3D Modeling
The virtual design of the molds was a critical step that was only possible with this technological methodology. This process made it possible to analyze the level, position, and depth of each of the fragments that make up the mold. This ensured a correct and precise construction of the digitized object. This was taken into account during the design at all times to ensure an easy and safe disassembly of the positive to be obtained with the waxy material. The Boolean tool greatly facilitated the 3D modeling making it possible to obtain a reliable copy of the fragments to be reproduced. The modeling proved to be very fluid while the whole process went quite fast once the necessary design for each piece had been established (Fig. 1). Finally, because the scanned model offers a 1:1 scale of work that is also applied to the molds, great volumetric precision was achieved.
3D Printing
The 3D printing tests of molds to determine the best position and orientation of the parts have given the best results when carried out with PLA filament. Firstly, the horizontal arrangement of the mold along its main axis, and with its base forming a 0º angle with the printing platform provided poor quality in the details of certain areas of the mold. This occurred because during the printing process some parts require support structures to hold areas that are not in contact with the plate and these supports are difficult to remove without leaving marks. Once the copy is finished, these supports are sometimes difficult to remove without leaving marks and for this reason it is preferable to look for positions in which the least amount of them is created. In this case, several threads in complex areas were not correctly deposited and fused despite the fact that the layers, in general, have been faithfully placed without highlighting the lines of each one of them (Fig. 2a). The horizontal position with the base at 45° to the printing platform gave very good results. Printed at this angle, the previously altered areas were not affected, as the deposition of the filament was carried out with great precision, without the need for structures. Regarding the printing lines of each layer, these are practically unnoticeable (Fig. 2b). The horizontal position of the printing platform with the base at 90° made it possible to obtain adequate results in terms of detailing complex areas. However, by depositing the filament perpendicularly on most of the mold, the lines were very conspicuous, greatly reducing surface detail (Fig. 2c).
Regarding the vertical position with the various angles of 0º, 45º and 90º concerning the printing base, the results were very similar to those obtained in the horizontal position. The angle at 0º generated some more visible imperfections compared to the horizontal position, negatively affecting the quality of the mold (Fig. 2d). As for the 45º angle, good results were achieved in general, although it should be noted that the filament in this position was more evident than in the horizontal position, producing irregularities (Fig. 2e). Finally, using an angle of 90º, the filament was very marked, practically the same as in the horizontal position (Fig. 2f). The printing times for the different angles were almost identical in the two positions. The 90° angle was the fastest by taking 651 min to print. The 0º angle took 888 min to print, and the 45º angle was the longest, taking 1051 min.
Once the first phase was completed, the casting positives obtained from the molds were analyzed. The horizontal positive at 0º recorded several lines of the mold impression, not achieving adequate results for the purpose of this study (Fig. 3a and d). However, with the horizontal position at 45º, a uniform surface finish was achieved, with hardly any imperfections in the mold (Fig. 3b and e). On the other hand, the 90° positive recorded the obvious lines of the impression (Fig. 3c and f), considerably reducing the surface details of the model. Drawing from these characteristics, its use was discarded. As for the vertical positions, all three recorded the imperfections of the molds. The 0° and 90° positions show the most irregularities and impression lines. Of the three vertical positions, the one with the best results was the 45º position. However, some small flaws in the details became visible. Drawing from these data, the use of the vertical position with its respective angles was discarded because the expected results were not obtained. In all six cases, the use of petroleum jelly made possible the removal of the positive from the mold very easily while avoiding the use of force or any other operation during the handling of the parts to remove the casting positive that could cause minor damage or breakage.
Drawing from obtained data that includes both the analysis of the molds and the wax positives, it was confirmed that the molds printed in the horizontal and vertical position of 45º met the appropriate characteristics to solve the case study proposed in this paper. They both achieved optimum results in terms of the quality of the deposition of the filament per layer and the reproduction of the details. Of the two positions, the horizontal position provided the best performance. At a 45° angle, there is very little surface area on the inner side of the mold perpendicular to the printing base, which means that the layer lines are not noticeably marked. This is how a high level of detail was achieved over most of the mold. For these reasons, it was considered appropriate to use the horizontal 45° position for testing the different materials (Table 1).
