Mould making is the crucial step in the production of
castings. Making a mould properly means a casting half done. A sand
mould is formed by packing sand into each half of the mould. The sand is
packed around the pattern, which is a replica of the external shape of
the casting. When the pattern is removed, the cavity that will form the
casting remains. Any internal features of the casting that cannot be
formed by the pattern are formed by separate cores which are made of
sand prior to the formation of the mould.
Sand
is the most commonly used mould material for casting. Other materials
used include metals, plaster and ceramics. Sand moulds are made in
moulding boxes or flasks which are metal frames with sufficient adhesive
area to support sand. In large flasks additional adhesive area is made
available by providing cross bars in the flask.
Most
moulds are made in two part flasks, the top part called “Cope” and the
bottom part called “Drag”. Copes and Drags are held in definite relation
to each other by aligning pins and lugs. For some moulds additional
intermediate boxes called “Cheeks” may be required. If the production
rates are high, moulds may be arranged in the form of stack moulds with a
number of such intermediated boxes.
Also
lubrication is often applied to the surfaces of the mould cavity in
order to facilitate removal of the casting. The use of a lubricant also
improves the flow the metal and can improve the surface finish of the
casting. The lubricant that is used is chosen based upon the sand and
molten metal temperature.
Characteristics:
1. The mould should be strong enough to resist erosion by the flow of the metal and to take the weight of the metal.
2.
The mould material should not produce too much of gases as the gases
may enter the mould cavity and get entrapped with metal or violently
boil out the metal.
3. The mould should be made in such a way that gases generated are vented out of the mould.
4. The mould should be refractory enough to withstand the high temperature of liquid metal.
5. The cores should be collapsible enough to permit contraction of the metal after solidification.
6.
The passages for the flow of liquid metal into the mould should be
designed to provide smooth non turbulent flow of metal, effective
trapping of slag and proper directional solidification.
7.
There should be a proper riser system in the mould to provide
sufficient extra metal to account for the liquid and solidification
shrinkage.
8. The mould material and moulding process should be such that cleaning of castings is facilitated.
No comments:
Post a Comment