Process for the production of filters, and a filter produced by the process

A process for the production of a filter having an active filtering part made up of a folded filter material web, said web being delimited by side edges at which the folds terminate, and end edges at which a first or last fold is located, comprising the steps of:

• first producing a blank by creasing the filter material web and pushing the same together to form the folds with formation of predetermined spacings between the folds during advancement of the folded web;
• joining said side edges of the web by strips, such that the ends of the folds are covered over and fixed by the strips;
• cutting off the blank which comprises the folded material web and the strips arranged on its side edges, to form filter units;
• maintaining the spacings between the folds constant on application of the strips and during advancement by means of threads of at least one screw element pushing down between the folds, said threads thus form spacing elements;
• rotating the at least one screw element, such that the threads follow the folds during the advancement;
• and assembling several filter units in a casing to form a filter having a filter surface which corresponds to the combined surface of the units included, said casing having a box shape with a first side and a second side corresponding thereto, openings in each side, the filter units being arranged sealingly between said sides so that between said units spaces are formed which extend between the said first and second sides;
• each filter unit partly bordering on a space which is in communication with the openings on the first side and, at its second side, on a space which is in communication with the openings on the second side, so that a medium, which flows in through the openings on one side and out through the openings on the other side, must pass at least one wall formed by the filter material web of at least one of the filter units.