Metal casting is a procedure that is used to solidify molten metal material in a preformed die or mold. Casting methods vary, but generally involve a process of developing a mold or die shape that determines the final desired geometry of the piece, melting the metal to molten form, and filling the mold. The molten metal undergoes a cooling procedure, involving temperature or chemical controls, and solidifies in the shape of the mold. Because there are different molding and curing procedures, the ability to repeat mold shapes in large runs or produce complex geometries also varies.
Aluminum is a common metal used in casting, in both its standard and alloy forms. Aluminum is lightweight but sturdy, allowing it to properly function in multiple application roles. When casting aluminum, there are curing concerns related to shrinkage and cracks, but following industry guidelines can help achieve best results.
Casting Methods and Procedures
Casting aluminum is a multi-step process, but the standard procedure is very simple. Manufacturers must first determine the mold geometry that will shape the final product. For many basic molds, the cast is designed in two parts that connect to define the shape. For instance, a screw mold will define one half of the screw’s shape on one side, and the other half on the other side, but in the inverse. When metal is poured into the mold, it will fill in all of the negative mold space and solidify into that shape during curing.
Once the pattern is developed and the mold is prepared, the metal is melted to liquid form and poured into the mold housing. Generally, there is a small opening in the mold called a gate or a riser, which represents a reservoir into which the molten metal is poured and a piece of the pattern that will be machined off of the final product. Additionally, molds include extra volume for allowances. Allowances are special modifications made to mold patterns to account for chemical changes to the metal during cooling. For instance, a mold made of sand will impart a rough texture to the final product, and this rough texture allowance will be smoothed off in the final steps.
Once the mold is finalized, the molten metal is added. In some instances, a chemical compound is applied to the interior surface of the mold, which will act as a lubricant to ease extraction of parts. After the metal is poured, it will cool naturally (sitting in the mold until its temperature drops to a point where the liquid is solid) or through chemical or temperature treatment. The part can then be ejected from the mold to undergo machining, which compensates for allowances made in the original mold. The part is cured through a heat treatment, and then undergoes final procedures to remove any additional allowances.
Types of Casting
There are a variety of casting methods,aluminum casting,magnesium casting,zinc casting etc, but aluminum castings respond best to a few methods. The major difference between the two classifications of casting is the size of the part run. For parts that require a limited number of iterations produced, expendable casting is a method that relies on a mold that will eventually wear down through use. Non-expendable casting is more appropriate for parts needed in larger runs, because the mold itself will not wear down as quickly despite heavy use.
Some of the more common methods of expendable casting used with aluminum include sand casting, plaster mold casting, shell molding, and lost-wax casting. These methods are expendable because the materials used to form the molds—sand, plaster mold, and wax—wear down faster than molds made of metal. In general, the casting method described above applies to these casting techniques, with the variations most apparent in the cast material. Expendable casting methods are most used when the number of parts produced is small, and when costs need to be kept lower. Expendable methods are common in a broad range of industries, and are accessible for casting hobbyists and non-professionals.
Non-Expendable casting, the more permanent of the two classifications, relies on sturdier materials for use in mold structures. The molds are often made of steel, but other metals are commonly used. Permanent mold casting, die casting, semi-solid metal casting and continuous metal casting are all common non-expendable casting techniques. These methods are desirable to manufacturers wishing to make a large initial investment on molding technology that will last for a longer amount of time than most expendable casting methods.
Because of its lightweight yet robust qualities, aluminum is one of the most widely used and produced metals in the world. It is used in the automobile, aerospace, packaging, currency, construction, electric, dye and paint industries. Aluminum is relatively easy to form because it has good fluidity and responsive grain structure, and final treatments allow the manufacturer to control the strength and toughness of the final product. However, some of aluminum’s qualities require extra steps considerations when casting.
Aluminum has a relatively high shrinkage percentage compared to other metals, in the range of 2 to 8 percent. This means that the final product will contract between 2 and 8 percent smaller than the original mold. Shrinkage can be dangerous or disadvantageous, because the final part may contain shrinkage cracks and porosity. Shrinkage is should be taken into consideration when casting aluminum structural parts, because the defects may lead to failure.
Aluminum also has a low melting point, which can be a benefit for energy needs during production. However, aluminum is also susceptible to burning if treated with overly high temperatures. Depending on the geometry of the mold, burning can occur in lopsided distributions, as thinner sections of a mold might burn while others do not. It is typically not recommended to preheat a mold for aluminum casting parts, because it heightens the probability of burning.
Additionally, aluminum can be prone to hot cracks, which are fusion cracks that form during solidification. They occur when a metal cools in an unbalanced distribution. If aluminum cools in a non-uniform pattern, these cracks can develop due to the low ductility of the metal. Because the metal does not deform well under pressure in this nascent state, it may just crack. Special cooling procedures must be followed to prevent this outcome.