which are connected to the cover plate 20 on one side an air inlet 22 is formed at the center of the cover plate 20 the outer radial ends of the vanes 30 extend beyond the outer periphery of the cover plate 20 preferably the body is formed as a single piece item or monolithic construction by injection molding the body 10 further comprises an outer ring 40 which is connected to the outer radial ends of the vanes 30 the outer ring 40 is in an axial extended tubular structure the back plate 60 is disposed inside the outer ring 40 and may be fixedly connected with the inner surface of the outer ring 40 by bonding welding or preferably by plastic deformation of the outer ring preferably the outer ring 40 is provided with an axial end the end remote from the cover plate 20 which extends beyond the back plate 60 in the axial direction this arrangement prevents the outer edge of the back plate from accidentally making contact with the volute the inner radius of the outer ring 40 is larger than the outer radius of the cover plate 20 to facilitate removing the body 10 from the molding die the vanes 30 are provided with protrusions 32 on the side remote from the cover plate 20 the back plate 60 is provided with through holes 62 corresponding to the protrusions 32 for passage of the protrusions 32 preferably the protrusions 32 extend through the holes 62 beyond the back plate 60 and are fixedly connected with the back plate 60 preferably the protrusions 32 are hot staked or otherwise plastically deformed to fix the back plate to.