In this article, results obtained from an investigation focusing on the application of diamond coatings on inserts for thermoplastic injection moulding applications are reported. The injection moulding industry can potentially benefit significantly from the use of diamond coatings on moulds typically employed in the injection moulding systems. Diamond films were coated on steel and onto silicon inserts and subsequently tested in real operating conditions. The moulded objects were analysed and the results were compared between bare steel and silicon inserts. The as-grown diamond coatings were characterised prior to and post the routine injection cycles using a number of techniques, such as Raman spectroscopy and SEM. Furthermore, the polymer finishing quality was assessed by optical microscopy for each material and coating used, and the results are presented and discussed in this article.