Tool Design Meets Challenges
Click here to see this article as it appears in Modern Casting.
At first glance, the automotive electrical housing appeared to be a standard aluminum die casting with few features that would cause concern during production. The 1.74-lb. part houses the circuit board and stepper motors that control the shifting of the transmission and has an annual volume of 1.2 million. Design-wise, it doesn’t seem to be overly complex, but the leak tightness and surface finish requirements proved to be stumbling blocks in achieving series production for the new part.
Imperial Die Casting (Liberty, South Carolina), a division of RCM Industries Inc., agreed to take on the challenge of producing the part, but several difficulties originated with the print. As-cast surfaces are referenced back to multiple machining surfaces. Furthermore, these multiple machining surfaces are referenced independently to cast features, creating issues with the dimensional tolerances. Due to multiple tiers and worldwide part usage, design changes were severely limited. Because so much is already invested once the part is machined, the customer wanted the variations in the casting to be minimized across multiple casting tools, as well as 32 different machining fixtures, all in production at the same time.
The customer’s superior functional performance demanded stringent dimensional requirements and several positional and dimensional features must be held within a tight window:
The 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. These as-cast positional dimensions are called back to a machined profile that houses the stator motor assembly.
A positional specification is required 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 imperfection must be fixed before more castings are made. The 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. This ensures no loose particles will find their way into the stepper motor or electronics after the assembly phase.
The outside mounting 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 no way to vent these mounting bosses, making this feature without any defects proved challenging.
Porosity specifications on the machined surfaces also played a significant role in the quality of the casting. Specifically, the thicker section of the bore core is machined and cannot show any porosity that exceeds a certain amount. These bores, as well as the part, must be leak tight and are leak tested. These parts house electrical components, so no moisture can be allowed to breach the housing.
The center bore cores also have a maximum machining stock on each side to reduce run out and chatter. This tight window for machining stock can cause problems with non-cleanup if the diecast dimensions have excessive variation.
The customer gave several different 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. Surface 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 casting 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. Imperial requested to make the ribs on the mounting bosses thicker so the bosses would be easier to fill, reducing any chance of non-fill. Because the part was already in production, with finishing and assembly processes in place based on the original design, this request was denied. Thus the challenge was left to tool design and the casting process at Imperial to ensure a more than adequate casting 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 designs were suggested and modeled to find an appropriate way to fill the casting. Due to the as-cast sealing surface specifications, it was challenging to gate the part in a manner that would provide sufficient fill while maintaining the specifications. The as-cast seal groove surrounds the entire part, and gating directly into the standing steel that forms this feature would cause accelerated erosion. The customer design included a portion of the flange with a thicker wall, which would allow for gating above the seal groove steel instead of directly into it.
This design was locked, so the area for gating was limited.
Once a few potential tooling designs were chosen that might address the challenge, casting process modeling simulations were run to find an optimal runner that would fill the part adequately. Each runner design was simulated with different variables such as: tip diameter, die temperature, slow shot velocity, fast shot velocity, and start of fast shot. After the simulation, the optimal design was chosen to produce the tool so production could start.
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. Standard practices at Imperial dictate thermal oil lines are used to pre-heat and stabilize the cavity temperatures, and water lines are used to cool the shot blocks and runner blocks. Initial simulations revealed areas of the cavities that would absorb more heat than others, indicating a need for additional cooling in these areas, such as in the standing steel near the gates.
The use of cooling lines helps ensure as little shock as possible to the steel during the initial startup of the die and also maintains the temperature during production. Due to tight dimensional tolerances, all the cores in the tool were covered with a chromium-based coating to help prevent tool wear and reduce soldering. This coating also allowed the dimensions throughout the cores to be held within the required tolerance. A vacuum also was applied to reduce any gas porosity that may occur.
After the initial startup of the tool, minor porosity was discovered on the outside of the two snouts after machining. The amount of machining stock was 0.02 in. (0.5 mm) on each side of the bore cores. The snouts have a considerable amount of thickness, so shrink porosity would be a common finding. To reduce that amount of porosity exposed by machining, stock was added on both the inner and outer diameters of the cores. This reduced the amount of machining stock and reduced the chances of exposing porosity.
Though the part looks simple, significant challenges stemmed from the tolerances, low porosity, leak tightness and surface finish requirements, with limited changes allowed by the customer. This brought on challenges in design and process that required extensive analysis of the part.
With a focus on tool design and quality production processes, Imperial met these obstacles, and has been producing the part at high volume, serial production rates since 2011.