Surface damage of mold steel and its influence on surface roughness of injection molded plastic parts

Plastic injection molds are an important economic sector due to the increasing use of injection molded plastic components. The selection of mold steel material, its processing conditions, and the way it is machined and hardened all have an impact on how it performs during use across its lifecycle for injection molding. For this study, injection molding conditions seen during high volume manufacturing were used to induce wear that would be typical during processing. A 50 wt.% glass fiber reinforced Polybutylene Terephthalate (PBT) was used for the research. After 3000 injection operations, surface damage was found only on the steel used for the stationary half of the mold cavity and the corresponding surface of the molded parts. In contrast, the moving half showed no surface damage or increase in surface roughness. Hardness and microstructural determinations showed that the steel of the stationary half presented an mixed microstructure with a hardness of 31 HRC, while the moving half showed a martensitic structure with a hardness of 53 HRC, due to the quenching and tempering heat treatment which it had received. The lower hardness and different microstructure used for the stationary half were the reasons for its lower service life. The evolution of surface roughness both on the steel surfaces and on the injected parts with the number of injection operations and its location have been characterized by contact and optical profilometry and microscopy observations.