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3D printing: State of the art and future perspectives
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Review
3D printing: State of the art and future perspectives Caterina Balletti ∗ , Martina Ballarin , Francesco Guerra Dipartimento di Architettura Costruzione Conservazione, Università Iuav di Venezia, s. croce 191, 30135 Venezia, Italy
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Article history: Received 31 August 2016 Accepted 17 February 2017 Available online xxx Keywords: 3D printing Cultural heritage Preservation Explorations Accessibility Museums
a b s t r a c t In the last years, the development of 3D technologies applied to the field of Cultural Heritage (CH) has led to results of the utmost importance from the point of view of preservation, valorisation, communication and fruition of our assets. In particular, we experienced many interdisciplinary projects in which, thanks to the cooperation of different fields of research, incredible results have been obtained, through the technological collaboration of computer graphics and documentation, of industrial engineering and preservation and access of CH. This paper aims at drawing attention to the actual technologies in use for solid printing (digital fabrication) used for the realization of material copies, therefore tangible, of three-dimensional digital virtual models. Even though ulterior developments to these technologies are possibilities to be expected, the process of 3D printing has gradually gained levels of accuracy, which can nowadays be deemed as satisfying. This is even more true in the industrial field (from the manufacturing industry to the design industry), but also in other fields, such as the medical one, for example, for the realization of artificial limbs, and the CH field, which can benefit from new instruments for the restoration and preservation of cultural assets in museums. The metric characteristics of precision and accuracy of the model printed with 3D technology are the fundaments for everything concerning Geomatics, and have to be related with the same characteristics of the digital model obtained through the survey analysis. In other terms, the precision of the printed product must be evaluated in relation to the precision of the instruments used in the analysis. Thus, in the CH field there is the possibility of new systems of access, cataloguing and study, where the models, both virtual and tangible, represent the fundament of visualization and analysis of the form (also from the metric point of view) of each artefact of artistic and historical interest. © 2017 Elsevier Masson SAS. All rights reserved.
1. Research aim In this paper, we present the state of art and the potential and large spectrum of applications of fabrication technologies in the CH. Through a brief history and characterization of the most common 3D printer technologies, we try to present a review of the applications of 3D printing on CH, considering our experiences or those of other researcher. In the final part of this paper, a particular aspect of solid printing is analysed: the level of accuracy reachable in the creation of material models. This level of precision must be related to that of the instruments of analysis through which the artefacts are converted into digital format. We must not forget that the process that leads to the realisation of a material copy must go through a numeric model and that in this process there is a progressive loss of definition, both from the qualitative and quantitative points of view. The simplification operations that the digital data
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must undergo might cause a difference between the geometry of the printed object and that of the original object. 2. Introduction Digital technologies are able to offer an essential contribution to the documentation, analysis and subsequent use of the cultural assets, as they can be used in different forms and for different aims: study and research, diagnosis, repair, preservation, protection, communication-divulgation, fruition and formation of the cultural heritage. In the last years the use of electronic and Information Technologies (IT) has increased exponentially, creating new sceneries and possibilities in the field of the CH. This evolution of instruments and methods is in partnership with a diffusion of instrumental techniques for surveys; in particular, the 3D scan, which allows observation of complex geometries impossible to analyse through traditional methods. Solid printing has gained a special role in this technological development. The rapid creation of prototypes is a technique that
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allows the production of material copies of objects with complex geometries directly from the mathematical model in relatively short periods of time and often without being expensive. In recent years, this technique has experienced a very strong development thanks to the large diffusion on the market of desktop 3D printers, printers which are quite cheap and whose dimensions are reasonable. They use a FDM technology (Fuse Deposition Modelling), an additive kind of technology, as will be described later, which creates material models through the superimposition of material layers. This innovative system has allowed a rapid growth of this technology for ‘commercial’ uses, allowing less expert users to enter the world of 3D printing, creating a community of makers, comparable to the one which formed after the introduction of Arduino [1] to the general market. The material objects are realised quickly and with quite low budgets, starting from numeric models, often with complex geometries, converted in G-code, which is the programming language of the machines working with numeric control (CNC). We are also experiencing a reduction in size and cost of these printers (Desktop 3D printer), a process which contributed to the growth of this technology and favoured its commercialisation. The opening of FabLab in Italy (laboratories including machines for digital creation, able to transform ideas in prototypes and products and open to the general public) proves the development of this technology: in a study published at the beginning of 2015, Make in Italy reports in detail on the birth of FabLab and of makerspaces in our country and its growth in the last two years. There are more than 70 active labs spread throughout Italy, a community of more than 1000 FabLabs distributed throughout 78 countries according to the Fab Foundation, an organization that emerged from MIT’s Center for Bits & Atoms Fab Lab Program (http://www.fabfoundation.org). Thousands of people, along with associations, industries, museums, universities and institutions of all kind, are investing time, resources and energies in order to open laboratories including machines for digital fabrication to the public. 