**1. Introduction**

Unlike conventional sand-casting using wooden molds, full mold casting offers the advantage of near-net shaping without the need for a draft angle in the product shape. As a result, the number of person-hours in post-casting processing can be decreased, reducing cost and shortening delivery times. However, the occurrence of residue defects, which is a phenomenon unique to the full mold casting method, is a major problem in the manufacture of large castings by this method. This is a serious issue that significantly impairs the quality of the product because the unresolved part of the foam model is mixed in with the product.

As a measure against defects in residue at the casting site, the pouring temperature can be increased to facilitate the thermal decomposition of a foam model. However, due to the high temperature inside the product, the volume shrinkage due to solidification increases and shrinkage nests tend to occur. On the other hand, if the pouring temperature is lowered to prevent shrinkage cavity, the foam model cannot be thermally decomposed efficiently and residue defects are generated. Although there is also a method of installing multiple gates, which are the inlets to the product, to improve the molten metal turnover, a flow of metal that rolls in the foam model is generated, and residue defects are more likely to occur.

Traditionally, these casting conditions and schemes have been determined based on the experience and intuition of skilled workers. In recent years, however, the use of computational fluid dynamics (CFD)-based simulations has made it possible to study appropriate conditions and solutions in advance based on the analytical results of the melt flow and solidification processes, thereby reducing the cost and time required for trial manufacture and experimentation. Particularly, solidification simulations are used to study casting conditions and solutions for shrinkage cavity defects, which can be estimated with high accuracy by comparison and verification with experiments [1]. The authors have also been optimizing the shapes of the runners in die castings by using CFD simulation and the shape optimization method for molten metal flow [2].

In recent years, the full mold casting method and the vanishing model casting method have been actively researched. Maruyama et al. have collected, analyzed, and investigated the pyrolysis products generated during the filling of molten metal in the vanishing model casting process [3]. Koroyasu et al. reported on the adiabatic properties of a coating in the vanishing model casting method and conducted a simple simulation of molten aluminum alloy flow in that method to investigate the influence of the air permeability of the coating, the presence or absence of depressurization, and casting methods on molten metal flows [4]. Karimian et al. investigated the influences of the coating thickness on product quality. When the coating film is thin, the amount of pyrolysis gas emitted from the foamed model is higher, and the amount of residue due to pyrolysis is reduced [5]. In addition, studies on the analysis of molten metal flow behavior in vanishing model casting have been actively conducted using CFD simulation [6, 7].

This study proposes a new casting design for the production of large castings by full mold casting. In this method, a residue trap is installed at the product part to prevent the occurrence of residue defects, and the optimal design of the residue trap is realized by using CFD simulation and the shape optimization method. The effectiveness of this method is demonstrated through casting experiments using an actual full mold casting.
