Method of making a cross-flow honeycomb structure

The present invention is directed to a novel method for extruding a honeycomb structure. The method comprises the use of an extrusion die with primary and secondary feed holes and associated discharge slots, wherein extruded material exits the primary discharge slots at a faster rate than the secondary discharge slots. The faster moving extruded material buckles and forms a corrugated layer in between the two secondary extruded layers. The secondary extruded layers form straight cells. The material to be extruded maybe ceramic, plastic or metal.

BACKGROUND OF THE INVENTION 
The present invention relates to cellular cross-flow bodies, as well as an 
extrusion die and method for making such bodies. In particular, the 
invention relates to a die and method of extruding cross-flow honeycomb 
structures useful for various applications such as heat exchange, 
filtration, catalysis, oxygen production, and energy production. In 
cross-flow applications, gases or fluids flow in more than one direction 
through the structure. 
Honeycomb monolithic structures are generally composed of straight 
flow-through cells. However, such flow-through structures are not 
appropriate for certain applications such as where it is desirable to have 
the fluid make several passes through the channels before it is 
discharged, where cross-flow channels are desired such as in heat 
exchangers, or when increased turbulence would be beneficial. Multiple 
passes and prolonged contact lead to more thorough heating and/or cleaning 
as the fluid is allowed prolonged contact with the heat exchanger, 
catalyst or filter. 
Traditional methods for making structures with non-parallel channels or 
passages generally require multiple steps. For example, in one approach a 
cellular ceramic body is cut and plugged so as to form non-parallel flow 
directions. In another approach often used for cross-directional flow 
structures such as heat exchangers and fuel cells, layers of green or 
fired sub-assemblies are formed by frit-bonding. This is the method often 
used for fuel cells where monolithic and planar structures contain 
non-parallel channels for fuel and air such as found in heat exchangers. 
Typical cross-flow structures (e.g., heat exchangers), are formed by first 
extruding a honeycomb-like body of ceramic material from a die orifice. 
This extrusion results in a block of ceramic material having straight-flow 
channels or cells which are generally of square or other rectangular 
cross-section, arranged parallel and adjacent to one another along the 
axis of extrusion. To form a cross-flow structure, portions of the sides 
of the extruded ceramic block are cut away to convert the ceramic block 
having straight-through passages into a composite block having alternating 
rows of straight-through flow, and Z-flow, L-flow, U-flow or other similar 
cross directional flow through the ceramic block. The cross-flow (Z-flow, 
L-flow, etc.) channels are then made by sawing into the sides of some of 
the channels in the ceramic block and afterwards sealing the ends of these 
channels, thereby forming the cross-flow channels. 
Various methods have been suggested for making cross-flow structures for 
example, by sawing and stacking. In general, the suggested methods have 
required multiple steps. For example, in one approach a cellular ceramic 
body is cut and plugged so as to form non-parallel flow directions. In 
another approach, green or fired sub-assemblies of ceramic material are 
stacked and bonded together by sintering or frit-bonding. It has also been 
suggested to use the sawing technique to produce an L-flow cross-flow heat 
exchanger in which both flow directions through the heat exchanger follow 
an L-shaped path. When such sawing techniques are utilized to make 
cross-flow heat exchangers, very high precision extrusion geometries are 
required, as well as high precision cutting equipment, to arrive at a good 
quality finished cross-flow heat exchanger. Imprecision in either the 
extrusion or the cutting equipment can result in leakage paths between 
channels, which has a deleterious effect on heat exchanger performance. 
Further, because such heat exchangers are typically made by sawing into 
the side of the extruded ceramic body, it is very difficult to 
consistently achieve precise uniform cutting of the ends of the ceramic 
body. Such inconsistencies can result in undesirable leakage paths between 
adjacent channels. 
Cross-flow heat exchangers having straight through flow channels in two 
directions have been disclosed in which layers having upstanding ribs 
thereon are laid one on top of another to form a heat exchanger having 
alternating layers of straight through flow channels, every other layer 
being arranged in a transverse direction to the one before it. The 
upstanding ribs of these layers in the green state are relatively weak, 
due in large part to their relative lack of support. Consequently, these 
methods sometimes result in the ribs being bent either prior to or during 
the stacking process. Furthermore, because each directional flow layer 
consists only of one layer of channels, the manufacturing process is 
relatively time consuming and labor intensive. To date, traditional 
extrusion dies have proved inappropriate for producing cellular structures 
of the type described above where the channels are not necessarily 
parallel. Such methods have proved both difficult and expensive for 
non-parallel, cross-flow dies due to the many processing steps often 
required to produce useful dies. 