3D printers have demonstrated effectiveness in many other fields of application, in particular that of the Cultural Heritage. Thanks to the recent innovations in the IT technology and multimedia it is now possible to develop new forms of analysis and fruition of the Cultural Heritage, which are used along with more traditional methods. Models, first digital, then material, have introduced new possibilities of access, cataloguing and study of the cultural assets as they form the basis for both the visualization and the metric analysis of any artefact both from the artistic and historical point of view. As far as museums are concerned, for example, there is the possibility of creating identical replicas, both digital and material, of spaces and three-dimensional objects. Exhibitions and collections can take advantage from digital fabrication as the access to their information becomes customised according to the user, the content and the complexity of the given message. All this brought 3D printing to repute as one of the most important possible outputs, at the same level of more traditional digital or paper.
3. The subtraction and addition process In order to realise an object we might use two techniques radically different from each other: the subtractive, referring today to Computer Numerically Controlled (CNC) machining, and the additive, concerning the Additive Manufacturing (AM) processes [2]. AM, popularly called 3D printing, technologies today are used by makers all over the world, but its inception can be traced back in the 1980s, at which time it was called Rapid Prototyping (RP) [3]. RP was conceived as a fast and more cost-effective method for prototypes realization for product development within the industry [4].
Before describing AM main technologies, we want to fix some important dates to tell shortly how 3D printing was born: • in 1984 – Chuck Hull invented and patented a sterolithography aparatus (SLA) machine. Hull went on to co-found 3D Systems, the first organization nowadays operating in 3D printing. The STL format file was born; • in 1986 – Carl Deckard, Joe Beaman and Paul Forderhase (with other researchers) developed the ideas of Chuck Hull and filed a patent in the US for the selective Laser Sintering (SLS); • in 1988 Crump patented the Fused deposition modelling – which is printing with fuse material. This technique does not involve the use of laser or dust and uses fused plastic to spread in strata to create the object. Crump also founded Stratasys, another leading business in the field; • 1993 – was patented the Electron beam melting (EBM); • 2005 – Mcor Technologies Ltd – an Irish company – starts the Paper 3D laminated printing: a machine, which superimposes sheets of paper and prints on them. The result is an additive method, which includes the use of colours; • in 2005, thanks to the technology of the Self replicating rapid Prototyper a 3D printer which prints itself is first realised (opensource RepRap and FAB@Home projects). The RepRap Project [5] is an abbreviation Replicating Rapid Prototyper, and it aims to develop a 3D printer, which prints on its own the majority of its own components. All the products created with this project are published with open source licences; • in 2008, Bre Pettis, Adam Mayer, and Zach “Hoeken” Smith found MakerBot Industries. Digital fabrication technology is characterized by the basic physical process employed for the tangible object to be obtained. The subtraction process consists in removing the unnecessary material from a block to obtain the final object. The lathe is a tool that allows to remove the exceeding material from a block placed on a rotating platform, thanks to a series of tips of different shapes. It has been used for centuries: the first hydraulic lathe is more than 500 years old, but the Ancient Greeks and Egyptians used it as well, creating the first tools which allowed them to rotate a plate using a pivot with their feet. Modern lathes are more complicated and versatile; they use engines instead of human strength and can have quite a high level of automatism. However, they are based on the same principle with which our ancestors created the first vases regularly shaped. Milling machines are more modern machines, which allow the realization of complex products. They work very similarly to drills, but instead of creating a hole, thus using the tip of the tool, they cut laterally. The simplest milling machines work on three axes, while the most complex ones work on five or even six axes, supporting rotation around multiple axes, so that they can reach almost any part of the product. Milling machines can work with a vast range of resistant materials, from wood to metals. Lathes and milling machines are born as tools controlled manually, but in the meantime some numerically controlled machines were created, called CNC (Computer Numerical Control), and controlled by a computer which handles the passage from a 3D model realised through a CAD application (Computer-Aided Design) to the production of the object. The linking chain is a new kind of software called CAM (Computer-Aided Manufacturing), which transforms the digital model of the object in a series of commands to impart to the machine. The file format is a standard one called G-code, which includes information like N40; G82; X1500, automatically generated by specialised software. 3D printers are also CNC machines; the only difference is that instead of removing material they add it, which makes them an
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Fig. 1. FDM approach applied to the fac¸ade of S. Giorgio’s church in Venice. Supports for protruding parts are designed in the slicer software and will be removed then from the final printed model.
