D


ie castings off er exceptional design freedom while still achieving necessary mechanical properties in a product. However, some features can cause quality problems and others can add unnecessary cost to the casting. T is article provides some pointers on how to maximize value.


Flat Is Good Before starting a design, study the required features and develop an idea of the


part’s general size and shape. Figure 1 shows some hypothetical requirements for a casting that must encase a mechanism, hold in a fl uid and keep anything else out. T e casting might look like that shown in Figure 2. T e required features as shown in the left illustration in Figure 3 should be


rotated to a position that results in the smallest height, as shown in the right-hand illustration in Figure 3, before starting the casting design. If a good design is not possible at that rotation, then the features must be rotated to where a good design is possible. Reduced cavity depth generally improves die cost and casting machine operating factors. T e “fl at is good” rule is not a hard-and-fast requirement, but it is a good starting point.


Wall Thickness One often parroted guideline is, “T e wall thickness should be constant.” Actu-


ally, that rule is not exactly right. T e wall thicknesses should best meet the part function. T e diff erence in wall thickness from one area to another is not what in- duces casting problems; a sudden change in thickness can make it diffi cult to cast. A casting wall should change thickness at a rate of at least one to fi ve, as shown


in the left-hand illustration in Figure 4. If a sharp corner with the possibility of some fl ash sticking up is allowable, or if the cost of removing the fl ash is acceptable, a sud- den change in casting thickness as shown in the right-hand illustration of Figure 4 often can be accommodated. T e “insert seam” in the die cavity maintains the severe temperature gradient in the die required to properly solidify the diff erent thicknesses. T ere are various casting wall thickness recommendations in literature and some companies have their own standards. No material property limits the thinness of a


casting wall. As the wall is made thinner, the metal injection power required of the diecasting machine increases, and usually the die clamping tonnage also increases. Die castings have been made in all alloys at 0.5 mm thickness and some at 0.3 mm. A recent study commissioned by the U.S. Council for Automotive Research, Southfi eld, Mich., showed castings as big as 1 m2


could be made 1


mm thick. It concluded the diecasting industrial culture, not material properties, negate the feasibility of such castings. Limiting thinness is a particular


casting supplier’s practice. When a casting looks a lot like another but has a thinner wall, the casting machine power requirement increases signifi - cantly and the required metalcasting acumen can take a quantum leap. T e casting should be designed with


the minimum wall thicknesses required for product function, then increased as required by the diecaster for its capability.


Dimensioning Datums Unlike a machined part, a die casting


is not created by controlling a cutter through a series of X-Y-Z coordinates from a known coordinate system (e.g., the classical 3-2-1 locating system). It is made within two or more blocks of steel, as shown in Figure 5, that have been carved into the shape of the desired casting using such a coordinate system. T e diff erence is subtle but both real and important. T e base of the coordinate system (i.e., the 3-2-1 locating points)


Fig. 1. A casting must be designed to contain some fl uid around the mechanism and keep dirt and water out.


Fig. 2. The casting for the requirements shown in Figure 1 might look as shown. The features would be designed to the die pull arrow.


Nov/Dec 2013 | METAL CASTING DESIGN & PURCHASING | 31


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