Properties of ceramic injection moulding formulations

The oxidative and thermal degradation of nine suspensions, with polypropylene as the main organic component and a fixed silicon powder loading, were studied by thermogravimetry. Samples were in the form of finely divided shavings and moulded bodies. The acceleration of weight loss in oxidizing atmospheres was dependent on sample size, indicating oxygen diffusion control. Minor additions in the formulations exerted considerable influence on the form of the thermograms. Samples heated in nitrogen presented an exfoliated skin defect which did not appear in oxidizing atmospheres. An important observation was that the form of the thermogram could not be directly related to the incidence of defects in the powder assembly and was therefore an incomplete guide in the screening of potential blends. Nevertheless, a low temperature dependence of weight loss was shown to be desirable for process control.     

Rheology of composite ceramic injection moulding suspensions

Ceramic injection moulding compositions incorporating sinterable silicon nitride powder and silicon carbide whiskers were prepared by twin screw extrusion using a polypropylene-based organic vehicle. Their viscosities in the shear rate range 100 to 1400 sec^–1 were measured by capillary rheometry. The relative viscosity ( _r)-ceramic volume loading (V) curve fitted the Chong equation

Selection of a powder for ceramic injection moulding

Ceramic suspensions were prepared from six alumina powders using the same high molecular weight organic vehicle. 18 mm diameter cylinders were compression moulded from four of these suspensions which were suitable for ceramic injection moulding. The organic vehicle in each cylinder was removed by thermal degradation according to the same temperature ramp. The defects present in the cylinders after removal of the organic vehicle are discussed in terms of the powder characteristics. Criteria for the selection of ceramic powders that could be used together with a high molecular weight organic vehicle for this shape-forming method, are deduced from the results obtained.     

Properties of ceramic injection-moulding formulations

Five ceramic injection moulding formulations with 65vol% powder and polypropylene as the main organic component were injection -moulded under identical conditions. The extent of mould filling and the production of moulding defects in thick sections were examined in the light of rheological and shrinkage properties of the formulations.     

The use of silane coupling agents in ceramic injection moulding

Ceramic injection-moulding blends containing 56 vol % fine silicon nitride and 65 vol % coarse silicon powder in a polypropylene and wax vehicle were prepared by dispersive mixing with and without 2 p.p.h. silane coupling agent based on the weight of powder. The addition of silane reduced the viscosity and the pseudoplasticity of the suspensions considerably. The mechanical strength of the moulding compositions, which may influence the tendency to crack during solidification in the cavity, was marginally increased. The treated silicon powder was strongly hydrophobic but this was not the case with the silicon nitride powder.[/p]     

The control of sprue solidification time in ceramic injection moulding

The ability to control the sprue solidification time by using modulated pressure was investigated over a wide range of mould temperatures using a silicon nitride suspension. The mould temperature needed for infinite sprue solidification time was reduced from 158 to 75° C by the application of a pressure of 132 MPa modulated at 0.5 Hz. Under similar moulding conditions, macroscopic moulding defects in the hub of a solid rotor were avoided. Such defects were present in all static pressure mouldings. The thermal power input to the sprue resulting from oscillatory flow was estimated.On leave from Department of Materials Engineering, Nanjing Institute of Technology, China.     

Computer modelling of the origin of defects in ceramic injection moulding

Thermal stress profiles were calculated for injection-moulded flat discs of a zirconia-polystyrene suspension by considering the heat flow in, and elastic properties of, the suspension in the solid state. The calculated maximum tensile stress in the centre of the moulded discs could be used to predict the incidence of cracking in moulding experiments. The computer model allows the influence of material and machine parameters on residual stress profile to be explored for the simple case of an infinite flat plate. In particular, high tensile stresses are induced by low mould temperatures and high injection pressures.     

Computer modelling of the origin of defects in ceramic injection moulding

The acquisition of thermal property data for the subsequent computer modelling of solidification-induced defects in the injection moulding of ceramic suspensions is described. Thermal diffusivity of a polystyrene-zirconia suspension was measured between 80 and 160 °C. Volume thermal expansion and specific heat were also recorded as a function of temperature and the equation of state was derived. All the experiments use standard laboratory equipment making the procedure widely applicable. In subsequent work, these data will be used to predict the origin of voids and cracks in ceramic moulded bodies.     

Computer modelling of the origin of defects in ceramic injection moulding

A finite difference method for the calculation of sprue closure time is described and compared with measured cavity pressure fall for a zirconia-polystyrene suspension. The disparity between measured and calculated values is explained by the balance of volume shrinkage rate in the cavity and flow rate in the sprue during the final stage of mould packing and cooling.     

Debinding and sintering defects from particle orientation in ceramic injection moulding

The causes of the defects in injection-moulded ceramic bars during debinding and sintering were investigated using density and shrinkage measurements in combination with macroscopic observation. Differential shrinkage during sintering is the main cause of post-debinding defects. Particle shape, mould geometry and gate position have a large effect on this differential shrinkage, while the volumetric shrinkage is almost constant and the influences of moulding conditions are relatively small. Particles with plate-like morphology orientate during moulding and contribute markedly to defect formation during debinding and sintering.     

An oscillating pressure unit for ceramic injection moulding: Part A

When oscillating pressure moulding equipment which was designed for polymers, was applied to ceramic suspensions, severe localised wear rendered it useless after only a few shots and damaged expensive components of the high pressure valve. The equipment was redesigned to receive inexpensive wear resistant inserts and to allow for rapid replacement of worn parts. The selection of materials and the construction of the modified oscillator are described in this article and the performance of the unit will be discussed in Part B.     

The effect of silane contents on fluidity and green strength for ceramic injection moulding

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Improvement of the rheological properties of the zirconia/polypropylene system for ceramic injection moulding using coupling agents

In order to improve the rheological properties of the zirconia/polypropylene system for ceramic injection moulding, the zirconia powder was surface-treated with aluminate (A), silane (S) and titanate (T) coupling agents dissolved in toluene. 2% additions of these coupling agents decreased the torque,T _h, for compounding the zirconia filled-polypropylene after 1 h at 180°C. The surface treatment also reduced the viscosity, η_a, of the compound, the effectiveness of the coupling agents on viscosity reduction being T > S > A. On the contrary, the apparent activation energy,E _a, for flow of the compound increased when fluidity was promoted by surface treatment. Rheological measurements in systems with different titanate concentrations showed that the three parameters,T _h, η_a, andE _a, remained nearly constant for coupling agent additions of over 2%. This critical concentration of 2% showed good correspondence to the optimum concentration evaluated from thermogravimetric analysis of the powders.     

Organic fibre modified ceramic shell moulding for investment casting

Abstract

Investment casting research is being carried out by the University of Birmingham sponsored by the EPSRC and a consortium of industrial companies. The programme is aimed at developing a fundamental understanding of the process, with a view to routinely producing sound, net shape castings. The casting of liquid metals to produce solid objects is a manufacturing process, which has been practised for over five thousand years, with investment casting being one of the oldest known metal shaping methods. The technique itself has tremendous advantages in the production of quality components and key benefits of accuracy, versatility and integrity. As a result the process is one of the most economic methods of forming a wide range of metal components. Environmental and economic pressures have, however, resulted in a need for the industry to improve current casting quality, reduce manufacturing costs and explore new markets for the process. Optimisation of the mechanical and physical properties of the ceramic shell will be fundamental to achieving these aims. This paper reports on the work carried out to determine the applicability of the use of fibres as reinforcement media for investment shells and to explore methods by which the use of organic fibres can improve ceramic shell performance.