additive technology, allowing three-dimensional objects to be created by adding material to material layer by layer (AM). The flux followed in the additive process is analogue to the subtractive one: it begins with a numeric model in 3D and then a G-code is generated. After the model is ‘sliced’ in many thin layers with a specialised software called slicer, the G-code commands allow the machine to build the object superimposing the single strata, reaching levels of complexity which are impossible to replicate with other technologies: for example, with a milling machine it is impossible to realise an empty sphere. There are various techniques used in 3D printing; here we describe some of them briefly. The most widely used technique among printers, especially for domestic ones is the Fused Filament Fabrication type (FFF), which was born from another previous technique Fused Deposition Modeling (FDM) developed by Scott Crump at the end of the 1980s and commercialised in the 1990s by Stratasys [6]. Both techniques are based on the stratification of liquid plastic materials. Casting is based on additive principle, so that each strata once deposited is covered by the following one, forming superimpositions which can create roughness, depending on the material, on the casting dimension, on the nozzle and on the casting precision. The functioning is quite easy: a nozzle is heated in order to melt the material (filament) and can be moved on three axes, horizontally (axes X and Y) and vertically (axis Z), from an numerically controlled engine, commanded by a CAM software. The plastic filament (solid) is unrolled from the reel in which it has been put, in order to pass through a heated area with a high temperature. From here, thanks to the push of a pinion linked to the stepper engine, the liquid plastic is expelled from the nozzle, where the flux can be started and stopped. The framework which supports the structure of the plate (where the material to form the 3D object is collected) and of the engines must be controlled on a control sheet with a driver for the so called stepper motors. These engines are used to obtain increased velocity and precision and because they guarantee an elevated torque motor and a very satisfying twisting movement. There are different parameters that cooperate in creating the quality of the finished product. First of all is the kind of material used: PolyLactic Acid (PLA, which is a vegetable, biodegradable plastic), or ABS (Acrylonitrile butadiene styrene, a petrol derived from plastic, useful to realise mechanic objects thanks to its high resistance), and the section of the filament, obtaining resolutions ranging from 100 to 300 micron. It is also possible to use printers with more than one nozzle, which allow the use of different materials and colours. The FDM technique also requires the creation of a support structure (Fig. 1) during the creation of the object, especially for the empty spaces or projections. These are supports projected directly
from the software of the printer. At the end of the production, it will be necessary to remove all the additional supports generated and not included in the original drawing. The printers with one nozzle create supports with the same material of the final object, which implies that the supports must be removed once the print is complete, meaning that this removal might not be precise and sometimes it might damage the quality of the final product. The printers with two nozzles on the other hand might create the additional supports with filaments made of different material from the principal object, which means they are not fused and thus more easily removable, even manually. Some materials used to create the supports can also be made out of soluble materials, which would dissolve in water or other liquid substances. In this case, the removal of the support would be ‘chemical’. The result of a FDM print is not comparable to the quality level obtained with other industrial techniques involving laser, but the value for money is surely closer to the expectation of the public. Another industrial technique is