Case Studies in Green Sand Casting
The green sand casting process is the most widely-used process for casting ferrous and nonferrous metals. The process has evolved from a manually intensive operation to a highly mechanized and fully automated process.
In terms of weight, complexity and alloy composition, a wide range of castings can be produced via green sand casting. In terms of shape, the process can be used to cast parts as simple as ingots and as complex as engine blocks. It is commonly used for ferrous alloys; however, it can be used for almost any alloy, including those of aluminum, copper, magnesium and nickel.
One of the major advantages of green sand casting is its low cost, especially when mass-producing castings on highly automated molding and coremaking equipment. Another major advantage of green sand is its versatility with respect to production. The process can be economically applied to small and large production runs. In addition, the production rate can be increased by making multiple mold cavities in a single mold.
The green sand process uses molds made from a compacted or compressed green sand mixture. The green sand mixture consists of sand, clay, water and various additives used to obtain desired properties, such as improved surface finish.
Castings weighing from several ounces to 500 lbs. are common, while some as large as 7,000 lbs. can be made.
Properties achieved in green sand casting vary by supplier, but in general, the castings can be produced with walls as thin as 0.09 in. with no maximum limit. When thin walls are required, the metalcasting facility will take special precautions with respect to the shrinkage allowance in patterns, mold preparation, venting, pouring technique and other factors. A tolerance of +/-0.03 in. can be cast, but such close tolerances can cause a higher rejection rate. A tolerance of +/- 0.06 in. is practical for small castings, but increased tolerances are required for larger castings. A tolerance of +/- 0.03 in. for the first inch is possible in special cases, but at an additional cost. In general, the closest tolerances can be kept only for the dimensions that lie entirely in one part of the mold. Greater tolerances are required for dimensions across the parting line, which is subject to variations in the closing of the mold. Casting dimensions also are a function of the degree of compaction of the mold, method of pattern removal from the mold, as well as the pouring temperature and pouring rate.
Here are three green sand castings where the process led to significant increases in value and cost efficiency to the customer. Each example takes advantage of the strengths of the process in different ways.
Cab Mount Bracket
Tight space and high load requirements for a multifunction mount bracket for a semi-truck cab made the green sand casting process attractive for a supplier to a major semi-truck manufacturer. The single iron component, produced by AFS Corporate Member Dotson Iron Castings (Mankato, Minnesota) supports and ties the fuel tank strap and rear sleeper components to the frame. Aside from meeting the performance requirements, cost savings was an important factor in determining the final solution. Dotson Iron Castings collaborated with its customer and OEM to develop a plan to reduce cost. This included doubling core production, automating core placement, and reducing riser waste along with the use of an automated trim press at Dotson. Ultimately, the cast version of the bracket reduced the cost of the system by 10% compared to the steel weldment version.
When designing for green sand, the structural integrity of all mounting locations is considered to determine whether to core or cast solid holes. Key factors driving the as-cored vs. as-cast solid holes decision are desired soundness in the area, hole diameter to thickness/depth, the pitch or distance between them, and parting plane orientation. In this case, critical mounting locations required soundness and dimensional tolerance that could not be achieved with cored holes. Additionally, the locations of these features would not allow the placement of cores, even if one desired it.
The sand casting’s cored pockets provide stiffness and aid weight reduction. Overall dimensional tolerances achieved in casting are better over fabrication. Each weldment can vary dimensionally from distortion, and is process sensitive, meaning the number of weld pass, welding speed, current input, weld wire size, etc. can have an effect. In casting, once the tooling is tweaked, the process will produce consistent dimensions within well predicted dimensional tolerances.
Cast Iron Cooker
AFS Corporate Member Goldens’ Foundry and Machine Co. (Columbus, Georgia) converted the standard ash grate, firebox and fire ring found in a typical kamado grill made out of ceramic into a single cast iron component. It’s a thick-walled cast iron shape that provides durability and resiliency. The design allows for air insulation between the heat source and the exterior of the cooker, extending the heat cycle life.
The cooker, which was prototyped in nobake sand before being cast for production in green sand, has a 20.5 in. diameter and 330 sq. in. cooking area. The 100% cast iron construction is made in Goldens’ casting facility with a powder-coated exterior. Split-design cooking grates with legs allow cooking close to or away from the heat source.
Cast iron has many strengths and admirable qualities, but low weight isn’t one of them. For all the hopes Goldens’ had for its new cooker, they would be rendered moot if the product wasn’t light enough. It also would not live up to their hopes if it were light, but not durable. After testing and development, the cooker’s design features a 3/8 in. wall thickness for a total weight of 330 lbs., with a 120-lb. cart, which makes moving the grill easier. No, it’s not light like a feather, but light enough to be portable while staying strong enough to take a beating from repeated usage. A hinge helps take the strain off of lifting the iron lid.
Four-Cylinder Intake Manifold
The manifold was designed for the high-performance automotive aftermarket, specifically the Mitsubishi Evolution.
Casting in aluminum added strength to the part and allowed customer AMS Performance/Bullseye Power (West Chicago, Illinois) to incorporate larger bell horns, larger plenum and matched cylinder plot angles, which helped improve performance.
AFS Corporate Member Eagle Aluminum Cast Products (Muskegon, Michigan) used intricate shell cores placed in the green sand molds to produce the internal bell horns. The casting was 30% cheaper than the original sheet metal weldment.
Click here to see this story as it appears in the September/October 2018 issue of MCDP.