The role of additive manufacturing in the investment casting process

. The aim of this study was to systematically describe the additive manufacturing (AM) technologies used for various casting technologies and how rapid investment casting (RIC) is changing the field of casting. The capability and effectiveness of additive manufacturing to provide investment casting production solutions has been investigated. The traditional investment casting method is less efficient in terms of cost and time to develop new wax models of solid tooling for low volume production or prototypes. To overcome this problem, it is proposed to introduce additive manufacturing to these processes to produce investment casting models. This paper discusses specific applications of rapid prototyping in this area. The study concluded that the use of additive manufacturing for investment casting instead of the traditional method is more cost-and time-efficient. Terms and designations: RP – Rapid Prototyping, IC, RIC – (Rapid) Investment Casting, PM – Project Manager, AT – Additive technologies, AM – Additive manufacturing


Introduction
Additive manufacturing is defined by a range of technologies that can convert virtual solidmodel data into physical models. ATs produce parts by polymerising, fusing or sintering materials in predetermined layers without the use of tools. An exact copy of the final product is created in CAD software, and the CAD data is then split into a series of twodimensional cross sections of finite thickness. These cross sections are loaded onto 3D printers to combine them, adding them to each other layer by layer to form the product [1]. In this way, the geometry of the part is reproduced on the AM machine. This principle is the basis of almost all AP machines, with variations in each technology in terms of the methods used to create the layers and their adhesion. AMs have improved to the point where many manufacturers use the output of AM machines to produce their final product. Direct digital manufacturing opens the door to new applications that were normally considered impossible, infeasible, or uneconomical. There is now a significant opportunity for mass customisation, where a product can be produced to the consumer's requirements but at an economically viable price [2].
The term rapid investment casting (RIC) refers to the use of additive manufacturing (AM) technologies in investment casting. This technology enables cost-effective development and production of master models. In addition, the implementation of additive manufacturing reduces casting time while maintaining the same product quality. It is also cost effective for single or small batch production. Numerous cost-effective solutions have been described in the literature for the investment of orthopaedic implants using additive technology [3].
Rapid investment casting can be divided into three approaches: 1. RP-manufactured IC patterns -manufactured moulds for wax injection. 2. Direct fabrication of ceramic moulds for case fabrication. 3. RP-manufactured injection moulds, further classified into wax and non-wax, and RPmanufactured wax injection moulds for tooling [2].
The impact of 3D printing on the manufacturing sector has been significant. The development of AT technology has ushered in a new era characterised by a wide range of materials, higher accuracy and lower relative manufacturing costs [4].
AM is a manufacturing method with great potential to revolutionise traditional manufacturing methods in the future. Its characteristics, such as no tooling required for mass customisation, short lead times, relative affordability for mass production with complex configurations and the ability to minimise material wastage, make it highly advantageous. Advances in technology have also impressed designers and artists, as it allows them to accurately realise their ideas at the most challenging levels of complexity. Moreover, the final product can be obtained without involving a large number of skilled craftsmen and long lead times [2]. Traditional methods face various challenges. Mould fabrication using traditional methods such as machining has limitations including restrictions on minimum wall thickness, the need to eliminate sharp corners and undercuts which require increased angles of inclination and result in increased fabrication costs [5]. These limitations are further exacerbated when dealing with parts of higher design complexity. As a consequence, non-functional design modifications (increased casting weight) are often required and additional machining steps may be required after casting. The ultimate goal of this paper is to analyse the feasibility and effectiveness of rapid prototyping as a viable solution for investment casting production. In the next section, we will review the types, applications, advantages and limitations of additive manufacturing in the field of metal casting. In the methodology section, the workflow design of this research will be proposed and several case studies will be presented as supporting evidence. Finally, the conclusions and key results will be summarised in the final section [6].

