Injection Molding of Magnesium Parts (Thixomolding®)
Thixomolding® is a new production process for magnesium parts. Similar to the injection molding technology for plastics, magnesium granules are metered from a feeding hopper into the screw of the thixomolder. Within the screw the magnesium material is heated up to the semi-solid state while it is transported to the nozzle. Due to shearing the solid phase is transformed from a dendritic to a globulitic shape. As a result viscosity is reduced. In addition the melt is shear thinning.
Processing Magnesium in the semi-solid state has several advantages concerning the quality of the parts and the economics compared to conventional die casting. Due to a closed flow front entrapment of gases and oxides is reduced. The mechanical properties are improved and the parts can be heat treated and welded. In addition the reduced solidification shrinkage leads to better tolerances. Lower energy consumption and reduced thermal strain of the mold is reducing the costs.
Up to now Thixomolding is mainly used for producing thin-walled housings in Asia and North America. Most of the parts are produced from AZ91.
At NMF the first Thixomolding® machine in Europe from Japan Steel Works was set up in March 2002. The machine has a clamping force of 220 t, the maximum shot weight is 660 g Magnesium. For fully automatic operation auxiliary equipment, as for example a take-out robot and spray-robot, was added.
Several demonstration components of different wall thickness were cast for a wide range of processing parameters. The properties of the components (fraction solid, mechanical properties and porosity) were evaluated. It has been proven that the usage of low processing temperatures (and therefore high solid phase contents) reduces the porosity significantly.
Processing at low temperatures (and therefore high solid phase contents) can reduce the porosity of thick-walled parts significantly.
In the near future a comparison of thixomolded® components with die cast parts will be carried out. Especially the potential of the new technique for automotive components will be investigated. One focus will be on small lot production. Furthermore, the injection molding process will be enhanced at NMF. For example, the development of new magnesium alloys, the production of metall-matrix composites, and the design of hot runner systems will be investigated.