4.3 Effect of Number of Shells
According to the ANOVA table, the p-value for the number of shells is
less than 0.05, indicating that the number of shells at the 95 percent
level is statistically significant for all materials. The tensile
strength increases as the number of shells decreases, as shown in Figure
9. Tensile strength drops rapidly as the number of shells increases from
Level 1 to Level 3. The more shells, the less time it takes to cool
because of the more voids between the outer shells’ subsequent layers.
This scenario reduces the bonding between the outer layers. When the
number of shells is increased, more space is occupied by the number of
shells alone, and the infill required to build the object is reduced,
resulting in a decrease in tensile strength. It is clearly observed in
figure.10.For all materials, level 1 shells provide greater tensile
strength than level 3 shells.Also, the more outer shells contain more
voids between the overlapping shells.
<<<Figure.9 Stress-strain Relationship for
Number of Shells (NS) a).3 Nos. b).4 Nos. c) 5 Nos.
>>>
The PLA has maximum and minimum tensile strengths of 51.75 MPa and 45
MPa, respectively. The PLA/ceramic composite yields 44 MPa and 41.25
MPa, respectively. PLA/Cu has a maximum and minimum tensile strength of
32.75 MPa and 27 MPa, respectively. The PLA/CF composite has maximum and
minimum tensile strengths of 26 MPa and 25 MPa, respectively. PLA and
Ceramic-SW have maximum and minimum tensile strengths of 59.6 MPa and
55.5 MPa, respectively. PLA/Cu-SW has maximum and minimum tensile
strengths of 55 MPa and 52 MPa, respectively, while PLA/CF-SW has
maximum and minimum tensile strengths of 53.25 MPa and 50.25 MPa. In
terms of the outer shell, increasing the outer shell width reduces the
tensile strength because the outer shell occupies more voids between the
subsequent shells, which reduces the tensile strength. When the two
subsequent shells are deposited, bonding between the outer peripheries
develops, resulting in less bonding between the shells and a reduction
in tensile strength. The voids between the layers act as a stress
concentrator, which may aid in the propagation of the crack during
loading and cause the tensile strength to decrease. All of the
materials, composites, and sandwiches show that increasing the number of
shells reduces the tensile strength of the printed parts. Further
microscopical examination reveals that the air gap is greater in the
subsequent outer layer than in the infill layers. Microscopical
examination of figure.10 clearly shows that the number of outer layers
increases the voids between the outer layers and adjacent layers,
reducing the strength. This can be controlled considerably by increasing
the overlapping of outer shells.
<<<Figure.10 Microscopic Examination of
Number of Shells>>>