After you have evaluated the characteristics of gray iron and selected it as your casting’s ideal material, consider the following nine tips when you start designing the part.
1
CONSIDER Shrinkage
Most metals and alloys shrink during the casting pro-
cess. T e three common shrinkage types are liquid, solidi- fi cation and solid. Each occurs in a diff erent casting stage. To off set liquid shrinkage, design components so all parts progressively increase in dimension to one or more suitable areas where feeder heads (risers) can be placed. Feeding compensates by supplying additional molten metal to a freshly poured mold. Gray iron exhibits surface concavity in areas where solidifi cation shrinkage is not taken into ac- count. To compensate for solid shrinkage, the patternmaker must apply the shrink rule when designing the pattern. T e rule makes the dimensional adjustments between a cast- ing’s measurements at room temperature and just above the melting point. Gray iron parts may show varying amounts of dispersed
micro-shrinkage in the same areas. Much of the “open grain” in some gray iron castings is dispersed shrinkage. However, high-carbon, low-strength gray irons under 30,000 psi tensile strength typically expand rather than contract on solidifi cation due to graphite rejection.
2
AVOID Hot Spots
Hot spots are the most common defect in a casting
design. T ey can cause localized shrinkage or hot tears, which can result in structural weakness. When two or more sections conjoin, free cooling is interrupted, causing a hotspot. Following are several ways to avoid hot spots in gray iron castings: • Round corners to 10% of the section thickness to reduce heat and stress concentration. Be sure the added radii aren’t too large, as this can result in shrinkage (Fig. 1). Rounded corners, also called fi llets, can help eliminate cracks, tears and draws at reentry angles.
• A well-designed casting brings the minimum number of sections together at one point. By adding a section (forming a “T”), a hot spot is created at the junction and will cool like a wall that is 50% larger. Stagger sec- tions so they do not all gather at one point to minimize the hot spot’s eff ects and reduce distortion (Fig. 2).
• If staggering sections is not possible, core a hole through the center of the junction to help speed solidi- fi cation and avoid hotspots. Use oval-shaped holes with the longest dimension in the direction of the stresses. Holes should be as large as possible, consistent with strength and stiff ness. Avoid rectangular-shaped cored holes in ribs or webs.
3
DESIGN Uniform Sections
Generally, gray iron sections over 1 in. in thickness should be cast no thicker than absolutely necessary. It’s also important to adjust the C.E. (carbon equivalent) to be ap- propriate for the thickest section being cast.Having a casting with some walls that are thicker than others can result in shrinkage voids. Design uniform sections that solidify evenly, or have the heavy sections accessible to feeding from risers. T e diff erence in thickness for adjoining sections should not exceed a 2:1 ratio (Fig. 3). If it is less than 2:1, the transition should take the form of a fi llet. For ratios greater than 2:1, the recommended transition is a wedge.
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Sept/Oct 2011 | METAL CASTING DESIGN & PURCHASING | 35
Fig. 2. To prevent uneven cooling, stagger the sections so no more than two sections conjoin. If the sections have to intersect, create a circular web.
Fig. 1. Designs that have sharp corners result in structural weak- nesses from localized hot spots at the junction. Incorporate rounded corners to avoid hot spots and improve strength. Note: The shaded gray area is the metal.
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