details DESIGN
Hollow structures provide more stiffness with less weight, especially in aluminum, where thicker sections are diffi cult to cast with high integrity and a slower cooling rate results in lower strength.
• Solid knuckles contain signifi cant material un- necessary for the structural requirements. Most of the unnecessary material is in the gooseneck, which must be very stiff to meet vehicle handling requirements.
• A second area of structural ineffi ciency in solid castings is the hoop, which is most often designed to be strong as well as stiff .
• Sand core technology and a unique and fl exible gating arrangement possible with (VRC/PRC), allowed the designers of the Model S knuckle to put the material in the most structurally effi cient locations, resulting in a design with structural effi ciency near 100%.
Incorporating mounting brackets and other appendages results in a one-piece design.
• Careful selection of the parting plane of the metal mold, gating system and stable internal body core makes it possible to include mounting bracket and ribs, which provide additional stiff ness.
A fl exible gating system allowed progressive fi lling for a clean and sound casting.
• T e VRC/PRC process produced as-cast, near net shaped castings with excellent surface fi nish and dimensional tolerances.
• By eliminating risers and their connec- tions, the casting surface fi nish is better and the need for secondary cutting and grinding operations is reduced.
The self-cored hoop section, which extends around the control arm ball joints, was strong and stiff and helped achieve further weight reduction.
• Establishing the casting parting plane and orientation as a fi rst step makes optimizing the design easier using self-cored features.
• Self-cored features eliminate the need for additional sand cores and reduce the overall cycle time.
Jiten Shah is president of Product Development & Analysis (PDA) LLC, Naperville, Ill., and a 25-year casting design and manufacturing veteran.
Jan/Feb 2013 | METAL CASTING DESIGN & PURCHASING | 19
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