Sand Casting
Sand casting is the oldest method of casting. The process is basically a simple one, with a few universal requirements. A mold cavity is formed around the pattern in a two-part box of sand; the halves are called the cope (upper) and the drag (see Figure 2.18). Fitted together, these form the complete mold. To withdraw the pattern from the mold, the pattern must be tapered and have no undercuts. This taper is called draft and usually has an angle of 3°. The most common molding material used in sand casting is green sand, which is a mixture, usually of ordinary silica sand, clay, water, and other binding materials. The molding sand is placed in the lower half of a two-part form and is packed around half the pattern (up to the parting line). Then a dry molding powder i$ sprinkled over the pattern and the sand in the cope as a release agent, the upper half of the form is put in place on top of the drag, and the remaining sand is packed into place. When the sand is compacted sufficiently, the cope and drag are separated and the pattern is removed (see Figure 2.19). The two halves are then rejoined, and the metal is poured in through an opening (sprue) into the cavity. The size of the pattern must be adjusted to allow for the shrinkage of the metal as it cools. Aluminum and steel shrinks about '/4 in./foot, depending on the thickness of the part—the thicker parts shrinking less. Molten metal may flow from the sprue to various entry ports (gates) in the cavity. Risers at each gate act as reservoirs of molten metal that feed the casting during solidification. A sprue and riser can be seen in Figure 2.20. Aluminum castings need larger gates and risers than other metal castings, thus the average weight of metal poured is about two or three times the weight of the finished casting. Figure 2.21 shows the weight comparison of equal volumes of various metals. The importance of proper gating has been demonstrated by the fact that redesign and relocation on the gates feeding a previously improperly gated casting have increased the strength of the casting as much as 50 to 100%.
Cores
A hollow casting is made by inserting a core of sand into the cavity before pouring. Figure 2.22 shows the maksing and use of cores in a sand casting mold. Any leakage between the halves of the mold cavity forms thin ridges of flash. Protuberances left from risers and sprues must be removed from the part, along with the flash, after it has solidified and the sand has been cleaned off.
Patterns
When more than one part is to be made, the pattern is split along the parting line and each half is mounted on a pattern board or match plate as shown in Figure 2.23. One half of the impression is made in the cope, the other half in the drag. You can see the locating holes for the core in both the cope and drag. The mold is then opened up and the pattern board removed, the core installed, and the mold reclosed for pouring.

General
Sand casting is the most common casting process used with aluminum alloys. Casting has the lowest initial cost and permits flexibility in design, choice of alloy, and design changes during production. For these reasons, sand casting is used to make a small number of cast pieces, or to make a moderate number that require fast delivery with the likelihood of repeat production. A sand-cast part will not be as uniform in dimensions as one produced by other casting methods, so greater machining allowances must be made. Surface finish can be controlled somewhat by varying the quality of sand in contact with the metal and by applying special sealers to the sand surfaces. Hot shortness of some aluminum alloys is also important. Certain alloys have low strength at temperatures just below solidification. The casting may crack if these alloys are cast in a mold that offers resistance to contraction as the metal solidifies. This is called hot cracking. Hot shortness varies with the alloy used. Aluminum-silicon alloys show considerably less hot shortness than aluminum-copper alloys. The wide range of aluminum alloys available enables the designer to choose an aluminum alloy and avoid hot cracking when this factor is important.

Basic Design Considerations
Often a casting is designed by someone who does not have the experience in how the casting is to be produced, and the foundry will then inherit problems caused by poor design. Someone who has no knowledge of the foundry and its requirements should talk with a local casting expert or the foundry before starting design. Some basic considerations are the following.
1.    On castings that must present a cosmetic appearance along with function, the exterior should be designed to follow simple, flowing lines with a minimum of projections.
2.    Avoid irregular or complicated parting lines whenever possible. Design for partings to be in one plane.
3.    Use ample but not excessive draft. Avoid any no-draft vertical surfaces unless there is no other way out.
Avoid long, slender cores through heavy metal sections or long spans. When unavoidable, they should be straight and well anchored to the mold. Avoid the use of pattern letters on any surface other than one parallel to the parting.
Avoid sudden changes in section thicknesses that will create hot spots. Use ribs to stiffen or strengthen castings, thus reducing weight. Avoid all sharp corners and fillet all junctions.
Stagger crossing ribs so that the junction will not create a hot spot which could shrink.

As larger quantities are called for, the point where it becomes economical to go to permanent-mold or die casting depends on the size and complexity of the casting and other factors.

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