in a method for crystallising plastic granules with a tendency to conglutinate , in particular pet and pu granules , the granules are subjected to vibratory excitation in a vibrating trough transversely to the direction of longitudinal extension of the vibrating trough , at around their reaction temperature . the flow of granules follows a spiral path through the trough , achieving a narrow dwell time spectrum and a homogenous temperature gradient for the granules . conglutination is also reliably prevented .