MXPA97004561A - Fl investment homogenization assembly - Google Patents

Abstract

A method and apparatus for homogenizing a flow stream of plasticized material supplied by an extruder, to reduce the viscosity gradients that cause defects in the extruded products, wherein a flow reversing homogenizer placed in the flow stream and incorporating a plurality of interlacing channels to simultaneously transfer the portions of the flow stream from the central flow to the peripheral flow locations and other portions of the flow stream from the peripheral flow stream to the central flow locations

Description

FLOW INVESTMENT HOnOGENIZER ASSEMBLY BACKGROUND OF THE INVENTION The present invention relates to an apparatus for extruding ceramic products and more particularly to apparatuses for extruding finely structured ceramic parts such as ceramic honeycomb structures at increased speeds and with fewer structural defects. A method of twin worms for extruding complex cross-sectional structures of plastified mixtures of inorganic powders and suitable binders is described in the U.S. Pat. No. 4,551, 295 to Gardner et al. In this process, a charge mixture consisting of inorganic powders of clay, talc and alumina is combined with organic binders and water, and the resulting mixture is plasticized in the extruder. The plasticized filler mixture is then fed to a honeycomb extrusion die mounted at the end of the extruder. It is also known to use perforating or perforating plates or the so-called breakers in the extrusion apparatus, to control the back pressure in the worm section and / or to support screens to remove aggregates from the flow stream. The Patent of E.U.A. No. 2,771,636 describes such use for the extrusion of polymer from a single-worm extruder, while the U.S. Pat. Do not.

No. 3,888,963 discloses a similar homogenizer for use in the extrusion of hammer-shaped ceramic ram. In the latter patent there is also described a flow homogenizer having a mixing function, said homogenizer which incorporates angularly arranged flow passages instead of flow passages parallel to the extrusion direction. This flow homogenizer must promote the localized mixing of the filler material as it is introduced into a tapered compaction chamber upstream of the extrusion die. The feature of the procedure of the U.S. Patent. No. 4,551,295 above is the extensive work of the powder mix that can be extruded which occurs as the mixture is plasticized within the extruder. It requires a complete work of mixing inorganic powders, water and organic binder materials in order to provide a truly homogenous plasticized load. For more efficient work, a co-rotating twinworm extruder is used, the action of the twinworms in these extruders being a particularly effective means of converting the rotational energy into high shear mixing. A fundamental difficulty with the adaptation of worm extrusion processes to the production of finely structured ceramics is the non-even mixing, which has to produce a plasticized mixture of non-even viscosity. A by-product of the mixing process is the generation of heat within the plasticized load. With most of the organic binder systems currently employed in these processes, this heat causes a change in the viscosity of the charge, for example by reducing the viscosity of the charge within the domains of the highest temperature within the mixture. . The unpaired mix also generates areas of cumulative shear nonuniformity (ie areas of different shear history) within the load, with the areas subjected to shear stress more extensively tending to exhibit lower viscosity than the other zones inside the mixture. Because the load is not evenly heated and includes regions of different shear history, some portions of the load fed to the extrusion die will be relatively stiff and will be difficult to extrude, while others, softer portions of the load ex will rush more quickly. In fact, both of these effects tend to decrease the viscosity and increase the extrusion rate at the center of the flow stream through the extrusion die, with respect to the viscosity and speed of extrusion around the periphery of the flow stream. Since the industry has strived to increase production and decrease the cost of producing extruded ceramic honeycombs,. Much effort has been focused on measures to increase the extrusion speed of ceramic loading materials. Unfortunately, attempts to increase the production of twinworm extruders through the use of higher extruder operating speeds causes higher temperature and shear stress disparities within the load, so that the extrusion rates for the materials of Load on the central (axial) portions of the flow stream increases much faster than the speeds for the peripheral portions of the flow stream. The defects of the product such as "snake-shaped" bands and / or the distortion of the contour of the product on the front faces of the extruded structures, in this way increase markedly at said speeds. Other adverse consequences of the higher speeds of operation of the extruder may include persistent residual molding effects of otherwise normal flow interruptions introduced into the filler material flowing through the conventional homogenizer or sieve apparatus. At more conventional extrusion speeds these molding effects are not a problem since the patterns are erased as the loading material passes through the extrusion die. The control of the loading temperature and the maintenance of the uniformity of the loading temperature are in fact endemic problems for the twin worm ceramic sturgeon procedures. Many proposals have been made to address these problems, one of which has been to simply operate the extruder at slower worm speeds, to allow load temperature equilibrium. However, such speed reductions are not economical in the market today since they reduce the production of the process. However, as previously suggested, the operation of the extruder at higher speed also results in reduced production due to the increased incidence of shape distortion in the extruded products. A number of other measures have been tested to alleviate the effects of uneven load heating and defects of the resulting product including proposals such as cooling the barrel and / or extruder worms during the process, changing the load speeds, change the design of the extruder worm, and change the set points of the procedure. In general, however, none of these attempts has succeeded in providing a truly homogenous load. It is evident from a study of the previous restrictions of the procedure that an extrusion method would have a significant economic value, which would allow the faster production of extruded products, without increasing the homogeneities of the loading temperature and / or unmatched viscosities of the load and extrusion speeds. Therefore, a main object of the present invention is to provide an extruder apparatus that improves the homogeneity of the temperature and the uniformity of the viscosity in the plasticized powder loading materials processed in the extruder. Another object of the invention is to provide an extrusion apparatus and an extrusion method that allows the use of faster speeds of the extruder worm and higher extrusion speeds without increasing the susceptibility of extruded preforms for distortion or damage after extrusion. . Other objects and advantages of the invention will be apparent from the description thereof.

