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DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

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structure before and after drying, with a filament size of 200 µm. Figure 3.6a illustrates<br />

the lattice structure immediately after removal from the oil bath. An aluminum oxide<br />

plate is used as the substrate during printing. The structure consists of 20 layers with each<br />

layer having a perimeter box with a thickness of 3 filaments (c.a., 0.6 mm) and an array<br />

of x or y oriented rods spaced at 0.5mm to leave a 0.3 mm gap. Upon removal from the<br />

oil bath, the structure retains some oil in the interstitial space of the lattice.<br />

In figure 3.6b, the water and oil have evaporated under ambient conditions. Shape<br />

retention of the lattice structure is reasonably well without visually discernible cracking<br />

of local structures. It implies good interlocking of particles to resist the capillary force<br />

exerted by liquid meniscus during drying. The structure in this state can be handled easily<br />

with forceps or fingers and is comparable in strength to a ceramic green body that has<br />

low binder content.<br />

3.3.6 Shape Retention and Drying Shrinkage<br />

The carbon black ink, though with a lower shear elastic modulus (G') magnitude<br />

in the plateau region compared to the HA ink, displays good shape retention during<br />

printing and subsequent drying processes; but for each ceramic ink, a carbon black ink<br />

has to be tailored to confer compatible drying shrinkage.<br />

For assembling a fugitive support structure with spanning feature, the ink<br />

elasticity is directly related to the shape of spanning features along with the buoyant force<br />

compensated weight of the span. 30 The self weight of the span is partially offset by the<br />

difference in specific gravity of the ink filament and the fluid bath into which it is printed<br />

(i.e., the oil bath in Robocasting). Since the carbon black ink has a lower specific gravity<br />

98

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