# Realization of Impression Evidence with Reverse Engineering and Additive Manufacturing
## 3 Results
### 3D Reconstructions of Plastic Footwear Impression
#### 3.1.4 3D Printed Impression Models  
As-built FDM-printed impression replicas varied in build time, taking 19.5 h for the handheld LS model to 17.15 h for the iLiDAR model, reflecting differences in geometrical complexity. A layer thickness of 0.1 mm was chosen for detailed accuracy, albeit resulting in longer print times. Estimated printing costs averaged approximately USD 10 per 3D printed model. Conventional POP casting, in contrast, took 30 h and costed USD 5.  
Figure 22 depicts a comparative view of 3D printed impression replicas from models obtained by the five scanning techniques alongside a reference photo of the reference footwear outsole (Figure 22a), photo of the impression (Figure 22b), and the POP cast (Figure 22c). The surface quality of 3D printed models varied depending on the quality of the master CAD model file, mirroring the geometric clarity of the corresponding scan model. In Figure 22d, the FDMed SL scanner model exhibited superior detail levels, with clear class and individual characteristics in the 3D impression, providing confidence in correlating them with those of the reference footwear outsole.  
Figure 22: Cont.  
The 3D printed impression from the handheld LS model (Figure 22e) effectively captured characteristics with clear depth and shape. However, the photogrammetry model (Figure 22f) displayed limited clarity in detail, though maintaining the same characteristic presence and position as the reference footwear. The 3D printed Kinect v2 replica (Figure 22g) struggled to distinguish characteristics beyond the tread design. Due to the limited resolution of FDM compared to other AM techniques like SLA and SLS, and the low geometric detail of the original iiLiDAR model, the 3D printed impression replica from the iiLiDAR model performed the poorest (Figure 22h). Conversely, the casted replica (Figure 22c) could not replicate all geometric features, as it suffered from characteristic loss during the evidence recovery process, attributed to destructive pouring effects, POP properties, artifacts, and surface porosity, as previously mentioned.  
In addition to their non-destructive nature and high geometric accuracy, 3D printed impressions offer the advantage of easily remanufacturing the model many times at various scales upon request and facilitating data sharing for remote 3D printing.  
On the other hand, while FDM is often the most affordable and easy to source its materials, it is also the slowest method and tends to have higher surface roughness compared to other 3D printing processes like SLA or SLS (e.g., FDM production rate per hour \(\approx\)1.11, SLA \(\approx\) 7.09, SLS \(\approx\) 17.66). Plastic impressions are freeform, featuring tread elements with varying depths and wall angles. The freeform nature of plastic impression surfaces coupled with the inherent stair-step effect of the FDM technique contributes significantly to the surface roughness. Figure 23b provides a magnified view of a region with differing angles, highlighting distinct layer lines and contour intervals. Figure 23c shows a representation of corresponding stair-step effect inherent to FDM at different build inclinations. Stair-steps with high build inclination was represented by red circles, while stair-steps with low build inclination are denoted by green circles.  
Figure 22: A side-by-side comparison of 3D printed impression replicas build from models acquired by: (**d**) EinScan-SP structured-light 3D scanner; (**e**) HandySCAN 3D handheld laser scanner; (**f**) smartphone photogrammetry; (**g**) MS Kinect v2 RGB-D sensor; and (**h**) iPhone’s LiDAR sensor contrasted against (**a**) the reference footwear outside; (**b**) the impression; and (**c**) the POP cast.