Selective Laser Sintering (SLS): it is a technique which uses a laser as a sintering source of plastic material (polycarbonate, nylon, ABS), ceramic dust or a metallic one covered with thermoplastic resins which mingle with each other if heated (laser beam). The laser is conducted in the points of space defined by a 3D model in order to create a solid structure through sintering. Different from the process of fusion, the thermic process of sintering is intended for the use of dusts. The final product (Fig. 2) is completely smooth without the conventional roughness (though minimal) of the FDM technique. The main advantages of these techniques are: • the possibility to use different materials; • support structures are not necessary to create pieces with undercuts as the very dusts are deposited in strata and are not sintered to work as a support. The un-sintered dusts are to be removed at the end of the production process, though they are partly reusable. Sterolythography (SLA) is a printing technique that uses a tank filled with a particular liquid resin able to polymerize if exposed to light. A laser is projected through a system of mirrors in order to complete a scan of the surface. A piston lowers the tank with the liquid resin for the following scans. The scanning process repeats itself until the three-dimensional object is completed (Fig. 3). At the end, the object is taken from the liquid resin and exposed to ultraviolet light to complete the polymerization process. As for the FDM technique, a limited set of materials can be used and it requires the removal of supports from the finished product. Laminated Object Manufacturing (LOM): this technology uses a millworking system to obtain thin layers of material, later selected
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Fig. 2. The hypothetical reconstruction of Guggenheim Museum in Venice (Ca’ Venier dei Leoni). The tactile model was printed with a SLS technology with a high level of decorative details.
and united. The material is formed by special sheets of paper, cut according to the need and glued one on top of the other on the previous sheet. Dimensions are quite extended, according to the product volume. The support is formed by exceeding paper and requires an ulterior passage in order to remove the unnecessary material with tools for woodcarving. In Scopigno et al. [7] authors summarize, as can be seen in Fig. 1, the fabrication technologies that we described and they present a brief qualitative evaluation, with which we agree just considering our personal direct experience. They compare the different techniques in terms of material costs, use, resolution and geometrical accuracy especially considering the CH application (Fig. 4). From personal experience and from those described in essays, it is clear that for the Cultural Heritage, these technologies have reached a good level of quality, but one has to choose the technology
which is more apt for the needed type of application, as also explained in the examples that follow.
4. Applications in the contest of CH 3D printing is leading a revolution across many sectors and a wide range of applications, including those that until a few years ago would not have even been considered appropriate for this technology: industrial production, design, health. In the medical field some important results have been achieved: engineers and physicians are able to develop 3D-printable prosthetics that are fully customized to the wearer; e.g. the German para-cycling champion Denise Schindler worked with Autodesk to become the first to compete with a fully 3D-printed prosthetic leg at the Rio
Fig. 3. SLA printing techniques: the reproduction of the bust of Francesco II Gonzaga, in the City Museum of Mantova, is taken from the liquid resin once completed and exposed to ultraviolet light to complete the polymerization process.
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Fig. 4. The comparison done by Scopigno et al. (2015) of the different techniques in terms of material costs, use, resolution and geometrical accuracy especially considering the CH application.
