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Put All Requirements Up Front
Perform design preplanning of all parts of an assembly in tandem to assure lowest casting costs.
Engineering practice often calls for the critical components of a new prod- uct assembly to be designed in detail first. Often, key internal components (CPU boards, etc.) move from evaluation to final hardware designs while external housings are still concept sketches. Thin, rigid housing features can be diecast, but finalized designs of internal units may preclude such a housing or drive up costs. By completing mating part designs—with diecasting process input—at the same time as the critical components, engineers often can make tradeoff s early and simply in all mated parts for sizable total cost savings. Additional cost drivers hinge on providing the housing producer with full details up front on all further specifi ed operations, whether they are made by the diecaster, the third party or the OEM. Post-casting machining details impact die design options and final ma-
chining costs. Specific finishes, especially cosmetic-level surfaces, call for alternate approaches to initial die tool and trim tool construction decisions, as well as the type of part deburring to be specified. All post-casting operations should be planned for at the earliest stages. Post-casting surprises can lead to unnecessary costs.
3 Avoid Overdesign
When converting to diecasting, avoid overdesign on previous feature holdovers.
Match die casting material proper- ties to the required performance specifi cations, rather than the pos- sibleyexcessive material specs from a previous manufacturing process. Take advantage of the design free-
dom and benefi ts unique to diecast- ing manufacturing as they apply to the function and performance of the
proposed new component: • T ink thin: An overdesigned structural feature, such as wall thickness, may remain in a part redesign based on the require- ments of an earlier process.
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• T ink tapped threads: Molded plastic parts may require metal inserts or through-bolts, while di- ecasting can employ tapped threads or utilize self-threading fasteners.
• T ink cast-in threads: Die cast- ings can be designed with as-cast external threads for cost savings.
Be alert to the signifi cant cost reductions possible through combining multiple parts.
2 Advanced diecasting can achieve
intricate, complex shapes diffi cult to execute in any other high-speed met- alforming process, so consider part assembly consolidations. Most designers and engineers
are familiar with the value of plastic molding part consolidations when new molds are required. Similar consolidation principles and re- sulting benefi ts are realized when incorporating die castings, including fewer individual parts, fasteners and assembly steps. Analyze a new diecast component
together with the entire assembly plan to identify consolidation opportunities.
Limit Differing Wall Thicknesses
Understand the cost impact of widely varying wall thicknesses on production cycle time.
Diecasting production cycle time is directly related to the uniformity of a part’s
wall thicknesses. Designing adjacent thick-to-thin walls hinders effi cient produc- tion, making smooth metal fl ow and high cavity fi ll rates diffi cult or impossible, depending on the alloy being cast. Talk to a custom die casting engineer early about the ideal wall thickness range for
your design in aluminum, magnesium or zinc. When wall thickness variations are es- sential, use transitions to avoid abrupt changes. While modern diecasting can produce thickness variations that were once diffi cult, they should be specifi ed with care.
Mind the Parting Line
Make an early decision on parting line placement for your diecast component.
Unless your diecast part normally will be hidden from the end-user or assembled
as an internal component, parting line placement is high on the list of cost drivers. Good designs for parts with cosmetic appearance requirements make parting line concealment a primary consideration along with the key features of the part. Allowing parting line placement to come after all other design decisions can
result in a visible metal bead requiring costly operations such as shave trimming of the part and/or hand polishing, thus sacrifi cing the automated production savings inherent to diecasting. Even with non-cosmetic parts, uninformed parting line placement could result in the need for a stepped parting line with more costly two-stage trimming.
Sept/Oct 2011 | METAL CASTING DESIGN & PURCHASING | 29
Consolidate Parts
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