Sand casting involves many sequences and there are as follows:
Design and manufacturing the casting. When doing this all vertical edges must be tapered as to make it easier to lift the pattern out of the mould. -
Contraction allowances have to be added to the pattern because it shrinks in size in all dimensions. In this case it’s Aluminium, which has a contraction allowance of 1.3%. -
Machining allowance: Castings are usually machined so that they fit perfectly with mating components. So therefore extra material should be added for machining. This can range from 1 - 35mm. -
The pattern should be thick enough to avoid misrun defects (+2mm thick) -
Abrupt changes in the pattern should be avoided as it can cause shrinkage defects. Sharp corners should be replaced with fillets to reduce stress concentrations also. -
Put a rapping and drawing hole in the pattern. A spike is placed in the rapping hole and gently struck; this loosens the sand enough so that it can be removed with a bolt, which is screwed into the drawing hole. -
Treating the molten metal: Remove dross using cleansing flux, Measure the temperature using an Immersion Pyrometer (Aluminium 750°C) Bubble gas into it to remove hydrogen, Check chemical composition, Add sodium; this modifies the shape of the grain structure to improve ductility. -
Pour the molten metal into the mould.
Once cooled, remove casting from the mould and cut off the excess metal using a band saw (fettling)
The size of moulding boxes required: 225x225x100 (mm)
The pouring temperature for the aluminium: 750°C
Aluminium was used for the pattern.
Draft angles: 1° for outside edge, 5° for inside edge.
Castings should be quality checked, externally and internally, before machining.
Visual inspection (shrinkage, sand inclusion, porosity, cracks etc.) Dye Penetrant test and Magnetic particle inspection.
Radiography (X-ray or Gamma, for thick walled...
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