Bianchi, Maria Floriana
(2020)
An investigation into the thermal management and control of ceramic injection moulding die-tools.
PhD thesis, University of Nottingham.
Abstract
Ceramic Injection Moulding (CIM) is an extensively used process in the mass production of complex components, consisting of three stages: injection moulding, debinding (i.e. binder removal) and sintering. In the aerospace industry, CIM is employed to make ceramic cores, which are sacrificial components used to shape the internal cooling channels of high-pressure turbine blades and vanes, manufactured by investment casting. The injection moulding of cores presents significant challenges due to their geometry, characterised by adjoined features having uneven wall thickness: this determines differential cooling rates along the part arising throughout the moulding process, which, in turn, cause several flaws in finished cores (e.g. warpage, cracking and poor surface quality). Even higher mouldability challenges are brought by the increasing requirements of turbine working temperatures, calling for higher complexity of core geometries. To cope with this, suitable due-tool thermal control approaches have to be employed. However, state-of-the-art temperature control methods present strong limitations, as they aim at achieving a uniform cavity temperature, which do not prevent the development of uneven cooling rates in parts with differential wall thickness.
To address these shortcomings, the aim of this project is to develop and demonstrate a novel thermal control system for die-tools to improve the capability of manufacturing complex components characterised by uneven wall thickness through CIM. To achieve this, the present work addresses the following objectives: (i) to develop a methodology to analyse the performance of thermal control systems for CIM; (ii) to explore the effects of state-of-the-art thermal control approaches upon the quality of features having different wall thickness; (iii) to design and demonstrate a novel die-tool with a tailored and optimised temperature control system; (iv) to construct guidelines for applying and operating the novel proposed thermal management system with generic complex components.
In the first part of the work, a methodological framework for the performance evaluation of thermal control systems for CIM die-tools was developed. This methodology relates macro- and micro-structural outcomes observed in green and sintered components to the heat transfer phenomena occurring while employing a thermal control system. This was achieved with the support of a developed simulation of the injection moulding filling process. Then, a novel study was carried out to analyse the performance of Rapid Heat Cycle Moulding (RHCM) applied to CIM. The RHCM approach was found to promote a more uniform microstructure, in terms of particle orientation and packing, compared to the use of a constant ambient mould temperature. However, the RHCM approach determines an increase of mould-part adhesion, thus compromising the demoulding process. Based on the understanding achieved from these studies, a novel system was developed, having a regional control of die-tool temperatures. This was achieved through the development of a thermal control model, having the objective to minimise cooling rate gradients in the part, and demonstrated using a prototype mould equipped with actively controlled thermoelectric elements. The novel system ensures simultaneous solidification of features having different thicknesses, hence promoting higher dimensional accuracy compared to the RHCM approach, while keeping its advantages related to feature replication capability and surface integrity. Finally, the thermal model was applied to a case study core geometry and guidelines for the were elaborated for the use of the novel proposed system in industrial contexts.
The work developed in this thesis hence contributes to the understanding of how heat transfer phenomena in CIM affect the quality of ceramic moulded components. Moreover, the development and validation of moulds with regional temperature control push the boundaries of injection moulding to manufacture components characterised by uneven wall thickness, which has the potential of highly expanding the process flexibility and its possible fields of application.
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