Methods and Materials
Papers were selected based on keywords related to the role of additive manufacturing (AM) in casting. The keywords used included "Role of additive manufacturing in casting", "additive manufacturing in casting", "additive manufacturing in various casting processes", "3D printing in casting", "3D printing in various casting applications", "Rapid Manufacturing in casting Casting", "RM in various casting processes", "Rapid investment casting", "RP moulds for wax injection", "Direct fabrication of ceramic IC shell moulds", "Indirect tooling using AT" and "Direct machining using AT". No inclusion or exclusion criteria were applied in the selection process [7].
Research Questions. The aim of this systematic review is to explain additive manufacturing and rapid investment casting. The main research questions underlying our study are as follows: 1) How has rapid investment casting (RIC) changed the field of casting? 2) What are the limitations of conventional investment casting process? 3) What are the advantages of adopting additive manufacturing technology in investment casting? Eligibility Criteria. Studies eligible for inclusion in this systematic review must fulfil the following criteria: 1) Studies related to additive manufacturing. 2) Studies related to rapid investment casting.
3) Studies that evaluate the application of AT in the field of casting. 4) Studies published in English, Russian languages.
The exclusion criteria for this study are as follows: 1) Research papers of less than 4 pages. 2) Cases of AT in fields other than casting.
3) Studies lacking appropriate statistical and reliable data. 4) Studies published before 1994.
To refine the search and obtain accurate answers to our research questions, the authors followed the following steps: -Initially, abstracts of research papers were considered as the main part of the first step. -In the second step, full papers were reviewed to obtain more detailed information from the remaining papers [8].
A systematic electronic search was conducted by four independent authors using databases such as ELibrary, Google Scholar, ResearchGate, ScienceDirect and Mendeley to collect articles published up to September 2021 according to eligibility criteria. The following keywords were used in the search: "Role of additive manufacturing in casting", "additive manufacturing in casting", "additive manufacturing in various casting processes", "3D printing in casting", "3D printing in various casting applications", "Rapid Manufacturing in casting", "RM in various casting processes", "Rapid investment casting", "RP wax casting moulds", "Direct manufacturing of ceramic IC shell moulds", "Indirect processing using AT" and "Direct processing using AT". According to these keywords, many research papers were found [9][10][11][12].
During the selection process, the titles and abstracts of the studies were assessed for eligibility. Where two researchers disagreed, the two remaining researchers were consulted and consensus was reached at the meeting. Data were extracted in such a way as to provide correct and comprehensive answers to the research questions. All authors contributed equally to the collection of information from different research papers [13].

Results
This section presents the findings of the study with a comprehensive discussion of the solutions to each question. Given that different authors have different perspectives, thorough discussions were held between all authors that led to the conclusions of the study. Investment casting. Investment casting, is one of the oldest known casting methods and is supported by moulding wax models. It has been used in a variety of moulds over the past thousands of years. Originally, beeswax was mainly used to create models in the casting process. However, it has now been replaced by more advanced waxes, refractory materials and special alloys. Investment casting is highly regarded for its ability to produce components with accuracy, versatility, repeatability and integrity from a variety of metals and alloys [14][15][16].
The thin wax models used in investment casting must have sufficient strength to withstand the forces encountered during mould making. The term "investment casting" comes from the fact that the model is surrounded by a refractory material. Suitable materials for investment casting include stainless steel alloys, brass, aluminium, steel and glass [17].
The process involves pouring material into a cavity within a refractory material that replicates the desired part. Currently, the two main investment casting methods are liquid glass and silica sol. These methods differ mainly in surface roughness and cost of production. The ceramic mould in the liquid glass method is made from liquid glass silica sand, and the liquid glass method involves dewaxing in high temperature water. The silicic acid sol method is more expensive but provides a less rough surface compared to the liquid glass method [18].
Limitations of traditional investment casting method. The traditional investment casting method has several limitations, particularly in terms of cost, especially for low production castings. High costs are associated with specialised equipment, expensive refractories and binders, numerous operations required to create the mould, high labour costs and occasional minor defects. Casting objects requiring cores is fraught with difficulties. Holes produced by this method should not be smaller than 1.6 mm and should not exceed a depth of approximately 1.5 times the diameter. In addition, the production cycle of this technique is longer compared to other methods [19].
3D printing technology. 3D printing involves the creation of a three-dimensional object based on a CAD model. The term "3D printing" encompasses various processes in which material is applied, bonded, or solidified to create a 3D object, usually by layer-by-layer application. In the early 1980s, 3D printing techniques were mainly used to create practical or aesthetic prototypes, and at the time it was called rapid prototyping.
Advances in additive manufacturing have led to commercially available rapid prototyping (RP) printers capable of producing highly detailed models. One additive manufacturing process involves print heads (similar to those used in inkjet printers) applying thin layers of photopolymer, which are then cured to create the final object, layer by layer. The first step in the printing process is to design the model using software such as SOLIDWORKS. The virtual part is saved in stereolithographic file (STL) format, and the data is sent to the printer as individual slice images. For each slice, the printer applies a 16 micron thick layer of photosensitive resin with a nominal cross-sectional resolution of 42 microns. [20] The printer applies two types of resin: support material and model building material. The support material is a gel-like UV resin specifically designed to support complex geometry and subsequent layers during printing. A variety of model building materials are available, varying in colour, hardness and flexibility. After each cut is applied, the resin is cured in a UV lamp and then the tray is lowered to 16 microns to repeat the process. Once printing is complete, the part is removed from the tray and the support material is removed manually by scraping and hydro-jetting. In hard-to-reach areas, soaking in an aqueous solution of 10-20% NaOH followed by water jet rinsing can be used to remove residual support material. The high resolution and large printing capabilities of 3D printers allow us to design and print custom shapes and parts required for aperture fabrication [21][22].
Once the assembled moulds are printed, the fabrication process involves two casting processes: (1) cold casting of the supporting aperture structure using a tungsten-epoxy mixture and (2) investment casting or investment casting of the point inserts. We found that this is a cost-effective method of aperture fabrication. However, it is important to note a significant cost difference between traditional machining and rapid prototyping: the cost of traditional machining is mainly determined by the operations required to machine the part, while the cost of rapid prototyping is mainly dependent on the size of the part rather than its complexity. Rapid prototyping enables complex geometries such as printing parts with square holes or curved channels embedded in a solid block. Given the geometric complexity of the imaging systems we develop, traditional processing methods would be prohibitively expensive or even impossible. However, with 3D printing combined with casting, we can easily fabricate complex parts [7].
Advantages of additive manufacturing technology. Additive manufacturing has several advantages. Firstly, the components or products produced using this method are lightweight and cost-effective. This allows parts to be manufactured with precise dimensions, and production cycle times are greatly reduced. Designers can visually assess the parts and evaluate their advantages and disadvantages.
In additive manufacturing, there is no need for assembly in multi-component manufacturing. Solids and shells can be manufactured simultaneously, resulting in less material usage and increased structural rigidity. Once the digital file is sent to the 3D printer, the printing process is fully automated, eliminating the need to change tools during operation. This reduces processing time by eliminating the need for additional fixtures, moulds or jigs. Due to the nature of 3D printing, there is little or no material loss. In addition, 3D printers can be installed in non-industrial areas [23][24].
'Rapid' aspect of this technology goes beyond the speed of manufacturing parts. This also applies to the process of developing casts or models, which relies heavily on computers. Additive manufacturing starts with computer-aided design that provides a relatively smooth transfer process. Regardless of the complexity of the product, it can be manufactured in a single operation. In contrast, products manufactured using traditional technologies often require several intermediate stages before they reach the final consumption stage. The introduction of additive manufacturing can significantly reduce the number of intermediate stages and the amount of materials and operations required, simplifying or eliminating many multi-step processes [1].
Limitations of additive manufacturing. Additive manufacturing also has some limitations. Firstly, it depends on the materials used and the choice of materials is limited. The design of components or patterns is limited by the available materials. Cycle time can be a limiting factor, and the resolution of 3D printing technology is typically around 50 microns. The accuracy achieved depends on the process used and varies from 50 to 300 microns depending on software capabilities and material properties. There are temperature limitations for material processing: operating temperatures are typically below 100°C for materials such as poly lactic acid (PLA) and acrylonitrile butadiene styrene (ABS). Component size is limited by the size of the printing platform, which can cause problems during assembly. Printing large parts can take a significant amount of time. While 3D printing is commonly used for prototyping plastic materials such as PLA or ABS, it may not be suitable for mass production. Finally, the mechanical properties of the product are usually anisotropic before it undergoes the finishing process [25].