BRIEF DESCRIPTION OF THE INVENTION The invention provides an extrusion apparatus and a method for extruding loads of plasticized inorganic powder which offer a significant improvement in the thermal homogeneity of a plasticized filler material which is to be extruded to a degree of extrusion. other means of charging configuration. The result of this improvement is that a viscous plasticized filler material can be obtained more uniformly exhibiting a more uniform flow in and / or through a die or other configuration means. As noted above, the difficulties associated with the operation of the high speed extruder in the prior art have resulted in heating greater than the excess of the central portions of the flow stream, causing an unacceptably low viscosity, coupled with temperatures significantly lower in the peripheral portions, the load current resulting in an undesirably high viscosity. The objects of the invention are achieved through the use of the extrusion apparatus comprising an improved flow homogenizer upstream of the extrusion die, said homogenizer performs cross-transfer in substantial mass of the loading material from the central portions towards the peripheral ones, and from the peripheral to the central portions, of the load flow stream. This mixing or cross-transfer is effective in overcoming the problems associated with uneven loading heating in twinworm extrusion processes as well as other extrusion processes. Then, in a first aspect, the apparatus of the invention is a modification of a conventional extruder, the latter typically comprising a worm section containing one or more worms that can be rotated inside a barrel to mix and transporting or pumping extrudable material, such as a mixture of plasticized inorganic powder, through the extruder. The extruder supplies a flow stream of the plasticized inorganic powder mixture to an extrusion die or other configurator means located downstream of the worm or extruder worms, typically fixed to or otherwise placed at the outlet end of the extruder. According to the invention, the apparatus further comprises an improved homogenizer called a flow inversion homogenizer, said homogenizer having the form of a plate or other body disposed in the flow stream between the upstream worm section and the medium configurator downstream of the extruder. For the purposes of the following description the terms "upstream" "downstream" are applied to the locations, directions, or components of the extrusion apparatus described herein in order to define the positions with respect to the flow direction of the load material stream that can be extruded through the extruder. The flow reversal homogenizer of the invention includes an upstream face, a downstream face, and a plurality of through channels passing through the homogenizer between the upstream and downstream faces. The channels include a first plurality of divergent channels and a second plurality of convergent channels. Each divergent channel has an entry opening in a central portion of the upstream face of the flow reversing homogenizer and a discharge opening in a peripheral portion of the downstream face of the homogenizer, to transfer a portion of the charge mixture. plasticized from a central trajectory to a more peripheral one within the load flow stream.

