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

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.     

Properties of ceramic injection moulding formulations

The rheological properties of nine ceramic injection moulding compositions based on polypropylene with a fled silicon powder loading were measured. Minor components of the polymer-ceramic blend were shown to have a considerable effect on the properties of the melt. An attempt was made to derive, from capillary rheometer flow curves, the parameters which are thought to influence moulding quality. !n particular, the shear rate dependence of viscosity at the nozzle temperature, fluidity ate shear rate of 100 sec^?1, the temperature dependence of viscosity in the region of the nozzle temperature and at o shear rate of 100 sec^?1 and the yield stress are discussed. !n Part 2 the influence of these variables and other properties of the formulations, on quality of moulding will be considered.     

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.     

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.     

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.     

Measuring and modelling air mass flow rate in the injection stretch blow moulding process

The injection stretch blow moulding process involves the inflation and stretching of a hot preform into a mould to form bottles. A critical process variable and an essential input for process simulations is the rate of pressure increase within the preform during forming, which is regulated by an air flow restrictor valve. The paper describes a set of experiments for measuring the air flow rate within an industrial ISBM machine and the subsequent modelling of it with the FEA package ABAQUS. Two rigid containers were inserted into a Sidel SBO1 blow moulding machine and subjected to different supply pressures and air flow restrictor settings. The pressure and air temperature were recorded for each experiment enabling the mass flow rate of air to be determined along with an important machine characteristic known as the ‘dead volume’. The experimental setup was simulated within the commercial FEA package ABAQUS/Explicit using a combination of structural, fluid and fluid link elements that idealize the air flowing through an orifice behaving as an ideal gas under isothermal conditions. Results between experiment and simulation are compared and show a good correlation.     

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.     

Process modelling for liquid moulding of braided preforms

Prediction of the fibre architecture within reinforcement preforms is required to model subsequent processing and performance characteristics. In recent years, a number of studies has been published in which forming or draping of fabric reinforcements has been examined, and links between draping and subsequent processing and mechanical properties have been established. Similar work on 2D braiding is limited, with existing models restricted to cylindrical geometries or surfaces of revolution. This paper presents a model for the braiding of a general mandrel cross-section, which is composed of a number of flat facets. A relatively simple geometric procedure is used, based on the paths of key braid tows that bound each flat facet. Locking or jamming of the braid is accounted for by considering the effects of tow spacing and braid angle on the fibre architecture. Experimental results are presented for a range of geometries to demonstrate the validity of the model, including a range of rectangular mandrels and a prototype automotive component. In all cases agreement is reasonably good, given the appropriate locking angle. The consequences of variations in fibre architecture on subsequent flow are demonstrated. The relationship between ply angle and the principal permeabilities is shown to be different from that observed for woven fabric reinforcements. At low braid angles, flow within the large inter-tow spacings appears to dominate the measured permeability. Finally the effect of fibre architecture on the subsequent eprocessing is demonstrated using a PC-based flow model. This demonstrates that the associated variations in permeability and porosity may result in a non-uniform flow pattern during liquid moulding.     

Computer modelling of point defects in ABO3 perovskites and MgO

We present results for basic intrinsic defects: F-type electron centers (O vacancy which trapped one or two electrons) and hole polarons bound to Mg or K vacancy in ionic MgO and partly covalent KNbO₃ perovskite, respectively. We demonstrate that a considerable covalency of the perovskite chemical bonding makes the F-type centers therein much more similar to defects in partly-covalent quartz-type oxides rather than the conventional F centers in alkali halides and ionic MgO. Both one-site (atomic) and two-site (molecular) polarons are expected to coexist in KNbO₃ characterized by close absorption energies. Our calculations confirm existence of the self-trapped electron polarons in KNbO₃, KTaO₃, BaTiO₃, and PbTiO₃ crystals. The self-trapped electron is mostly localized on B-type ion due to a combination of breathing and Jahn–Teller modes of nearest six oxygen ion displacements. The relevant lattice relaxation energies are typically 0.2–0.3 eV, whereas the optical absorption energies 0.7–0.8 eV, respectively. According to our calculations, the absorption energy of a bound electron polaron in KNbO₃ by 0.1 eV exceeds that for the self-trapped electron polaron and equals 0.88 eV.