To overcome some of the above problems, recently in co-pending, co-assigned 
U.S. Ser. No. 08/132,923 (Faber et at.) filed Oct. 7, 1993, now U.S. Pat. 
No. 5,458,834, a method has been suggested for forming self-supporting 
cellular structures by extruding relatively soft batches into a drying 
medium or by contacting the formed structure with a drying liquid 
immediately as the structure exits the extrusion die. 
More recently, in co-pending, co-assigned U.S. Ser. No. 08/341,667 (St. 
Julien) filed Nov. 17, 1994, now U.S. Pat. No. 5,525,291, a moveable die 
is disclosed which is capable of producing honeycomb structures some of 
whose cells may be perpendicular to the axis of extrusion, and which 
structures may be used as cross-flow bodies. 
There continues to be a need for easier, more effective and less expensive 
methods for making cross-directional flow structures and other cellular 
structures in which the cells are not always parallel to the axis of 
extrusion. Accordingly, the object of the present invention is to provide 
an extrusion die and method of making geometrically complex cell 
directions such as cross-flow structures in which the cell directions are 
not always parallel to the axis of extrusion. 
SUMMARY OF THE INVENTION 
Briefly, the present invention relates to an honeycomb structure having a 
matrix of straight cell walls defining a plurality of straight flow cells 
running generally parallel to the axis of extrusion, as well as a matrix 
of cross-directional or corrugated cell walls defining a plurality of flow 
channels some of which may be at an angle, preferably perpendicular to the 
axis of extrusion or to the straight flow channels. Additionally, 
described herein is an extrusion die apparatus and method for extruding 
these unique honeycomb structures. The apparatus is generally comprised of 
a extrusion die having a primary and secondary feed holes and associated 
discharge slots. In a preferred embodiment the die also comprises 
extrudate buckler for causing buckling of the extrudate which exits the 
primary slots. 
In one particularly useful aspect, the invention relates to an extrusion 
die for extruding a honeycomb structure having a matrix of straight cell 
walls defining a plurality of straight cells running generally parallel to 
the axis of extrusion, and a matrix of cross-directional or corrugated 
cell walls defining a plurality of cross-directional cells running 
generally perpendicular to the axis of extrusion. The die comprises an 
inlet portion, an outlet portion with the inlet portion possessing a group 
of primary feed holes for forming the cross-directional cells and a group 
of secondary feed holes for forming the straight cells. Correspondingly, 
the outlet portion possesses a group of primary slots for forming 
cross-directional cells which communicate with the primary feed holes, and 
a second group of associated and intersecting secondary slots for forming 
straight cells which communicate with the secondary feed holes. 
The method for forming the unique honeycomb structure comprises the 
following steps: 
(a) providing an extrusion die having an inlet portion, an outlet portion, 
the inlet portion having primary feed holes and secondary feed holes, the 
outlet portion having a primary slots for forming cross-directional cells 
which communicate with the primary feed holes and secondary associated and 
intersecting slots for forming straight cells which communicate with the 
secondary feed hole; and (b) passing extrudable material from (i) the 
inlet portion to the outlet portion of each secondary feed hole, and 
subsequently to the associated straight cell slot intersection, and 
thereafter downwardly from each associated slot intersection, and (ii) the 
inlet portion to the outlet portion of each primary feed hole, and 
subsequently downwardly to the associated non-intersecting primary slots, 
to form an secondary and primary extrudate, respectively. Furthermore, the 
primary extrudate which exits through the primary slots is caused to 
buckle and form corrugated or cross-directional cell walls to thereby form 
channels or cells which run generally at an angle to the axis of extrusion 
and the extrudate which exits through the straight cell slots form 
straight cells.

DETAILED DESCRIPTION OF THE INVENTION 
The main object of the invention, that is, producing a unitary structure 10 
as depicted in FIG. 1 having cross-flow cells or channels 11, can be 
achieved by using a buckling inducing extrusion die 20 as illustrated on 
FIGS. 2 and 3. This inventive extrusion die 20 is comprised of an inlet 
portion 22 and an outlet portion 24. The inlet portion 22 possesses a 
first group of, or primary, feed holes 28 for forming cross-directional 
cells and a second group of, or secondary, feed holes 30 for forming 
straight cells. The outlet portion 24 possesses a first group of, or 
primary, slots 32 for forming cross-directional cells; these primary slots 
32 communicate with the primary feed holes 28 and are non-intersecting 
with each other. Additionally, the outlet portion possesses a second group 
of, or secondary, slots 34 for forming straight cells; these associated 
and intersecting secondary slots 34 communicate with the secondary feed 
holes 30. 
