Extrusion honing using mixtures of polyborosiloxanes and grit
Extrusion honing is a technique used for the finish machining of engineering components. The process may be used to deburr, radius or polish depending on the specific application. Extrusion honing utilises a polyborosiloxane polymer, which is mixed with fine abrasive particles and forced through or around components using an extrusion honing machine to achieve the machining action. It is evident that the machining process is very dependent upon the physical and mechanical characteristics of the polyborosiloxane. However, the process to date has largely been developed empirically, and further developments will require a greater understanding of the relationship between the characteristics of the polymer and the machining action. The rheology of the C-11 polyborosiloxane has been investigated using a concentric cylinder, a slit-die and a capillary rheometer. The shear history of the polyborosiloxane prior to testing with the capillary rheometer was found to significantly affect the results produced. At shear strain rates up to 100 s-1 and a temperature of 30°C the rheology of the polyborosiloxane has been approximated to a power law equation (K.=989KPas; n=0.935). At shear strain rates greater than 100 s-1 the pseudoplasticity of the polyborosiloxane increases significantly. Using this power law equation the velocity, shear strain rate and the shear stress within the polyborosiloxane in an extrusion die has been modelled mathematically. In some cases the extrusion barrel pressure and the mass flow rate through the extrusion die have both been shown to vary considerably during the constant ram velocity extrusion of polyborosiloxane in the capillary rheometer. It is tentatively proposed that such variations in the extrusion barrel pressure during capillary flow are a result of periodic slipping and sticking of the polyborosiloxane to the extrusion die wall. This mechanism is thought to be considerably influenced by the polyborosiloxane temperature, the extrusion ram velocity and the flow passage geometry. It is proposed that the measured variations in the mass flow rate of the polyborosiloxane through the extrusion dies are a result of pressure induced density variations within the extrusion barrel. This mechanism has been mathematically modelled. Temperature profiles within the C-11 polyborosiloxane have been measured on an extrusion honing machine and the results suggest significant heat generation towards the periphery of the restriction. These results have been used to model mathematically the heat generation and transfer within the polyborosiloxane during operation of the extrusion honing machine. It is proposed that heat generation during extrusion honing is by friction at the medium/workpiece interface and internal shear within the medium. Both of these mechanisms would cause the greatest heat generation towards the periphery of an orifice restriction.