Sand casting
In spite of its innocuous name, sand casting is one of the most major industrial metal casting processes. Sand casting accounts for over 90% of all metal poured for casting.
The process of sand casting begins by fabricating a pattern for the final object. The pattern is often two piece due to the construction of the mold. The pattern can be made from virtually any substance including wood, foam, clay and plastic.
The mold which contains the sand is called a flask. It consists of two pieces, the top or cope and the bottom or drag. The centerline divides the cope from the drag. Holes called sprues feed molten metal into the flask and holes called risers allow air bubbles to escape.
To begin the casting process, the flask is broken into its two pieces. The pattern is inserted into the flask and the flask reassembled. Sand is packed tightly around the pattern. Then the flask is opened and the pattern removed. The sand imprint is checked carefully, and appropriate risers and sprues added (if not contained on the original pattern). Then the flask is closed and molten metal poured into the sprues until it emerges from the risers.
After the metal has cooled, the flask is broken open and the cast part removed. The sand is cleaned and recycled back for the next casting operation. The sprues and risers are removed and the part is cleaned.
Tricks:
Either "green" sand (actually black) or dry sand is used for casting. In green sand casting, the sand binder is kept moist with water. The part is cast as soon as possible after the pattern is removed. In dry sand casting, an organic binder is used -- and the mold is baked after the pattern is removed. Green sand casting is more economical, dry sand casting has better dimensional tolerances.
To create a hole in the middle of a casting, a baked sand part called a core is inserted in the mold after the pattern has been removed. The core will be removed destructively after the casting is complete -- leaving a hole in the middle of the part.
Polystyrene or Styrofoam can be used to create a one-time pattern for a specialty casting. In this process, the pattern is inserted into the flask and left there. When the molten metal is poured over the pattern, it vaporizes and the vapor escapes from the riser holes.
Materials:
Any metal that can be melted. Common metals include cast iron, steel, brass, bronze, aluminum alloys, and magnesium alloys.
Advantages:
• Exceptionally economical
• Virtually no materials waste, as leftovers can be remelted and used again
• The castings can range from a few ounces to thousands of pounds
• The castings are isotropic
• Virtually unlimited freedom of shape
Disadvantages:
• Dimensional tolerances of 1/16" are typical -- this is large for many applications
• The castings have a work hardened (chilled) surface and cause significant tool wear in post casting machining.
In spite of its innocuous name, sand casting is one of the most major industrial metal casting processes. Sand casting accounts for over 90% of all metal poured for casting.
The process of sand casting begins by fabricating a pattern for the final object. The pattern is often two piece due to the construction of the mold. The pattern can be made from virtually any substance including wood, foam, clay and plastic.
The mold which contains the sand is called a flask. It consists of two pieces, the top or cope and the bottom or drag. The centerline divides the cope from the drag. Holes called sprues feed molten metal into the flask and holes called risers allow air bubbles to escape.
To begin the casting process, the flask is broken into its two pieces. The pattern is inserted into the flask and the flask reassembled. Sand is packed tightly around the pattern. Then the flask is opened and the pattern removed. The sand imprint is checked carefully, and appropriate risers and sprues added (if not contained on the original pattern). Then the flask is closed and molten metal poured into the sprues until it emerges from the risers.
After the metal has cooled, the flask is broken open and the cast part removed. The sand is cleaned and recycled back for the next casting operation. The sprues and risers are removed and the part is cleaned.
Tricks:
Either "green" sand (actually black) or dry sand is used for casting. In green sand casting, the sand binder is kept moist with water. The part is cast as soon as possible after the pattern is removed. In dry sand casting, an organic binder is used -- and the mold is baked after the pattern is removed. Green sand casting is more economical, dry sand casting has better dimensional tolerances.
To create a hole in the middle of a casting, a baked sand part called a core is inserted in the mold after the pattern has been removed. The core will be removed destructively after the casting is complete -- leaving a hole in the middle of the part.
Polystyrene or Styrofoam can be used to create a one-time pattern for a specialty casting. In this process, the pattern is inserted into the flask and left there. When the molten metal is poured over the pattern, it vaporizes and the vapor escapes from the riser holes.
Materials:
Any metal that can be melted. Common metals include cast iron, steel, brass, bronze, aluminum alloys, and magnesium alloys.
Advantages:
• Exceptionally economical
• Virtually no materials waste, as leftovers can be remelted and used again
• The castings can range from a few ounces to thousands of pounds
• The castings are isotropic
• Virtually unlimited freedom of shape
Disadvantages:
• Dimensional tolerances of 1/16" are typical -- this is large for many applications
• The castings have a work hardened (chilled) surface and cause significant tool wear in post casting machining.
