Abstract: A device for blending different fibres in a desired ratio to make envelopes for drain pipes has at least two pressure passages in a parallel relation, with a small space therebetween. Different fibres are introduced into the first ends of each pressure passage by a conveying fan outlet, and the fibres so introduced build up in each pressure passage into material columns of homogeneously compressed fibres. Each pressure passage has at least one exit port for venting pressurized air in front of its column. Conveyors are provided at the second ends of the pressure passages to feed the compressed fibres into a longitudinal opening in one side of a stationary drum which extends over the width of the second ends of the pressure passages. A pin-feed drum is rotatably arranged within the stationary drum and is driven to convey the compressed fibres to a common suction passage on the other side of the stationary drum, which in turn is connected to a suction tube.