Every year there is an Increase in the amount of plastics formed by all types of processes, such as extrusion, blow moulding, injection moulding, etc The recycling of waste plastic materials 1s developing at a manufacturing process scale, and rts arm is to increase further the sales o f plastrc products by reducing raw material costs However thrs economic argtunent rs only one of the reasons why it is important to recycle these materials. The other is environmental. Each time an item of plastic material passes through the manufacturing process, its quality is reduced. Some plastic material can be recycled over and over again. This research concerns the number of times polypropylene (PP) formed by the injection mouldlng process can be recycled. The work focuses on the quality of the materlal dter each recycling operation Material was passed through the forming process up to three times Also, since it is posslble to improve the quallty and reliability of the polypropylene, by addlng a percentage of fresh "raw" rnatenal, the study included an rnvest~gatront o optirnlse the mechanical properties using this approach. The moulding process was used to produce products whose mechanical properties could be measured to provide an indication of the quality of the material at that stage. The relationship between the mouldlng process parameters and the qualify of the subsequent products was analysed, and thls was extended for cases where the materlal was recycled twlce or three times through the equipment The powders ploduced m these trlals were then mixed with diffelent percentages of raw material, and passed through the equrpment again to produce products whose mechanical propert~esc ould be measured. To reduce the number of permutations a Design of Experiments (DOE) method was implemented to define the moulding process conditions whlch resulted In the optimum product quality for material belng recycled once, twice or three tlmes. A 2 Factorla1 Design (FD) techque was also used. Different mathematical models were obtained to show the relationship between the process parameters, the number of times the material was recycled, and the product quality Thls was extended to show the effect of adding a percentage of raw material to the powder mix. All the mathematical models were verified by further experimentation The resuIts show that these models can predict the mechanical properties of the products adequately, once the processing parameters have been defined.