## 4 Discussion
Secondly, reheating as the PLLA layer is deposited (T\({}_{\text{ext}}\) = 215 \({}^{\circ}\)C) is likely to push the PVA above its glass transition temperature (T\({}_{\text{g}}\) = 80 \({}^{\circ}\)C), and therefore, this layer might be expected to deform slightly under the force exerted by PLLA deposition. However, this is likely to be tempered by friction between the polymer and print surface, which might restrict the expansion in the width of the deposited layers. The distinction in thickness between PLLA layers printed on PVA at \(e_{PVA}\) = 0.04 and 0.06 might be understood by inspecting the cross-sectional optical microscopy presented in Figure 8c. Here, PLLA thickness is fairly uniform across a cross section for \(e_{PVA}\) = 0.04 but shows a pronounced central dip for \(e_{PVA}\) = 0.06 (likewise a central bump is seen in the peeled PVA). This is likely to be related to the increased extrusion of PVA at higher extrusion factors, though this effect is not observed in the unpeeled PVA layers included in Figure 5. It might be suggested that the increased width of the sacrificial layer at the higher extrusion factor experiences a greater resistance to any expansion that may be driven by the deposition of the PLLA layer, with the outer regions less restricted to expansion than the central. For a more analytical consideration of these observations, the spreading of extruded material deposited on a translating surface during 3D printing has been recently modelled through the finite element method by [PERSON] et al. (2018). Similarly, this restricted expansion can be used to explain the difference in proportionalities in PVA printed at \(e_{PVA}\) = 0.04 and 0.06 in Figure 5. A drop from 0.98 \(\pm\) 0.03 to 0.80 \(\pm\) 0.05 on increasing extrusion factor may be indicative of resistance to expansion, and results in a dependency of \(\varphi_{Layer2}\) on \(L_{PVA}\)--lower layer separations yield lower values of \(\varphi\). This is depicted in Figure 12. Two views of the nozzle moving towards (Figure 12 Left) and to the right (Figure 12 Right) of the page reference are shown. As \(L_{PVA}\) is reduced, the width of the deposited layer increases to maintain a constant volume and experiences a greater resistance to further expansion, where frictional forces scale with the contact area. Whilst the relatively uniform values of \(\varphi_{Layer2}\) presented in Figure 7 for \(e_{PVA}\) = 0.04 somewhat highlights a lack of this effect at lower extrusion factors, it is clearly visible in the linear proportionality of \(\varphi\) at \(L_{PLLA}\) < 225 \(\upmu\)m for \(e_{PVA}\) = 0.06. Similar variation in \(\varphi_{\text{layer2}}\) is evident in the variation in offset between idealised nozzle motion and layer cross section reported by [PERSON] et al. (2018). In the experiments reported here, the printer bed is only nominally level. Further, this printer incorporates an inbuilt'mesh-bed-levelling' function that automatically adjusts for deviations in bed calibration, but this could vary by tens of microns between prints. A lack of first layer consistency, the reason for the suggestion of this technique, is also an issue--it is difficult to measure an exact point for z = 0. A zero point with an error of \(\pm\)30 \(\upmu\)m might serve as a rough estimate. Aside from the clear demonstration of repeatable sample thicknesses, there is, therefore, a limitation on what might be concluded from the absolute measurements of layer thickness and therefore \(\varphi_{Layer2}\) in this paper. Rather, the finer points of discussion of layer offsets are presented conceptually--fully quantitative analysis could be achieved with a more precise and accurate printer. The technique of bed levelling is emphasised in Figure 9, where sacrificial PVA layers were used to support the printing of PLLA on a damaged Bakelite block. Here, uniform PLLA layers are produced despite exaggerated surface indentation (\(\sim\)1 mm).