Each of the converging channels "in the homogenizer has an inlet opening in a peripheral portion of the upstream face and a discharge opening in a central portion of the downstream side thus acting to transfer a portion of the charge mixture. from the peripheral path to the most central or axial path within the load flow stream., the homogenizer is designed to perform a cross-transfer of the mass of the filler material completely within the flow stream and in a relatively short section of the extruder between the worm section and the die or other configurator means provided therewith. In a second aspect, the invention includes a method for homogenizing a flow stream of a plasticized inorganic powder charge mixture to be fed from the worm section of an extruder to a forming means such as a die extrusion disposed downstream thereof. The method comprises transferring the first functions of the plasticized powder loading mixture from the peripheral flow paths to the more central ones within the flow stream, and concurrently transferring second portions of the mixture from the central locations to the more peripheral ones in the flow stream. This method of cross-transferring portions of the plasticized load, ie, exchanging the locations by portions of the plasticized load, introduces higher temperature, centrally flowing load material subjected to higher shear stress, and colder peripheral flow paths. In this way, the cross transfer is very effective to achieve thermal homogeneity and substantially improved shear history in the load flow stream before delivering the load to an extrusion die or other configurating means for configuring the charge in the final product or preform configurations The flow reversal homogenizer of the invention can be used as the sole means for homogenizing charge in the extruder or can be used in combination with another tool for mixing or sieving of the type used presently in the art to improve the quality and uniformity of an extrus load ion before forming with a mold or die. Examples of such optional tool, which may be placed upstream and / or downstream of the flow reversal homogenizer in the load flow stream, include conventional homogenizing or disrupting plates, particle screens, screen support plates, dotted plates, and restriction plates of flow area. A particular preferred combination according to the invention, as fully described below, is a combination of a screen support homogenizer plate and a screen placed immediately upstream of the flow reversing homogenizer.

DESCRIPTION OF THE DRAWINGS The invention can be better understood by reference to the drawings in which Figure 1 illustrates schematically a worm extruder incorporating a flow reversal homogenizer according to the invention; Figure 2 is a schematic perspective view of a useful flow reversal homogenizer in the extrusion of Figure 1; Figure 3 is a schematic top flat vieta of a second embodiment of a flow reversal homogenizer useful in the extruder of Figure 1; Figure 4 is a schematic side cross sectional view of the flow reversal homogenizer of Figure 3;; Figure 5 is a second schematic side cross-sectional view of the flow reversal homogenizer of Figure 3; Figure 6 is a schematic top view of a third embodiment of a flow reversal homogenizer useful in the extruder of Figure 1.

