What is Core in Casting?
Core is defined as sand bodies which are used to form a hollow portion of cavities of desired shape and size in casting.
The cores are not molded with the pattern, but the separate core boxes are used to mould these sand cores.
You can see wooden core boxes for different shapes and sizes of sand cores (see below image).
In the casting process, the cores should be collapsible after the pouring of metal.
This property of collapsibility is present in the sand cores. Hence sand cores are widely used in casting.
Beside the formation of internal cavity, the sand cores also have few other properties.
- Sometimes there may be some difficulty in making the molds of difficult shapes. In such cases, the cores are placed in the mould after the pattern is drawn from the mould.
- The mold surface becomes strong by placing a core in the mold.
- Cores also provides some benefits to the gating system of the casting.
- Strainer core is used to prevent the entry of foreign materials.
- Pouring basin core provides a cup or basin at the top of the mould for the entry of the molten metal.
- Splash core is used to prevent the erosion in gates and runners of the mould.
The foundry sand core has to resist the extreme heat of the molten metal.
Hence it should have properties like high strength, high permeability, high refractoriness, collapsibility, etc.
When the cores are made up of molding sands in the moist state, they are known as green sand cores.
When the cores are made up by baking the mixture of sand and core binders, they are known as dry sand cores.
There are various sizes of sand available for manufacturing the core, but those sand selection depends upon the size of core as well as temperature of the molten metal.
If the cores are made up of small grain sand, then it will develop a smooth surface on the final casting.
Types of Core in Casting
5 main types of cores in casting are mentioned below.
Let me tell you few things about each of these sand casting cores in brief.
#1) Horizontal core in casting
- Horizontal core is one of the simplest core in casting.
- As the name suggests, the horizontal core is placed in the horizontal direction in the core prints.
- The horizontal cores are mostly placed at the parting line of the mould in such a way that one half of the core remains in the cope and other half remains in the drag.
- The horizontal cores are generally in cylindrical shape, but other shapes are also prepared as per requirements. (Above image shows a horizontal core which is in cylindrical shape).
#2) Vertical core in casting
- Vertical cores are similar to horizontal cores, but the only difference is in their positions.
- The vertical cores are placed in vertical direction in the sand mould.
- The tapers are provided on the sand casting cores, so that it can be easily arranged without tearing the sand of cope and drag.
#3) Balanced core in casting
- In casting, the balanced cores are the cores which are supported only from the one end, and the other end of the core is hanging. (See above image).
- The limitations of balanced core is that it can be made of smaller lengths to avoid its breaking.
- Balanced cores are used to make holes in horizontal direction in the casting.
- If the balanced core is longer in length, then it is supported by using chaplets.
#4) Drop core
- Drop core (aka wing core or stop off core) are used when a cavity or a hole is not in line with the parting surface.
- Drop cores are kept either above the parting line or below the parting line.
#5) Cover core or Hanging core
- Cover core (also known as hanging core) is that type of core which is hanging vertically from the top and has no support from the bottom.
- Hanging core is used when the entire mould is prepared in the drag only.
- As shown in the image above, the entire mould is prepared by ramming the pattern in the mould and then the upper part of the mould cavity is covered using a hanging core.
- This type of core requires a passage from the top for the entry of the molten metal into the mould cavity.
- Cover core is also called as hanging core because it does not have any support from the bottom.