An expansion of the core body generally along its axis is known as print. The design of this print is dependent upon the axis direction and the quantity of openings present. Each of these opening for the core support, are related to individual print.
Core print design requires various considerations. They are :
- To hold the core in place during the mold assembly process, the print design is required to maintain a balance in the body.
- The print must be strong enough to resist the buoyancy force of the molten metal during casting process.
- The print should be stable enough during the process of filling the mold.
- The deflection of the core must be minimized.
- The transfer of the heat from the core to the mold should be maximized.
- There should be a provision for escaping the internal gases within the core.
- To avoid inaccurate assembly structure, unsymmetrical holes should have infallible prints.
- Combining the prints of adjacent cores to form single structure.
Furthermore, design of the core print depends on the type of core classified as:
- Horizontal simply supported core
- Horizontal overhanging core
- Vertical doubly supported core
- Vertical hanging core
- Vertical bottom core
Since, cored holes and undercuts increase the cost of the tooling and production time; hence, they should be minimized in terms of quantity. They also direct quality problems, which can be avoided by tracking following criterions:
Diameter of the core : The process of removing and cleaning small sand cores, generally in thick sections of a casting, is a difficult process. Hence, this is done by accumulation of high heat within the core enclosed by molten metal. In the process of die-casting, the molten metal contracts on to the core of the metal, absorbing it tightly, and thus make it difficult to remove the core. Another important consideration is that it may lead to cost-savings to machine small holes rather than producing them by cores. The factor that affects the core size depends on the part, metal casting process, measurement of thickness of the section, and the length of the core.
Core aspect ratio : The cores having larger lengths as compared to their diameter are likely to become distorted during the process of mold filling, under high pressure. Sand cores, which are narrow in nature, may even break down. The limiting aspect ratio of the cores depends on the type of core, material of the core, cast metal and pressure range while filling process.
Inter-core distance : There are many considerations affecting the limiting distance between two or more cores. Too close holes lead to thin sections and thus having following issues :
- The metal may not completely fill the section.
- Due to shrinkage porosity, hot spots may be formed.
- A marginal shift in the cores positioning reduces the thickness of the walls and hence exaggerating the above two issues. The wall thickness of the sections should always be greater than the diameter of the core.