DETAIL DESCRIPTION As will be apparent from the description of the invention provided herein, the method and apparatus of the present invention have application in any of a wide variety of worm or ram extrusion processes. In this way, any extrusion process in which variations in temperature or history of shear stress through a flow stream or other load of extrudable filler material can cause viscosity variations that adversely affect the configuration properties of the extrudate. load, will benefit from the flow reversal as described herein at least from the viewpoint of improved viscosity homogeneity of the load. However, since the invention can be used for particular advantage in the extrusion of twin worms that form cellular honeycomb structures, for example as described in US Patents. Nos. 3,790,654, 3,885,977 and 4,551,295, the following description will refer specifically to those procedures although the invention is not limited in its application thereto The flow reversal provided by the flow horogenizer of the invention has been surprisingly effective to obtain equilibrium both in the temperature and in the viscosity of the charge current in these procedures of the extrusion of twin worms.The efficiency is demonstrated by the fact that the homogenizer has allowed a significant increase in the feed speed of the load Through the extrusion with equal or superior quality in the tool being extruded Through the use of this apparatus, the majority of the deformation problems referring to the different central flow, including shadows in pattern or bands, have been eliminated. swollen in the extruded products, in addition, these defects of the product do not appear again Extrusion rates are significantly higher than those normally used in these processes. In this way, with the homogenizer's own design, the cross-transfer of the charge material within the flow stream is sufficiently extensive that any residual temperature gradient or charge flow homogeneity within the flow stream are not large enough to cause Notable deformations in the structures extruded from the dice. This result is particularly unexpected in view of the difficulty of extruding thin honeycomb structures without defects even from bottom loads not subjected to fast plasticizing processing. As already noted, the flow reversal homogenizer of the invention as described above finds particular application in single-worm design extruders or, more preferably, twin worms. A schematic cross-section elevation view of a twinworm design extruder is illustrated schematically in Figure 1 of the drawing. Referring more particularly to Figure 1, the extruder 10 includes a worm section comprising two extrusion worms 12 arranged inside a barrel 14, the worms being driven by the motor 16 at the entrance end of the barrel. The barrel 14 is provided with an inlet port 18 for introducing the loading material to be mixed and plastified in the extruder. A flow reversing homogenizer placed downstream of the worm section is contained within a cartridge 22 mounted on the outlet end of barrel 14 of the extruder. A screen 24 and a screen support homogenizer 26 are also disposed within the cartridge 22, these being located in front of or upstream of the homogenizer 20 with respect to the flow direction of the loading material being pumped from the worm section. A die 28 of honeycomb extrusion is mounted downstream of the flow reversing homogenizer at the outlet end of the cartridge. In the operation of this apparatus the plasticized loading material pumped from the barrel of the extruder 14 by the worms 12 passes first through the sieve 24 and the sieve support plate 26, then through the flow reversing homogenizer 20, and finally outside the extrusion as a honeycomb structure through the honeycomb extrusion die 28. Figure 2 of the drawing provides a schematic perspective view of a basic design flow reversal homogenizer. As shown in Figure 2 the homogenizer 20 is provided with a plurality of passage channels 31 and 32, those channels that pass through the homogenizer of an inlet or upstream face 27 to a downstream outlet or face 29 of the homogenizer (not shown) located on the opposite side of the homogenizer body. The channels in the homogenizer 20 include a first plurality of centrally disposed (diverging) channels 31, directed radially outwardly. In the operation of the homogenizer, the channels 31 transfer the charge material by colliding in the central portions of the upstream face 27 of the homogenizer of the central or axial portions of the flow stream outward to the peripheral positions within the flow stream leaving the homogenizer on the downstream side 29 thereof. The channels in the homogenizer 20 further include a second plurality of peripherally arranged, convergent channels 32, directed radially inwardly, these channels being directed to the channel loading material which collides in the peripheral portions of the upstream stream 27 of the homogenizer from the peripheral portions of the flow stream inward to the axial or central positions within the flow stream leaving the homogenizer at the downstream face 29 thereof. In the embodiment shown, the converging channels 32 have approximately the same total cross-sectional area as the diverging channels 31, so that the volume of the material transferred to the central portions of the flow stream is approximately equal to the amount of material transferred. to the peripheral portions of the flow streams. For the purposes of the present description, the central paths or flow stream portions within the flow stream passing the flow reversal homogenizer are paths or portions located between the center of the flow stream and one half of the distance to the flow stream. edge of the flow stream, that is, at the radial locations below R / 2, where R is the radius of the flow stream. The peripheral positions or flow-stream portions are those located at radial distances between R / 2 and R from the center of the flow stream. Another example of a flow reversal homogenizer according to the invention is shown in Figure 3 of the drawing, which provides a schematic top plan view of said homogenizer. Referring more particularly to Figure 3, the homogenizer 40 comprises a plurality of channels 42 that open at the peripheral locations on the upstream or inlet side 47 of the homogenizer. The channels 42 direct the loading material of the portions disposed outwardly of a flow stream that strikes on the inlet side 47 inward toward the central portions of the flow stream leaving the homogenizer at the outlets of the channels 42 shown in fading. The homogenizer further comprises a plurality of channels 41 that open at the centrally disposed locations on the inlet face 47, the channels 41 being offset to direct the charge material which collides in the central portions of the entrance face in directions outside. In this way, the channels 41 transfer the loading material to the peripheral portions of the flow stream leaving the homogenizer at the outlets of the channels 41 on the opposite face of the homogenizer, shown in fading. The homogenizer 40 comprises additional features to improve the efficiency of the cross-transfer of the material in the design of the apparatus. These include the portions 43 of relief or recessed surface on the entrance face 47 of the homogenizer, located adjacent and at least partially hollowing the inlets to each of the converging channels 42. The recesses 43 help to collect and direct the material of load striking the peripheral portions of the entrance surface 47 in the channels 42, reducing the material feedback in the channels 41. The portion 45 of relief or recessed surface on the entrance face 47 is also provided, said portion of surface at least covering or recessing the inlets to the diverging channels 42. The recessed surface 45 helps to collect and direct the material that collides in the central portion of the