Pattern is always larger in size than that of final casting. Because, certain allowances are provided due to metallurgical (shrinkage) and mechanical (machining draft, shake and distortion) reasons.
The various allowances in pattern are
(iii) Draft / Taper
(iv) Distortion / Camper
(v) Shake / Rapping
Practically all metals shrink except bismuth.
There are two types of shrinkage such as
(i) liquid and
Liquid shrinkage is the reduction in volume from liquid to solid stage at solidus temperature.
Risers are provided in mould to compensate this.
Solid shrinkage is the reduction in volume when metal looses temperature in solid state.
Shrinkage allowance is provided to compensate this.
Difference metals shrink at different rates.
For this, patternmaker is provided with a special rule.
This rule has two scales on each side.
The graduation is oversized proportionately eg. on one size scale, 1mm for each 100 mm.
This contraction is still a guideline because actual contraction depends on several factors such as
(i) composition of metal and impurity present
(ii) moulding method
(iii) pouring temperature
(iv) complexity of design
Finish and accuracy is normally less in sand casting. So machining is sometimes required to improve these parameters.
Scales generated on ferrous materials are to be removed. So extra material is provided which is removed by either machining or cleaning
Machining allowance depends on
– metal to be cast
– type of moulding
– accuracy required
– surface complexity
Machining allowance may vary from 2 to 20 mm. Another classical way to reduce it is to keep the casting in drag flask totally so that dimensional variation and other defects due to parting plane are reduced to minimum.
At the time of removal of pattern from sand mould, the vertical faces of the pattern are continuously in contact with the sand. So there is chance of damage of mould cavity. To reduce this, vertical surfaces are tapered from parting line. It is called draft allowance.
It is given to all surfaces perpendicular to parting line.
It is given for easy removal of pattern without damaging mould cavity.
Amount of taper depends on
– shape and size of pattern vertically
– moulding method
– moulding materials
It is provided on both external and internal surfaces.
External -10 to 25 mm/m
Internal – 40 to 65 mm/m
A pattern is rapped by striking with a wooden piece. It is done so that the pattern is loosened a little in mold for easy removal. This activity increases the size of mould cavity a little which results in larger size casting.
For this reason a negative allowance is provided in pattern. This is called shake allowance.
It is generally given in big castings and neglected in small ones.