Sand Casting Introduction
Sand casting is a metal casting process that pours molten metal into a refractory sand mould. After solidified, the desired objects is taken out from mould either by breaking mould or taking mould apart. The solidified object is called casting, and the process is also called foundry.
The discovery of the casting process was probably around c 3500 BC in Mesopotamia, and casting technology has been greatly improved by China around 1500 BC.
Casting Advantage and limitation
The casting process is extensively used in manufacturing because of its many advantages
- It is possible that any object with an intricate shape can be cast.
- the material almost has no limitations, either ferrous or nonferrous
- Tools for casting mold are simple and inexpensive, so for trial or small lot production, sand casting is a better choice.
- Casting solidified uniformly, so the metallurgies of casting have no directional properties.
- Casting is almost net shape production according to the 3D math model, so it is the most economical way comparing with machining
- Casting any size and weight, almost no up limit
However, sand casting has also dimensional accuracy and surface finish limitation. As such, it is not suitable for the final application in many cases. Because of this, many other casting methods, such as die casting, precision casting, have been developed.
Sand casting has applications in almost all industries. The classic application is the engine block, cylinder head, crankshaft, pistons, and piston ring in the automotive industry. Furthermore, iron castings, such as anchorage block and anchor head, play a key function in construction and mining industry. Besides, cast iron fry pan and cast iron pot have got much favor in the kitchen cooking.
Sand Casting Mould Structure
A set of two parts mould consist of the flask, cope, drag, sprue, runner, mould cavity, sand core and riser basically.
Mould cavity is made from a pattern, and the pattern is the replica of the object to be made. The most common pattern material is wood because of its easy availability. However, it is not the only choice. Other materials, such as cast iron, brass, aluminium, may be used too. Choice of pattern material depends essentially on the size of the casting, the number of castings to be made from a pattern, and the dimensional accuracy required. The sand core is used for making hollow cavities in castings.
The following picture gives a visual impression of related casting terms.
Mould and Cores Materials
The sand material of mould and core composite of silica grains, clay binder and moisture mainly. Besides, the properties of moulding sand are dependent to a great extent on a number of variables, such as grain shape and size, clay type and amount, moisture content and method of preparing sand mould.
Sand Mould Making Procedure
While materials, tool and equipment are all ready for production, the first thing is to make sand mould for casting. The below steps are a general procedure to follow:
Mold Cavity Patterning
- A bottom board is placed on a moulding platform, drag moulding flask kept upside down on the board along with drag part of a pattern at the centre of the flask.
- Dry facing sand sprinkled over the board and pattern to provide a non-sticky layer
- Pour freshly prepared moulding sand of requisite quality into drag and pattern to a thickness of 30~50mm, rest of drag filled with backup sand and uniformly rammed to compact the sand
- Use vent wire make the vent holes in the drag and pattern to the full depth of flask
- Roll finish drag flask over to the bottom of board exposing the pattern, place cope half pattern over the drag pattern, align cope flask with drag flask with the help of pin and lug.
- Sprinkle dry parting sand over the cope and pattern, locate sprue pin and riser pin if necessary in the proper location from the pattern
- Similar to that of the drag flask, freshly prepared moulding sand and backup sand is spread over the cope half pattern, then sand rammed thoroughly.
- Withdraw sprue pin and riser pin carefully, cut pouring basin near the top of the sprue.
- Separate the cope from drag, withdraw pattern using spikes and rapping pattern all around to enlarge mould cavity so that the mould walls are not spoiled
- Cut the runners and gates in the mould carefully
- Apply facing sand in the form of paste all over the mould cavity and runners
- Put dry sand core into the mould if necessary.
- Replace cope on the drag, aligning the two with the pins
The mould now, as figure showed, is ready for pouring.
Casting Design Consideration
In designing a casting, it is always the consideration to realize product function economically and eliminate defects through the concept of designing for manufacturing.
For example, it would simplify the molding procedure to keep the parting line straight on the design stage. In addition, an even wall thickness design would eliminate stress occurring, and defect of shrinkage and crack.
Special Casting Process
As mentioned before, sand casting has its limitations on either quality requirements or economic cost in many applications. In such cases, a special casting process would be more appropriate.
- Shell molding
- Precision investment casting
- Permanent mold casting or gravity die casting
- Die casting or pressure die casting
- Centrifugal casting
These special casting processes will be introduced in separate article individually so that we may have an overall and comprehensive understanding casting process.
So far, a simple introduction of the sand casting process has finished. In sum, each casting process has its proper application. Casting material, design structure, quality requirement, and usage volume may have a major impact on the selection of the casting process.
In conclusion, sand casting is a highly versatile casting process that is capable to make metal intricated objects at a relatively low cost.
To learn more, visit the American Foundry Society to get more inside on the theory and practice of the casting process.