I designed and refined an extrusion-printed brace to retrofit my company's instrument with. Without the brace, 2mm diameter cantilevered steel pins were subject to plastic deformation that would render the instrument unable to push fluids around a disposable cartridge as intended. A medium likelihood, high severity hazard.
The brace served as structural support on internal pins and was designed for the following user needs and design inputs:
- Install into instrument with narrow clearances
- Prevent users from permanently bending pins in the instrument. Translates to design input of preventing bending beyond 2mm.
- Injection mold or 3D printed for manufacturing
- Allow for pins to retain existing degrees of freedom for movement in XYZ directions.
With these in mind, I iterated through 10+ prototypes before settling on a solution that was validated for manufacturing variability in mind, withstand user applied forces, and dimensions considering material and positional tolerance stack ups to avoid interference during use. A few examples are below.
Prototype. Failed at reducing amount pins cantilevered over the brace part (black sheath). Too thick and required a cut-out (left most pin) to maintain clearances with existing tolerances.
Close to final design. Adjusted fillets for 3D printing manufacturability and thickened boss to handle higher loads.
Digital force gauge data below shows how well final design improvement meets design inputs. Improvement from ~5lbs to ~20lbs needed to damage pins enough to cause a harm.
Force gauge data I collected shows improvement in bending stress required to fail for each cantilevered pin from 4.6 to ~20 lbs.
The final prototype and subassembly is below. Met design inputs. Features include the ability to:
- Install with existing hardware
- Prevent stress from reaching weakest areas of brace at its mounting features
- Fully protect all pins from one direction of failure mode. Remaining failure mode mitigated by reducing likelihood of pin bending causing issues.
- Risks of failure modes successfully mitigated and added to Design for Failure Modes and Effects Analysis (DFMEA) documents
- Retain existing pin movement (+/- 1mm radially from pin center) even with worst-case tolerance stack up interferences.
- Suitable for 3D printing with nylon. No delamination between printed layers observed. Injection molding not possible since volume is inconsistent across the part. Vetted a supplier to be evaluated against supplier quality list.
I learned how to design a part around complex existing design constraints while still adhering to design input/output goals. Although this end result is straightforward, it was a structural puzzle governed by tight internal instrument clearances. I stopped iterating once all design inputs and needs were met.
Final design as a part (left) and installed on its subassembly (right).
