FIELD

[0001] The processes and apparatus disclosed herein are in the field of technical ceramics and are particularly applicable to the manufacture of ceramic structures such as ceramic bodies like honeycombs by processes that include the extrusion of plasticized ceramic- forming powder mixtures through extrusion dies.

BACKGROUND

[0002] The manufacture of ceramic honeycomb structures by processes that include the preparation of plasticized ceramic-forming powder mixtures, the extrusion of the mixtures through honeycomb extrusion dies to form honeycomb extrudates, and the drying and firing of the extrudates to produce strong, refractory ceramic honeycomb structures, is known. Ceramic honeycomb structures thus manufactured are now widely used as substrates for the support of catalysts to treat combustion exhaust gases, e.g., from motor vehicles, and as porous wall flow filters for trapping exhaust particulates from sources such as diesel engines. [0003] Initial use of such honeycombs was for the treatment of automotive exhaust gases, where relatively small and light structures were adequate to support sufficient catalyst for the substantial removal of carbon monoxide, nitrogen oxides and unburned hydrocarbons from the exhaust stream. The successful production of much larger honeycombs by extrusion methods has required substantial advances in the art.

[0004] Among the problems arising in the production of larger honeycombs is that of maintaining control over the geometry and integrity of the products. Structural defects including cell or channel distortions, internal channel wall defects, and surface defects such as surface or skin cracking can frequently arise, and can be particularly difficult to control at the larger extruded honeycomb sizes.

[0005] In the production of larger honeycombs, large, radially oriented surface or skin cracks can develop in the wet extrudate, which cracks can extend relatively deeply from the surface into the honeycomb interior. Diagnosing the cause of this defect is complicated by the fact that cracking problems can be intermittent, seldom appearing at the start of a production run but often arising later, and increasing in severity, as the production run continues. SUMMARY

[0006] The methods disclosed herein include a method for forming an extruded body comprising the steps of forcing a ceramic-forming material in an extrusion direction through a tubular enclosure and through an extrusion die, the tubular enclosure comprising a friction- reducing inner surface. Particular embodiments of those methods include forming a honeycomb extrudate from a plasticized ceramic-forming powder mixture by the step of conveying the plastic mixture from an extruder to a honeycomb extrusion die through a connecting enclosure having a friction-reducing inner surface.

[0007] The methods disclosed herein include embodiments wherein the friction-reducing inner surface comprises a plurality of protrusions defining a plurality of troughs or indentations on the inner surface. The protrusions may comprise ridges, the troughs may comprise grooves, and the indentations may comprise dimples. These methods of delivering plastic mixtures from an extruder to an extrusion die can reduce or eliminate changes in material flow of plasticized ceramic-forming material that can arise in connecting enclosures, whether from contact with enclosure walls or from other causes.

[0008] Apparatuses for achieving the described process improvements include modifications to equipment for the manufacture of honeycomb extrudate from a plasticized ceramic- forming powder mixture, i.e., equipment including an extruder, a honeycomb extrusion die, and a connecting enclosure joining the extruder with the die for the conveyance of plasticized ceramic-forming mixture therebetween. Embodiments of the apparatus include a connecting enclosure incorporating a friction-reducing inner surface.

[0009] In some embodiments, the friction-reducing inner surface will be provided by a surfacing layer such as a coating or enclosure liner that is formed of a solid material differing in composition from the body or subsurface of the enclosure. The material used will have a coefficient of friction lower than the body or subsurface of the enclosure. In other embodiments, the friction-reducing inner surface will be a textured inner surface, the texture being designed to reduce the contact area between the plasticized ceramic-forming mixture and the enclosure inner surface as the mixture is conveyed to the extrusion die. In still other embodiments, a combination of an inner surfacing layer of low- friction material provided with a textured surface can be used. DESCRIPTION OF THE DRAWINGS

[0010] The methods and apparatus herein disclosed are further described below with reference to the appended drawings, wherein:

[0011] Fig. 1 is a schematic illustration of conventional apparatus for the production of honeycomb extrudate by extrusion through a honeycomb die;

[0012] Fig. 2 is a schematic illustration of an embodiment of a connecting enclosure as described herein;

[0013] Figs. 3a-3c present enlarged schematic illustrations of sections of friction-reducing surfaces useful in combination with the enclosure of Fig. 2;

