Influence of the Recyclate Content on the Process Stability and Part Quality of Injection Moulded Post-Consumer Polyolefins

In recent years, the drive to adopt sustainable practices and develop eco-friendly processes and products has gained significant momentum in the global polymer industry. A key component of this shift is the increased use of recycled materials, which not only align with environmental goals, but also offer considerable economic advantages. Integrating post-consumer recyclates (PCRs) into manufacturing processes, particularly in injection moulding, holds great potential for reducing CO₂ emissions, decreasing reliance on virgin materials, mitigating waste and promoting a circular economy. Nevertheless, a switch to 100% recyclate has not yet been effective or economical in many areas of application, so mixtures of virgin and recyclate material represent a promising approach and must be analysed further. Therefore, this study examines the impact of different virgin/recyclate-mixture ratios on both injection moulding process stability and resulting part quality, focusing on high-density polyethylene (HDPE) and polypropylene (PP) blends. For this purpose, virgin/PCR mixtures are compounded, rheologically analysed using high-pressure capillary rheometry and processed via injection moulding. The process data are analysed, and the produced parts are mechanically and geometrically evaluated. The findings show that for PP, an increasing recyclate content results in a nearly linear improvement in tensile strength and modulus of elasticity without significantly affecting material viscosity, ensuring stable processing conditions. However, part warpage increases with higher recyclate content. In contrast, for HDPE, a higher recyclate content decreases the mixture viscosity, leading to decreased injection pressure and dosing torque during processing. Despite this, the tensile strength and modulus of elasticity improve, while part warpage decreases for HDPE. For both materials, though tensile strength and elasticity increase, higher recyclate contents negatively affect fracture behaviour, as evidenced by breakage patterns and strain at break. The study also demonstrates that the linear mixing rule can be applied to process parameters and part geometry characteristics for virgin/recyclate mixtures, facilitating the integration of recyclate content into product development.