INJECTION-MOLDING OF WC-6CO POWDER USING 2 NEW BINDER SYSTEMS BASED ON MONTANESTER WAXES AND WATER-SOLUBLE GELLING POLYMERS

The successful use of two new binder systems for the injection mouldin g of WC-6Co powder and details for each stage of this process as appli ed to hardmetals are described. The binder systems were mixtures of a montanester wax (MEW) blend with liquid paraffin (LP) and water solubl e polyvinyl alcohol (PVA) with a gelling agent, LP, and water: these p rovided satisfactory mixing: moulding, and demoulding properties with very short debinding times, as low as 3 h for the PVA system. Optimum powder-binder compositions/ formulations, and related rheological and thermogravimetric properties, were also investigated. It was found tha t the moulding feedstocks were predominantly pseudoplastic in their be haviour having viscosities of 50-250 Pa s and 10-20000 Pa s for the wa x and water based systems respectively, within the shear rate range of 20-1200 s(-1) which is acceptable for powder injection moulding (PIM) . A relatively high hardmetal volume loading of 65% with suitable rheo logical properties for injection moulding was therefore developed usin g the wax based binder system which resulted in sintered densities >99 % of theoretical. Thermogravimetric analysis for each binder showed th at the MEW has a wider burning range (almost linear in the range 380-6 50 degrees C) than other waxes and polymeric binders. This was a very useful property for an easy and defect free burn out which also helped to reduce debinding time to 28 h for 6 mm thick samples. (C) 1997 The Institute of Materials.