A core is a separate piece that is often created from moulding sand and placed inside of the mould to form cavities and openings which pattern alone can make it. To lower the final cost of the castings, VR foundries work on the design to reduce or eliminate the number of cores required.
There are 3 factors that decide the minimum diameter of a core:
- The length of the core
- The thickness of the metal part around the core
- The special procedures and precautions adopted by the supplier foundry
The core is subjected to adverse thermal conditions which increase in severity as the core diameter decreases and metal thickness around the core increases. The amount of heat from the heavy section increases and should be dissipated via the core. As the thermal conditions increase in severity, it becomes difficult and costly to clean up the castings and perform removal of the core.
The core length and the thickness of the metal section around the core both impact the bending stresses induced in the metal core by buoyancy forces, and therefore how the supplier foundry can achieve the tolerances required. If the size of the core is sufficiently large, rods can often be utilized to increase the core’s strength. Obviously, as the core length and metal thickness increases, the amount of reinforcement needed in order to resist the bending stresses considerably increases. This means the minimum diameter core must be increased so as to put up the additional reinforcing.
The cost of removing cores from casting cavities can become inhibitory when the areas that are to be cleaned are unreachable. The casting design should be in such a way so as to allow for openings adequately large for removing cores.
There are certain areas that are difficult to cover which makes it difficult to remove cores, thereby increasing the project cost. We design the castings with openings adequately large enough that help eliminate cores. Castings are generally shipped with un-machined, as-cast surfaces unless explicitly specified by the customer. We have a thorough understanding of which surfaces to be machined and the location of datum points. The permissible dimensional tolerances must be indicated on the drawing. Tolerances are generally agreed upon by agreement between the customer and supplier foundry.
There must be a good working relationship between the customer’s design team and the supplier foundry for optimizing the casting design. A qualified foundry professional should first check the casting design before production to ascertain the feasibility and success of the casting project.