To form the honeycomb structure of the invention, an embodiment of which is 
depicted in FIG. 1, extrudable material, preferably, plastically 
deformable material, is passed from the inlet portion 22 to the outlet 
portion 24 of each primary feed hole 28 and each secondary feed hole 30 
forming a primary and secondary extrudate. The secondary extrudate 
thereafter passes into the associated secondary slots' 34 intersections 31 
and subsequently passes downwardly from each associated slot intersection, 
whereupon this secondary extrudate exits through the secondary slots 34 to 
form straight cells. 
The primary extrudate thereafter passes downwardly to the associated 
non-intersecting primary slots 32, to form an extrudate. Upon exiting the 
primary slots 32 the primary extrudate is caused to buckle to form 
cross-directional cell walls, i.e., to thereby form channels or cells 
which run generally at an angle to the axis of extrusion. Once the 
adjacent extrudates, primary and secondary, are exiting simultaneously, 
the adjacent cell walls, i.e., the cross-directional cell walls and the 
straight cell walls, are caused to become bonded together. To ensure that 
the cross-directional cell-forming walls bond sufficiently to the adjacent 
straight cell walls, bonding material, preferably a liquid such as water 
or a slip (thin slurry of the extrudable material), can be sprayed down 
the cross-directional cell-forming walls as such wall exits the die to 
improve bonding. Once bonded sufficiently, the extrudate can then be dried 
to form a self-supporting structure and optionally, fired or sintered to 
form a sintered structure. The result is a structure having cells formed 
by the cross-directional cell-forming or primary extrudate which are at an 
angle to the axis of extrusion and straight flow channels or cells which 
are oriented vertically and formed by the straight cell forming or 
secondary extrudate. 
Preferably, the respective exit flow rates, primary and secondary, should 
be controlled such that a differential linear exit flow rate is maintained 
between the primary extrudate and the secondary extrudate, i.e., the 
exiting primary extrudate 41 should do so at a faster linear rate than the 
exiting secondary extrudate 42. Stated another way, the invention 
contemplates that the linear flow of the extrudable material which exits 
through the cross-directional cell slots should be controlled such that it 
is faster than the linear flow of the extrudable material which exits 
through the secondary feed holes. As a result, the extrudate which exits 
through the intersecting and straight slot segments will form straight 
cells and the extrudate exiting through the cross-directional cell slots 
is caused to buckle and to form corrugated or cross-directional cell walls 
thereby forming channels or cells which run generally at an angle to the 
axis of extrusion. 
There are numerous factors which contribute to and can be used to control 
the linear flow rate of the extrudate which exits the cross-directional 
cell and straight cell discharge slots. Among these variables are the 
amount of material which is fed into the respective feed holes, viscosity 
of the material to be extruded, area of the respective feed holes and the 
associated slots feed hole length and speed of extrusion; itself, 
controlled by various factors known to those skilled in the art. Lastly, 
it should be noted that the linear flow exit rate can also be controlled 
by the polishing or roughening of the extrusion walls, i.e., the feed hole 
or discharge slot extrusion wall surfaces. Suffice it to say that one 
skilled in the art would be able to precisely enough control these 
variables to obtain the desired exit flow rate differential, i.e., 
extrudate exiting the primary slot at a faster rate than exiting the 
secondary discharge slot. 
In another embodiment, the die 20 possesses a extrudate buckler 45 for 
causing the buckling effect (i.e., resultant cross-directional cells) to 
be imparted to the extrudate which exits the primary slots, i.e, a device 
for restricting flow of the extrudate at the outset of extrusion. FIGS. 
4A-4D illustrate the use of an extrudate buckler 45 to cause the buckling 
effect necessary to form the cross-directional cells. Specifically, the 
buckling effect is imparted to the primary extrudate 41 exiting the 
primary slots 32 by initially by holding a plate or a flat surface 
apparatus or the like 45 against the die face at the at the outset of the 
extrusion. This extrudate buckler prevents the faster-exiting primary 
extrudate 41, from getting ahead, in a linear sense, of the secondary 
extrudate 42. As a result of being constrained or restricted, the primary 
extrudate 41 necessarily deforms, bunches up or buckles up to produce a 
buckled structure having open cells or channels which run generally in a 
direction perpendicular to the axis of extrusion. The simplest and lowest 
order deformation is a buckling mode as depicted in FIGS. 4A-4D. Once the 
desired buckling is initiated and the cross-directional cell/straight cell 
wall bonding is obtained, the buckler may be removed. The ensuing 
extrusion thereafter continuously produces the inventive honeycomb 
structure, one of a cross-flow configuration as shown in FIG. 1. 
