Pattern materials :
The common materials of which the patterns are made are the following :
1) Wood :
It is the most common material used for pattern making because of the following Advantages :
(i) It is cheap and available in abundance.
(ii) It can be easily shaped into different forms and intricate designs.
(iii) Its manipulation is easy because of lightness in weight.
On the other hand, it has certain disadvantages also as follows:
(i) It wears out quickly due to its low resistance to sand abrasion. As such, a wooden pattern cannot stand a long constant use.
(ii) It is very susceptible to moisture, which may lead to its warping or splitting. This needs its careful storing in a dry place and the application of preservatives.
(iii) Its life, owing to the above reasons, is short as compared to other pattern materials. This confines its use to such cases only when a small number of castings are required.
Metals are used with advantage, as pattern material, only when the number of castings to be made is very high and a closer dimensional accuracy is desired. They have a much longer life than wooden patterns and eliminate the inherent disadvantages of wood to a great extent.
But they also carry the following Disadvantages :
(i) They are costlier than wood and, therefore, cannot be used with advantage, where a smaller number of
castings is to be made.
(ii) For giving different shapes and fine surface finish they need machining. This again adds to their cost.
(iii) Most of them are very heavy and in case of large castings the weight of the pattern always poses a problem in its manipulation.
(iv) A large number of them have a tendency to get rusted.
Plaster of Paris or gypsum cement is advantageously used as a pattern material since it can be easily casted into intricate shapes and can be easily worked also. Its expansion can be easily controlled and it carries a very high compression strength. Its specific use is in making small patterns and core boxes involving
intricate shapes and closer dimensional control. A marked feature of this cement is that contrary to the action of metals, it expands on being solidified. Thus, if a cement of proper coefficient of expansion is selected, the effect of shrinkage of casting can be automatically neutralized.
Plastics are gradually gaining favor as pattern materials due to their following specific characteristics :
1. Lightness in weight.
2. High strength.
3. High resistance to wear.
4. High resistance to corrosion due to moisture.
5. Fine surface finish.
6. Low solid shrinkage.
7. Very reasonable cost.
The plastics used as pattern materials are thermosetting resins. Phenolic resin plastic and foam plastic suit best for this purpose. For making the pattern, first the moulds are made, usually from plaster of Paris. The resin is then poured into these moulds and the two heated. At a specific temperature, the resin solidifies to give the plastic pattern.
Wax patterns are exclusively used in investment casting. For this a die or metal mould is made in two
halves into which the heated wax is poured. The die is kept cool by circulating water around it. As the wax
sets on cooling, the die parts are separated and the wax pattern taken out.