Author(s): Sterling Allred
Mentor(s): Philippe Passeraub, Elizabeth Clark
Institution BYU
FFF 3D printing is a promising process for distributed manufacturing, of which, proximity to the end user is critical. The design of the product to be printed can be shared via an online repository with controlled versions. Local FFF 3D printers can then be used to manufacture the desired products. The quality of printed parts regarding the stability of their size and of their mechanical functionalities is critical, especially for medical devices where risk reduction is required. We are developing an FFF 3D printable emergency drug administration device to be ultimately manufactured in a decentralized manner with different printers. A key mechanical part of this device is the protection cap of the prefilled syringe needle. This part must perform two friction functions at the same time: (1) to attach with a transition fit to the device body, and (2) to grip the sterile needle protection with an interference force fit. This allows the simultaneous removal of the cap and of the needle protection with the same gesture. The goal of this work is to present a robust design and printing method to limit the effect of low precision FFF 3D printing, especially in regard to distributed manufacturing. Two cylindrical features within the cap enable these aims. First, the outer portion of the cap can be attached with transition fit to the end of a tube designed to contain a prefilled syringe. The feature designed to grip the needle cover is a cylindrical cavity with rough surfaces that attach with interference fit to the needle’s protective cover. The cap was printed in several samples, to reach sufficient statistical significance, using several models of FFF 3D printers. To limit the geometrical printing error of the parts, the results from a previous study on friction and fit for FFF 3D-printed piston assemblies were used. Two different design strategies were compared for the transition fit function: (1) using the internal surface of the second part, and (2) using its external surface consequently with a larger diameter. This second approach showed less variability in the attachment force to correspond to the desired type of fit. The hole to attach the needle protective cover showed a reliable strong attachment force. These two friction functions working together made it possible to insert the cap in the final assembly of the medical device, and when removing it to detach the needle protective cover from the syringe.