inlet surface 47 in the channels 41, reducing the material feedback in the channels 42. Figure 4 of the drawing is a schematic cross-sectional view of the flow reversal homogenizer 40 throughout of line 4-4 of figure 3. The relief surfaces 43 are shown more clearly in figure 4 and the orientations of the channels 42 directed inwards, said channels crossing the flow reversal homogenizer 40 from a location towards outside on the entrance face 47 thereof to a more central location on the downstream side or exit 49 thereof. Figure 5 of the drawing establishes a schematic cross-sectional view of the flow reversal homogenizer 40 along the line 5-5 of Figure 3. Said figure shows the arrangement of the recess 45 as well as the orientation of the directed channels 41 outwardly, the latter passing through the homogenizer 40 from the central locations on the inlet side 47 to the peripheral locations on the downstream side 49 thereof. Yet another design for a flow reversal homogenizer according to the invention is set forth in Figure 6 of the drawing, which is a schematic top plan view of the upstream or inlet surface of the homogenizer. As shown in Figure 6, the furnace 50 shows a large number of flow channels than the homogenizer of Figure 3, including "10 diverging channels 51 alternating with 10 converging channels 52. An important aspect of the design of the homogenizer 50 is which equalizes the area of the portion of the entrance surface within the boundary line 55, which must supply the channels 51, with the area of the entrance surface outside the boundary 55, which must supply the channels 52. The current portions of the load flowing on the edge that collide at the points outside the limit 55 will tend to flow towards the converging channels 52, with decreased probability of entering the divergent channels 51. The rest of the load current will then be construed to enter the channels divergent 51. At least regions 53 and 53a adjacent to channels 51 and 52, respectively, are recessed, the recesses 53 and 53a helping to collect and direct portions of the flow stream that collide thereon in their respective associated channels 51 and 52. In the particular embodiment shown, a further gap in the entrance surface of the homogenizer 50 is provided as a circumferential groove v, limited at its inner edge by the boundary 55, at its outer edge by the boundary 55a, and with its vertex which follows the center line of slot 55b. This slot v is superimposed on and connects the recessed portions 53a, acting to further reduce any tendency for the load current portions colliding at the edge to enter the diverging channels 51 located centrally. A key functional feature of the invention's flow reversal generators, derived from the arrangement of multiple channels directed within an individual body, is the advantageous effect of "a simultaneous large-scale cross-transfer of the loading material within? It will be evident that, for this purpose, not all channels have to be within a solid structure, although it is very preferred, alternatively ensalee of interlacing tubes can be used between the multiple opposing plates, or other structures designed to perform approximately one-to-one cross-transfer of nearly equivalent volumes of material within a flow stream.The essential requirement is to ensure cross-distribution of significant volumes of load material over a continuous individual length of the flow path in order to obtain an orderly exchange of materi to high temperature due to low temperature in the continuous extrusion environment. By taking into account equivalent cross-sectional areas of convergent and divergent channel, the homogenization of the viscosity can be improved without the need to induce turbulent flow to force the proper mixing. Another important feature of the homogenizers of the invention is that the size of the channel, which directly impacts the proportion of the flow stream subjected to the realignment of the flow path. For the useful kiln-heating it is considered that the aggregate cross-sectional area of the channels (the sum of the individual cross-sections of the channel) in the arrangement must constitute at least 10% of the total cross-sectional area of the flow stream. In the homogenizer, the cross-sectional area of the flow stream is considered to be substantially the area of the face upstream of the homogenizer, More desirably, an aggregate cross-sectional area of the channel constituting 15-30% will be provided. In general, the reduction in the viscosity gradient through the flow stream will be approximately proportional to the volume of the material exchanged, it is expected that they will be equally important to other factors, such as the efficiency of the homogenizer design to prevent undesirable upstream cross-feeding of the axial segments of the flow back into the converging channels (axially directed) of the homogenizer. The flow reversing homogenizers provided in accordance with the invention can also be characterized in terms of an inversion efficiency factor, a factor that reflects the magnitude of the radial displacement of the material towards or away from the central axis of the flow stream carried out by the homogenizer The large transfer factors result in lower thermal / residual viscosity gradients in the homogenized charge. That is, since the colder, more viscous segments of the flow stream that collide on the upstream face of the generator are those closest to the barrel of the extruder., and since the hotter, less viscous segments are in the axis of the flow stream, the largest radial repositioning that can be practiced of each of these segments of the flow stream is generally desired. The investment efficiency factor Ei of a homogenizer can be calculated from the expression: so ? Ei = where t is the thickness of the homogenizer plate,? is the longest of the angles of the convergent and divergent channels of the homogenizer with respect to the flow axis of the charge current (usually but not necessarily the same), and r is the radius of the flow of current from the worm section (the largest radius in the case of oval or other non-circular currents) that collides in the homogenizer. The investment efficiency factor is limited practically by the considerations that relate to the strength of the homogenizer plate, flow impedance, and channel wear velocity, each of which will have an impact on channel sizes, angles "of the selected channel and channel thicknesses, however, the efficiency factors of at least approximately 0.25, more typically 0.3-0.5, can be practically obtained and these are the values that will normally be selected for better results. , the flow homogenization according to the invention can be carried out without requiring sieving, mixing or other processing of the charging current, if desired, however, while not suitable as a sieve support itself, the Flow inverter can be used very effectively to add support to a sieve and a sieve support plate arranged in the flow stream The latter is normally required in any case to properly screen and condition the extrusion load through the downstream extrusion die. A particularly preferred flow reversal assembly for the operation of the high speed extruder according to the invention is a combination of the flow reversing homogenizer with a thin, highly perforated homogenizer / ta tand carrier plate, the last plate with its sieve or support screens being placed against and just upstream of the flow inversion homogenizer in the flow stream of the worm section of the extruder. The screen support plate will more preferably comprise a plurality of small parallel holes, widely spaced, across the entire cross section of the flow stream, the close spacing of the holes being allowed by the support to the plate offered by the homogenizer adjacent. The invention can be further understood by reference to the following example of the operation thereof.