Discussion
The main question addressed in this study is how rapid investment casting (RIC) will change the field of casting. The study focusses on a review of specific applications of additive manufacturing (AM) in casting. One of the main problems encountered in lowproduction investment casting is the labour-intensive and expensive nature of the process. However, the development of additive manufacturing technology has propelled it into the next era, offering a wide range of materials, higher precision, and lower manufacturing costs. AT show great potential to improve traditional manufacturing methods in the future [26][27][28].
The unique advantages of 3D printing in mould manufacturing include reduced lead times, flexibility and the ability to validate prototypes in CAD software. It also generates minimal noise and waste, creating a clean manufacturing environment suitable for installation in non-industrial settings. Traditional mould-making methods often face limitations in terms of minimum wall thickness, lack of sharp corners, and undercuts, resulting in higher gradient angles and increased manufacturing costs. These constraints are further exacerbated when tooling complex parts, often requiring modifications to the part design and additional post-casting machining steps [29].
This review aims to investigate the capabilities and effectiveness of AT in providing an efficient solution for customisable and small batch production of investment casting. Conventional methods involving the development of new wax models of solid tools for small batch production and prototypes are inefficient in terms of cost and time [30][31][32][33][34][35][36].

Conclusions
Summarising, additive manufacturing offers a cost-effective solution for small-scale production. 3D printing resin models is a viable alternative to traditional wax model making. It offers several advantages over traditional methods, including faster production and high accuracy, with the potential to eventually replace traditional methods. However, for large-scale production, traditional metal mould technology is still superior to polymer moulds in terms of durability and performance. Additive manufacturing holds great promise and has the potential to contribute significantly to a more sustainable industrial system. Although it has already demonstrated successful applications in various fields including moulding, its current limitations restrict its use in certain circumstances. Further research in this area may open up new possibilities and allow 3D printing to find applications in everyday life.