PSI - Issue 42

Guilherme Opinião et al. / Procedia Structural Integrity 42 (2022) 1266–1273 Guilherme Opinião / Structural Integrity Procedia 00 (2022) 000 – 000

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inclination is also intended at avoiding the collinearity between the direction of loads applied and the principal axis of the build process. To accommodate this orientation and due to the needed detail and small feature size, polyvinyl alcohol (PVA) was used as support material in a BB 0.4 print core. The main printing parameters used were a layer height of 0.1 mm, 100% of infill density, zig-zag infill pattern of both extruders, a printing temperature of 205ºC, a print speed of 55 mm/s, and a support speed of 25 mm/s. Figure 3(B) shows a representative example of a batch of similar samples.

(A) (B) Fig. 3. Fused filament fabrication of the specimens. (A) Slicing software preview of the build with models in the middle (in red) of the support material (blue). (B) A batch of 3 specimens, a representative example of each one of the studied geometric parameters. 4. Results and discussion Torque insertion tests were performed with FFF screws on a pre-perforated polyurethane foam plate, used to simulate the bone. This foam is produced by the American company Sawbones in accordance with the ASTM F-1839 08 standard “Sta ndard Specification for Rigid Polyurethane Foam for Uses as a Standard Material for Testing Orthopedic Devices and Instruments”. During insertion, screws were tested in holes with diameters of 6; 6.5; 7 and 7.5 mm. When inserting the samples into the 6.0 mm and 6.5 mm holes, all samples fractured when subjected to a torque of less than 1 Nm. When inserting in the 7.0 mm holes, the model samples with a pitch of 5 mm and 17 holes broke with torques of less than 1 Nm, while samples from the other models were successfully inserted with a torque inferior to 1 Nm. When inserting into the 7.5 mm holes, all samples were successfully inserted with torques below 1 Nm. Figure 4(A) shows a representative image of a screw that fractured in mid-length during tightening.

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(B) Figure 4. Representation of (A) a broken FFF screw specimen after the insertion test, and (B) FFF specimen defects and layer effect observation, image obtained with an AM7025X DinoEye eyepiece camera mounted on an Optika SZM trinocular stereo microscope. Insertion tests are affected by screw geometry and the torsion resistance of the implants. Naturally, increasing the perforated area of the screw decreases its mechanical strength. Samples whose holes were enlarged broke when subjected to a torque of 1.5 Nm, while a similar screw with slightly smaller holes fractured with a torque of 2.2 Nm. Most fractures started in the areas close to one of the holes of the implants and propagated through the thread until it reaches the next hole. Furthermore, the fractured surface follows the inclination of the thread.