In the context of casting footwear impressions, the dimensional stability and compressive strength of foams are of paramount importance. Figure 3 displays the average values of dimensional stability in three directions (length, width, and thickness) of specimens. It is evident that lower temperatures result in slightly higher dimensional changes. When temperatures vary above zero, the dimensional changes in three directions for the polymer are significantly smaller. Relative changes in length and width range from 0.1–0.13%, and in thickness from 0.2–0.22%. The lowest linear changes occur at 23°C and 70% relative humidity. For an expansion rate of 0.22% (the maximum value), a 300mm long cast would expand by 0.66mm. This minor expansion is negligible and undetectable in practical tests. Additionally, all linear changes in foam rise (thickness) and traverse (width and thickness) directions show no significant differences, indicating that the cellular structure is predominantly spherical (Fig. 5b). For all practical purposes, the polymer foam casting material is dimensionally accurate when the foam cast is removed outdoors and stored indoors.
The compressive strength of polyurethane foams, tested at -10°C, 0°C, 23°C, 40°C, 70°C, and 70% relative humidity for 48 hours, is shown in Fig. 4. It is observed that temperature variations do not have a significant effect on the compression strength (at 10% strain). Compressive strength heavily depends on the cellular structure of the resulting foam [12–14] and is a measure of the foam's ability to deform under loading. Figure 5b illustrates a foaming cross-linking network provided by the polymerization reaction with the formation of polyurea segments [15], resulting in stronger hydrogen bonding interactions and high stiffness [16].
Figure 4 Plot of compressive strength of the rigid polyurethane foam to temperature which has been at 70% relative humidity for 2 days
Molded foam casts were produced by pouring the reaction mixture into a covered mold. As the rising foam reached the mold cover and impressions in the soil, its progress was restricted. Overfilling of the mold resulted in a measured positive pressure inside the mold as the foam gelled and cured [17]. The pressure in the mold and the equalization of this pressure throughout the closed mold before the foam hardened accurately produced the shape of footwear impressions in the soil. The produced foams were lightweight, with density variations between the top and middle sections [12, 18]. Micrographs of the foam's surface and closed cellular structure mid-section are shown in Fig. 5. Figure 5a displays a dense outer layer at the foam surface, with flaw sizes decreasing to 100 µm, forming the basis of the casting design for recovering a 3D footwear impression. The dense surfaces of foam blocks are caused by the existence at the foam-impressions interface of a thin layer of noncellular polymer [19–24]. The uniformly distributed, typical closed-cell cross-section beneath the skin layer is shown in Fig. 5b. The porous mid-section can be explained by a nucleation and growth mechanism, where the polymer blowing reaction rate is faster than formation [18, 20].
With only photographs, an examiner would have limited views of the features. Making a cast of 3D impressions should be a regular practice, even in cases where photographed impressions appear to have limited details [5, 21–23]. The foam possesses positive features such as high dimensional and mechanical properties stability, ease of deformation and handling, making it an alternative material for the accurate reproduction of 3D footwear impressions. The raw data of foam casting on the characteristics of soil-origin shoeprints are shown in Fig. 6. For soil, the foams perform most successfully, with a greater number of features identified using polyurethane foams present on the shoeprints.
The images in Figs. 7 and 8 depict two of these casts to demonstrate the versatility of this new process and material. The footwear marks developed on each casting were compared with the twenty-eight characteristics present on the known shoe in Fig. 1. Overall, all the impression characteristics developed using polyurethane foams were entirely visible in Fig. 7. In a comprehensive comparison, characteristics obtained by the plaster of Paris casting were lighter and more faded around the perimeter in Fig. 8. For example, in Fig. 7 No.1,11, the tiny lines were clearly seen with greater detail in the cast than in Fig. 8. Additionally, the embossments No.16,17, and 18 were completely shadowed out in Fig. 8. The results show that the more tiny footwear marks deposited on soil, the clearer the polyurethane foam casting lifted.
FIG. 7 Characteristics on cast prepared by the rigid polyurethane foam
Plaster of Paris is the beta form of calcium sulfate hemihydrate (CaSO4 × ½ H2O); adding water will result in gypsum (CaSO4 ×2H2O). Casting materials such as gypsum have sufficient weight to erode and destroy valuable details when poured onto the impression, especially critical in the case of impressions in fragile soils [5, 20, 24]. However, the foam casting material was manufactured by the reaction of isocyanate and polyol, which has gas expansion inside, then solidified in foam state [16, 20].
The 3D polyurethane network was formed to create the footwear impression cast. The overall results indicate a noticeable difference in the impressions left on the foam casts. The rigid polyurethane foam demonstrated excellent stability in both dimensional and mechanical aspects, and its easy moldability allowed for the revelation of details that were previously hidden in the original footwear impressions.