2016 Paralympic Games [8]. Or today complex surgical procedures can be pre-planned using 3D printed organs [9–12]. Special attention should be given to the realization reducing times and costs of CH all types of reproductions: archaeological finds, sculptures, architectural elements, paintings and artworks in general. Replicas can be used in many ways: • for study and research; • to set up alternative museum exhibitions, such as tactile museum tours for the blind and visually impaired; • for restoration, re-creating missing portions of an object; • to organize workshops with schools; • for museum merchandising (e.g. producing, with cheap reproduction technologies, accurate small-scale replicas of the artworks conserved in a museum or in a CH institution). The realization of replicas usually is through the use of rubber moulds over the original artworks, which were then used for the subsequent production of gypsum or resin copies. This process requires a direct contact with the object, sometimes risking compromising its preservation. Now, thanks to the 3D surveying methods, from laser scanning to photogrammetry [13–23], digital models are realized with a high accuracy, and 3D printing gives the possibility to obtain true copies of the original, ensuring safety of the artwork. A special consideration should be addressed on the materials to be utilized in the final use of replicas, as in the particular case of archaeological finds: these objects are characterized by a complex geometry and a high level of details, that must be preserved in order to be explored. In this case, the FDM printing is not suitable because the final result can be damaged by the inability to adequately remove the supports necessary to print the most protruding parts of the model, affecting the “tactile” quality of the replica. These difficulties are compounded by the desire to create copies that are true to the original not only in form but also in materials, to create a tactile experience as complete as possible. For this it is preferred the use of SLS technique with marble powders, which make the physical model more mimetic to the original. Moreover, the visual accuracy of the reproduction can have a key role in many CH applications. Today few 3D printing devices are able to produce coloured replicas; therefore, their colouring has to be made manually to obtain good quality results [24]. Therefore, any improvement in the colour reproduction features of available technology could be really beneficial for CH applications.
Digital fabrication of tangible 3D replicas can replace any CH artwork which has to be removed from its original position, e.g. in a museum if the original object has to be restored or lent to a temporary exhibition; or the replacement can be permanent, for example to protect the originals from further degradation caused [25]. Since preservation involves keeping statues or architectural elements in their existing state and preventing them from decaying further and reducing the damage caused by the flow of water, chemical agents and different types of pests and micro-organisms, digital fabrication demonstrates that it has many potentiality at this level of intervention. Preservation suggests moving these works from their original position to more protect environments, such as exhibition rooms or museum warehouses. But, just to revive the original concept or legibility of the historical site, 3d printed copies are made. In [26] authors describe the potentialities and criticalities of prototyping systems in a replica of the statue of a Prophet, originally located beside one of the entry to the Florence Cathedral. For this purpose, different technologies and materials were tested. Two solid model of the whole statue, at 1:1 scale, were made: a first one in polystyrene and a second one in marble, evaluating their accuracy, in terms of in geometric details, and resistance. 3D printing technologies can contribute also to CH restoration methodologies reproducing the missing components of a statue or other artwork such as for the Ebe statue of Canova [27], where prototyping allows to study possible static strains and stress applied on supporting critical points, or the restoration of the Madonna of Pietranico, a terracotta statue fragmented in several pieces due to the earthquake in Abruzzo [28]. A reconstructive hypothesis of the statue was built by working on the digital models of the scanned parts; then the reassembly of the pieces was helped by the use of 3D printed innovative supporting structures. The structural properties (e.g. minimal visual impact, resistance to vibrations and transportation hazards) were considered while designing a proper holding structure. Within the scope of regenerating geometries, that cannot be easily generated via traditional techniques, the Sala di Cristoforo Sorte, a room of the Palazzo Ducale of Mantua, is a good example: the room had a very damaged double crown moulding of the sixteenth century, decorated with Ionic and lesbian kyma, with numerous and large gaps. The reconstruction [29] has focused on the integration of those missing elements alongside the traditional restoration methods with reverse engineering and rapid prototyping technologies. Two different approaches were tested using a FFF printer with PLA filament: the first, printing the missing parts and apply them within the gaps; the second, creating and printing casts of the