In general, the ABS material achieved good results. The deposition of the filament is slightly marked and, as a result, certain fine details were not sufficiently well rendered (Fig. 4a). However, the PLA material, in contrast to ABS, was able to reproduce them. The deposition of the filament was practically unnoticeable, offering a slight improvement in the surface finish and, therefore, in the rendering of details (Fig. 4b). In turn, PLA Tough achieved slightly better results than PLA in terms of surface relief definition (Fig. 4c). In the case of PLA 3D870, as with the two previous materials, good surface finishes have been achieved, with the exception of some small imperfections (Fig. 4j).
The INNOVATEFIL PA HT material, in general, has captured the details with good quality, however, it has generated certain imperfections during the deposition of the filament in complex areas, not being able to obtain the existing details (Fig. 4d). On the other hand, the HIPS material in general has achieved a good finish, but as in the previous material, the details have been somewhat reduced due to the deposition of the filament leaving slight marks between layers (Fig. 4e).
For the PETG material, it has generally achieved good finishes, however, the deposition of the yarns again in each layer has been marked, resulting in the loss of subtle details (Fig. 4i). Another of the materials tested, ASA, has achieved very similar results to PETG. Despite achieving good overall quality, some of the details have been lost, as the deposition of the filament is very evident (Fig. 4k).
In one of the semi-rigid materials, specifically, PP, when depositing the filament, these lines did not achieve a uniform finish, causing some loss of detail (Fig. 4f). Some stringing has also occurred in certain areas despite setting the temperature and shrinkage rate recommended by the manufacturer. This phenomenon occurs because during printing movements the extruder tip drips a small amount of material and this causes the generation of fine threads that are distributed over the entire printing area. Flexible materials, such as TPU95 A and FLEX, generated constant stringing during printing (Fig. 4g and h). Despite setting the temperature and the shrinkage speed recommended by the manufacturer, this problem has remained constant during the printing process. As a result, a significant loss of detail has occurred. As for the INNOVATEFIL POLYCARBONATE material, it has not provided valid results, as the deposition of the filament is again very marked (Fig. 4l).
NYLSTRONG is one of the materials that achieved really good finishes (Fig. 4m). The big drawback of this product is the constant generation of cracking, or delaminations, between the layers. The printing parameters recommended by the manufacturer made possible a perfect fusion. However, it has been observed that temperature is a factor to be taken into account, as fluctuations in this value favor the delamination process and increase the tendency of the mold to crack. Despite the good results in the reproduction of details, the difficulty of printing was very high. Regarding the PVA material, it provided finishes that were below expectations because it requires very specific storage conditions. It was observed that any small variation in temperature or humidity slightly modifies the surface finishes (Fig. 4n). Finally, the printing time for each of the molds in the horizontal position with the base at 45º to the printing platform ranged between 747 and 1169 min. Although the position selected at a 45º angle took the longest to print compared to the other angles that were tested, it is undoubtedly the one with the best quality on the final finishes.
