Fig. 2. The casting is welded to a brace on one end and bolted to a construction frame on the other.
• The uniformity and the +/- tolerance of the center slot opening. If it were too narrow, the gusset wouldn’t fi t into the slot; too wide and the bolted plates wouldn’t have surface contact with the gusset.
• The position, alignment and diam- eter of the bolt holes on the two plates.
• The angle, depth and uniformity of the weld shoulder on the cylinder. (The dimensional tolerances for
the castings were specifi ed by the ISO 8062 Grade CT8. The A grade specifi es the tolerances per the basic casting dimension.) Cast Connex and Pacifi c Steel engi-
neers considered two types of molding methods to meet the requirements for the steel connector. Green Sand Mold—Moist, clay-
bonded sand is tightly packed around a wood or metal pattern in mold boxes to form a rigid mold cavity. The two pattern halves are removed, and the mold is assembled with cores. Nobake Sand Mold—Chemically
bonded sand is packed around a wood or metal pattern in mold boxes. At room temperature, the sand mold becomes rigid. Pattern halves are removed, and the mold is assembled with cores. Each of the two molding methods
has relative capabilities, advantages and costs (Table 1). The design team attempted to choose a molding method that took into account the as-cast toler- ance and fi nish requirements and level of detail while achieving the lowest possible cost. Green sand molding can meet the
baseline requirements for the connec- tor for surface fi nish, detail level and cost. But for the longer 27-in. heavy connector, green sand molding has a larger dimensional tolerance (0.12 in.) than the target tolerance requirement of 0.1 in. Nobake sand molding also can
meet the baseline requirements and offers improved surface finish. In
32 METAL CASTING DESIGN AND PURCHASING
addition, nobake molding can hold tighter dimensional tolerances at larger dimensions than green sand. It meets the target tolerance of 0.1 in. for the larger dimension, so the designers determined it was the best choice for the connector. 3. Casting Design and Mold Orientation One of the advantages of the casting
process is that detailed features can be produced in a near-net shape confi gu- ration. This capability can reduce or eliminate expensive machining steps. The center slot of the connector is
a key feature because the gusset has to fi t cleanly but tightly into the slot for fi eld assembly. Rather than ma- chining 10-in to 18-in. cuts, the center slot was formed with a sand core. Cores are sand shapes inserted into the mold to produce interior features in the casting. Another design choice for the con-
nector was whether and how to pro- duce the bolt holes in the connector plates. Bolt holes can be drilled into the fi nished casting or produced in the casting with features on the sand core
Target
and then fi nish drilled after casting. The connector did not have a standard bolt- hole confi guration. In actual use, each structural designer chooses the bolt size, count and confi guration based on the structural loads with which he must work. With that in mind, the designers decided to produce the con- nector without bolt holes and let the fi eld engineers drill the holes required for their design. Finally, in mold design, the orien-
tation of the part in the mold is an important factor in producing a sound casting. The part should be oriented in the mold so that metal fl ow is uni- form and even in the different sec- tions of the casting and the core can be securely positioned in the mold. In the case of the steel connector, the metalcasting engineer had two options for orientation in the mold—vertically or horizontally. In the horizontal orientation, metal
would fl ow fi rst into the lower plate and then into the top plate, producing non-uniform fl ow and fi ll into the two plate sections. In addition, the core forming the center slot would be ori- ented horizontally and tend to “fl oat” in the melt, producing variation in the slot geometry. In the vertical orientation, the de-
signers determined metal fl ow would be uniform, with the two vertical plate sections fi lling evenly together. The center core also was oriented verti- cally and securely seated in core prints (seating slots) in the top and bottom molds. The designers therefore deter- mined this was the preferable orienta- tion for producing a fl aw-free casting.
Table 1. Capabilities of the Green Sand and Nobake Processes Capabilities
As-Cast Dimensional Tolerance (+/-) Across 4 in.
As-Cast Dimensional Tolerance (+/-) Across 25 in.
Nominal Surface Finish (RMS) Minimum Section Thickness (in.) Intricacy of Detail Tool/Pattern Cost Mold Material Cost
ISO8062 CT-8 1.6/0.06
ISO8062 CT-8 2.6/0.1
450 0.5 Fair Low Low
Green Sand Typical:
1.6/0.06 Typical:
3/0.12
400-900 0.25 Fair Low Low
Nobake Typical:
1.4/0.05 Typical:
2/0.08
300-900 0.19
Good Low
Medium JANUARY/FEBRUARY 2010
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