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Title: The characterisation of wax injection moulding and de-wax processes in investment casting
Author: Cendrowicz, Aleksander.
ISNI:       0000 0001 3525 6178
Awarding Body: University of Birmingham
Current Institution: University of Birmingham
Date of Award: 2003
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The wax injection moulding and de-wax processes in investment casting were investigated; by measuring the relevant material properties of two commercially available pattern waxes; performing experimentation on industrial equipment; and using analytical and numerical techniques to solve some of the fluid mechanics, heat transfer and solid mechanics equations associated with each process. The rheological behaviour of the molten pattern waxes was found to be Newtonian at higher temperatures and shear-thinning at lower temperatures. The viscosity was also found to depend on shear and thermal history. Samples pre-sheared by both "screw" and "tank" type injection moulding machines were found to give lower vi~cosities than freshly melted pellets. The extensional viscosity was higher at lower temperatures and lower shear rates than that expected for a Newtonian liquid. The pressureflowrate- time relationship for the flow through a tube of a power-law fluid with exponential dependency with temperature was obtained by solving the coupled dimensionless heattransfer and fluid mechanics equations. The method was used to correct capillary viscometry results for viscous heating. Predictions of the pressure-drop through cold cylindrical running systems during injection moulding were found to agree qualitatively with experimentally measured values, although quantitative agreement was limited by the constitutive model that was used. The formation of surface defects during the injection of patterns was investigated by using flow-visualisation and computational fluid dynamics. It was found that substantial flow-lines occur when the flow-front velocity is below 10 mm.s-l, and significant air entrapment occurs at flow-front velocities above 1 m.s-1 A model was developed to predict the freezing time of running systems during the holding stage of the injection moulding cycle and the subsequent shrinkage behaviour of the pattern, which were found to be influenced most by the wax temperature, hold pressure, hold time, and runner diameter for thick unrestrained pattern geometries. The gas-assisted injection moulding process was adapted for wax pattern production, and was observed to reduce shrinkage by a factor of two. A onedimensional dynamic model was developed to calculate the stress in a cylindrical investment shell during de-waxing. The relevant physical properties of un-sintered ceramic shells were measured, including the pore size distribution, wax permeability, thermal expansion and hot & wet stress-strain behaviour. It was shown how the geometry, material properties and operating conditions can affect the frequency of shell cracking during the de-wax process.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available