# Effects of Post-Treatment to Improve the Surface Quality of 3D Printing Cement Mold Casting
## 1 Introduction  
Metal casting has been steadily researched and developed in today's machinery and manufacturing fields as a method of pouring molten metal in which aluminum, cast iron, steel, special steel, etc. are melted into a mold to obtain the desired shape and characteristics [1]. The most traditional method, sand casting, has undergone many developments over a long history of 4000 years, and still occupies a large share in the modern casting market [2]. The powder bed method is a 3D printing technology that has received a great deal of attention in the sandcasting market. It is easy to apply to sand casting. Moreover, products applied to existing organic binders, inorganic binders, and cement binders have been developed. It is also notable in the casting market because mold making for large products is simply possible in an automatic process [3]. The current casting sand is specialized for large castings, which has an economic advantage, such as for marine engine parts. Accordingly, processing and storage costs are incurred, which makes the process inefficient and not economical [3, 4]. In particular, it is difficult to supplement and apply the product because it is difficult to make a prototype. Even after casting, several post-processing processes are required to improve product quality; therefore, it takes a long time to secure the actual product [5]. On the other hand, if the powder bed method 3D printing process is applied, it is possible to produce an output, as well as complement the drawings that have been converted into data by omitting thewooden mold and mold production simultaneously, which enhances the prototype production process. Not only shortening, but also improving the surface roughness simplifies the post-casting post-treatment process [3]. In addition, through process automation, it is possible to secure uniform products and cast complex shapes that have not been applied to existing products [3; 6]. This is an advantage that high-quality castings offer (can be secured in a single process), and it can also be economical not only for large castings but also for the high-quality and high-value-added casting industry [3]. ExOne and VoxelJet, the current leaders in the 3D printing casting market, prefer to use organic binders in the sand used for existing castings [6]. However, similar to the phenolic and furan binders used mainly in traditional sand casting, environmental pollution problems such as the generation of toxic gases occur [3]. Inorganic binders, such as cement and water glass, are essential to reduce the potential carbon footprint by reducing the gas evolution process [7]. The use of water glass, which is an inorganic binder, as an alternative, has the advantage of minimizing gas generation, but due to the difficulty in ensuring air permeability and the problem of recycling after use, product development is limited. On the other hand, cement, which is an aqueous binder, has attracted attention as a substitute for organic and inorganic binders, by minimizing the generation of organic gas, and refractory cement can minimize thermochemical reactivity to improve casting quality and ensure economic efficiency [8]. The cement powder as a porous binder offers significant advantages, such as easy integration of core and gating systems in the production of parts with complex internal geometries, as they do not interfere with rheological design elements without a vent structure [8]. In addition, the cement-bonded molded core of appropriate strength enables the design of complex shapes in the CPC aluminum process with an easy de-powdering function [9]. In this study, alumina cement, which is a high-temperature casting refractory cement, was used as a binder to select refractory spherical sand to produce 3D printing powder, and a mold was produced through a powder bed 3D printing process.