4.1 Effect of Raster Angle
According to the ANOVA Table.7, the p-value of the raster angle is less than 0.05, indicating that the raster angle is significant at the 95 % confidence interval. Furthermore, as shown in Figure. 3, tensile strength increases as the raster angle rises from 0° to 90°, as does the number of shells. The increased number of shells in the printing process may reduce the need for infill, allowing the outer shells to carry the load. The shells of 3D-printed parts are typically overlapped to create a stronger part. This overlap angle improves the bonding between the shells and reduces voids between the layers. When the angle is increased from 0° to 90°, stress is transferred from the inner layer to the outer shells, increasing the tensile strength along with the angle and number of shells while maintaining uniform layer thickness. A 90° angle forms cross sandwiches and distributes the load to the outer shells, resulting in greater ductility.
<<<Figure.3 Stress Strain Relationship curve for Raster Angle (RA) a) 0°, b) 45° & c) 90°>>>
Because of the molecular bonding in the material, the virgin PLA material has a high elongation at a 0° angle. With the increased bonding created between different materials in the sandwiches, the composite material showed less elongation than the composite sandwich material. Because of the spaces between the layers and improper bonding, the PLA/CF has the lowest tensile strength of all materials for all angles of inclination. Short fibers, irrespective of stress, may result from incorrect layer bonding, causing fibers to be pulled out during tensile load application; this was observed by microscopic examination as shown in figure.4. This can be prevented if continuous carbon fibers are used to make the specimens. Premature failure was observed during testing due to stress concentration at the fillet areas, as shown in Figure.5. because the raster ends at the fillet profile in ASTM standard test specimens. A number of researchers have observed the same type of failure in the testing [37, 38, 39, and 40]. The greatest average strength of the PLA material was 52 MPa at a 90° angle, and the minimum average strength was 47 MPa at a 0° angle. The average tensile strength of PLA material was increased from 0° to 90°. Because the overlapping increases as the angle increases, the strength is improved. The tensile strength of PLA/Cu composite material increased by 15.62 percent (27.67 to 32 MPa), PLA/Cu composites increased by 15.66 percent (27.66 to 32 MPa), and PLA/Al2O3 increased by 4.76 percent (42 to 44 MPa), but the composite sandwich material’s average tensile strength remained unchanged. But the tensile strength increases for all angles of inclination in the material order of PLA/CF <PLA/Cu <PLA/Al2O3 <PLA <PLA-PLA/CF-SW <PLA-PLA/Cu-SW <PLA-PLA/Al2O3.
When the angle changes from 0° to 90°, the failure mode in the specimen changes from ductile to brittle. For an angle of 0°, the maximum and minimum elongations of the material are 18% and 6.98%, respectively. The maximum and minimum elongations of a 90° angle are 16 and 7.2 percent, respectively. When the angle is changed from 0° to 90°, it is clear that the ductile failure transforms into brittle failure. It was clearly observed from the figure.3 diagrams of stress-strain relationships.
<<<Figure.4 Microscopic Examination on Raster Angle 90° >>>
<<<Figure.5 Stress Concentration and Failure in printed samples>>>