Casting Process Part 1

Transcription

1 Mech Zone Casting Process Part 1 (SSC JE Mechanical/ GATE/ONGC/SAIL BHEL/HPCL/IOCL)

2 Refractory mold pour liquid metal solidify, remove finish Casting - Process of Producing Metallic Parts by Pouring Molten Metal in to a Mould having a Cavity of the Part to be Cast and then Allowing the Molten Metal to Solidify. Moulding Process of Making a Mould Using a Pattern. Pattern A Model or Replica of the Part to be Cast.

3 Advantages Complex forms, low cost Certain shapes cannot be machined One piece parts vs. multiple piece parts Design changes are easily incorporated High volume, low skilled labor Large, heavy parts can be made easily Disadvantages Problems with internal porosity Dimensional variations due to shrinkage Trapped impurities, solids and gasses High-tolerance, smooth surfaces not possible More costly than stamping or extruding in some cases

4 Sand Mould Casting Molten Metal is Poured into the Cavity of the Mould without Applying External Pressure (Gravity Feed). Permanent Mould Casting - Gravity Die Casting Die Casting Molten Metal is Forced under very high Pressure in to the Die (Metallic Mould). Hot Chamber Die Casting Cold Chamber Die Casting

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6 Forms of Casting and Terminology Flask : The box containing the mold Cope : The top half of any part of a 2-part mold Drag : The bottom half of any part of a 2-part mold Core: A shape inserted into the mold to form internal cavities Core Print : A region used to support the core Mold Cavity: The hollow mold area in which metal solidifies into the part Riser: An extra cavity to store additional metal to prevent shrinkage Gating System: Channels used to deliver metal into the mold cavity Pouring Cup: The part of the gating system that receives poured metal Sprue: Vertical channel Runners: Horizontal channels Parting Line / Parting Surface: Interface that separates the cope and drag of a 2-part mold

7 Draft: Taper on a pattern or casting that allows removal from the mold Core Box: Mold or die used to produce cores Casting: The process and product of solidifying metal in a mold Metal Solidification

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9 Metal Solidification Dendrites Tree-like structures that form during the solidification of alloys Slow cooling rates produce dendrites with larger branch spacing; faster cooling rates produce finer spacing; very fast cooling rates produce no dendrites or grains

10 Shrinkage or Contraction Tp Liquid shrinkage Tf solidification shrinkage solid shrinkage Ta

11 Fluid Flow Metal is poured through a pouring cup Risers hold and supply metal to prevent shrinking during solidification Gates are designed to prevent contaminants from reaching the mold cavity

12 Fluidity of Molten Metal Fluidity The capability of a molten metal to fill mold cavities Viscosity Higher viscosity decreases fluidity Surface tension Decreases fluidity; often caused by oxide film Inclusions Insoluble particles can increase viscosity, reducing fluidity Solidification pattern Fluidity is inversely proportional to the freezing temperature range

13 Fluidity of Molten Metal Heat Transfer The metal that solidifies first is at the wall of the mold; this solid layer thickens as time passes Shrinkage during cooling can change the part dimensions and sometimes cause cracking; it is caused by the metal s thermal expansion properties and the phase change between liquid and solid.

14 Classification of Casting Processes Process Advantages Disadvantages Examples Sand Shell mold Expendable pattern Plaster mold Ceramic mold many metals, sizes, shapes, cheap better accuracy, finish, higher production rate Wide range of metals, sizes, shapes complex shapes, good surface finish complex shapes, high accuracy, good finish poor finish & tolerance limited part size patterns have low strength non-ferrous metals, low production rate small sizes engine blocks, cylinder heads connecting rods, gear housings cylinder heads, brake components prototypes of mechanical parts impellers, injection mold tooling Investment complex shapes, excellent finish small parts, expensive jewellery Permanent mold good finish, low porosity, high production rate Costly mold, simpler shapes only gears, gear housings Die Excellent dimensional accuracy, high production rate costly dies, small parts, non-ferrous metals gears, camera bodies, car wheels Centrifugal Large cylindrical parts, good quality Expensive, few shapes pipes, boilers, flywheels

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