Fluid filter and method of manufacture

In a process for the manufacture of a filter for liquid or gaseous media, especially for exhaust gases of combustions engines, the filter is formed from one or several filter plates out of a mesh, network, reticulation or fabric of metal wires. The filter body of a plurality of layers or plies of wires is brought between the electrodes of a resistance welding unit and subsequently a resistance welding is performed for partial connection of the wires to one another by spot welding.

TECHNICAL FIELD 
The invention relates to a process for the manufacture of a filter for 
liquid or gaseous media, especially for exhaust gases of combustion 
engines, which consists of one or several layers, plies or plates made out 
of a mesh, network, reticulation or fabric of metal wires; and to a filter 
thereby manufactured. 
BACKGROUND OF THE INVENTION 
Filters are already known which consist of a plurality of layers or plies 
of metal fibers which are sintered together. Sintering is generally done 
in a furnace or oven under pressure and at a suitably high temperature, 
care being taken at the same time to provide that a vacuum is present. 
A disadvantage of such a process is, however, that this method or process 
is relatively expensive and frequently, depending on the medium to be 
filtered, there also exists the problem that arises when the filter plugs 
or clogs up. Then the filter must either be replaced or cleaned in a 
costly and/or time-consuming way. 
SUMMARY OF THE INVENTION 
An object of the present invention is, therefore, to create a process or 
method for the manufacture or production of a filter, and to thereby 
provide a filter which is easy to manufacture or produce and which has a 
good filtering performance or capacity. 
According to the invention, this object is solved in that a filter plate 
consisting of a plurality of plies or layers of wires is placed between 
the electrodes of a resistance welding device or unit, the wires are 
pressed against one another, and subsequently a resistance welding 
operation is performed for partial joining or connecting of the wires to 
one another. 
A filter manufactured or produced in this way can be made in a much simpler 
and less expensive way than known filters. In the inventive process the 
manufacture or production can take place in a single operation, with 
resistance welding devices or units of a known type. During resistance 
welding, the individual or single wires are spot welded together 
essentially solely in individual lumps, dots or points in a punctiform 
structure. Due to this structure, they do on the one hand form a compact 
unit, but on the other hand, a high flow-through capacity is ensured, for 
a multiplicity of pores or openings are left available for the filtering 
operation. 
In a simple way, following resistance welding, the filter body can be 
brought into the desired filter form or shape and, as the case may be, may 
be formed by connecting several plates into a single filter unit 
In an improvement of the invention, provision is made for the individual 
layers or plies to consist of wires having different thicknesses. By this 
measure, which can be implemented without any problems in the production 
process according to the invention, the filters can each be optimally 
matched to the demands made upon them and to the fields of application. 
They can, for example, be used in the area of the inlet side of thinner 
and/or finer wires, whereby, in particular in the case of a gaseous 
medium, entrained particles can immediately be held back or retained on 
the surface and only a reduced portion or percentage of the smaller 
particles will be penetrated into the filter medium at all. 
This measure has the advantage that a reduction of the filter performance 
or capacity is thereby avoided, as is the case in the state of the art due 
to plugging, clogging or fouling of the filter with corresponding 
particles. On the other hand, coarser and thicker or stronger wires in the 
central area and/or in the area of the outlet side produce a corresponding 
strength for the filter body. 
It goes without saying that also any other combination of wire thicknesses 
or strengths can, however, be used when required. 
In a simple way, with the process according to the invention, wires with 
differing material compositions and/or differing coatings can also be 
welded to one another, which further extends or expands the range of 
application of the filter according to the invention. In particular, 
different effects can thereby be exercised on the medium to be filtered. 
For example, catalytic effects can be achieved, which is of great 
advantage, particularly for the cleaning of exhaust gases of combustion 
engines. In this way, other environment polluting substances, such as 
carbon monoxide, hydrocarbons and nitrogen oxides, can also be removed 
from the exhaust gas, for example, in addition to soot particles from a 
diesel engine. For this purpose it is solely necessary to provide at least 
single plies or layers of the filter body with coatings of platinum, 
rhodium, vanadium or another material or substance having a catalytic 
effect. 
In a further very advantageous improvement of the invention, provision can 
be made for powdery, grainy or chip-like particles to be applied to the 
surface of the individual layers or plies or their wires, preferably by a 
sintering operation. 
Through the additionally applied particles, the separating effect is 
significantly increased. Due to the particles, a distinct enlargement of 
the wire surface is achieved, with which a many times larger adsorption 
area, and thus a significantly larger separating area, is made available. 
At the same sintering capacity, the thickness of the individual filter 
plies or layers can, for example, in this way be significantly reduced. 
A further advantage through the application of the particles lies in the 
fact that where required, depending upon the planned application, the 
particles to be applied can also be correspondingly selected. These may, 
for example, consist of metal, of plastic, or of ceramic materials, or 
even of mixtures thereof. 
In an application as a catalytic converter body, particles are used, for 
example, which have catalytic properties, such as platinum, rhodium or 
vanadium. 
The particles themselves can be introduced into the filter body in any way 
desired. For example, they can be introduced through a viscous carrier 
liquid. Likewise, shaking-in is possible or an introduction by means of an 
electrical and/or magnetic charging of the layers or plies and a 
subsequent doping with the particles. 
In case of need, the sintering operation can be selected so that in 
addition to a sintering-on of the individual particles onto the surface of 
the layers or plies or wires, the wires are also sintered at points or 
spots to one another and in this way form a more stable unit or whole. 
Resistance welding of the wires to one another can take place in different 
ways. 
If plates or bodies of a well-defined or definite size are manufactured, 
for production in one single operation electrodes can be provided, which 
are at least as large as the filter plate to be formed. If then the filter 
plate or body to be welded is brought between the two electrodes, as the 
electrodes come near to each other and press the individual or single 
layers or plies, a filter plate or body of a well-defined or definite size 
can be produced in one single operation. 
In the same way, the manufacture of a plurality of plates or bodies from 
one band or strip is possible, which is then pushed through stepwise or 
cyclewise between the electrodes. In this way any desired size of filter 
plate can be obtained by an appropriate separation of individual plates or 
units. 
If the electrodes are formed as rotating rolls, a band or strip of any 
desired length can be formed in a continuous operation, with this strip 
having to be pulled through between the two opposing rolls solely under 
pressure. 
A very advantageous and not obvious improvement of the invention consists 
in that the wires are pressed, prior to resistance welding, out of their 
generally round or circular cross-sectional shape into a flat shape and 
more preferably into an at least approximately rectangular form or shape. 
In this way one obtains a considerable surface enlargement and thus a 
larger filtering area, and from this in turn there results a high 
filtering capacity or performance. This can be achieved with little effort 
or expense. 
A further increase in the filtering capacity or performance is produced, if 
provision is made for the flat-pressed surfaces also to be profiled. In 
this way, for example, a wave-shaped profile can be pressed in, with which 
the surface becomes even larger.