EXAMPLE A twin screw extruder used in routine commercial production for the fabrication of cordierite ceramic honeycomb (MgO-A1203-SiO2) structures of a plasticized filler mixture comprising clay, talc is selected for modification. , alumina, organic additives and water. This extruder is retroaused with a cartridge incorporating a flow reversing homogenizer assembly substantially as shown in Figure 1 of the drawing. The flow reversal homogenizer used in the cartridge assembly has a structure corresponding to that schematically illustrated in Figure 3 of the drawing, which comprises 8 cross flow channels having a cross-sectional area of the aggregate channel "comprising about 25% of the cross-sectional area of the flow channel through the cartridge The efficiency factor of inversion of the homogenizer is approximately 0.29.

O E A perforated screen support plate is placed in the cartridge upstream of the flow reversal homogenizer, which supports a screen assembly to remove the particles from the charge flow stream. The screen support plate incorporates a plurality of holes of 6.3 nm (diameter) punched in a very separate hole pattern across the entire cross section of the flow channel. The die mounted on the output end of the cartridge is the same as it was used for conventional commercial production. The installation of the described flow reversal homogenizer assembly provides a significant improvement in the extrusion production capacity. During the operation of the extrusion without the assembly, increasing the speed of the operation of the extruder to provide bending of the honeycomb extrusion speed increases the typical defect rate for honeycomb defects related to viscosity such as "snake-shaped bands" by approximately 100%. However, with the assembly in operation, and maintaining the normal extrusion rate at double, the defect velocity is reduced to less than 10% of the typical defect velocity incurred at normal speeds, and less than 5% of the velocities of Defects found when operating the extruder at the higher extrusion speeds without the flow reversal assembly installed.

