3D Printing a Metal Shift Knob for Faster Cooling

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Metal 3D printing and design thinking enabled an engineer to create a shift knob designed to cool rapidly inside a hot car.

Additive manufacturing is commonly used to control temperature in moldmaking applications through conformal cooling channels—that is, complex passageways that use water or coolant to carry heat out of the mold during injection molding. But additive manufacturing can also be applied in such a way that the component itself dissipates heat without the use of internal fluids. An open geometric form enabled by additive manufacturing can provide dramatic thermal advantages over the same component manufactured as a solid object.

When challenged by Concept Laser to create a sample design that could only be produced through metal additive manufacturing, Nathan Huber, simulation services engineer at Phoenix Analysis & Design Technologies (PADT), referenced his past experience designing airflow solutions for refrigeration. He saw an opportunity to apply the same design thinking to a shift knob for his Subaru STI. By designing a hollow knob with a network of intersecting struts, Huber theorized that he could create a knob that would cool more quickly than typical knobs when the hot car had been parked in the Arizona sun.

In addition to the goal of thermal control, the design also needed to be 1) possible to make only with additive manufacturing and 2) largely self-supporting (in other words, built with minimal use of sacrificial support structures) to minimize postprocessing time and effort. Huber also knew that the  knob would be built on a Concept Laser Mlab cusing machine stocked with Remanium Star CL, a medical-grade cobalt-chrome alloy typically used for dental applications.

Huber based the design for the case study knob on an existing 50.8-mm-diameter knob made of solid stainless steel, which weighs 1.1 lbs. Using ANSYS Spaceclaim Direct Modeler, he designed a knob with a grid-like web of struts joining at right angles at its outer skin. This first design fell within the desired size and weight for the part, but it wouldn't have been self-supporting in manufacturing; furthermore, the right angles of the struts would have required substantial postprocessing, enough to be cost prohibitive.

The second iteration of the design took a different approach, using only vertical supports to reduce the amount of postprocessing that would be necessary. However, the lower material density of 0.0086 g/mm3 meant that the knob fell short of the 1-lb weight goal.

The final iteration of the knob, and the one which was actually printed, combined the two approaches with triangular cells. This geometry allowed the structure to be almost completely self-supporting (a ring of support material was necessary around the bottom of the knob to compensate for the overhang) while minimizing postprocessing. The knob weighed 1.04 lbs following support removal and bead blasting.

To make the part fully functional, it was necessary to tap a hole in its base for installation into the vehicle. Here, says Huber, the material proved to be "overkill" for the application. Remanium is incredibly hard, and tapping it proved difficult. Instead, PADT bored a hole in the knob with a carbide cutting tool and installed a brass insert, which was then tapped, completing the knob. (Fixtures 3D-printed on PADT's PolyJet machine were used to hold the knob during these processes.)

Finally, it was necessary to run a simulation to see whether the new metal AM design truly made a difference in thermal control for the shift knob. Using ANSYS Transient Thermal, Huber simulated both the AM knob and the original solid stainless steel knob with an initialized temperature of 150°F, a temperature that can be quickly reached in a parked car in Arizona, he says. A convection heat transfer boundary condition was placed on the outer surface of each knob, assuming a slightly raised film coefficient of 50 W/m2C to account for air movement and setting the ambient temperature in the car at 70°F.

The simulations ran for 5 minutes. The final temperature of the solid spherical knob after this time was about 115°F. Meanwhile, the 3D-printed knob cooled to about 84°F. The increased surface area in the 3D-printed web-like design helped to dissipate the heat faster and more efficiently than the solid design, says Huber. Watch the simulation in the video below: 

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