FIG. 1 shows schematically the manufacture of an individual or single 
filter plate or body in a resistance welding unit 1 having a positive 
electrode 2 and a negative electrode 3, each in plate form. The two 
electrode plates or bodies 2 and 3 possess a size which corresponds to the 
size of the filter plate or body 4 to be manufactured which lies between 
the two electrodes. The filter plate 4 consists of a plurality of layers 
or plies 6 of single or individual wires 5, each of which are connected to 
the others in every position by longitudinal and transverse wires to form 
a mesh, network, reticulation or fabric. 
If the two electrode plates 2 and 3 are now brought closer together, and a 
pressure is exerted on the interposed filter plate or body by the 
electrode plates 2 and 3, with the resistance welding taking place 
simultaneously, the individual or single wires 5 join together in the 
layers or plies. Also, the individual layers 6 join together during the 
welding. Thus a firm or solid and uniform filter plate or body 4 is 
formed. This filter plate 4 can subsequently be brought into the desired 
filter form or shape. 
FIG. 2 shows a manufacturing or production process of plates or bodies 4 
out of a continuous band or strip 40 pushed through continuously between 
two rotating electrode rolls or rollers 20 and 30 under pressure in the 
direction of the arrow. The band or strip is likewise built up from a 
plurality of layers or plies 6 with longitudinal and transverse wires, the 
only difference to the manufacturing or production process shown in FIG. 1 
being that filter plates or bodies can be subsequently separated from the 
band or strip in the desired size and shape or form. 
FIG. 3 shows the shape or form of an individual or single wire 5, which is 
brought out of a wire with a round or circular cross-section (shown in 
broken lines) into an essentially rectangular form or shape prior to 
resistance welding. This can be done in any desired way. As can be further 
seen with FIG. 3, the effective filter wire surfaces 7 and 8 are also 
profiled in the shape of waves as inflow and, as the case may be, outflow 
sides (see direction of arrow), resulting in a further surface 
enlargement. 
Also shown in FIG. 3 by means of points or dots is a doping of the layers 
or plies 6 or their single or individual wires 5 with small particles 9, 
which are applied to the filter wire surfaces 7 and 8, for example, by a 
"flooding-in" by means of a viscous liquid. The connection of the 
particles 9 to the individual or single wires 5 can be accomplished by a 
sintering operation in a way known per se. 
FIG. 4 shows a composite filter body 50 made of wires having different 
thicknesses. The filter body is made of three individual plates or 
sections 51, 53 and 55. Section 51 is made of a plurality of longitudinal 
wires 52L and transverse wires 52T which have the smallest diameter, 
section 53 is made of a plurality of longitudinal wires 54L and transverse 
wires 54T which have an intermediate diameter, and section 55 is made of a 
plurality of longitudinal wires 56L and transverse wires 56T which have 
the largest diameter. The spaces between the wires of the wire network are 
of the largest size in section 51, of intermediate size in section 53, and 
of the smallest size in section 55. Thus, the flow of gas containing 
entrained particles, as shown by the arrow, will enter section 51 which 
retains the largest particles with some intermediate and small particles, 
pass through section 53 which retains intermediate size particles with 
some small particles, and exit from section 55 which retains the smallest 
particles.