PSI - Issue 64

Zohaib Hassan et al. / Procedia Structural Integrity 64 (2024) 1184–1191 Hassan / Structural Integrity Procedia 00 (2024) 000 – 000

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superplasticizer. Some commercial superplasticizers yield short open time as compared to others. Figure 6 shows the printing of hollow square geometry using superplasticizer Type-2. Material consistency remained the same during printing, even with a delayed start, 30 minutes after mixing. No layer shortening or breaking of the layer was observed. Hence, the superplasticizer yielding large open time was found to be necessary to minimize the layer shorting and breaking during printing at constant accelerator dosage optimized at the start of printing.

Fig. 3. Flow vs. Time (adopted from Hassan et al., 2024)

3.2. Inconsistency in printing The second lab scale print was performed with variable accelerator dosages to regulate the flow during the printing of hollow cylindrical geometry after 30 minutes when mixture rheology was continuously changing. Accelerator dosage was decreased to keep the print quality high by monitoring the surface cracking and wetness. As the variation in rheology is not linear, neither is the change in accelerator dosage. These two parameters should be well synchronized to get consistent material after extrusion. This is practically challenging and leads to extra accelerator dosage than required, causing inconsistent printing observed during the second lab-scale print, as shown in Figure 5. A complete failure was observed in the middle of the second lab-scale printing after 40% of the print when the decrease in accelerator dosage caused a large flow, making it challenging to continue printing. This problem was rectified by avoiding the change in accelerator dosage during printing in the third lab-scale print using the superplasticizer, which yields a large open time. 3.3. Dry layer with surface cracking A non-wet surface is another issue observed when the accelerator becomes higher than optimum, mainly in the second half of the first lab-scale print, as shown in Figure 4. The rheology of the mixture started changing at a fast rate during the second half of the printing. Just before the shortening and breaking of the layers, a defect described in section 3.1, the surface of the filament turned visibly dry with some surface cracking. These cracks were only on the surface of the layer. Dry and non-wet surfaces may create a weaker bond than laying a wet layer on the wet surface. Again, a large open time with less influence of rheology change to avoid the regulation of accelerator dosage during printing is one of the solutions. The same solution was exercised in the third lab-scale print, where the surface of the printing layer was wet without any visible cracks, as shown in Figure 6. This was achieved by using the optimum dosage of accelerator admixture set at the start of printing.