DETAILED DESCRIPTION

[0014] We have now identified a likely contributing cause of surface cracking in large honeycomb extrudate, i.e., a larger than expected decrease in the flow rate or extrusion speed of extrudate exiting the periphery of the die to form the peripheral segments of the shaped honeycomb extrudate. A relatively low rate of extrudate flow to surface or skin portions of the extrudate, combined with a relatively high rate of flow through central portions of the extrusion die, can produce surface cracking if the differential in flow rate is large enough. Further investigations based on this finding revealed a tendency of the plasticized powder mixtures to develop domains of slow flow or even plugging at locations around the outer regions of the honeycomb extrusion die inlet face, even after relatively short intervals of production. This effect was found to increase at slower extrusion rates. Yet initial flow rates through the dies were determined to be relatively even across the entire diameters of the die inlet faces.

[0015] Present analyses suggest that flow effects arising upstream of the honeycomb extrusion dies are likely involved in the development of slow peripheral flow and die plugging. While not intending to be bound by theory, it is speculated that a change from plug flow at the extruder outlet to a more laminar flow mode in the course of transit through connecting hardware to the inlet of the extrusion die, as the result of friction between the plasticized ceramic-forming mixture and the walls of the connector, plays a major role. [0016] Regardless of the underlying cause, the methods and apparatuses herein disclosed have been found to effectively reduce or substantially eliminate extrudate surface cracking. These results are obtained through steps to modify the flow of plasticized ceramic-forming powder mixture from the extruder outlet to the die, the modifications being effected through improvements to the structures of the conduits or connecting enclosures customarily provided to connect extruder outlets to extrusion die inlets.

[0017] While not limited in its application thereto, the methods and apparatuses disclosed herein have particular benefits for the production of large ceramic honeycomb structures, e.g., structures with honeycomb extrudate dimensions or cross-sections transverse to the axis of material conveyance through the apparatus that are in excess of 25 cm in diameter, or in excess of 750 cm ² in area. The above-described radial cracking problems can appear more frequently in the production of such structures than in the production of smaller honeycomb shapes.

[0018] An apparatus suitable for the manufacture of both large and small honeycombs in accordance with the disclosed methods is schematically illustrated in elevational cross- section in Fig. 1 of the drawings. Apparatus 10 includes extruder 12, honeycomb extrusion die section 14, and a connecting enclosure 16 joining extruder 10 to die 14. A plasticized ceramic-forming powder mixture (not shown) is mixed and conveyed by twin extruder screws 18 from the extruder 12 toward honeycomb extrusion die section 14. The function of connecting enclosure 16 is to channel and shape the plasticized mixture for delivery to die section 14. Although shown as a separate machine element in Fig. 1, alternative arrangements of the apparatus wherein the enclosure 16 is an extension of, or integral with, either extruder 12 or die section 14, are also contemplated herein. Methods and apparatuses disclosed herein include plasticized mixtures of ceramic-forming material that are forced through the enclosure via a single screw or twin screw extruder or a ram extruder are useful. [0019] Known connecting enclosures used in apparatuses arranged as above described constitute a source of difficulties with respect to flow velocity at the periphery of the plastic extrudate issuing from the die. Known enclosures are typically formed of pressure- and wear-resistant materials such as steel which we have found to offer significant impedance to the flow of peripheral segments of the flowing mixture, due to frictional interactions between the steel surface and the plasticized mixtures. As a consequence, the center of the flowing charge of plasticized ceramic-forming mixture tends to travel faster than the periphery, translating into a lack of a sufficient flow of mixture to peripheral regions of the die. [0020] Slow peripheral flow, particularly at slow extrusion rates and/or during the extrusion of large-diameter parts, appears to be caused by "dead zones" in the peripheral flow stream, or even plugging of edge portions of the die. The faster flow of core material then creates peripheral tension manifested in the development of radial surface cracks. Particularly in the case of larger honeycombs, relatively wide cracks extending from the skin into sub-skin sections of the honeycomb extrudate have been found to form.

