details DESIGN
Diffi cult Part Relies on Design for Solidifi cation
JITEN SHAH, PRODUCT DEVELOPMENT & ANALYSIS, NAPERVILLE, ILLINOIS CASTING PROFILE
Cast Component: Caterpillar Class 8 truck CX31 transmis- sion housing.
Material: A356-T6. Weight: 107.5 lbs. Dimensions: 17 x 20 x 19 in. Process: Permanent mold. Application: Heavy truck.
tion at every stage of development between the OEM design engineers with in-depth knowledge about the component requirements and the metalcasting facility’s experts in process parameters and capabilities.
S Top/Cope
In aluminum, mechanical properties are directly propor- tional to the local cooling rate, which is driven by the section thickness. Critical areas must be defi ned early.
• Identifying the critical areas from the high operating structural stresses and relating their sensitivity to fl aws, such as shrinkage, inclusions and slag, at the beginning of the casting design and development are essential to achieve sound castings consistently.
• Parting line orientation was key to the success of high integrity quality casting production. Correctly determining how to orient features in the cope and drag halves of the mold facilitates the placement of the risers and gates to achieve the directional solidi- fi cation necessary for sound castings.
tahl Specialty Co., Kingsville, Mo., delivered an aluminum main transmission housing to Caterpillar that provided a 17% cost reduction in 2013 alone. T e customer wanted to produce the part in aluminum rather than cast iron as originally designed to reduce weight. Key to the success of the project was intense collabora-
Casting confi guration, especially section thickness, determines directional solidifi cation.
• Stahl Specialty Co. paid close attention to the casting risering and gating system, along with the permanent mold heating and cooling systems, to achieve the desired thermal balance to promote directional solidifi cation.
Bottom/ Drag
• Section thicknesses were varied so they are thicker near the risers, which remain liquid longer than the sections they are attached to, in order to feed liquid metal to the section during solidifi cation.
• Computer simulation predicted the isolated hot spots which, if not fed, would lead to shrinkage and porosity. External feeding aids, such as localized air chills, reduce hot spots when carefully designed.
16 | METAL CASTING DESIGN & PURCHASING | May/Jun 2014
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