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mould by using 3D scanning, used for reproducing the decoration avoiding any contact with the original. In [30] authors describe the Opus Digitale project: an interesting application of high resolution laser scanning survey, with photogrammetry data integration, in order to create, noninvasively and without any contact with tesserae, a digital 3D model of a portion of a Byzantine mosaic later reproduced by rapid prototyping technology. Digital technologies are combined with skilful craftsmanship, creating a production of plaster polychrome replicas of historic mosaics, faithful to the original. The goal of the project is the preservation and future dissemination of our cultural heritage: new technologies will perpetuate as long as possible both the matter and the essence of artworks. Today physical model, which was traditionally used to anticipate and see what had not yet been realized, becomes a true copy that will replace the original, becoming a monument itself. 3D printed replicas can replace a lost heritage to prevent its memory, as it was planned for the city of Palmira, in Syria, where monuments were recently destroyed. A very ambitious project plans to recreate the ruins of the archaeological site by replacing them with their physical copies made through robotics and 3D printing. Projects like this make it obvious that there is the need to assess how much the printed model corresponds to the original. As an example, in the Project Mosul [31] 3D models of artefacts, destroyed by Islamic State (Isis), are reconstructed with a pioneering crowdsourcing system. The items are 3D prints of virtual models of four ancient pieces created by a group of archaeologists by applying digital technology to simple tourist photos. Scale reproductions were displayed at a private event at the Museum of Arts and Design in Manhattan. The Million Image Database Project at the Oxford Institute [32] for Digital Archaeology distributed cameras to volunteers across the Middle East to collect 3D photos of sites. As well as creating 3D models, they recreated full-scale artefacts, sites, and architectural features using cement-based 3D printing techniques [33]. This started with a recreation of the arch from Palmyra’s Temple of Bel, placed in London in April 2016. A peculiar application is the tactile exploration supporting blind and visually impaired people (haptic exploration): the transmission
of knowledge for them occurs prevalently through touch and the use of alphabetical forms, such as the Braille alphabet. Just because some researches demonstrated that physical models have been shown to enhance learning experience among student populations as they include their sense of touch [34], 3D printed copies can be a support to allow everybody to know and explore CH with fingers, from sculptures to paintings, to bring even two-dimensional art closer to being accessible. This can be done just producing a tangible replica or integrating the surface of the printed object with other sensors to obtain a multisensory experience (Fig. 5), for example with contextual audio information, relevant to the part of the object that is being touched in a specific moment. Tooteko [35] is a smart wearable device that combines touch and hearing to help the visually impaired to visualize objects that they could not experience otherwise. It enables blind people to perceive the world through touch and hearing. Tactile models of artworks, obtained by laserscanning or photogrammetry, are transformed into speaking models by the use of NFC tags, thus allowing an interactive and independent exploration. Haptic experience heavily depend on the question of scale, but digital fabrication brings objects to a human scale, so tactile models fit in our hand and details are adapted to the size of our fingers [36]. The project Tactile Painting [37] present a computer-assisted workflow for the creation of tactile representations of paintings, suitable to be used as a learning tool in the context of guided tours in museums or galleries. Starting from high-resolution images of original paintings, data suitable for rapid prototyping machines are generated to produce the physical touch tools. CNC-milled textured reliefs render fine details, like brush strokes and texture suitable for the sense of touch, so the haptic output is quite faithful to the original paintings. Finally, 3D printing technologies can contribute also to contemporary art: for example, Marc Quinn, in collaboration with Factum Arte, realized some of his sculptures [38] on the base of real shells forms. An STL file of the found form is obtained from scanning the shell in three dimensions. Then, it is enlarged using a 3D printer and cast in aluminium, concrete, stainless steel or bronze [39].
Fig. 5. Tooteko multisensory system allows to navigate any 3D surface with finger tips and get in return an audio content that is relevant in relation to which part of the surface you are touching. Tooteko is made of three elements: a high-tech ring that detects and reads the NFC tags and communicates in wireless mode with the smart device; a tactile surface tagged with NFC sensors and an app for tablet or smartphone. During the tactile navigation of the surface, when the finger reaches a hotspot, the ring identifies the NFC tag and activates, through the app, the audio track related to that hotspot. Thus, a relevant audio content relates to each hotspot.
