Effect of Section Thickness of Closed and Open Hollow Structure

Figures 6-33 to 6-39 show the comparison of mechanical properties and thermal properties of different outline/cross-section thickness and solid specimens of Nylon 12 laser sintered material.

It can be seen that the outline/cross-section thickness influences the mechanical properties. The figures demonstrated that the tensile properties increased when the thickness of the outline/cross-sections were greater.

As shown in Figure 6.5(d) on page 134, the variation in the maximum tensile strength of the solid sample is not huge. However, the variation in the elongation at break of the same sample is huge for the solid sample.

The layer-to-layer bonding region in solid specimen is larger than for hollow specimen. Thus greater energy is needed to break the layer-to-layer bonding. In addition the laser scan lines on the solid samples are longer than the close-hollow samples. Longer scan lines allows the previously scanned line time to cool and solidify while shorter laser scan lines limits the cooling time between lines. This resulted in stronger fusion of particle-to-particle bonds that increases ductility and are more difficult to break [Nelson J.A., et al, 2014]. Furthermore in the region of effect of end of vector (EoV) for hollow specimens are very together which exposure the specimens to longer and higher intensity laser. This will causes greater sintering and

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increased densification and affect the mechanical properties [Ajoku, U. et al, 2006]. This is illustrated by Figure 6-40 below.

(a) Hollow structure (b) Solid structure

Figure 6-40. End of vector effect (a). Hollow structure (b). Solid structure

Moreover, the variation in the tensile modulus changes with the different section thickness. Laser sintered materials such as Nylon 12 exhibits anisotropy behaviour (Faes, M. et al, 2015, Ajoku, U. et al, 2006b] due to the influence of processing parameter. This is different from most polymer materials that are manufacture from bulk material through moulding or extrusion processes that tend to produce isotropy behaviour [Griskey, 1995]. Beside the orientation, secondary effect of end-of-vector (EoV) during the sintering process will produce different tensile modulus for varying cross section [Ajoku, U. et al, 2006b]. When the cross section of sintering area (as view from above toward the build platform) is bigger, the tensile strength and modulus will be different from smaller cross section as EoV causes greater sintering and increased densification. And in more prevalent in small feature or crossection [Ajoku, U. et al, 2006b]. This behaviour occurs due to the build orientation of the components and different temperature changes which occur within the parts during the LS process.

However, as shown in Figure 6-36 for sintered material, it was confirmed from the crystallinity characterisation results there was no significant difference for each section thickness and solid for either non-heat treated or heat treated specimens. This signifies that the crystallinity was not affected by the different section thickness or solid. In contrast, the crystallinity became increasingly higher when each of individual specimens was heat treated compared to non-heat treated specimens.

In addition, the peak melting temperature, as revealed in Figure 6-37 for the sintered material, illustrated that for each different section thickness and solid, non-

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heat treated or heat treated specimens, the peak melting temperature was significantly different and the peak temperature of the heat treated specimens were higher compared to non-heat treated specimens. This is possible, because of the variation in crystallinity of each different section thickness and solid. The peak melting temperature of heat treated specimens was higher due to the increase in crystallinity when the specimens were heat treated.

Regarding the closed and open hollow structures, there was no significant difference because in the un-sintered area from both specimens there were no melted particles due to the design of the specimens. This means the original (input) powders were still in the same particle condition (see Figure 6-18, morphology of particle), even though the specimens were heat treated. In the SLS process, the laser will scan the design area only (the outline parameter/profile of the parts), as a sintering process to melt a powder particle when components or parts are produced.

Figures 6-11 to 6-13 show the results of the 2mm section thickness; the closed and open hollow structure confirm that there was no impact, as there was no significant difference of their mechanical properties for both results. It can be described that the mechanical properties occurs as an effect of force on the sintered area. In the closed hollow structure, the original powder material is enclosed in the hollow area; it is not sintered. This indicates, that this material can be observed as the original powder. Meanwhile, for the open hollow structure, the original powder was also not sintered during the SLS process. This trapped powder, which was the original powder that was removed prior to conducting the tensile test. Therefore, it can be seen that the force on the sintered area was similar. As a result, with respect to the un- sintered area, this area has become un-melted (particle) material. In addition, this un- melted material does not fuse with the sintered material area; therefore, when the reaction force occurred during the tensile test, there was no distribution of force on this area.

In this research, the specimens that were used were round bars with hollow structures, which confirmed that the larger sintered area gave higher tensile properties. This case is different with the results demonstrated by Majewski and Hopkinson [2011] that used dog bone as the specimens in their investigation. Their results demonstrated that there was no influence of section thickness on the mechanical properties [Majewski and Hopkinson, 2011].

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However, from Majewski and Hopkinson [2011] and this particular research, the results confirm that there were no effects on the tensile properties, even though there was a small increase in crystallinity at the larger section thickness. This is because slight increase in crystallinity is not enough to have a significant effect on them.