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September 2017 • The PCB Magazine 69 3D PRINTED ELECTRONICS FOR PRINTED CIRCUIT STRUCTURES tensile specimens were printed in the direction of force (0° specimens) and perpendicular to the direction of force (90° specimens). The 0° spec- imens failed at an average of 47.028 MPa UTS while the 90° specimens failed at a much low- er force of 13.572 MPa, a ~72% loss of strength. The difference in observed UTS is drastic and shows the effect that print direction has on strength. This difference is again due to the fact that, since the direction of force is perpendicu- lar to the 90° specimen's print direction, the ad- hesion between lines is the only thing provid- ing resistance rather than the material itself. Ideally, there would be strength in every di- rection as there can be multiple directions of force in certain applications. It is common in 3D printing to not print 0° or 90° infills, but rath- er to print 45° as this would provide strength in multiple directions [7,8] . However, this is a compromise as this decreases the overall ten- sile strength when compared to printing sole- ly in the direction of force. Furthermore, if the direction of force is in the Z-direction, the same problem arises because layer adhesion is the predominant factor in determining how much force a part can be subjected to before failure. Since layer and side-by-side line adhesion are as- pects of 3D printing, there needs to be a way to increase the force they can sustain. To do this, a "reflow" experiment was performed. The ap- proach was to use a temperature-controlled la- ser to heat and essentially "reflow" the already- printed plastic. A 30W laser was mounted on the machine and was placed just above the 90° tensile part. Using a non-contact thermometer, the temperature of the plastic part was elevat- ed and held to 120°C while the laser traveled across the surface of the specimen. The laser completed three passes over the middle section of the part, reflowing the plastic and increasing the adhesion between layers. Tensile testing these "reflowed" samples yielded an average 21.574 MPa UTS compared to the untreated samples' average 13.572 MPa UTS, a 58% increase. While the strength increase is much im- proved, it is still below the UTS of parts with infill printed in the direction of force. This is important because it can enable the printing of stronger parts that exhibit force in multiple di- rections. While this was performed to essential- ly increase side-by-side line adhesion, it could also be applied to layer adhesion. By placing the laser in front of the printing head, the previous layer of deposited material would be reheated while simultaneously being printed on top of, creating a more solid part. This will increase lay- er adhesion with the added benefit of possibly removing voids, resulting in a stronger overall part; this is for future work. Figure 12: Average UTS of reflowed and untreated 90° tensile parts. Figure 11: Laser "reflowed" tensile part.