Additive Manufacturing (AM) has proved to be a great technology for manufacturing 3D objects that can serve many purposes such as prototypes, fully functional parts, educational tools, or tools that facilitate other types of manufacturing technologies. In particular, the AM was significantly developed to be implemented in the investment casting (IC) industry during the last decades. When AM is used to produce the patterns for metal casting, the manufacturing process is referred to as the rapid investment casting or RIC. Munich et al [1] have mentioned eight types of commercially available AM technologies that are used to produce either wax or non-wax patterns for the IC. Among those methods, the FDM technology is of particular interest for it is a low-cost and widely available product. Additional advantages include the larger build plate and simplicity of the manufacturing process. During the manufacturing process, a wire of plastic/filament is melted and immediately deposited on a flat platform creating a cross-sectional feature of an intended object. The process is continuously repeated, and layers build upon layers creating a solid object. The capabilities of the FDM technology to produce the IC patterns have been studied extensively. Different materials such as general plastics and wax-based filaments are available in the market to be used as patterns for RIC. Among non-wax materials, the ABS showed better performance due to a better surface finish compared to wax filament[2]. The advantage of using non-wax material is in the superior mechanical properties that prevent the pattern from failing during handling or transportation[3][4]. Nevertheless, several downsides have been identified when using non-wax Rapid Prototyping (RP) patterns. They include large thermal expansion during pattern burnout leading to ceramic shell cracking and a higher amount of residual ash [5][6][7][8]. Moreover, the gases generated from burning non-wax patterns, significantly contaminate the mold material, which in turn affects the casting quality. As far as the critical pattern burnout temperature is concerned, when using non-wax patterns, the ceramic shell cracking occurs when the pattern reaches its glass transition temperature (Tg) according to [6]. For ABS the Tg is around 100°C [9] [10] with melting temperature (Tm) ranging from 200 to 240°C. From the physical properties of the ABS filament, it is evident that the burnout procedure is the only proper means for removing the pattern from ceramic mold. In such a case, the material had a negative environmental effect due to wastage and high energy consumption.
As for wax-based filaments, the first materials were developed in the 1990s by Stratasys and had the commercial names ICW04 and MW01. In 1994 Comb et al designed several experiments aiming to modify the composition of wax filaments by mixing wax-based materials ICW04 and MW01 with plastic formulations to improve physical and mechanical properties [11]. In addition, the authors briefly described how the two factorial design was implemented to analyze the effect of printing and envelope temperatures on the surface quality of manufactured parts. Although the study contains meaningful information on the material’s properties, the study is void of important details on printability, dimensional accuracy, and surface roughness of parts made of castable wax. The other mention of wax-based FDM material can be found in the review work done by Cheah et al in 2005 [4] where the wax filament was described as the material having inferior mechanical properties. Considering the current market, the list of commercially available “made for casting” filament products with some characteristics is shown in Table 1.
Table 1
Commercially available castable FDM filaments
Commercial Name | Density, g/cm3 | Nozzle Temperature, C | Cost for 1 kg, $ |
Print2Cast | 0.91 | 140–150 | 70 |
MoldLay | N/A | 170–180 | 65 |
Think3D | N/A | 170–180 | 33 |
Blue Wax PLA Polycast | 1.2–1.43 1.1 | 195–240 190–230 | 40 80 |
Wax3D | 0.98 | 100–110 | 34 |
To the authors' best knowledge, the reports on the effectiveness of the commercial FDM waxes listed in Table 1 are unavailable except for the MoldLay. In 2019 Wang et al performed a topology optimization of a RIC pattern using the MoldLay wax filament [12]. Among a few drawbacks associated with 3D printing wax, a low solidification of material and layer curling were identified as the major downsides. As can be seen from Table 1, most of the castable filaments have printing/melting temperatures much higher than the melting temperature of traditional waxes used in IC. For example, most of the traditional IC waxes have a range of melting temperatures of 60–100°C. Nevertheless, it can be argued that the Wax3D having Tm around 100 ˚C is a potentially better candidate to be used in RIC. Moreover, when working with the material, it has been noticed that it has low viscosity in the molten state which is beneficial for the dewaxing process. As for mechanical properties, the filament has shown high flexibility and softness. Due to low tensile strength and elastic modulus, the manufacturing process is associated with challenges related to finding proper process parameters. Therefore, having considered all the properties of the wax-based FDM filament, the authors have found it necessary to identify the optimal process parameters that can result in successful manufacturing RIC patterns. To achieve the goal, an Orthogonal Array (OA) L9 Taguchi optimization method was implemented with the focus to achieve higher dimensional accuracy. Higher-dimensional accuracy along with low surface roughness are two main requirements established for the IC patterns. Thus, in addition to dimensional accuracy optimization, the appropriate surface treatment was selected. The research concludes with the work dedicated to finding mechanical, rheological, and thermal properties of the wax filament.