A pattern may be defined as a replica or model of the desired casting which when packed or embedded in a suitable moulding material produces a cavity called mould. A pattern differs from the actual component is the following ways.
a. It carries pattern allowances
b. It has provision of core prinks
Fine details which cannot be obtained by casting are eliminated.
5.5.1 Pattern allowances
The dimensions of the pattern are different from the final dimensions of the casting required. This is due to the various reasons mentioned below.
(i) Shrinkage allowance
Most of the metals used in casting work contract during cooling from pouring temperature to room temperature. These contractions take place in three forms viz. Liquid contraction, solidifying contraction and solid contraction. The first two are compensated by gates and risers and the third one by providing adequate allowances in the pattern.
(ii) Finish or machining allowance
The finish and accuracy achieved in sand casting are generally poor. Therefore when good surface finish and dimensional accessory is required it is generally achieved by subsequent machining. Also ferrous materials would have scales on the skin which are to be removed by cleaning. Hence extra material should be provided as these surface which is to be subsequently removed by machining or cleaning. The corresponding portion or surfaces on the pattern are given adequate allowance by increasing the metal thickness there to compensate for loss of metal due to machining on these surfaces. The amount of this allowance depends upon the metal of casting, method of machining to be employed, method of casting used, size and shape of the casting and degree of surface finish required on the machined portion. Ferrous metals need more allowance than non ferrous metals. Similarly large and slender castings need more allowance than this shorter ones. This allowance varies from 1.5 mm to 16 mm. But 3 mm allowance is quite common for small and medium size castings.
(iii) Draft allowance
At the time of withdrawing the pattern from the sand mould the vertical faces of the pattern are in continuous contact with the sand. This may damage the mould cavity. To reduce the chance of this happening the vertical faces of the pattern are always tapered from the parting line. This provision is called draft allowance. Draft allowance varies with the complexity of the job. In general internal surfaces require more draft than external surfaces.
Again more draft should be provided for hand moulding compared to machine moulding. Usually the amount of draft varies from 10 mm to 25mm per meter on external surfaces and from 40mm to 70mm.
(iv) Rapping or shake allowance
When a pattern is to be withdrawn from the mould it is first rapped or shaken by striking over it from side to side, so that its surface may be free from the adjoining sand wall of the mould. As a result of this the size of the mould cavity increases a little. Hence a negative allowance is to be provided in the pattern to compensate the same. However it is negligible for small and medium sized castings.
(v) Distortion allowance
When a metal has just solidified it is very weak and therefore likely to be distortion prone. This is particularly for weaker sections, such as long flat portions, V, U sections or in a complicated casting which may have thin and long sections which are connected to thick sections. To eliminate the effect of distortion extra material is provided at these locations. Sometimes the shape of the pattern itself is given an equal amount of distortion in the opposite direction of likely distortion.
(vi) Mould-wall movement allowance
The surface layer of sand which comes in contact with the molten metal is subjected to excessive heat and static pressure. As a result movement of mould walls occur. This movement of mould walls affects the ultimate size of the castings and should be compensated by providing corresponding allowance in the pattern.