Injection Moulding of WC–6Co Powder Using Two New Binder Systems Based on Montanester Waxes and Water Soluble Gelling Polymers

Abstract

The successful use of two new binder systems for the injection moulding of WC–6Co powder and details for each stage of this process as applied to hardmetals are described. The binder systems were mixtures of a montanester wax (MEW) blend with liquid paraffin (LP) and water soluble polyvinyl alcohol (PVA) with a gelling agent, LP, and water: these provided satisfactory mixing, moulding, and demoulding properties with very short debinding times, as low as 3 h for the PYA system. Optimum powder-binder compositions/ formulations, and related rheological and thermogravimetric properties, were also investigated. It was found that the moulding feedstocks were predominantly pseudoplastic in their behaviour having viscosities of 50–250 Pa sand 10–20000 Pa s for the wax 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 rheological properties for injection moulding was therefore developed using the wax based binder system which resulted in sintered densities > 99% of theoretical. Thermogravimetric analysis for each binder showed that the MEW has a wider burning range (almost linear in the range 380—650°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. PM/0726