The present invention relates to a method for the production of bodies in plastic comprising two portions hinged to each other by a single rotation pin. The method comprises the steps: a) predisposing a mold with two distinct forming chambers for the two portions; the mold comprises a third chamber for the single pin, made and positioned in such a way that the pin is formed already aligned along the hinge axis; the mold is provided with a pair of pegs axially distanced from each other to form the third chamber and sliding along the axis between a first and a second operating position; b) positioning the pegs in the first position; c) injecting plastic material inside the chambers; d) shifting the pegs from the first to the second position, bringing the pin to engage inside the seats; e) opening the mold and extracting the two assembled portions.