Claims (15)

NOVELTY OF THE INVENTION

1. - Extrusion apparatus comprising, in combination: an extruder means for providing a flow stream of a mixture of plasticized inorganic powder in an extrusion die placed downstream of the extruder means; and a flow inversion furnace placed in the flow stream between the die assembly and the extruder means.

2. Extrusion apparatus comprising, in combination: an extruder means for providing a flow stream of a plasticized inorganic powder mixture in a forming means placed downstream of the extruder means; and a flow inversion homogenizer placed in the flow stream between the forming means and the extruder means? the flow reversing homogenizer comprising a plurality of passage channels for transferring a portion of the plasticized mixture from a peripheral path to a central within the flow stream, and simultaneously transferring a portion of the plasticized mixture from a central path

3. The extrusion apparatus according to claim 2, further characterized in that the forming means is a honeycomb extrusion die and wherein the flow inversion homogenizer is a body. which comprises an upstream face, a downstream face, and a plurality of passage channels that traverse the body between the upstream and downstream faces, the passageways comprising a first plurality of diverging channels that terminate at the peripheral locations within the flow stream and a second plurality of convergent channels ending in the central locations within the flow stream.

4. Extrusion apparatus according to claim 2, further characterized in that each divergent channel has an entry opening in a central portion of the upstream side and a discharge opening in a peripheral portion of the downstream side, and wherein each converging channel has an entry opening in a peripheral portion of the upstream face and a discharge opening in a central portion of the downstream side.

5. Extrusion apparatus according to claim 2, further characterized in that the extruder means is selected from the group consisting of a single-twin extruder, twinworm extruders, and ram extruders.

6. Extrusion apparatus according to claim 2, further characterized in that the extruder means is a twinworm extruder.

7. Extrusion apparatus according to claim 3, further characterized in that the plurality of convergent channels has an aggregate cross sectional area equivalent to the plurality of divergent channels. 8.- Extrusion apparatus according to claim 3, further characterized in that the plurality of passage channels has an area in aggregate cross section that constitutes at least 10% of the cross-sectional area of the upstream side of the channel. flow inversion homogenizer. 9.- Extrusion apparatus according to claim 8, further characterized in that the plurality of passage channels has an aggregate cross-sectional area constituting 15-30% of the cross-sectional area of the upstream face of the reversing homogenizer of flow. 10. Extrusion apparatus according to claim 8, further characterized in that the flow reversal homogenizer has an inversion efficiency factor of at least 0.25. 11. A method for homogenizing a flow stream of a plasticized inorganic powder mixture comprising: transferring the first portions of the mixture from the peripheral to the central locations in the flow stream and concurrently transferring the second portions of the mixture from the central locations to the peripheral ones in the flow stream. 12. A method for homogenizing a flow stream of a plasticized inorganic powder charge mixture supplied from the worm section of a worm extrusion to the forming means placed downstream of the worm section comprising transferring the first portions of the plasticized powder loading mixture from the peripheral flow paths to the centrals within the flow stream, and concurrently transferring the second portions of the mixture from the central to the peripheral flow paths in the flow stream. 13. A method according to claim 12, further characterized in that the worm extruder is a twin-screw extruder, wherein the forming means is a honeycomb extrusion die, and wherein the transfer of the first and second portions of the charge mixture reduce the inhogeneity of the viscosity of the flow stream. 14.- A method according to the claim 13, further characterized in that the transfer of the first and second poeicionee of the flow stream is carried out by a flow reversal homogenizer comprising a plurality of interlacing channels for transferring the portions. 15.- A method in accordance with the claim 14, further characterized in that the first and second portions transferred by the flow reversal homogenizer comprise at least 10% of the flow stream.