimated matting pressure as a function of type of operation, and illustrating the disadvantage of a prior art method. 
   

   DETAILED DESCRIPTION 
   A flow chart diagramming a method for manufacturing a catalytic converter is shown in  FIG. 1 . The method will be described to some extent by reference to  FIGS. 2 ,  3 , and  4 . It has been found that by heating the material prior to sizing reduces the internal matting pressure against the fragile substrate and therefore reduces the likelihood of breakage thereof. 
   Thus, in the preferred process, in a first step  12 , intumescent matting  27  is wrapped around a substrate  25 , and the substrate  25  with matting  27  is stuffed into a housing  29 . If the substrate has a washcoat, it is left unfired prior to assembly. The stuffing operation is ordinarily conducted through the use of a stuffing cone  30 , as shown in  FIG. 2 . The stuffing cone compresses matting  27  to a diameter the same as or slightly smaller than the smallest potential diameter of housing  29 , according to manufacturing tolerances, thus allowing matting  27  and substrate  25  to slide into place within housing  29 . 
   The stuffing operation is done under low pressure, and low mount Gap Bulk Density (GBD). The GBD defines the level of mat compression in grams per cubic centimeter (g/cm 3 ). The preferred mount density for the stuffing operation is less than or equal to about 0.7 g/cm 3 . It is also preferred that the mount density be greater than or equal to about 0.6 g/cm 3 . 
   After stuffing, substrate  25  and matting  27  are positioned within housing  29  as shown in  FIG. 3 , and the assembly is heated in an oven to undergo burn-off and expand step  14  ( FIG. 1 ). It is preferred that the assembly be heated to a temperature greater than or about 500 degrees Celsius. The heating can comprise heating the converter to a maximum temperature less than or equal to about 600° C. Durig this step, organic binders are burned off and the vermiculite or other intumescent component of matting  27  swells. If the substrate has a washcoat, this burn-off and expand step also serves to fire the washcoat, thus seperately firing the washcoat is unnecessary. 
   Following burn-off/expand step  14 , housing  29  is sized by step  16  to bring the GBD to approximately 1.0 g/cm 3 . The target size for each housing may vary depending upon the size of the substrate. Alternatively, the target size may be a dimension that is common for all converters being manufactured that is optimally determined to satisfy GBD requirements within reasonable tolerances. 
   There are several known methods for sizing housing  29 . A preferred sizing technique is described in commonly-assigned U.S. patent application Ser. No. 09/141,299, filed Aug. 27, 1998 by Michael R. Foster, et al., which is incorporated herein by reference in its entirety. In this method, each substrate  25  is individually measured to determine its dimensions prior to stuffing into a housing that does not have any slits. The housing is then sized by compressing it from all directions in a radial press, thereby plastically deforming the housing until it reaches the target size. Such sizing devices are generally known for expanding and diametrically compressing pipes. 
   Another method that can be used, sometimes referred to as a tourniquet or shoebox method, is best for cylindrical housings having one or two slits and includes compressing or tightening the housing, e.g., with a strap or press, until the proper size is reached or the compressing force reaches a selected stop force, then welding the seem or seams. Housing  29  is reduced in size until the target size is reached or a selected stop-force sensed by the sizing machine. In this manner, sizing step may compensate for variations in the size of the substrate. 
   After the sizing step  16 , mat  27  undergoes a relaxation step  18  in which the mat material partially relaxes, reducing the internal pressure. Finally, the connection ends  32  are added to housing  29  by form/weld step  20 . After the sizing step  16 , the mat can be allowed to relax concurrently with adding connection ends to the housing. Form/weld step  20  may comprise any known method of forming connection ends onto housing  29 , either by welding them to the housing  29  or by deforming housing  29  to shape the connection ends. In one preferred embodiment, housing  29  extends some distance on either side of substrate  25  as shown in  FIG. 3 , and undergoes a spin-form process in which rollers progressively shape either end until connection ends  32  are formed as seen in  FIG. 4 . 
   EXAMPLE 
     FIG. 5  shows a diagram showing estimated pressure changes in an example according to the method described above. Initial mounting pressure is shown at the left side of the diagram to be less than 10 psi. Burn-off/expansion increases the pressure to about 80 psi. Subsequent to the burn-off expand step  14 , pressure increases again during sizing step  16 , during which internal mat pressure increases to about 150 psi. This same pressure is attained in the method shown in  FIG. 6  because, as noted above, the internal pressure subsequent to the sizing operation is dependent upon the GBD of the mat. Relaxation step  18  reduces internal pressure by about 50 psi to about 100 psi. Since matting  27  has already been expanded in expansion step  14 , subsequent heating in use of the device will not increase the pressure within the mat to such a degree that substrate  25  is likely to fail. 
   While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to a particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

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