When projects get complicated, one of the best tools for mapping out the design process is MindNode. It helps me break down complex designs into smaller parts that can be addressed individually. It also makes the ideation process a bit quicker because by the time you starting sketching you’ve already been thinking about things like target user, material, and usability issues.
After I’ve identified the design challenges that need to be addressed, I begin collecting reference and inspiration images to compile them on an artboard of sorts using an app called PureRef. I pull images from all product categories that might be relevant and try to establish some basic rules for form language. Throughout the design process, I continually reference this artboard to make sure I’m adhering to the original design intent.
This being the first project I’ve don’t that involved a more complex assembly, I wasn’t sure if I should tackle the enclosure or the internal mechanism first. I chose to tackle the body of the jet injector and then focus on the moving parts later. These are some quick thumbnail sketches done in Sketchbook Pro to realize the basic look of the device. The decision to design the enclosure first resulted in a fair amount of frustration later on when I was forced to constantly redesign the internal mechanism in CAD to make it fit within the existing geometry.
As soon as a basic form was established for the injector body, I realized I needed to design the the actual cartridge that it was meant to house. How the cartridge was inserted, retained, and then released was going to drive some of geometry of the device’s body. This is an early concept for the pre-filled injector cartridge. It was meant to be retained with a single detent, which could be withdrawn by the user, allowing the spent cartridge to drop out of the device cleanly. The design was dropped in favor of a more robust mechanism…
This is my first attempt at designing the cartridge release mechanism. Technically, it would work to retain the cartridge, but the mechanism has a fatal flaw. Any downward force on the retainer would cause it to move backwards and let the cartridge fall. It’s also likely to be susceptible to debris fouling, so the idea was dropped early in the design process.
My second attempt at designing the cartridge removal/release mechanism. I stopped pursuing a mechanism that would allow the cartridge to be loaded from the top and released out the bottom because the retaining method always proved too complex or fragile. In this iteration, the retainer was attached to a lifting mechanism. This was a more robust approach, but operating the lifter and pulling the spent cartridge out of the holder at the same time with the same hand would have been challenging for the user, which broke one of the core features I outlined in the MindNode mapping process…one-handed operation.
Probably the most difficult engineering challenge I had to solve was the plunger rod release/dry-fire safety mechanism. I had to design a reliable firing mechanism that could be interrupted when the device was either open and/or there was no cartridge present. The image above shows one approach involving a bent steel rod/hook for the safety and a sheet metal plunger rod lock that is operated against a small metal retainer with two compression springs. Not only would assembly be more complicated, but the small metal spring retainer would probably have to be machined or made using metal-injection molding…either method would have been quite expensive. I dropped this design in favor of a theoretically cheaper and more reliable, multi-pronged spring steel mechanism and a spring steel safety.
Here is the final design. There’s no reason this couldn’t be prototyped and tested, though a decent amount of simulation and testing that would be needed to figure out what main spring length, wire diameter, pitch, etc. would be required to completely force 0.5 mL of liquid out of the vial, through someone’s skin, and into the muscle without causing discomfort. There’s a possibility that the appropriate main spring size would make it too difficult for the average person to charge the device, in which case a secondary device would be needed to charge the plunger rod.
With every new project, new material shaders need to be created for rendering the final design. Although it may not look like it, sheet metal parts pose an interesting challenge because the shear lines from cutting the sheet stock exist in different orientations within the same plane. To achieve a realistic look, the shear lines would have to be isolated to the edges of the part and oriented correctly.
Fusion 360 offers little control over texture mapping, so the next best thing is to create offset faces that correspond to the various orientations of the shear lines so that materials could be assigned to each face.
Tri-planar material shaders were created for the sheet metal’s face, each shear orientation, and then blended together to give the final appearance.
I don’t have access to sheet metal equipment to prototype the internal firing mechanism, but at the very least the enclosure can be 3D printed in order to identify any issues with the ergonomics of the grip and the one-handed usability of the device. The geometry of the final design makes it a prime candidate for SLA, which can provide high resolution parts with complicated geometry. The silicone/TPU grip, originally intended to be overmolded on an injection molded body, would need to be printed separately and glued into place to give a better representation of the look and feel of the final design.