Abstract: Process for blending an additive such as colour pigment etc. with a plastic material, wherein the plastic material for instance in the shape of granules, pellets, grains or powder is introduced into a first hopper equipped with a stirrer. The first hopper is placed in a direct connection with a feed screw of a plastic moulding machine. The additive in the shape of granules, pellets, small tablets or big grains is charged into a second hopper which is joined to the first hopper by a dosing part including a dosing plate, which executes reciprocating motions and is equipped with at least one hole, and a dosing pipe, a dosing hose or the like. The amount of additive fed to the first hopper is intended to be regulated by varying the number of reciprocating motions of the dosing plate per unit of time and/or by varying the plate thickness and/or by varying the hole size of the dosing plate and/or by varying the number of holes in the dosing plate.