The free fall triboelectrostatic separation is widely used for the selective sorting of plastics from granular industrial waste. The electric charge per mass ratio of the granules is a critical parameter influencing the purity of recycled plastics and the efficiency of the electrostatic separation process. The aim of the present study is to validate an experimental procedure for the optimization of the vibratory-type tribocharging device for granular plastics. This tribocharger is composed of a metal plate covered with a thin PET (polyethylene terephthalate) layer. The amplitude of the vibratory motion of the plate can be adjusted using a potentiometer. The length of the vibratory feeder, the flow rate, and the velocity, at which the granules move on its surface, are the variables that can be controlled in order to optimize the tribocharging process. As particle size is an important physical factor influencing the charging process, the study was focused on two size classes (1 to 2 mm and 2 to 5 mm) of ABS (acrylonitrile butadiene styrene) granules originating from the recycling process of waste electrical and electronic equipment. The optimum operating conditions, obtained by using the response surface modeling methodology, differ between the two size classes of granules. Both the length of the vibratory tray and the velocity of the granules on its surface have a significant effect on the outcome of the tribocharging process.