predetermined diameter g this diameter can vary between four and eight millimeters however a diameter of five millimeters is preferred a replaceable round wire preferably of spring steel may be used for the production of the beater element 3 1 it can be seen from fig5 that a free end portion of each of the rods 10 and 11 has a predetermined angle beta to a tangential plane e parallel to the axis of rotation of the opening roller the point of tangency between the roller surface and the plane e is located at the fixing axis b of the screw 13 the angle beta is about 90 in the example shown so that a free end portion of each of the rods 10 and 11 opposite to a prolongation a 1 of the radius a of the cross section of the opening roller is inclined forwardly at an angle gamma in the direction of rotation d this angle gamma is the angle between the rear face of the outer end portion of a rod and a radius a of the roller so located as to touch the trailing surface of the rod an angle u03b3 of four degrees 4 u00b0 has been found to give good results however the selection of the angle beta and consequently of the angle gamma can be different according to the size of the flocks to be cleaned and the flock material and can best be.