Protective Device Casing
Problem Definition
During clinical testing of a pressure driven drug delivery device, a failure occurred during device activation due to a common mis-use. To initiate drug delivery, the patient must remove a plug from a delivery tube connected to a drug reservoir. The instructions for use specify that while removing the plug, the device should be held by the delivery tube; however, most users instinctually hold the device by the larger drug reservoir. This use creates a moment on an epoxy joint between the reservoir and the tube, resulted in failure of the joint in some cases.
Design
To prevent the mis-use condition of a moment being applied at the epoxy joint, I designed a 2-piece, injection moldable protective case. Prototypes were made to assess iterate on the size and shape of the case using an Ultimaker 3 FDM printer.
I designed the two pieces of the case to mimic the device geometry, with a split near the device centerline. They were joined with 2 snap-joints on the side which protects the delivery assembly of the device. On the opposite side, the parts are mated at a pivot point designed into the bottom piece of the case.
Prior to injection molding, I increased the draft angles of appropriate walls and identified preferred gate and eject pin locations in the part drawings. I selected the Polycarbonate (PC) Makrolon 2588 for molding; for PC’s flexibility and resilience making it an appropriate material for snap features, and for the grades conformance to ISO-10993 biocompatibility tests.
In addition to preventing relative motion between the housing and delivery assemblies during plug removal, the case also prevents premature ejection of the plug, and protects the device from accidental damage prior to its use.
Ergonomic Analysis
A critical design requirement for the delivery device was for it to be usable by the patient population; which extends to the protective case as well. The main concern related to this requirement is the action of opening the case to remove the drug delivery device.
The case is designed to be opened by using a ‘key-grip’ to pinch the top and bottom of the case together, causing the top piece to rotate about the pivot point and disengage from the cantilever snaps.
I researched the key-grip force capabilities for the patient population , and used the data set the maximum key-grip force for opening the case at 8 lbf. While the 3D printed prototypes met this criteria, I felt that for a re-design was important to ensure the molded parts also met this criteria for the injection molded PC.
Using a simple Free-Body Diagram (FBD), it can be determined that the key grip opening force relates to the cantilever separation force by the ratio of the distances of each force from the pivot point. (This assumes each force is aligned with the rotation). The separation force can then be related to the snap dimensions using cantilever tables and taking into account the friction between the two pieces and the material properties of the PC.
I used the aforementioned relations to determine the new snap feature dimensions, and then ran a SolidWorks Simulation to confirm the calculations and ensure assembly or disassembly would not break the snaps.
Resolution
We had our molding vendor create a 2-cavity prototype tool to mold the case with the re-designed snap. The top piece was molded using 2 side inserts, and the bottom piece was molded with 2 side inserts and a rotational insert.
Parts were ordered from the prototype tool; several were tested for the max opening force and found to not exceed the maximum force set at 8 lbf.
Unfortunately, the project was cancelled prior to shipping tests to validate the case’s ability protect the device. For future design iterations, I would have liked to focus on removing sections of the case that did not protect the delivery system; with the goal of reducing tool complexity and per piece material cost.
One-Handed Smartphone Case
Problem Definition
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