The invention relates to a process and apparatus for the production of a filter in the form of a ceramic honeycomb body monolith, namely starting from a blank of the honeycomb body monolith.
Such filters have found increasing usage, for example, as diesel soot filters during the course of public concerns regarding the environment in passenger automobiles, trucks, and buses propelled by means of a diesel engine.
The effect of such a filter is based on the feature that the ducts of the honeycomb-shaped monolith are sealed alternatingly on both sides in such a way that the exhaust gas is forced to flow through the porous walls between the ducts. In this connection, the walls serve as the actual filters for the soot, which latter occurs in particles, or--depending on the particular usage--for the material to be removed by filtration. The ducts will, of course, be clogged with soot or the like after some time; as a consequence, such a filter must be regenerated from time to time, i.e. normally heated, so that the retained particles are oxidized.
The blanks for the honeycomb bodies frequently consist of sintered glass-ceramic monoliths made available in most cases by an extrusion process with subsequent drying and sintering. The body shrinks on account of drying and sintering, this shrinkage being nonuniform. This nonuniform shrinkage results in an incalculable deviation of the actual geometry from the desired geometry of an idealized honeycomb body. Additionally, the extrusion process leads to warpage of the honeycomb structure.
The aforementioned deviations, then, exert a considerable influence on the further processing of the honeycomb body monolith, namely in case the objective resides in alternatingly sealing the ducts on both end faces.
The entire significance of the concomitant problem of large-scale industrial manufacture becomes clear considering a standard monolith with a diameter of 5.66" (14.4 cm). The rough size of the honeycomb amounts to about 3.6.times.3.6 mm so that there are about 1,400 honeycomb ducts per end face with a hole cross section of respectively 2.4.times.2.4 mm. Therefore, approximately 700 honeycomb openings, and thus filter ducts, must be sealed on each end face alternatingly. Based on the aforementioned deviations from the desired geometry of the monolith, it is impossible in this instance to seal the 700 openings per end face with a nozzle head with 700 nozzles all at once in one working step. Also sealing of each second duct with the so-called plugging compound manually is not feasible in view of the high labor costs arising.
Devices and methods for the alternating sealing of the honeycomb ducts of a honeycomb body monolith have been proposed in the state of the art. Attention is invited, for example, to the very illustrative U.S. Pat. No. 4,411,856, incorporated herein by reference. This reference suggests the use of a mask completely covering one end face of the monolith, this mask being provided with passages and with peg-like extensions on the side facing the monolith. The pegs are inserted in the ducts not to be sealed whereas the plugging compound is introduced through the passages in the mask into the end zones of the ducts to be sealed. An attempt is made to consider the above-mentioned deviation of the monolith from an idealized desired geometry by providing that the mask utilized consists of an elastic material. This method may be usable within narrow limits. However, there is the problem that the spacing of the aforementioned pegs with respect to the passages is fixed so that the compensation of the deviations of the monolith from the desired geometry, obtained by the elasticity of the mask, remains restricted to relatively low values.