Abstract: A forming assembly comprises two presser members (22,24) each having a corrugated surface for receiving a workpiece made up of generally diamond-shaped pockets (10). Each presser member is made up of sections (22a,22b; 24a, 24b) mounted on slide rods (38) for movement between a spaced-apart position, in which a workpiece can be located therebetween, and a closed position, movement into which, while the workpiece is held between the presser members, is effective to compress the workpiece laterally. For holding the pockets in their diamond shape formers (20) are inserted therein. A flange (18) is formed at one end of the workpiece and integral therewith by pressing outwardly folded material against a lateral end face of the presser members (22,24) using a presser plate (52). This forming method is especially suitable for forming self-supporting filter units.

Abstract: A rigid self-supporting filter unit is produced by placing two portions of fiber fabric in face-to-face contact and stitching them along parallel regions, using two rows of stitches (16), to form pockets into which formers (20) are inserted to shape the pockets into a generally diamond shape. This assembly is then placed between two presser members (22, 24) which are corrugated, the "valleys" of the corrugations receiving the diamond-shaped formed portions. The presser members are heated and the portions are thus treated by heat and pressure to cause the fabric to be rendered rigid. The formers can then be removed. Suitable fibers for this purpose include a rigidising fiber known as P84, polyphenylene sulphide fibers, polyester fibers either alone or mixed with polypropylene fibers, and bicomponent fibers.

Abstract: A process for precisely controlling the thickness of a heat-softenable sheet material (52) comprising heating the material to bring it to a softened condition, compressing the material while in the softened condition to the desired thickness or somewhat less and then cooling the material to a temperature below its softening point so that it sets while precisely maintained at the desired thickness. The material is cooled by passing it between substantially parallel cooling plate (32,34) rigidly spaced with a separation equal to the desired thickness of the material.