Sand Casting Design Guide and Considerations
Sand casting, the most widely used casting process, utilizes expendable sand molds to form complex metal parts that can be made of nearly any alloy. Because the sand mold must be destroyed in order to remove the part, called the casting, sand casting typically has a low production rate. The sand casting process involves the use of a furnace, metal, pattern, and sand mold. The metal is melted in the furnace and then ladled and poured into the cavity of the sand mold, which is formed by the pattern. The sand mold separates along a parting line and the solidified casting can be removed. The steps in this process are described in greater detail in the next section.
General:
Sand casting is the most popular casting process employed in industry. Sand casting molds use silica based sands. There are two general types of sand: naturally bonded and synthetic sands. that can be controlled better synthetic sand and, is the most common used by foundries. In general sand used in making sand cast molds is fine, round grains that can be closely packed and forms a smooth mold surface. Sand cast molds are designed to have a good collapsibility (the casting shrinks while cooling) to avoid defects in the casting, such as hot tearing and cracking. Clay is used to cohesively bond sand particles, giving the sand strength. Common recognizable parts made by sand casting are: engine blocks, cylinder heads, housings and similar enclosures.
Design:
The designer should take into account the following: the parting line, finish, the draft, the presence of ribs, bosses, webs, and recesses, and the machining allowance. In general uniform walls are preferred with a maximum wall thickness not to exceed 5 inches and a minimum wall thickness ,25 inches over a 5 inch span max.
Parting Line:
The parting line should be identified on the casting drawing. Determining the position of the parting line is a critical step in the casting design. Parting line mismatch, and parting line and seam flash extension allowances should nor exceed .020". Maximum parting line seam flash extension material is about .015".
Finish:
As cast surface finish is typically 200 - 500 RMS. Post machining operation should be accounted for in material allowances should better surface be required.
Draft:
To facilitate the removal of the pattern from the weak, brittle molding sand cast draft should be defined and accounted for. Standard draft for sand casting is 2 degrees with a minimum of about 1 degree for external and internal features.
Section Changes
The design should not contain abrupt section changes. Fillets and tapers are preferred to sharp steps. If a section change of over 2:1 thickness ratio is unavoidable, there are two alternatives: design two separate castings to be assembled together, or use a wedge form between the unequal sections. The taper of the wedged area should not exceed 1:4.
Ribs and Web:
Improper rib, web and corner design can lead to sink spots. In general web or wall thickness should as a minimum be 0.10" - Aluminum, .13 Magnesium and .13 for Steel. See Casting Rib Design for additional information.
Sand casting is used to produce a wide variety of metal components with complex geometries. These parts can vary greatly in size and weight, ranging from a couple ounces to several tons. Some smaller sand cast parts include components as gears, pulleys, crankshafts, connecting rods, and propellers. Larger applications include housings for large equipment and heavy machine bases. Sand casting is also common in producing automobile components, such as engine blocks, engine manifolds, cylinder heads, and transmission cases.
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