The plastics industry is one of the fastest growing major industries in the world. There is an increase in the amount of plastic used for all types of products due to its light weight and ability to reprocess. For this reason, the reprocessing of thermoplastics and the usability of reprocessed materials are gaining significance, and it is important to produce and consume plastic materials in an environmentally friendly way. In addition, rising raw material cost linked to the increased oil prices encouraged for reusing of the plastic materials. The aim of this research was to study and optimize the injection moulding process parameters to achieve a trade-off between the product cost and product quality, measured through mechanical properties and geometry, based on using regrind ratios. The work was underpinned by a comprehensive study of multiple reprocessing effects in order to evaluate the effect of process parameters, material behaviour, reprocessing effects and possible links between the processing parameters and key properties. Experimental investigations were carried out, in particular, focused on the melt preparation phase to identify key process parameters and settings. Multiple reprocessing stages were carried out; using two types of PP material: unfilled and short glass filled. A series of tests were used to examine product quality (mass, colour and shrinkage) and physical properties (density, crystallinity, thermal stability, fibre length, molecular weight, in-line and off-line viscosity, tensile strength, modulus of elasticity, elongation (%) and flexural strength). This investigation showed that the mouldability of the filled and unfilled PP materials, through the successive reprocessing stages (using 100 % regrind), was observed to be relatively consistent. Given the link between the processing parameters and key product and material properties, it is possible to manufacture products with minimal loss to part quality and mechanical properties. The final phase of the work focused on process optimisation study for short glass fibre filled PP material and the identified key process parameters (melt temperature, screw rotational speed, holding pressure, holding time and injection rate). A response surface experiment was planned and carried out for three reprocessing stages (0 %, 25 % and 50 % regrind). The fitted response surface models were utilised to carry out the trade-off analysis between the operating cost (material cost, energy cost and labour cost) and product quality (dimensions and tensile strength) Based on the optimal moulding conditions, the operating cost was reduced (from stage I as a reference), by 24% and 30 % for stage II and stage III respectively. A small, perhaps undetectable, change in product dimensions was noted. In addition, a small reduction in tensile strength was noted (from stage I as a reference), by 0.4% and 0.1 % for stage II and stage III respectively. The same data was applied in other countries (Australia, USA, Brazil, Libya and China) to manufacture the same product; and it was observed that the cost was reduced with increasing of regrind ratio. But the significant reduction of the cost, essentially, depended on those countries which have low wage rates (e.g. Brazil, Libya and China). For example, the cost of moulded product manufactured in China is £ 0.025 (using 50% of regrind), while the cost of the same product produced in Australia is £ 0.12, hence giving a total saving of 79 % and making it a valuable issue to be considered in industry.

Pour améliorer leur performance, les entreprises du pôle de compétitivité Arve Industries se sont naturellement orientées vers l'approche Lean qui semblait être une solution idéale. Jusqu'à présent, le transfert de la démarche Lean vers les entreprises de ce pôle de compétitivité a été réalisé par simple application. Cette stratégie a conduit à des difficultés de mise en oeuvre de certaines pratiques Lean. Après avoir établi le profil Lean des entreprises du pôle de compétitivité Arve Industries, notre travail de thèse a cherché quelles adaptions et ajustements étaient nécessaires à l'application de la démarche Lean. En effet, les spécificités locales relatives à l'histoire, la situation géographique et les aspects techniques et organisationnels des entreprises du pôle de compétitivité pourraient jouer un rôle prépondérant dans les freins à l'intégration des pratiques Lean. Ainsi, nous avons développé, au cours de ce travail de thèse, de nouvelles approches prenant en considération les spécificités locales des entreprises du pôle de compétitivité Arve Industries. Nous avons proposé, tout d'abord, d'introduire les principes de la culture européenne dans une démarche d'amélioration facilitant la réussite d'une démarche de progrès. Par ailleurs, afin de favoriser l'application des cartographies de chaîne de valeur, nous avons développé une nouvelle approche d'identification des ressources critiques. Un modèle analytique de la quantité optimale à produire a également été proposé pour répondre aux spécificités locales. Finalement, ces divers ajustements contribuent au développement d'un système de production Lean adapté aux entreprises du pôle de compétitivité Arve Industries.