Pattern in Casting

In casting, a pattern is a replica of the object to be cast, used to prepare the mould cavity into which molten metal will be poured. Patterns are made with modifications to account for shrinkage, machining, and ease of removal from the mould. The quality of the casting depends heavily on the accuracy and design of the pattern.

1. Definition

A pattern is a model of the casting, constructed in such a way that it can be used to form an impression called a mould in damp sand or other suitable material. It is the starting point of the casting process and determines the shape, size, and quality of the final product.

2. Functions of a Pattern

To produce the mould cavity of the correct size and shape.
To provide seats for cores in the moulds (via core prints).
To establish the parting surfaces and lines in the mould.
To locate points for accurate mould assembly.
To minimize casting defects by proper design.
To reduce the cost of casting by enabling efficient moulding.

3. Pattern Allowances

Patterns are not made exactly to the dimensions of the finished casting. Allowances are added or subtracted to compensate for various factors:

Shrinkage Allowance: Compensates for metal contraction during cooling.
Machining Allowance: Extra material provided for surfaces to be machined to final dimensions.
Draft Allowance: Taper given to vertical surfaces to allow easy removal from the mould.
Distortion Allowance: Adjustments made to counteract warping during cooling.
Rapping or Shake Allowance: Reduction in size to compensate for mould cavity enlargement during pattern withdrawal.

4. Materials for Patterns

The choice of material depends on the number of castings required, desired accuracy, and cost considerations.

Wood: Light, easy to shape, inexpensive; suitable for small runs but less durable.
Metals: Aluminium, brass, cast iron; durable and accurate, suitable for mass production.
Plastics: Moisture-resistant, smooth finish, moderate durability.
Wax: Used in investment casting; melts out during mould preparation.
Plaster/Resin: For fine detail and smooth surfaces in special applications.

5. Types of Patterns

Single Piece (Solid) Pattern: Made in one piece; used for simple shapes and low production.
Split Pattern: Made in two halves along the parting line; easier to remove from the mould.
Match Plate Pattern: Cope and drag patterns mounted on opposite sides of a plate; used in machine moulding.
Cope and Drag Pattern: Separate plates for cope and drag; used for large castings.
Gated Pattern: Multiple patterns connected by a gating system; increases productivity.
Loose Piece Pattern: Has detachable pieces to allow removal from complex moulds.
Sweep Pattern: Formed by rotating a template (sweep) to create symmetrical moulds.
Skeleton Pattern: Framework outlining the shape; filled and finished by the moulder.
Segmental Pattern: Built in segments for large circular castings.
Follow Board Pattern: Uses a board to support weak or irregular patterns during moulding.

6. Selection of Pattern Type

Complexity of the casting shape.
Number of castings to be produced.
Required dimensional accuracy and surface finish.
Type of moulding process (hand or machine moulding).
Cost and time constraints.

7. Conclusion

Patterns are a critical element in the casting process. Proper design, material selection, and allowance application ensure high-quality castings, reduce defects, and improve production efficiency. Understanding the functions, types, and materials of patterns helps in selecting the most suitable pattern for a given casting requirement.