## 3 Results
### Printting PVA Directly onto the Bed  
The effectiveness of a PVA sacrificial layer in reducing the dependency of layer thickness on print bed misalignment and/or poor calibration is explored in Figures 5-8. The variation in thickness of PVA printed directly onto the printer bed at layer separations, \(L_{PVA}\), between 150 \(\upmu\)m and 325 \(\upmu\)m is shown in Figure 5 for values of \(e_{PVA}\) = 0.04 and 0.06. Proportionalities of 0.98 \(\pm\) 0.03 and 0.80 \(\pm\) 0.05 were measured for \(e_{PVA}\) = 0.04 and 0.06, respectively. A greater degree of variation in sample thickness for a given layer separation was observed for \(e_{PVA}\) = 0.06, especially at larger layer separations. These values are different from those reported for PLLA in Figure 4, indicating a material and printing condition dependence for the effect of programmed layer separation on sample thickness.  
The change in the cross-sectional aspect ratio of the printed line with increasing \(L_{PVA}\) can be seen Figure 6. Sample width is reduced and surface curvature increased as layer separation is raised. This might limit the range of \(L_{PVA}\) that would successfully support a layer of PLLA. However, \(L_{PVA}\) of 150 \(\upmu\)m through to 275 \(\upmu\)m are shown to have sections flat and wide enough (\(\geq\)400 \(\upmu\)m) to provide a reasonable surface for printing. An increase in sample width was observed on increasing \(e_{PVA}\) from 0.04 to 0.06 due to a greater volume of polymer deposited.  
The PVA thicknesses presented in Figure 5 were used to calculate the variation in \(\varphi_{Layer2}\) with layer separation, \(L_{PVA}\), which is shown in Figure 7. First considering \(e_{PVA}\) = 0.04 at higher layer separations, \(\varphi_{Layer2}\) appears fairly uniform, with a mean value between 35 and 40 \(\upmu\)m--as would be expected given a thickness to layer separation proportionality of 0.98 \(\pm\) 0.03 reported in Figure 5. At lower layer separations, for example, \(L_{PVA}\) = 150 \(\upmu\)m,  
Figure 4: A plot to show the measured layer thickness of PLLA deposited directly onto the printer bed at layer separations \(L_{PLLA}\) = 150 to 275 \(\upmu\)m at \(e_{PLLA}\) = 0.02. A gradient of 0.89 \(\pm\) 0.03 in PLLA variation is shown, compared with a trend of one-to-one variation in thickness with layer separation (indicated by the black dotted line).  
\(\varphi_{Layer2}\) is reduced to a mean value close to 25 \(\upmu\)m. A similar trend is observed for \(e_{PVA}\) = 0.06, albeit with a much larger variance. At higher layer separations, \(L_{PVA}\) = 225 \(\upmu\)m, \(\varphi_{Layer2}\) approaches a value between 40 and 50 \(\upmu\)m. At lower layer separations, \(L_{PVA}\) < 225 \(\upmu\)m, \(\varphi_{Layer2}\) reduces linearly with decreasing layer separation.  
Figure 5: The variation in measured PVA thickness for layer separations (\(L_{PVA}\)) between 150 and 325 \(\upmu\)m. Gradients of 0.98 \(\pm\) 0.03 and 0.80 \(\pm\) 0.05 are shown for \(e_{PVA}\) = 0.04 and 0.06, respectively. A trend of one-to-one variation in thickness with layer separation is represented by a black dotted line.  
Figure 6: Cross-sectional optical microscopy of PVA layers, \(e_{PVA}\) = 0.04 and \(e_{PVA}\) = 0.06, for layer separations \(L_{PVA}\) = 150 to 325 \(\upmu\)m.