Development & Analysis LLC (PDA), Naperville, Ill., the values published in Figure 1 and Table 1 are drawn from a random sample of metalcasting facilities in the U.S. and abroad. PDA collected published capabilities from the participating metalcasting facilities and used them to create an average range. The extreme values are based on the lowest root mean square (RMS) in micro-inches values the company discovered published in the literature. You can use the published values to zero in on the appropri- ate process, but the next challenge is fi guring out who falls where on the provided spectrum. You’ll also likely have to pay for the surface fi nish improvements available in certain processes. T e nobake process is more expensive than green sand, but the resulting surface fi nish is generally improved. As an alternative to changing processes, you can
Sand makes up a large majority of the mold, so it’s an essential factor in the casting’s fi nal surface fi nish.
Mold faces also can be compromised when liquid metal
physically crashes into the sand. Increased turbulence in the melt can cause imperfections in the sand mold, which can lead to fl aws in the casting’s surface. ■
work with your metalcaster to see which finishing processes might be available to improve the final sur- face of your component. The cast component producer may be able to use a different shot blast material, such as glass beads, to smooth a rough surface. Machining certain surfaces might also present an economically viable alternative. Shah said a metalcasting facility can achieve the
lower end of the surface fi nish range—and even those extreme best case surface fi nishes—by controlling several factors: weight/section thickness, alloy and mold material treatment. T e fi rst two are straightforward. As weight and
section thickness increase, the quality of the surface fi nish decreases. And alloys with higher melting points produce lower RMS values than alloys with lower melting points. In other words, i ron castings will not be as smooth as aluminum castings, and steel cast- ings typically will be rougher still. According to the data gathered by Shah, aluminum can be cast to lower surface fi nishes than can iron. Copper falls somewhere in between those two materials. T e same is true for nonferrous alloys versus ferrous alloys in the shell molding process. Sand preparation is the foremost concern when
trying to limit roughness, but metalcasters must ad- dress the molten metal’s entry into a mold. Aside from achieving a metallurgically sound melt prior to pour- ing, casting suppliers should be wary of extremely high temperatures when trying to smooth out their surfaces. Radiant heat from the molten metal can lead to degra- dation of the mold surface, which can produce condi- tions that lead to poor surface fi nish.
Jan/Feb 2016 | METAL CASTING DESIGN & PURCHASING | 45
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