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Metal Casting Technologies : September 2005
n full mold casting process, EPS pattern and gating system of 1.6 to 2.0 lbs/ft.3 density are made and coated with a suitable thin refractory material. On drying the coating, the assembly is molded in loose unbonded sand. Liquid metal is poured into the mold, while the EPS pattern remains inside the sand mold. EPS pattern is burnt from the heat of liquid metal and it is gradually replaced by liquid metal. On solidification and cooling, the casting is easily taken out from loose sand mold. Since the mold remains full during pouring without any cavity as in conventional mold, the process is known as Full Mold Casting (FMC) process. Gas related defects are common in all castings including FMCs. Sources of gas causing these defects are dissolved gases in liquid metal, air entrapped during filling of castings, gases evolved from mold and core materials during and after pouring. Gases from these sources leads to defects in castings in various forms starting from fine gas porosity to large blow holes. In FMC, additional source of gas prevails due to burning of EPS pattern. Huge volume of gases such as styrene vapor, methane and hydrogen are generated by burning of EPS pattern during filling of mold. In additon, liquid polystyrene and graphite powder type of products are also formed by burning of EPS depending on temperature of liquid metal. A thin refractory coating is applied over EPS patterns with a few objectives. The coating improves surface finish of casting. It also prevents sand inclusion during filling of mold, since loose unbonded sand used in the process are likely to be included in flowing liquid metal easily in absence of a refractory coating. Lastly, the coating imparts strength to the mold to some extent and hence reduce the risk of mold collapse partially during or just after completion of pouring. In conventional mold, air present in mold cavity is escaped through riser, flow- off, parting line and permeable mold. Without removal of air from the mold cavity, it cannot be filled by liquid metal. The situation is quite different in FMC. No mold cavity, hence no air exists. Instead, gases are generated by burning of EPS during filling of the casting. All these gases escape through the coating at first and subsequently through loose sand into the atmosphere. Enough precautions are taken in the formulation of the coating material and application of the coating over pattern to allow the gases to escape. To control gas related defects in FMC, a novel approach is undertaken in the present study by providing vents in EPS pattern. Vents are commonly provided in sand molds and cores to allow the gases to escape. In FMC, loose unbonded sand has good permeability, no cores are used, on the other hand pattern is gradually burnt during pouring and huge volume of gases are generated by decomposition of EPS. To facilitate the escape of decomposition products of EPS, vents of suitable design have been provided in the patterns for the trial castings, so that a continuous path for gas flow ending at external surface of pattern is available to the gases formed by burning of EPS. EXPERIMENTS Patterns of 300 mm x 100 mm x 116 mm as shown in Fig.1 were prepared using EPS of 1.8 lbs/ft.3 density. Suitable gating system was designed with a gating ratio of 1:4:4 and it was attached to the pattern with adhesive. To provide vent inside the pattern, it was divided longitudinally into two equal parts. 5 mm dia. vent was grooved inside the patterns. The vents started from the runner and it extended to a top position of the pattern in such a way that a continuous path for flow of decomposition gases was available. The photograph of pattern assembly is shown in Fig.1. The patterns were coated with a slurry based on sodium silicate and silica flour. To develop thick coating over patterns, the coated patterns were stuccoed with silica sand of 80 AFS grain fineness no. The coating and stuccoing were repeated two times with intermittent drying in air to develop a shell thickness of 2-3 mm approx. The shell was formed over patterns to avoid mold collapse during or just after pouring. The photograph of coated and dried pattern assembly is shown in Fig.2. The coated and dried pattern assembly were molded with loose unbonded silica sand ( AFS grain fineness no. 48). TECHNICAL FEATURE P.C. Maity, Professor, Foundry Technology Department, National Institute of Foundry and Forge technology, Ranchi 834003 India E: email@example.com W: www.mainnotech.com Experiences on full mold casting (FMC) with vented expanded polystyrene (EPS) pattern I 1 2 3 56 www.metals.rala.com.au