Asymmetric Honeycomb Wall-Flow Filter Having Improved Structural Strength

Abstract

A honeycomb filter includes an array of interconnecting porous walls which define an array of first channels and second channels. The first channels are bordered on their sides by the second channels and have a larger hydraulic diameter than the second channels. The first channels have a square cross-section, with corners of the first channels having a shape, such as a bevel or fillet, such that the thickness, t 3 , of the porous walls adjoining the corners of the first channels is comparable to the thickness, t 4 , of the porous walls adjoining edges of the first and second channels. Embodiments having a corner fillet with a radius, R c , are also disclosed. Embodiments wherein 0.30 t 4 ≦R c ≦1.0 t 4 exhibit combinations of low wall pressure drop and low thermal stress.

1 .- 16 . (canceled) 
     
     
         17 . An extrusion die assembly for making a honeycomb filter, comprising:
 a cell forming die having a central region and a peripheral region, the central region comprising an array of discharge slots cut to define an array of first and second pins and an array of first feedholes in communication with the array of discharge slots, the peripheral region comprising at least a second feedhole, the first pins having a larger cross-sectional area than the second pins, a cross-sectional shape of the first pins selected such that the width of the discharge slots is substantially uniform; and   a skin forming mask mounted coaxially with the cell forming die and radially spaced from the cell forming die so as to define a skin slot that is in selective communication with the at least second feedhole.   
     
     
         18 . The extrusion die assembly of  claim 17 , wherein the cross-sectional shape of the first pins includes a square having filleted corners. 
     
     
         19 . The extrusion die assembly of  claim 17 , wherein the cross-sectional shape of the first pins includes a square having beveled corners. 
     
     
         20 . The extrusion die assembly of  claim 17 , further comprising a reservoir defined between the cell forming die and the skin forming mask, the reservoir being in communication with the at least second feedhole and the skin slot. 
     
     
         21 . The extrusion die assembly of  claim 17 , wherein a volume of the outer skin reservoir is adjustable to control rate of flow of batch material to the skin slot.