[0021] In the apparatuses and methods disclosed herein, the inner surface of the connecting enclosure between the mixing/conveying screw elements of the extruder and the die section for forming the mixture into honeycomb extrudate is "friction-reducing". That capability of reduction in friction increases material flow in peripheral portions of the flowing charge of plasticized ceramic-forming material being conveyed through the enclosure to the die. [0022] Reductions in frictional interactions between the plasticized mixture and the inner surface of the enclosure are attained through one or a combination of (i) changes in the material forming the inner surface of the enclosure and (ii) changes in the surface profile of those surface portions. By the inner surface of the enclosure is meant either the entire surface in contact with the plasticized ceramic-forming mixture, or lesser portions of the inner surface, if sufficient in area to achieve the required improvement in peripheral flow. [0023] Changes in the material forming the inner surface can be effected through the application of a surfacing layer of a material differing in composition from the body or subsurface of the enclosure and having a coefficient of friction lower than that subsurface. Durable polymer surfaces, provided by coatings or by liners composed of abrasion-resistant, low-friction polymeric material are examples. Changes in the surface profile of the inner surface to reduce surface friction involve impressing or otherwise forming a texture in that surface, the texture being one that can reduce the net area of contact between the flowing plastic mixture and the enclosure inner surface.

[0024] Fig. 2 of the drawings is an enlarged schematic illustration in elevational cross-section of one embodiment of a connecting enclosure useful in the practice of the methods herein described. In that particular embodiment, changes to both the material composition of the inner surface of the enclosure, and to the texture or surface profile of that surface, are adopted.

[0025] Referring more particularly to Fig. 2, enclosure 16 includes an outer shell 20 formed, for example of a high-tensile strength material such as steel, and an inner liner 22 formed, for example, of ultra-high molecular weight polyethylene (UHMWPE) polymer. The desired characteristics of enclosure surfaces such as shown in Fig. 2 include a low coefficient of friction and good wear resistance, since many of the plasticized ceramic-forming powder mixtures processed in accordance with these methods are abrasive in nature. Other low- friction polymers such as fluorocarbon polymers, or even reduced-friction inorganic surfacing materials such as nickel or tungsten carbide, may also be employed. Again, while the embodiment of Fig. 2 includes a liner fully covering the inner surface of the connecting enclosure, embodiments provided with reduced friction surfaces over only portions of that surface can also provide useful results.

[0026] A further feature of the enclosure of Fig. 2 is a textured surface 24 impressed upon liner 22. In the particular embodiment shown, textured surface 24 comprises a succession of grooves 26 oriented transversely to arrow 28 through the central opening of enclosure 22. Arrow 28 corresponds to the axis of honeycomb extrusion or direction of material conveyance along which a plasticized ceramic-forming mixture 30 flows through connecting enclosure 16 to an extrusion die section, not shown.

[0027] Grooves 26 in Fig. 2 may be provided as a series of annular grooves in surface 24, or as a single helical groove traversing that surface from the inlet to the outlet of liner 22 within the enclosure. In some embodiments both the annular and the helical grooves in liner 22 are oriented in generally circumferential directions that are predominantly transverse to the direction of material conveyance (28) in Fig. 2. By predominantly transverse is meant a direction of groove alignment that diverges from the direction of material conveyance by more than 45°.

[0028] While textured surfaces such as surface 24 of the particular embodiment of Fig. 2 comprises surface grooves, other textured surfaces could alternatively be used, examples of such surfaces including pebbled surfaces and surfaces comprising indentations in the form of dimples as well as troughs. Without intending to be bound by theory, it is thought that such textured surfaces may operate to reduce friction by, among other things, reducing the area of contact between plasticized ceramic-forming mixture and the surface of enclosure 16. [0029] Figs. 3a - 3c of the drawings provide schematic illustrations of sections of polymeric liners suitable for modifying connecting enclosures, those liners incorporating grooved surfaces that could be utilized as textured inner enclosure surfaces for reducing friction in the practice of the methods herein disclosed. Fig. 3a illustrates a section 22a of a polymeric liner incorporating a textured surface 24a that incorporates grooves and ridges of sinusoidal or corrugated configuration, while section 22b in Fig. 3b includes a surface 24b incorporating grooves of saw-toothed configuration and section 22c of Fig. 3c includes a surface 24c of truncated saw-toothed configuration.

[0030] Suitable dimensions for textural features such as the grooves as illustrated in Figs. 3a- 3c will depend on the particular composition and properties of the polymeric inserts, and also upon the properties of the plasticized ceramic-forming mixtures being processed, but may readily be determined by routine experiment. As one example, for the case of UHMWPE liners of the type hereinabove described, groove depths and groove spacings are both suitably in the range of about 0.3-0.7 mm.

