Additive manufacturing assisted investment casting: vat photopolymerisation of sacrificial casting patterns for highly precise and complex components

This research is in the field of additive manufacturing-assisted investment casting. The abstract is structured in the following main sections.

Problem Statement

the ever-increasing demand for fuel efficiency of gas-turbines and jet-engines is constantly striving for more complex geometries of hot section components (e.g. turbine blades). Besides, the after-market services and spare parts production of older and less complex geometries has proved problematic due to the small batches required, which are not often economical for mould-making. Hence, implementing additive manufacturing methods to assist the conventional investment casting, known as rapid investment casting, has attracted significant attention in turbine and get-engine industries. Among additive manufacturing methods, vat photopolymerisation is one of the most promising techniques for this application, due to its scalability, cost-efficiency, and high surface finish and dimensional accuracy. However, the poor performance of the current commercial solutions on the market (e.g., ceramic shell failure, especially in complex geometries with thin walls) are some of the factors limiting the widespread adaptation of this approach by foundries, despite its advantages.

Aims and objectives

the primary aim of this PhD thesis is to enable a large-scale manufacturing of sacrificial casting patterns of complex geometries using cost-efficient and scalable LCD-based vat photopolymerisation technique. To achieve this aim, three objectives are defined for this research. 1) to design a novel photoresin formulation for 3D printing patterns that can soften in and above 100°C and before the thermal expansion of the solid polymer damages the ceramic shell. 2) To improve the dimensional accuracy and stability of the casting patterns through minimising the deformation and warpage during the printing and post processing stages. 3) To understand the interaction of heat and the monomer choices during the photopolymerisation process to maximise the advantages of heat-induced vat-photopolymerisation on enhancing the dimensional accuracy of the printed patterns.

Thesis structure

The body of this thesis is structured into three main chapters, each formulated and written in the form of a manuscript for an academic publication. Each research paper has been designed to correspond and address one of the abovementioned research objectives. Therefore, chapter 2 (first research paper) presents the design and characterisation of a novel photoresin formulation based on a monofunctional thermoplastic-like monomer, namely acryloyl morpholine. The characterisation consists of both generic properties of parts printed by vat photopolymerisation, such as mechanical properties and degree of conversion, and more casting pattern related characteristics such as thermomechanical and dimensional properties,and burnout/firing performance. Chapter 3 (second research paper) highlights the impact of elevated temperatures during the vat photopolymerisation process (hot lithography) on some of the characteristics of the printed parts including the mechanical properties, degree of conversion, glass transition temperature, and more importantly, dimensional behaviour of the parts. Chapter 3 (third research paper) connects the different properties (e.g., chain length, type of network, and type of functional group) of monomers to the changes of their polymerisation kinetics under different photopolymerisation temperatures.

Conclusions and novelty

this research, for the first time, demonstrated the successful use of thermoplastic-like monofunctional monomer of acryloyl morpholine in photoresin formulations aimed for sacrificial casting patterns, compatible with main-stream, cost-efficient, and scalable LCD-based vat photopolymerisation 3D printers. This research also shed more light on hot lithography, and in particular the impact of heat during the vat photopolymerisation process on minimising the deformation and warpage of the parts during and after the 3D printing process. This practice helps to achieve the dimensional precision required for advanced applications such as casting patterns used in aerospace and industrial gas turbine components, where the significantly high demand for complexity and dimensional accuracy had previously made LCD 3D printing abundant as a viable direct rapid investment casting method. Finally, an in depth understanding of the interaction of heat and monomers during the photopolymerisation process is outlined. This understanding will assist researchers and practitioners in designing the right combinations of monomer mixtures and process temperature to maximise the benefits of hot lithography.