In order to obtain a good final product, it is necessary to consider the properties of the slurry and the process parameters. Depending on the material, different results could be obtained with respect to the possibility of printing holes of a certain size or overhangs.
In order to discover strengths and weaknesses of the slurry, some additional tests were performed, as summarized in Table 3.
Table 3. Virtual Tray of fab-file for the LithaBone 480E.
1. Wall Thickness
2. Aspect Ratio
96 3. Overhangs
4. Minimal Feature and
overpolymerization
3.4.1 Wall Thickness
The term "wall thickness" literally means the distance between a surface and its opposite. Referring to porous scaffolds, the concept can be exemplified by taking an anatomical term and, by wall thickness, indicating the thickness of a single "trabecula".
This parameter is important to ensure that the final 3D trabecular structure is not formed by trabeculae that are too fragile and unsuitable for the printing parameters. Low wall thickness values (i.e. very thin trabeculae) can be obtained if the slurry has good stiffness properties; however, thin trabeculae are very fragile and could break during printing. In order to evaluate the wall thickness, cylinders with a height of 10 mm and diameter from 1 mm to 10 mm were produced.
The minimum wall thickness was evaluated by considering the non-sintered cylinder with the smallest printed diameter without defects, while the greatest diameter of the non-sintered sample
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presenting no cracks after sintering was considered to calculate the maximum wall thickness.
Cracks and other defects can often be due to a non-volatile binder which fails to evaporate completely, thus generating stress that could lead to breakage.
3.4.2 Aspect Ratio
The aspect ratio is the ratio between the height and the diameter of the sample in question. In order to evaluate the maximum aspect ratio, 18 square base bars and 18 cylindrical base bars were constructed, all with a height of 15 mm and a diameter/perimeter from a minimum of 0.833 mm to a maximum of 1.5 mm, and an aspect ratio from 2 to 18.
The samples were examined under an optical microscope before sintering to evaluate the presence of cracks on the surface.
In order to increase the visibility of any defects, the light was directed obliquely on the samples.
The maximum aspect ratio will be considered to be that for which the sample has no surface blur.
3.4.3 Overhangs
The purpose of this test was to evaluate the achievability and printing limitations of overhangs and pores.
Overhangs represent one of the most difficult challenges for SLA because their presence increases the risk of obtaining an unstructured final object.
Two types of overhangs were considered:
- H-overhangs (Figure 16): the achievability of printing overhangs connected to a supporting structure on both sides was evaluated. The various overhangs differed from each other both in length and thickness. In this case, printed and sintered samples were checked for fractures and conformity to the desired shape without any detachment from the vertical supporting structure.
Figure 16. H-Design of overhangs. The numbers listed horizontally indicate the length of the different overhangs (in mm); the numbers listed vertically indicate their thickness (in mm).
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- T-overhangs (Figure 17): these overhangs are only connected to the supporting structure on one side and therefore have a much higher probability than the H - overhangs of deforming and detaching during printing.
In this case, the ability to maintain parallelism between the overhangs and the building platform was evaluated. If the speed of polymerization and the stiffness of the material is optimized, the overhangs will be printed correctly; otherwise, they may be structurally inaccurate, deformed (mainly at the outer end) and/or even broken.
Figure 17. T – Overhangs Design.
- Diameter (Figure 18): it is often necessary to make porous scaffolds, especially when they are intended for use in BTE, as in this case. It is therefore essential to understand to what extent small or large pores with a certain diameter may be achieved.
The purpose of this test was to ascertain whether it was possible to print the desired diameter with a precise circumference and no grooves. Obtaining holes with serrations is frequent when the diameters are relatively large, whereas small pores must be checked for occlusions.
Figure 18. Diameter Design.
3.4.4 Minimal feature and overpolymerization
In order to check Minimal Wall Thickness, two identical test bases with many small protrusions of increasing width from left to right were printed.
The theoretical width of the pillars (μm) is given by the number of pixels (Figure 19) multiplied by the pixel size (machine dependent);
Given the fragility and the small size of the protrusions, the sample, once printed, is not detached from the building platform but cleaned directly above it and then evaluated under an optical microscope.
99 This test assesses:
- Material stiffness: Low stiffness values mean that smaller pillars are not printed correctly.
The test is considered positive if all small protrusions adhere perfectly to the building platform and do not show any defects and/or deformations.
- Printing accuracy and overpolymerization: overpolymerization problems that may be due to an excessive amount of photoinitiator in the slurry formula or an overlong exposure to light.
Pixel-dependent overpolarization values are calculated with respect to the width measured in μm. If possible, the width of the pillars is measured at different points of the central area.
Figure 19 . Minimal features illustation (the numeration corresponds to the number of pixels).
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