What is a Pattern ?
In casting, a pattern is a replica of the object to be cast, used to prepare the cavity into which molten material will be poured during the casting process.
Patterns used in sand casting may be made of wood, metal, plastics or other materials. Patterns are made to exacting standards of construction, so that they can last for a reasonable length of time, according to the quality grade of the pattern being built, and so that they will repeatably provide a dimensionally acceptable casting.
Functions of Patterns
1. A pattern is used to prepare a mold cavity.
2. For hollow casting core prints are added to the pattern to support the core in the mould cavity.
3. Gates, Riser, and channels may become the part of the casting. However, in most of the cases they are not an integral part of the pattern.
4. Efforts should be made to provide all allowances in a proper manner on the pattern and the surface of the pattern need to be made smooth. It results into less casting defects and low cost.
5. A properly constructed pattern minimizes the overall cost of the castings.
Pattern Material
The pattern can be made out of various materials. Some of the materials used for making the patterns are wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins. The every material has its own advantages and disadvantages of being used as a pattern.
In general the pattern material should possess the following characteristics:
1. Light in weight
2. Can be worked, shaped, and joined easily
3. Strong, hard and durable
4. Resistant to wear and abrasion
5. Resistant to corrosion, and to chemical reactions
6. Dimensionally stable and unaffected by variations in temperature and humidity
7. Available at low cost
The wood is the most favorable pattern material because of its availability in abundance and low weight. Also, it can be easily shaped and is relatively cheap. The main disadvantage of wood is that it absorbs moisture, as a result there is possibility of distortion and dimensional changes in the pattern. Hence, if wood is being used as a pattern material, the wood need to be properly seasoned.