It is contemplated that the strength of the inventive cross-flow 
cross-directional cell honeycomb structure could be increased by 
increasing the buckling frequency of the cross-directional cell-forming 
wall such that a more compact buckled structure is formed as shown in FIG. 
5. In this embodiment, the amount of solid material in the 
cross-directional cell region as well as the coverage and bonding of the 
cross-directional cell-forming walls to the straight cell walls are both 
increased when compared to that embodiment illustrated in FIG. 1. A direct 
result of the cross-directional cell walls exhibiting a more pronounced 
folded effect is that they form more rounded cross-directional flow 
channels. This increased buckling may be accomplished by increasing the 
linear flow rate of the extrudate exiting the primary slots while 
maintaining all other variables, i.e., no change in the secondary 
extrudate exit flow rate or any factors which control this rate. 
Referring now to FIG. 6, illustrated therein is an enlarged view of the die 
depicted in FIG. 3 exhibiting a modified embodiment of the extrudate 
buckler. Specifically, the extrudate buckler is located in a recessed 
region 35 of the die and is comprised of a pair of movable or 
reciprocating diversion rods 51 and 52; the axis of each diversion 
preferably perpendicular to the axis of extrusion. These diversion rods 51 
and 52 function to divert or cause the primary extrudate 41 to buckle and 
to possibly bond with the secondary extrudate 42. 
It is however, contemplated that the diversion rods may, alternatively, be 
positioned at an angle whereby the rods' axis would not be perpendicular 
to the aforementioned axis of extrusion. The resultant honeycomb substrate 
as depicted in FIG. 7 would exhibit cell walls which are not parallel to 
the adjacent secondary cell walls, but are in sense, twisted. 
Specifically the rods function in the following way: Once the primary 
extrudate 41 has exited the primary discharge slot 28 such that the 
extrudate is positioned between the rods, the first rod, either 51 or 52, 
moves in the general direction of the secondary extrudate 42 which is 
simultaneously being extruded The paths of rods 51 and 52 are represented 
by the dotted arrow 51A or 52A. As the rod moves in the direction of the 
secondary extrudate 42 it contacts the primary extrudate 41 and thereafter 
causes the primary extrudate to move toward the secondary extrudate. The 
rod moves the primary extrudate 41 until the extrudate 41 either contacts 
and thereafter bonds, or comes in close proximity, with the secondary 
extrudate whereafter the rod returns to its original position. At a 
predetermined time interval following the return of the first diversion 
rod, the interval dependant upon the desired degree of buckling, the 
second diversion rod is moved in the direction opposite of that traveled 
by the first, i.e., toward the secondary extrudate, until the primary 
extrudate either contacts and is caused to bond, or comes in close 
proximity, with the secondary extrudate. It is self evident that a faster 
interval results in a higher degree of buckling. Once the extrudate is 
sufficiently buckled, the second diversion rod is thereafter returned to 
its original position. This diversion of the extrudate by reciprocating 
rods is thereafter repeated until the desired honeycomb structure 
possessing the desired degree of buckling is produced. 
In a preferred embodiment, the rate at which the primary extrudate 41 exits 
the primary discharge slot 28 and the rod's speed of travel towards the 
secondary extrudate are controlled such that the resultant thickness of 
the primary or buckled cell walls is nearly equivalent to that thickness 
exhibited by the secondary cells walls. 
Alternatively, the primary extrudate and the rod's speed of travel toward 
the secondary extrudate can be controlled such that, as a result, the 
primary extrudate is actually stretched resulting in primary or buckled 
cell walls which are thinner than those exhibited by the secondary cells. 
Other mechanisms contemplated for use as the extrudate buckler, in place of 
the diversion rods, include mechanical, hydraulic or piezoelectric 
flappers as well as, alternating air jets. 
Referring now to FIG. 8, depicted therein is another embodiment of the 
cross-flow structure of the instant invention, i.e., a round extruded 
honeycomb structure possessing both cross-directional flow channels 61 and 
straight-flow channels 62 which are spirally adjacent. The method for 
forming the honeycomb structure of FIG. 8, comprises the aforementioned 
disclosed method though utilizing a slightly modified die wherein the die 
consists of spiralling primary and secondary discharge slots. 
One final embodiment of the cross-flow structure of the instant invention 
(not shown) comprises a round honeycomb structure possessing both 
cross-directional flow and straight-flow channels, however with the 
adjacent cross-directional and straight-flow channels being concentric to 
each other. Again the method of forming this structure would be similar to 
that disclosed herein with the die modified to consist of a plurality of 
concentric circular sections, i.e., a variety cross-directional slots 
concentric to straight cell slots. 
