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Title: Extrusion and drying of a hardmetal paste
Author: Ferstl, Harald
ISNI:       0000 0004 6420 8417
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2017
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Paste extrusion via ram or screw extruders is a popular forming process used in the production of ceramic and hardmetal parts. The pastes used are a mixture of particulate solids and an organic binder phase, which has to be removed after extrusion in time-consuming drying and debinding processes. The need for high yield strength and minimising debinding time both favour paste formulations with solids loading near the maximum. The extruded products have to meet stringent quality standards, requiring a profound understanding of the materials used and the processes involved, in order to guarantee stable processing and defect-free products. This dissertation investigates the extrusion and drying behaviour of a tungsten carbide hardmetal paste with a viscoelastic binder phase, with particular interest in conditions which can cause extrusion defects. The Benbow-Bridgwater method was used to characterise the paste extrusion behaviour. The rheological properties of the proprietary alcohol-based binder system were determined separately by rotational rheometry. This showed that the binder behaved as a viscoelastic solid with a strong sensitivity to temperature. Over the temperature range employed in industrial processing (30-42 �C), the rheological parameters of the paste were linked to those of the binder: the elastic modulus G' of the binder showed a linear correlation to the paste initial bulk yield stress ~, whereas the binder shear viscosity was linearly related to the paste velocity dependence factors, a and p. The effect of temperature, extrusion velocity and tool design on the formation of external (surface fracture) and internal (lamination) defects during extrusion was investigated. Increasing extrusion pressure, either by lower temperatures, higher extrusion velocities or using longer dies, was identified as key to avoidin~ surface fracture. The distance between a flow divider and the die entry had the largest effect on the formation of a lamination plane in the extrudates. The drying behaviour of paste discs, rods and tubes was studied in an industrial drying oven at constant temperature. Diffusion was identified as the governing mechanism. The data could be fitted to a Fickian diffusion model. The observed dependency of the effective diffusion coefficient on the size of the part indicated that the model requires further refinement to allow truly predictive capability.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral