An injection moulding machine was used to produce injection moulded parts from polypropylene under invariant injection moulding conditions. Part of the injection mouldings were shredded and re-processed up to fifteen times. The mouldings and the shredded plastic were tested for selected strength properties: Young's modulus, maximum tensile stress, breaking stress, strain at maximum stress, strain at break, Shore hardness, impact strength, mass flow rate, and longitudinal and transverse shrinkage. Relevant relationships were identified and conclusions were drawn on the basis of the data obtained.