Extreme Thick Section ( in.)
6 3
36 36 1.5
Typical Higher Surface Finish (RMS)
900 900 600 600 150
Typical Lower Surface Finish (RMS)
250 250 150 150 75
(without machining) typically cost more than green sand, but the price difference is offset when machin- ing costs are reduced due to nobake’s tighter tolerances. Reduction in scrapped castings and rework also affects the bottom line for castings that overexert the green sand casting process’ capabilities. Green sand castings can be pro- duced with walls as thin as 0.09375 in. (0.238 cm) with no maximum limit. When thin walls are required, a met- alcasting facility will take special pre- cautions for shrinkage allowance, mold preparation, venting, pouring technique and other factors, all of which will add cost. Shrinkage allowance is a pat- ternmaker’s calculation that accounts for the contraction in metal volume as the liquid metal cools to solidification. Venting is the method of providing a small opening or passage in a mold to facilitate the escape of gases when molten metal is poured into it. Tolerances as tight as +/-0.03 in. (0.0762 cm) can be cast in green sand, but the tighter the tolerances, the higher the rejection rate. A tolerance of +/-0.0625 in. (0.158 cm) is practical for small castings, but increased toler- ances are required for large castings. As a general rule, nobake thin
wall section limits are +/-0.1 in. (2.54 mm), although it varies by metal. Common tolerances for nobake range from +/-0.03-0.47 in. (0.9-12 mm) because cured sand used in nobake molding is more structurally stable than green sand. If surface finish is a factor, no- bake sand casting produces smooth- er surface finish than green sand. Nobake’s finish can go as low as 150 root mean square (RMS) in typical operations compared to 250 RMS with horizontally parted green sand. Surface finish varies within each sand casting process, depending on the fineness of the sand being used,
Extreme Lower Surface Finish (RMS)
100 100 40 40 40
Typical
Lower Draft (degree)
2
1 1 1
0
Typical Upper Draft (degree)
5 2 2 2 2
bonding mixture, mold compaction and sand control methods. Met- alcasting facilities also may incor- porate mold coating techniques to further improve surface finish. How- ever, this will add cost to the part. Tooling costs also factor into
choosing between green sand and nobake casting. Green sand tooling must be able to withstand the compac- tion force during the molding process.
Typical
Production Rate (castings/hr.)
100-500
100 to 500 10 10
5 to 40
Extreme
Production Rate (castings/hr.)
1,000 1,500 15 15
60
Because nobake tooling doesn’t have to withstand this force (only light vibra- tion), the pattern equipment in nobake sand systems vary widely among wood, plastic and metal tools. In addition, the lack of compaction force in molding allows nobake molders to use loose pattern pieces and other innovative tooling options to increase casting complexity and add design features to the component.
Green sand casting is one of the most economical metal manufacturing methods. Sept/Oct 2014 | METAL CASTING DESIGN & PURCHASING | 35
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