generous fi llet. Even better, use a tapered surface to reduce stress and provide better metal feeding characteristics. T ick sections in castings can often
cause problems during solidifi cation, requiring additional feed metal to achieve a proper soundness level. T is is especially problematic when the heavy section is isolated, meaning that the section is located in an area of the casting that is diffi cult to reach with commonly used gating and risering techniques. T e metalcaster must then often resort to using additional risers, chills, or padding to produce a conforming casting. T ese requirements will increase the cost to produce the casting. A good design practice to address
concerns with section size and sound- ness is to perform solidifi cation model- ing on all casting designs unless the design is simple. Modeling will quickly show areas that are the last to solidify, the eff ect of isolated sections, feed paths and other critical features of the design. T e engineering team can then
work with the metalcaster on adjust- ing the design to reduce or eliminate any issues that are uncovered before a physical casting is actually made.
6
Finishing Requirements Is it clear what your fi nish
requirements are for the casting? Is
there the possibility the metalcaster will be performing more fi nishing work on it than is necessary to fulfi ll the design intent?
Several considerations are: • Grinding. Does the parting line need to be ground fl ush with the surrounding surface, or can it be left as-cast? Simi- larly, can riser and ingate connections be ground smooth but left as a raised pad instead of being completely removed?
• Cleaning. Normal processing would be to shot blast the casting after shake- out, then perform grinding operations. If you are requiring an additional shot blast operation after grinding, this will be adding cost to the casting that could possibly be eliminated.
• Packaging. T is area often is critical
• Core usage. While it is not a problem to use cores to create internal passage- ways throughout your casting, always look for ways to eliminate or minimize them. If your design requires a large amount of cores, is there a way to adjust the design to allow several cores to be incorporated into one? If cores are assembled before being placed in the mold, is the assembly robust? Having a weak or fragile core assembly will cause higher scrap levels and increase costs. If a core is required to allow the mold to be parted, or to provide a base for proper gating or risering, this is an opportunity to work on the design to eliminate these types of cores and reduce costs. T in or long cores may be prone to movement within the mold, requiring chaplets to hold the cores in place during solidifi cation, which can increase costs. Long cores also can increase costs due to diffi culty remov- ing core sand in the cleaning process.
and needs to be reviewed with the met- alcaster to make sure all requirements are understood. If there are specifi c types of shipping containers, rust pro- tection or packaging methods required, these should be covered during the quoting stage to make sure you receive castings the way you need them.
7
Inspection If you have additional inspec-
tion requirements that are outside
the normal industry parameters, this will drive costs higher. For instance, Brinell hardness testing typically is performed on an audit basis, so if your requirements state that a larger than normal percentage need to be checked, be prepared for costs to increase. Likewise, requiring other tests such as magnetic particle inspection, X- ray or pressure testing on a more frequent basis will increase the overall cost. Work closely with the metalcasting engineering team to determine what is best for your particular casting design and application. To be successful in your eff orts to eliminate casting cost drivers, all the items listed above will require detailed collaboration and communication with the metalcaster. Ideally, this should be done during the design stage and before the quote is fi nalized so you can be confi dent that the cost drivers have been addressed and you have received an ac- curate casting price.
28 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2014
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