Introduction
For the continuation of the development of the national key industry and new growth industry, the advancement of the casting industry, which is a representative root industry, is critical. In addition to the casting industry, forging, surface preparation, heat treatment, and molding industries are referred to as root technology. These industries have a lower added value and labor-intensive while being the root of automobile, shipbuilding, and machine engine industries.
In particular, the casting industry of Korea has been fulfilling its role as the rear industry. However, over 90% of all companies are small to medium-sized companies, and, as the industry is mainly about multi-product, small quantity production, the technological difference is severe with short product development cycle, leading to the weakness in proprietary technology development capability and quality improvement.
For cast parts, the automobile industry obtains information such as order information and production plan from customers to produce parts required for the assembly of each module, taking the lean production method that is appropriate for securing flexibility of production line with swift response to customer demands. However, in the case of a differential gear case, the delivery is being delayed due to an increased defect rate notwithstanding this production method. As such, the result of an analysis of the causes showed issues in the casting molding process.
According to Kim, K.S (2014), a differential gear case assembled to an automobile differential gear is a case part of the gear combination device that enables rotation speed to be different between the left and right wheels of a differential gear as a part of the automobile differential restriction device. Its form is complex with highly important cleanliness as well as measurement precision of the inner diameter part. Also, it is absolutely necessary to secure mechanical characteristics and suppress defects of case materials for securing the durability of the final product. Cast molding in the process of casting process of hollow case types is mostly conducted as the gas torch method of the heating casting mold, during which casting defect occurs to cause a significant loss due to serious quality issues (p.51-55).
According to Yu, R (2011), in general, the factors that influence the quality of casting mold parts are pressure, speed, location, time, and temperature of the mold, among which the temperature of the mold exerts the greatest influence on the appearance quality of a molded part. Thus, the temperature control of cast is determinative to external appearance quality as well as qualities such as measurement stability (p.2887-2893).
Mid-sized molded parts are manufactured via direct heating with a gas torching method. When it is completed as a final molded product after going through the next processes, a variety of defects such as flashes, fusion defects, measurement defects, and gas defects. According to Yu, R (2011), these defects cause delivery delays and production costs increase due to the lowering of productivity and quality (p.2887-2893). In addition, in the case of a differential case, it takes a significant weight because it is manufactured as solid in the form of casting.
According to Yun, G.B (2013), this research uses the Design for Additive Manufacturing (DfAM) design method for the differential gear case body part with the purpose of increasing fuel efficiency while maximizing structure strength and making it lightweight to solve the above issues and thereby conducted lightweight design (p.1-18). Because it is difficult for a traditional machine process method to solve these issues, the 3D printing process method is applied to enable manufacturing by integrating specific forms internally. In addition, by applying the 3D printing process method with the Powder Bed Fusion (PBF) using metal powder, a support structure is created, which structure should be removed via post-processing. For the post-processing, heat treatment is conducted for surface finishing and material strength increase to improve the additive surface quality.