“
With such tight tolerances on flatness and surface finish, cooling lines were placed in specific areas of the cavity so a uniform temperature could be achieved.”
• Te flatness profile around the seal groove for the entire part is held within a close tolerance with an additional tighter flatness specification for each each 1 x 1-in. (25 x 25-mm) section.
• A positional specification is placed on the four motor mount cores. Tese as-cast positional dimensions are called back to a machined profile that houses the stator motor assembly.
no way to vent these mounting bosses, making this feature without any defects proved challenging. Porosity specifications on the
• A positional specification is re- quired for the five cover mount cores. These as-cast positional dimensions are called back to a couple of machined cores that are in turn called back to the machined bore cores on the cover side, as well as the mounting bosses on the outer section of the part.
All other surfaces of the casting not
specifically called out could not exhibit any flash, heat check or washout that exceeded the tolerance. Any imperfec- tion must be fixed before more castings are made. Te customer expects the castings coming out of the die after 100,000 rounds to look like those that were made when the die was new. Furthermore, the ejector pin flash can not exceed a certain tolerance or be loose in any way. Tis ensures no loose particles will find their way into the stepper motor or electronics after the as- sembly phase.
Te outside mounting
34 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2016 This is a magnified view of the breather port.
bosses are to be nearly perfect, and the bosses are expected to be clean of any imperfections so their integrity does not come into question. Due to the restrictions of the print and virtually
machined surfaces also played a significant role in the quality of the casting. Specifically, the thicker sec- tion of the bore core is machined and cannot show any porosity that exceeds a certain amount. Tese bores, as well as the part, must be leak tight and are leak tested. Tese parts house electri- cal components, so no moisture can be allowed to breach the housing. Te center bore cores also have a
maximum machining stock on each side to reduce run out and chatter. Tis tight window for machining stock can cause problems with non- cleanup if the diecast dimensions have excessive variation. The customer gave several dif-
ferent call outs for the die casting’s surface finish. These surface finishes are critical for the seal surfaces and breather port to ensure the adhesives used on the surfaces adhere to the part sufficiently. No moisture must enter the assembled housing. Sur- face tension testing is also important when determining whether proper
adhesion will occur. Die spray is a critical component when adhesion may be an issue.
Part and Tool Design The electrical housing was
already in production when it came to Imperial, so almost no changes could be made to improve the cast- ing or its processing. The diecaster made a few requests to improve the castability of the part, with the only major request of changing some of the part’s geometry. Impe- rial requested to make the ribs on the mounting bosses thicker so the bosses would be easier to fill, reduc- ing any chance of non-fill. Because the part was already in production, with finishing and assembly process- es in place based on the original de- sign, this request was denied. Thus the challenge was left to tool design and the casting process at Imperial to ensure a more than adequate cast- ing would be produced. Imperial determined a two-cavity
die would be used so the high-volume part could be produced at low levels of scrap and high levels of productivity. From there, different runner de-
signs were suggested and modeled to find an appropriate way to fill the cast- ing. Due to the as-cast sealing surface specifications, it was challenging to gate the part in a manner that would provide sufficient fill while maintain- ing the specifications. Te as-cast seal groove surrounds the entire part, and gating directly into the standing steel that forms this feature would cause accelerated erosion. Te customer design included a por- tion of the flange with a thicker wall, which would allow for gating above the seal groove steel instead of directly into it. Tis design was locked, so the area for gating was limited. Once a few potential tooling designs were cho- sen that might address the challenge, casting process modeling simula- tions were run to find an
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