Materials for Moldmaking

Steels with a chromium content of at least 12% are generally considered corrosion-resistant. Martensitic tool steels have a high chromium content of 16% and a carbon content between 0.33%-0.38% that provides quenching and tempering during which only tempering carbides appear in the microstructure, leaving behind enough chromium to ensure corrosion resistance.

The resistance to corrosion is based on the formation of surface layers, protecting the metal from further reaction with oxygen. The most effective element is chromium. In combination with oxygen, it forms an oxide layer (chemical configuration: Cr₂O₃) on the surface, which shows a dense and amorphous structure as the chromium content increases. The formation of this Cr₂O₃ surface layer of approximately one nanometer promotes passivation, a process that protects the material against surface corrosion and ensures the functionality of the mold.

Alloying with molybdenum improves resistance to pitting corrosion, as it contributes to stabilizing the passivating Cr₂O₃ layer. In addition, molybdenum builds into the structure of the layer and strengthens it by preventing the removal of oxygen from the respective layer. Using stainless steels alloyed with molybdenum can also avoid corrosive attacks during mold cleaning, preventing hazardous products from entering the production process.

(Source: The Benefits of Alloying Tool Steel)

The successful use of recycled materials in injection molding depends on how the original parts are recycled and re-pelletized. The two most important factors for producing a quality pellet of recycled plastic with consistent material properties are:

1. Re-melting. Proper re-melting of the recycled material occurs under very low shear rates in the extruder and at the lower end of the melt temperature. The objective is to gently re-melt the original material, which ensures maintenance of the material properties.
2. Melt filtration. The proper filtration process will remove any contaminants in the melt like cellulose, metal or wood pieces. State of the art melt filtration is fully automated and does not require manual operation steps. The melt is filtered continuously at low pressure and can remove particles as small as 70 microns in diameter.

Today’s recycling systems can help produce consistent quality plastic pellets that moldmakers can use in a wide variety of injection-molded parts.

(Source: Putting Recycled Plastics to Work)