Short Answer:

Patterns are the models used in casting to give shape to the mold cavity. They are made from materials like wood, metal, plastic, or wax and represent the exact shape of the final casting. Patterns are slightly larger than the actual product to allow for shrinkage and machining.

There are different types of patterns used in casting based on the shape, size, and complexity of the component. Common types include single piece, split piece, match plate, loose piece, and gated patterns. Each type has specific applications depending on the type of casting process and production requirements.

Detailed Explanation:

Types of Patterns

Patterns are an essential part of the casting process. They determine the shape and size of the final cast product. The choice of pattern type depends on the complexity of the casting, the number of castings required, and the type of molding process used. Below are the main types of patterns commonly used in foundry practice:

1. Single Piece Pattern:
   A single piece pattern, also known as a solid pattern, is made in one single piece. It represents the shape of the final casting and is directly placed in the molding box to form the cavity.

• Uses: Suitable for simple shapes and small-scale production.
• Example: Simple parts like gears, pulleys, or pipe fittings.
• Advantages: Simple design and low cost.
• Disadvantages: Not suitable for complex or hollow castings.

2. Split Pattern (Two-Piece Pattern):
   A split pattern is divided into two parts along the parting line. Each half is molded in a separate flask (cope and drag). Alignment is ensured using dowel pins.

• Uses: Suitable for complex shapes where withdrawal of a single pattern is difficult.
• Example: Valves, machine components.
• Advantages: Easy removal from mold and suitable for detailed shapes.
• Disadvantages: Requires accurate alignment, slightly higher cost.

3. Match Plate Pattern:
   In this type, both halves of the split pattern are mounted on opposite sides of a metal plate called a match plate. The gating and riser systems are also attached to the plate.

• Uses: Ideal for high-volume and automated molding.
• Example: Used in die casting and production foundries.
• Advantages: High accuracy, reduced molding time, and good productivity.
• Disadvantages: Expensive to manufacture.

4. Loose Piece Pattern:
   Loose piece patterns have detachable parts that are removed separately after molding. They are used when a single piece or split pattern cannot be easily withdrawn from the mold.

• Uses: Suitable for complex castings with projections or undercuts.
• Example: Complex pipe joints, pump bodies.
• Advantages: Allows molding of difficult shapes.
• Disadvantages: Time-consuming and requires skilled labor.

5. Gated Pattern:
   A gated pattern includes one or more patterns connected by gates and runners. These connections allow molten metal to flow from one cavity to another during pouring.

• Uses: Best for mass production of small identical parts.
• Example: Bolts, nuts, or small mechanical parts.
• Advantages: Saves time, reduces molding effort, and ensures uniform filling.
• Disadvantages: Difficult to repair if damaged.

6. Cope and Drag Pattern:
   This pattern is similar to a split pattern but the cope and drag sections are mounted on separate plates. This allows the cope and drag halves to be handled independently.

• Uses: Large and heavy castings.
• Example: Large machinery housings or bases.
• Advantages: Easy handling of large molds.
• Disadvantages: High setup cost.

7. Sweep Pattern:
   A sweep pattern uses a wooden or metal board shaped like a section of the casting. It rotates around a central spindle to form a symmetrical mold cavity.

• Uses: Suitable for large, circular shapes.
• Example: Pipes, columns, or wheels.
• Advantages: Saves material and cost.
• Disadvantages: Limited to symmetrical shapes.

8. Skeleton Pattern:
   A skeleton pattern is a wooden frame that outlines the shape of the casting. The molder fills the sand inside and smoothens it to the required contour.

• Uses: Large and simple shapes made in small quantities.
• Example: Large pipes, tanks, or pits.
• Advantages: Saves wood and cost-effective.
• Disadvantages: Requires skilled workers for accurate shaping.

9. Shell Pattern:
   A shell pattern consists of a thin shell or hollow form that gives shape to the mold. It is used in processes where light patterns are preferred.

• Uses: Light or delicate mold making.
• Advantages: Easy to handle and reduces material use.
• Disadvantages: Limited strength and durability.

10. Segmental Pattern:
    When a complete pattern cannot be made due to size, segmental patterns are used. They form a portion of the mold cavity, which is repeated to complete the entire mold.

• Uses: For large circular or repetitive shapes.
• Example: Large pulleys or rings.
• Advantages: Convenient for large parts.
• Disadvantages: Time-consuming for alignment.

Conclusion:

Patterns are the backbone of the casting process. The selection of the right type of pattern depends on the complexity, production rate, and required accuracy of the casting. While single-piece and split patterns are used for simple work, match plate and gated patterns are preferred for mass production. Proper choice ensures cost-effectiveness, good surface finish, and dimensional accuracy of the final casting.