Table of Contents
- 1 Pattern types
- 1.1 Pattern types in casting or Types of Pattern in Casting
The pattern is a model or replica of the object to be manufactured around which moulding sand is packed to get a mould of desired shape and size.
The quality of casting in terms of dimensional accuracy, surface finish and mechanical properties largely depend on the material used for the patterns, types of pattern, design and construction of a pattern.
Functions of Patterns
- A Pattern prepares a mould cavity for the purpose of making a casting.
- A Pattern may contain projections known as core prints for the purpose if the casting requires a core and need to be made hollow.
- It should be made properly and having finished and smooth surfaces which reduces casting defects.
- Properly constructed patterns minimize overall cost of the casting.
The pattern material should be
- Easily worked,shaped and joined.
- Light in weight.
- Strong,hard and durable.
- Resistant to wear and abrasion .
- Resistant to corrosion,and to chemical reactions.
- Dimensionally stable and unaffected by variations in temperature and humidity.
- Available at low cost.
Pattern types in casting or Types of Pattern in Casting
In the metal casting process, various types of patterns can be used but it is necessary to select a particular type of pattern for the application. The various types of patterns commonly used are explained in the following sections:-
1. Solid or Single Piece Pattern
It is the simplest form of a pattern made without any joints, partings or loose pieces in its construction is called the solid or single piece pattern. It is inexpensive and generally used for large castings of simple shape.
2. Split pattern
This pattern type is used for making castings having intricate details. It is split along the parting surface. One half of the pattern is moulded in drag and the other half in cope box. The two halves of the pattern must be aligned properly by making use of dowel pins.
Also read: Sand Mould Casting
3. Match plate pattern
In the match plate pattern, each half of the patterns are mounted on opposite sides of a plate called a match plate. The gates and runner are also attached to the plate. Both sides of the patterns are fastened. The material used to make the pattern is metal.
Example- Match plate pattern is used to make piston rings of i.c. engine
4. Sweep pattern
This pattern is used for large castings of circular sections and symmetrical shapes. A sweep pattern consists of a wooden board with the outer contour similar to the casting’s form. The wooden board is fixed to a metal rod and rotated about the metal rod as the axis of rotation, for getting a complete mould cavity. It is economical.
Also read: Shell Mould Casting
5. Gated Pattern
When a large number of small castings are required, gated patterns are used.
6. Loose piece pattern
This pattern is provided with loose pieces which are used to make easier for the withdrawal of the pattern from the mould. Here loose pieces (A & B) are removed separately by turning or moving the loose pieces through the mould cavity formed after the main pattern has been removed.
Also read: Investment Casting
7. Cope and Drag pattern
The cope and drag pattern is used for very large castings, in case the complete mould is too heavy and difficult to handle by a single operator. The cope and drag pattern is made in two halves. Both are moulded separately and are then assembled to make the complete mould.
8. Follow Board Pattern
Follow board pattern is used for casting which has some structurally weak portions which need some support for avoiding breakage during the moulding process. Hence these weak portions are supported by means of a Board that will closely fit the contour of the weak pattern.
Also read: Pattern Allowances
9. Shell pattern
Shell patterns are widely used for large symmetrical castings such as drainage fittings and pipes. The shell pattern consists of hollow construction. The outside contour is used as a pattern to form the mould. The shell pattern is made of two halves which are correctly aligned together by dowel pins. The shell pattern is usually made of metals.
10. Skeleton Pattern
The Pattern is the skeleton of the desired shape. Skeleton is made from wooden strips and is filled with loam sand rammed.
This pattern is used for large castings such as turbine housings which have hollow construction. If we go for a solid pattern to manufacture casting of hollow construction, we are ended with the heaviest pattern which would be difficult to handle by a single operator.
So what we do, we manufacture a hollow pattern which outside contour would be just similar to the casting’s form would act as a skeleton of the desired shape.
This way we can eliminate the problem of using extra material which could be expensive and handling the pattern.