CASTING PROCESS
SAND CASTING Sand Casting In this process, sand mixed with binder and water is compacted around wood or metal pattern halves to produce a mould. The mould is removed from the pattern, assembled with cores, if necessary, and metal is poured into the resultant cavities. After cooling, moulds are broken to remove the castings. This process is suitable for a wide range of metals (both ferrous and non-ferrous), sizes and shape complexity.
STEPS INVOLVED IN MAKING CASTING Make Pattern out of wood, Metal or Plastic. In case of Sand Casting, select, test and prepare the necessary sand mixtures for mould and core making. With the help of patterns prepare the mould and necessary cores. Melt the metal/Alloy to be cast. Pour the molten metal/Alloy into the mould and remove the casting from the mould after metal solidifies. Clean and finish the casting. Test and inspect the casting. Remove the casting defects, if any. Relieve the casting stresses by Heat-treatment. Again inspect the casting. The casting is ready for shipping.
Work flow in typical sand-casting foundries [source: www.p2pays.org]
THE SAND PROCESS
STEPS INVOLVED IN MAKING CASTING ( FROM FIG.) The mold is made of two parts, the top half is called the cope, and bottom part is the drag. The liquid flows into the gap between the two parts, called the mold cavity. The geometry of the cavity is created by the use of a wooden shape, called the pattern. The shape of the patterns is (almost) identical to the shape of the part we need to make. A funnel shaped cavity; the top of the funnel is the pouring cup; the pipe-shaped neck of the funnel is the sprue – the liquid metal is poured into the pouring cup, and flows down the sprue. The runners are the horizontal hollow channels that connect the bottom of the sprue to the mould cavity. The region where any runner joins with the cavity is called the gate.
CONTD. Some extra cavities are made connecting to the top surface of the mold. Excess metal poured into the mould flows into these cavities, called risers. They act as reservoirs; as the metal solidifies inside the cavity, it shrinks, and the extra metal from the risers flows back down to avoid holes in the cast part. Vents are narrow holes connecting the cavity to the atmosphere to allow gasses and the air in the cavity to escape. Cores: Many cast parts have interior holes (hollow parts), or other cavities in their shape that are not directly accessible from either piece of the mold. Such interior surfaces are generated by inserts called cores. Cores are made by baking sand with some binder so that they can retain their shape when handled. The mold is assembled by placing the core into the cavity of the drag, and then placing the cope on top, and locking the mold. After the casting is done, the sand is shaken off, and the core is pulled away and usually broken off.
Schematic showing steps of the sand casting process [source: Kalpakjian and Schmid]
CONTD. (e) Gated Pattern (f ) Segmental Pattern (g) Skelton Pattern (h) Sweep Pattern (i) Loose Piece Pattern (j) Follow Board Pattern
PATTERN ALLOWANCES Machining Allowance Shrinkage Allowance Draft or Taper Allowance Distortion Allowance Shake or Rapping Allowance
Pattern, Finish Allowance, and Wall Thickness Metal Pattern Oversize Factor (each direction) Finish Allowance (smaller number for larger sizes) Min Wall mm (inches) Aluminum 1.08 - 1.12 0.5 to 1.0 % 4.75 (0.187) Copper alloys 1.05 - 1.06 2.3 (0.094) Gray Cast Iron 1.10 0.4 to 1.6 % 3.0 (0.125) Nickel alloys 1.05 N/A Steel 1.05 - 1.10 0.5 to 2 % 5 (0.20) Magnesium alloys 1.07 - 1.10 4.0 (0.157) Malleable Irons 1.06 - 1.19 0.6 to 1.6 %
SAND CASTING TERMINOLOGY
DEFINITIONS POURING CUP: This is where the metal is poured into the mold. SPRUE: The vertical channel from the top of the mold to the gating and riser system. Also, a generic term used to cover all gates, runners and risers. RUNNER: The portion of the gate assembly that connects the sprue to the casting in gate or riser. GATE: The end of the runner in a mold where molten metal enters the mold cavity. RISER: A reservoir of molten metal provided to compensate for the contraction of the metal as it solidifies. MOLD CAVITY: The impression in a mold produced by the removal of the pattern. When filled with molten metal it forms a casting. COPE: Upper or top most section of a flask, mold or pattern. PARTING LINE: A line on a pattern or casting corresponding to the separation between the parts of a mold. DRAG: Lower or bottom section of a flask, mold or pattern.
ADVANTAGES OF SAND CASTING Use is widespread; technology well developed. Materials are inexpensive, capable of holding detail and resist deformation when heated. Process is suitable for both ferrous and non-ferrous metal castings. Handles a more diverse range of products than any other casting method. Produces both small precision castings and large castings of up to 1 tonne. Can achieve very close tolerances if uniform compaction is achieved. Mould preparation time is relatively short in comparison to many other processes. The relative simplicity of the process makes it ideally suited to mechanisation. High levels of sand reuse are achievable
LIMITATIONS OF SAND CASTING Typically limited to one or a small number of moulds per box.. Sand:metal ratio is relatively high. High level of waste is typically generated, particularly sand, baghouse dust and spent shot.
GATING SYSTEM: Sprue, Runner, and Gate The term gating system refers to all passageways through which the molten metal passes to enter the mold cavity. The gating system is composed of Pouring cups and basin Sprue Runner Gates Risers
FUNCTION OF GATING SYSTEM Fill the mold cavity completely before freezing Introduce the liquid metal into the mold cavity with low velocity and little turbulence Help to promote temperature gradients favorable for proper directional solidification. Separation of non-metallic inclusions. Regulate the rate at which liquid metal enters into the mold caity. Consume least metal.
TYPES OF GATES TOP GATE BOTTOM GATE PARTING LINE GATE