Short Answer:

A core is a part of the mold used in the casting process to form hollow or internal cavities inside the casting. It is usually made from sand and placed inside the mold cavity before pouring the molten metal. The molten metal solidifies around the core, and when the casting cools, the core is removed, leaving the hollow portion.

A core print is a projection on the pattern that creates a seat or support in the mold for the core to rest on. It ensures proper alignment and positioning of the core inside the mold, helping to maintain the shape and dimensions of the cavity accurately during casting.

Detailed Explanation:

Core and Core Print

In the casting process, cores and core prints play a very important role when the casting involves hollow parts or internal cavities. They help in achieving complex internal shapes that cannot be directly formed using the pattern alone. Both elements work together — the core forms the internal space, and the core print holds and supports the core during molding and pouring.

1. Core

A core is a pre-shaped body, usually made from sand, that is inserted into the mold cavity before pouring the molten metal. It occupies the space inside the mold where the metal is not required to flow. Once the molten metal solidifies, the core is removed, leaving a hollow space in the casting.

Cores are mainly used to form:

• Hollow portions like pipes or engine cylinders.
• Internal passages like coolant channels in engine blocks.
• Holes or recesses that cannot be made directly with the pattern.

Cores must have the following characteristics:

• High strength: To withstand the pressure of molten metal.
• Good collapsibility: To break easily after casting for easy removal.
• Smooth surface finish: To give a good surface finish to the internal cavity.
• Refractoriness: To resist the high temperature of molten metal without deforming.

Core materials:
Cores are usually made from silica sand mixed with binders like clay, linseed oil, sodium silicate, or resin to provide strength and binding. For special castings, metallic or plaster cores may be used.

2. Types of Cores

Cores are classified based on their shape and function. The main types include:

• Green sand core: Made from green molding sand and used for simple castings.
• Dry sand core: Made separately, dried before use, and used for large or complex castings.
• CO₂ core: Made using sodium silicate as a binder and hardened with carbon dioxide gas.
• Oil bonded core: Made using linseed oil as a binder and baked to gain strength.
• Shell core: Made from a mixture of sand and thermosetting resin, providing a smooth surface and high strength.

Each type of core is chosen based on the nature of the casting and the desired internal finish.

3. Core Prints

A core print is a projection or extension on the pattern that forms a seat or recess in the mold to hold the core in the correct position. During molding, the impressions formed by the core prints in the sand mold support the ends of the core.

Without core prints, the core might move or float due to the pressure of molten metal, leading to defective castings. Thus, core prints ensure the core remains fixed during pouring and solidification.

Functions of Core Prints:

• Provide support and accurate positioning for the core.
• Maintain proper alignment inside the mold cavity.
• Prevent the core from floating due to metal pressure.
• Help in maintaining uniform wall thickness.

Design of Core Prints:
Core prints must be designed strong enough to support the weight of the core. The size of the core print depends on:

• Size and weight of the core.
• Type of sand used for core and mold.
• Position of the core in the mold (horizontal or vertical).

For heavy or large cores, additional supports like chaplets are used along with core prints to prevent movement.

4. Relationship Between Core and Core Print

The core and core print work together as a system. The core forms the internal cavity, while the core print provides a resting position and holds it securely. The core print cavity in the mold is formed when the pattern (with its core prints) is removed from the sand mold. During molding, the core fits perfectly into this cavity, ensuring accurate placement.

This relationship is crucial for achieving the required dimensions and preventing defects such as misalignment, blowholes, or uneven wall thickness. Inaccurate placement of cores can cause serious problems in the final casting, including internal voids and structural weaknesses.

5. Common Casting Defects Due to Improper Core or Core Print
   Improper core making or wrong placement can lead to:

• Core shift: Misalignment of core leading to uneven wall thickness.
• Gas holes: Poor permeability of the core causing trapped gases.
• Metal penetration: Weak core surface causing metal infiltration.
• Broken core: Due to inadequate strength or poor support from core prints.

Hence, it is important to design and place cores and core prints correctly for producing accurate, defect-free castings.

Conclusion:

A core forms the internal cavities of a casting, while a core print provides accurate support and positioning inside the mold. Both are essential for producing complex, hollow castings with precision. Proper core material selection, design, and support through well-designed core prints help in achieving strong, defect-free, and dimensionally accurate castings in mechanical and industrial applications.