[0031] As noted above, another friction-reducing effect may arise from the interaction between textured surfaces and plasticized mixtures such as currently employed for ceramic honeycomb production. Those mixtures can incorporate a major inorganic component (such as powder) and a minor fluid vehicle component, with the vehicle component often including one or more lubricants. Thus the indentations in the disclosed textured surfaces may function as reservoirs for lubricating fluid vehicle constituents, e.g., oil components, collected from the flowing plasticized ceramic-forming mixtures traversing the connectors. Alternatively or in addition, the indentations may act as reservoirs for the plasticized mixture itself. In either case the flow past the connector contact surface becomes a "batch-on-batch", batch-on-oil", or "batch-on-oil-on-batch" flow that significantly reduces the level of intimate contact between the mixture and the actual inner surface of the connector. [0032] Regardless of the mechanism underlying the observed function, the low-friction and/or textured surfaces disclosed herein, whether provided in the form of enclosure liners or otherwise, have the net effect of facilitating or otherwise modifying peripheral flow such that the flow profile is more uniform for the extrudate issuing from a die section as described above, and the extrudate remains substantially free of radial cracks and other forms of shape deformation.

[0033] As noted above, embodiments of the methods disclosed herein include methods wherein the friction-reducing inner surface comprises a plurality of protrusions defining a plurality of troughs on the inner surface, including embodiments wherein the troughs form grooves in the inner surface and wherein the protrusions are in the form of ridges. Particular examples of embodiments featuring a plurality of protrusions include embodiments wherein the protrusions are between 0.005 and 0.050 inches tall. Examples of the disclosed methods featuring grooves in the inner surface include embodiments incorporating annular or helical grooves.

[0034] Embodiments wherein the protrusions are ridges include those comprising ridges that are spaced apart by 0.005 to 0.050 inches, or spaced apart by 0.010 to 0.040 inches. Also included are embodiments wherein each ridge is comprised of a respective pair of inclined walls terminating in a respective crest. Examples of the latter embodiments include those wherein the paired inclined walls of at least some of the ridges form an angle of between 50° and 70° with respect to each other. The respective crests in any of those embodiments comprising inclined walls may be pointed, flattened or rounded. [0035] In those embodiments of the above-described methods wherein the protrusions in the friction-reducing inner surface are in the form of ridges, the height of the ridges and/or the distance between ridges may be adjusted based on the hydraulic diameter of the cavity defined by the friction-reducing inner surface of the tubular enclosure. Thus embodiments of the above-described methods include those wherein a ratio of the hydraulic diameter divided by the distance between two adjacent ridges is between 200 and 1000. Also included are methods wherein a ratio of the hydraulic diameter divided by the height of the ridges is between 200 and 2000, e.g., between 200 and 1200.

[0036] The present disclosure additionally provides methods of forming honeycomb extrudate wherein the extrudate has at least one of (i) a cross-sectional dimension transverse to the extrusion direction in excess of 25 cm in size, or (ii) a cross-sectional area transverse to the direction of extrusion in excess of 750 cm ² in area. Such methods provide honeycomb extrudate of the disclosed large cross-sections that are substantially free of radial surface cracks, and may include embodiments wherein the extrudate comprises a plasticized ceramic- forming mixture incorporating a major inorganic component and a minor fluid vehicle component, and wherein the fluid vehicle component includes at least one lubricant. [0037] Methods for forming honeycomb extrudate as described include embodiments wherein the friction-reducing inner surface is a textured surface, such as a surface that incorporates one or more surface grooves having groove orientations in directions predominantly transverse to the direction of extrusion or axis of conveyance of the plasticized ceramic-forming powder mixture through the enclosure. Examples of such surface grooves are grooves that form a corrugated or saw-toothed inner surface.

[0038] The benefits of the methods and apparatuses herein disclosed are not limited to the improvements in surface integrity illustrated above. Through the use of the apparatuses and methods disclosed herein, the extrusion of a wider range of plasticized ceramic-forming powder mixture compositions and properties becomes possible, and such extrusion can be practiced over a wider range of extrusion speeds and extrudate sizes than can be successfully employed, for example, for defect-free honeycomb extrusion. [0039] While the foregoing descriptions have been offered with respect to particular embodiments of the methods and apparatus herein disclosed, it will be appreciated that such embodiments have been included for purposes of illustration only, and that other embodiments may readily be practiced for particular purposes without departing from the scope of the appended claims.