The extrudable material is comprised of solid particles, binder and 
solvent. Useful solid particles for the invention include plastic, natural 
organic materials, metal, intermetallics, carbon, graphite, ceramic, 
glass, glass-ceramic, precursors and mixtures of these powders. 
Particularly useful solid particles include zirconia, titania, silica, 
rare earth metal oxides, alkaline earth metal oxides, first, second and 
third transition metal oxides, talc, clay, alumina, carbon, graphite, 
soluble salts such as alkali nitrates and chlorides, silicone, alkoxides, 
and mixtures of these. 
Useful ceramic powders for the invention include days, talcs, aluminas, 
zirconia, hafnia, titania, cordierite, mullite, spinel, magnesia, 
forsterite, enstatite, (proto)-enstatite, silicas, sapphirine, carbides, 
borides, nitrides and mixtures of these. Particularly useful ceramic 
powders include alpha- and beta-alumina, alumina-chromia solid solutions, 
chromia, mullite, aluminum mullite-chromium mullite solid solutions, 
chromium mullite, sialon, nasion, silicon carbide, silicon nitride, 
spinels, titanium carbide, titanium nitride, titanium diboride, zircon, 
zirconium carbide, zirconia/hafnia alloys, clays, talcs, titania, 
cordierite, magnesia, forsterite, enstatite, (proto)-enstatite, silicas, 
sapphirine, mullite, spinel and mixtures of these. 
Useful binders include precursors of the solid particles, polyvinyl 
butyral, methyl cellulose, ethyl cellulose, polyvinyl alcohol, polyvinyl 
acetate, poly methacrylate, silicone and mixtures of these. 
Useful solvents include, alcohols, glycols, ketones, ethers, aromatic 
hydrocarbons, chlorinated hydrocarbons, esters, dibasic esters, water, 
organic acids, ethanol, isopropanol, tetrahydrofuran, toluene and mixtures 
of these. 
Depending on the required application, the extrudable material may include 
additives such as plasticizers (e.g., high molecular weight alcohols, 
glycols, polyethylene, polypropylene glycols, dibutyl phthalate and butyl 
phenyl phthalate), dispersants (e.g., sodium stearate, fish oil, poly 
glycols, poly glycol esters, phosphates and phosphate poly ethers), 
flocculants and gellants (e.g., acetic acid, propionic acid, isobutyric 
acid, ammonia, ethyl amine, dimethyl amine, diethyl amine, triethyl amine, 
oleic acid, salts and alkoxides), catalytic compound (e.g., base metal, 
base metal oxide, noble metal or a combination of these), as well as other 
additives such as propylene glycol, waxes, oils and surfactants. 
In one preferred embodiment, the extrudable material includes solid 
particles of carbon, graphite and mixtures of carbon and graphite, and the 
binder is polyvinyl butyral, polyvinyl alcohol or a mixture of these. In a 
another preferred embodiment, the structure is formed from solid particles 
of ion exchange resins and polyvinyl butyral as the binder. 
In another embodiment the extrudable material is comprised of a 
pre-cordierite mixture of calcined and a hydride clay, talc, alumina, a 
binder comprising a cellulosic binder such as hydroxy methyl-cellulose, a 
lubricant such as sodium stearate and water as the solvent; for instance 
see U.S. Pat. No. 3,885,977 (Lachman et at.) which is hereby incorporated 
by reference. 
Although the description and illustrative embodiments of the present 
invention have been described in detail with reference to the accompanying 
drawings, it is to be understood that the present invention is not limited 
to those embodiments. Various changes or modifications may be effected by 
one skilled in the art without departing from the scope or spirit of the 
invention. 
For instance, although the instant method and apparatus is described in 
terms of square cell geometries it is contemplated that honeycomb 
structures possessing other cell geometries are within the scope of the 
inventions, i.e., cells with round, hexagonal, triangular, rectangular, 
trapezoidal, and pie-shaped cross sections. Additionally, although the 
instant invention is disclosed in terms of configurations having one set 
of corrugated cell sections for each set of straight cell sections it is 
contemplated that other configuration/permutations within the scope of the 
invention; i.e., one corrugated for two straight, one corrugated for five 
straight etc. Furthermore, it should be noted that the although the sets 
of straight cell sections disclosed above possessed two cell layers, it is 
contemplated that honeycomb structures could be produced which have either 
a one cell layer or multiple cell layers within each section; i.e., 
various configurations of differing straight cell layers are also within 
the scope of the invention.