A state-of-the-art digital light processing 3D printing material and process for production of investment casting sacrificial patterns

Investment Casting (IC) is one of the oldest and most versatile methods of manufacturing near net shape metal parts. IC can deliver parts with intricate geometries, thin walls, superb surface finish, and high dimensional accuracy with an ability to manufacture most metallic materials used in different industries. However, the main shortcoming of IC method is its rather long development lead-time and high toolmaking cost. Specifically, in high-value manufacturing where fast development, low order quantities, short product life cycles, geometrical complexities, and customised designs are in highest demand, conventional investment casting method is prohibitively expensive and slow. However, additive manufacturing (AM) is set to change this shortcoming by reducing the production lead time and cost of sacrificial patterns used in IC. Patterns can be directly produced by AM techniques to entirely eliminate the need for the injection moulding tools and the injection process, which often comprise up to one-third of the product development cost and up to half of the development lead-time. These estimations are even higher for customised products such as biomedical implants.
Among many AM methods, Vat Photopolymerization process in which a liquid photo-resin is selectively solidified under light illumination, is known for its low cost, excellent resolution and high scalability. Digital Light Processing (DLP) is one of the Vat Photopolymerisation techniques in which the whole cross-section of the part is light cured at once making the process faster compared to other techniques. This AM technique and its application in IC will be studied in this research.
Research Gap
Although material jetting and stereolithography printing methods have been long studied in IC applications, there is currently little academic literature and limited industrial reports on using DLP systems to produce cost effective, fast and accurate IC patterns. Thus, one needs to define a methodology to develop a set of processes and materials to enable a wide use of DLP method in the casting patterns production. However, the challenges are to develop a resin mix, print process and handling/assembling procedure to offer the following: 1) minimum shrinkage during printing and post printing processes; 2) minimum residual stress during printing process to avoid deformations after printing; 3) minimum ash-content after pattern burn-out process; 4) minimum thermal expansion during burn-out process to avoid cracking and breaking IC ceramic mould; 5) best print resolution, surface finish and geometrical printability; 6)best handling and storing conditions for AM-IC patterns and best method to embed a ceramic core into an AM-IC pattern when needed.
Aims and objectives
This research aims to design, verification, and validation of a DLP system and material to achieve the highest castability, dimensional and geometrical accuracy, and cost effectiveness for IC patterns.
To achieve this aim, the following objectives are defined:
- To develop a methodology to better study an understand burning AM-IC patterns and thermal and gas expansion during burn-out process. It may include physical and computer-aided finite element simulation methods.
- To modifying the photocurable resin material and the DLP 3D-printing process to achieve the best dimensional accuracy and geometrical printability by reducing the shrinkage and residual stress during printing and post-printing, in order to minimise/control deformation of the part during print and post-print stages.
- Enhancing pattern removal process capability by introducing additives to the resin, improving resign mix materials, and adjusting the pattern removal operations, in order to prevent damage to the ceramic mould.
- Develop and validate a method to best store, handle and assemble AM-IC patterns and to fit the ceramic cores inside the patterns while maintaining its dimensional accuracy and functionality.

Additive Manufacturing (AM) is the direct production of end-use component parts made using additive layer manufacturing technologies. AM enables the manufacture of geometrically complex, low to medium volume production components in a range of materials, with little, if any, fixed tooling or manual intervention beyond the initial product design. It enables a number of value chain configurations, such as personalised component part manufacture but also economic low volume production within high cost base economies. The concept of AM is to use the layer approach to add value to a component part during manufacture. This Centre for Doctoral Training in Additive Manufacturing has been developed to bring about a formalised and innovative training structure to make the best of the human capital being graduated from Universities and allow them to embark on a tailored programme of doctoral training specifically in AM. Current training for AM is disparate and sporadic with no formal doctoral training available in the UK. However, AM is recognised by government and industry as being a key technology for the UK, and one that will allow the UK to maintain and grow its high-value manufacturing sector. AM provides a basis for long-term innovation within UK manufacturing and in particular, the concept of 'growing' entire components offers significant benefit to the high-value manufacturing sector, based on innovative design solutions. AM is cross-sectoral in nature, and with the development of the multi-functional AM is increasing in its diversity and the requirements for multi-disciplinary research are increasing, engagement with students and academics from varying disciplines, Chemistry/Biological Sciences/Physics etc. is a key requirement to make the most of the UK's research lead. In concert, industrial contacts are broadening, previous engagement from industries such as automotive and aerospace continues. However, basic materials companies, software and specialist manufacturing companies are now seeing AM as a route to market or exploitation of their products and / or services.
The current AM market place for machine tools, materials and services (such as software) is valued at just over $1.2Billion. However, it must be acknowledged that AM is an enabling technology. But, fundamentally, AM adds the greatest value in its application and taking an AM philosophy can result, through design freedoms, production flexibility and supply chain economics changes to the traditional manufacturing model that can provide business benefits unrealisable with conventional manufacturing technology. In the medium to long term, the opportunities for AM are significant and diverse, and through this philosophy, the CDT's industrial partners want to enable the next generation of AM processes, materials, software tools and supply chains.
It can thus be seen that there will be multiple beneficiaries from the CDT's establishment. The CDT's industrial supporters represent a variety of end user industries - both multinational and SMEs - together with the equipment and systems manufacturers and suppliers representing the value chain. The benefits from trained scientists and engineers and new AM technology and processes enables new product and market opportunities in diverse sectors, and economies in production of existing components and systems, whilst supplying the ready-made human resources to augment these developments within industry and academia. The requirement for students to undertake internships at these companies during their studies means that these companies will have a pipeline of talent to work with and employ in the future. From a wider perspective, the UK will benefit through increased competitive advantage of its manufacturing industries, and the public will gain through new products at economic costs. For society the AM offers the environmental gain of lower carbon footprints and more sustainable use of precious raw materials.