During the processing of the wax positive, it was possible to corroborate the data obtained previously (Fig. 5). In the case of PLA (Fig. 5b), PLA Tough (Fig. 5c), and PLA 3D870 (Fig. 5j), the results were very similar. Of the three, the PLA Tough performed the best because it shows a slightly better finish than the other two. Consequently, materials such as ABS (Fig. 5a), HIPS (Fig. 5e), PETG (Fig. 5i), ASA (Fig. 5k), and INNOVATEFIL POLYCARBONATE (Fig. 5l), tend to record filament at greater or lesser extent causing the loss of small details. Other materials such as INNOVATEFIL PA HT (Fig. 5d) have shown good detail in almost all areas of the mold, with the exception of a few finer and more complex parts. Regarding flexible materials, in the TPU 95 A (Fig. 5g) and in FLEX (Fig. 5h), the generation of a stringing caused a significant loss of definition, the reason why these products were discarded. In the case of NYLSTRONG, the details obtained were very good, but the constant generation of cracking is a clear reason why its use was discarded (Fig. 5m). Finally, water-soluble PVA filament was tested to see if it could be used to make reproductions using the lost mold technique, that is, by dissolving the cast to extract the copy. After several tests, the mold was worn and the surface was altered due to the temperature of the wax during pouring. This resulted in a considerable loss of detail, which led to discarding it as a possible option (Figs. 5n and 6).
Drawing from the obtained results, it was concluded that the thermoplastic materials (PLA, PLA Thought and PLA 3D870) offered the best finishes. Among them, PLA Thought has shown the best performance both during mold printing and through the process of obtaining the wax positive. The complete reproduction of details and the ease with which the positive can be extracted from the mold make it a very suitable material for 3D printing molds, ensuring high quality and fidelity in the production of the wax facsimile.
One factor that was taken into account during this study was the possible alteration of the properties of the materials used to create the molds due to thermal degradation. This factor is determinant when 3D printed objects have a support or mechanical function where its use is acting under different pressure forces. Large forces are required depending on the printed volume to alter the mechanical properties by thermal degradation until breakage. This is analyzed by mechanical tests, either tensile or flexural. When a certain force is applied at a given point the material undergoes a deformation and therefore a temperature increase that eventually produces a permanent deformation in the material [23, 24]. However, in our study, the molds were not subjected to bending or tensile forces at any time, so deformation was not expected. However, since the reproduction material we were going to use in the molds was molten wax, we performed a temperature test using specimens of different wall thickness to analyze the deformation of the different materials that had given the best results in terms of reproduction accuracy. The wax was poured at 75ºC on 0.5, 1 and 1.5 mm specimens (This last mentioned thickness is the one that was established to the wall of the molds), and no degradation or deformation of the material was observed (Figs. 7 and 8) considering that PLA melts at 180ºC approximately.
Creation of the facsimile
The elaboration of the facsimile of the anatomical artifact consists of six fragments. These are: (1) little finger, (2) ring finger, (3) middle finger, (4) right foot of the fetus, (5) left hand, and (6) the protruding part of the fetus, vagina, uterus and placenta (Fig. 6a1). Because not all fragments present the same difficulty in reproducing their volume, the less complex and smaller fragments have been developed in two pieces (Fig. 9b1, b2, c1, c2, d1, d2, e1 and e2), with the more complicated parts consisting of 9 (Fig. 9f1 and f2) and 14 (Fig. 9g1 and g2) pieces.
The use of elastic tapes and plasticine on the outer joints of each piece that made up the mold (Fig. 10a and b) has provided great resistance to the whole assembly. This prevented certain movements during handling that could cause the pieces to dislodge minimally. With regard to the wax casting in layers in the corresponding fragments of each mold, the wax was correctly distributed over the entire surface until the solid piece was achieved (Fig. 10a–c).
The result obtained has greatly exceeded the initial expectations, as all the details, even the most minute ones, have been recorded with high quality. Once all the molds had been removed (Fig. 10d), some small cracks in the joining areas were reworked with different instruments (Fig. 10e). The results obtained in the facsimile show a very high level of similarity with the original work (Fig. 10f and g), reproducing all the surface Details.
Obtaining the facsimile has allowed a deeper understanding of the problems of intervention, the methods of approach, and needs and difficulties when dealing with each treatment.
This made it easier to make decisions regarding the treatment of the real work. The tools used in this research can help professionals in the field of conservation and restoration of cultural heritage to reduce manipulations during interventions, to better understand the constructive technique of the models, and to test and plan conservation strategies with determination.
