Metal Casting Technologies : MCT-1STQRT-2017
TECHNICAL FEATURE Comparison of Lost Foam Casting (LFC) and Sheet Metal Mould Casting (SMMC) Processes P. C. Maity, Metal Casting and Materials Engineer E-Mail: email@example.com Introduction etal castings are produced mostly in bonded sand based moulds due to its capability to produce wide range of weight and shape of castings and relatively M low cost of production. Although castings having required quality are produced by sand casting process, it suffers from some drawbacks such as chances of sand fusion and sand inclusion defects in castings; pollution in the shop floor from fines in sand and emissions from binders; problem and cost associated with used sand reclamation and disposal. To overcome these problems associated with sand casting process, lost foam casting (LFC) was developed in 1950’s, also named as full mould process, that uses an expanded polystyrene (EPS) pattern, coated with a thin refractory layer and embedded in loose unbounded sand. During pouring, the liquid metal replaces the EPS pattern by burning. Lost foam casting has been used in foundries to produce complex shaped castings and it has been reported that production of aluminium and iron alloy castings by LFC process has increased by more than 1000% over the past two decades. The process is more suitable and economic for complex shaped castings and hence cylinder head and block are being produced in mass scale by this process. Still the number of foundries using LFC is negligible in comparison to the total number of foundries in any country probably due to difficulty in handling and storage of fragile EPS patterns and risk of mould collapse during or just after pouring. Sheet Metal Mould casting (SMMC) process, has been introduced in the recent past that uses sheet metal to form the mould cavity and loose unbounded sand as the mould material. In the first step, the surface of a casting to be produced is divided into two or more parts depending on the complexity of the casting in such a manner that each part of the surface can be produced of a sheet metal by sheet metal working. Each part is produced from sheet metal by cutting to required dimensions and subsequently by bending, drawing or combination of the two. The gating system and risers are also produced from the sheet metal. These parts are assembled to produce a sheet metal mould. The whole assembly is embedded in loose unbounded sand contained in a mould box and compacted. The subsequent steps are similar to conventional sand casting. Due to use of loose unbounded sand as the moulding material, risk of mould collapse prevails in this casting process too. LFC is being used in a limited number of foundries, whereas SMMC is a new concept and still to be used in foundries. Hence comparison of the characteristics of the two processes with respect to sand casting is apparently useful for adoption in metal casting industries. Basic steps of LFC and SMMC Lost Foam Casting (LFC) 1. Patterns are produced from Expanded Polystyrene (EPS) either by machining EPS blocks manually or by moulding in a aluminium die filled with EPS beads and by passing steam into the die. 2. Pattern is assembled with gating system also made of EPS. 3. A thin refractory coating formulated specially for this process is applied over pattern assembly. 4. The coated pattern assembly is placed in a moulding box and filled with loose unbounded sand so that a few inches of sand are present at the bottom of pattern assembly. The loose sand in the mould box is vibrated to have better compaction of loose sand. 5. Liquid metal is poured into the mould. During pouring, liquid metal replaces the EPS pattern by burning. 6. Cooling and shake out. METAL Casting Technologies 1st Quarter 2017 25 Sheet Metal Mould Casting (SMMC) 1. The surface of the casting to be produced is divided into a minimum number of parts in such a way that each part can be produced from sheet metal by cutting, bending, drawing etc. 2. Gating system is also produced from sheet metal by sheet metal working. 3. Sheet metal shapes for the casting and gating system are assembled. 4. The sheet metal assembly is placed in a moulding box similar to LFC and filled with loose unbounded sand so that a few inches of sand are present at the bottom of pattern assembly. The loose sand in the mould box is vibrated to have better compaction of loose sand. 5. Liquid metal is poured into the mould. 6. Cooling and shake out.