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5. Case study: considerations about 3D printing precision The widespread use of solid printers has highlighted, on the one hand, the problem of building digital models that need to be printed and, on the other hand, that of the conformity of the copy to the original. To identify the different applications of such reproductions, it is now necessary to determine and verify metric accuracy of the digital and physical models: only in this way will the copy effectively replace the original in various fields, especially in the event that this must be preserved and its usability is limited to exposure. Physical models, in fact, are more and more frequently used on the occasions in which physical contact is necessary; for example, museum exhibits devoted to children or the visually impaired. This contribution is part of a broader research, with the purpose to optimize the path that goes from the acquisition of a point cloud of an object (i.e. vase, statue, architecture, urban area) to its representation in a scaled 3D model. In this process, one must of course take into consideration, on one hand, the accuracy of the acquisition tools (multi image photogrammetry, TOF laser scanner, triangulation laser scanner) and, on the other, that of the final output device: FDM, CNC, SLA, SLS printers, etc. to avoid diseconomies dictated by the different instrumental precision and resolution. The main purpose of this first phase of the research is to analyse the metrical characteristics of the printed model in relation to the original object. We want to focus, in particular, on the methodology used: the acquisition in digital format of an artefact has given rise to two physical models, created by two different devices. Obviously, the procedure below should be extended using other printers, supporting other materials, technologies. That is why we would like to share our data (the original scanned model) with those interested in the testing. Both models have undergone a scan process and compared with the original. Since the object of our study was the precision that can be reached through these solid-printing systems, the test was designed so as to maintain unchanged conditions for the acquisition and processing of products as much as possible. 6. Data acquisition and 3D printing The scanned object is a vase of dimensions such as to be printed on a 1:1 scale. The artefact was first acquired in digital form by means of a triangulation laser scanner, the Konica Minolta Range 7 [40], which allows reaching sub millimetrical precisions (up to 40 microns) (Fig. 6a). The tool allows us to scan small objects from a distance between 450 and 800 mm and can use two different lenses, tele and wide-angle, which guarantee the acquisition of a portion of variable reality, based on the optics used and on the distance from the object, in a range which goes from 79 × 99 to 267 × 334 mm (on the XY plane). In the case presented here, a “tele” optics was used, subjected beforehand to a special calibration process. To ensure the scanning
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of the object in its entirety, a rotating stage has been used which, through the Minolta software Range Viewer, allows obtaining a first alignment of the different scans using the ICP algorithm (Iterative Closest Point). During the acquisition phase, the objects were positioned at a distance between 600 and 700 mm. By imposing the rotary plate on a 30◦ angular step, we obtained 12 scans for each object and ensured a sufficient overlap for its orientation. The processing phase of the digital product has been carried out using the Geomagic Studio software. In this environment, the numerical model obtained was subjected to processing that has optimized the data for printing, maintaining the original geometry of the object as much as possible. First, each single scan – that the instrument returns already in the form of mesh – was cleaned and the marginal portions have been eliminated, in correspondence of which there is a degradation of precision. Second, the orientation of the scans has been refined by applying an additional ICP algorithm. As a result of these operations, it was necessary to close the surface, by completing via software the model where the data had not been acquired. The last operation carried out was a decimation of the final mesh, because the printers’ software products do not allow you to work on models consisting of a number of triangles much greater than one million. The printers used in this test phase were chosen not only because they use two different printing systems, but also because they belong to two diametrically opposed worlds. The first is a CraftBot Plus (CraftUnique) [41], a 3D desktop printer, which is very affordable and uses a FDM system, an additive manufacturing technology in which the plastic material – in this case a PLA – is melted using an extruder (Fig. 6b). This extruder is moved on two axes (X and Y) by stepper motors, which deposit the filament in different positions. The displacement along the Z-axis is guaranteed by a third motor of the same type; it affects the plate on which the material is deposited and determines the layer’s thickness. The manufacturer guarantees an accuracy of displacements of a few microns, while the layer can be of different thicknesses: from a maximum of 300 to a minimum of 100 microns. The size of the printing area is quite small: 250 × 200 × 200 mm. The second printer used is EnvisionTEC Ultra [42], which uses a SLA technique and, therefore, a system of projections of ultraviolet light sections on a volume of liquid resin (Fig. 6c). In this case, the device consists of two distinct elements: an optical-projective system determines the creation of the pattern on the XY plane, while a mechanical system moves the plate on which the pattern is formed, ensuring its three-dimensionality. The cost of the tool is significantly higher than the previous one and the resolution of the final model arrives to a maximum of 25 micron in Z and 45 micron in XY. The maximum size of the printing area remains rather limited: 264 × 164 × 203 mm. Another difference compared to the previous device consists of the fact that, after printing, the model must be immersed in a bath of alcohol to remove the residue of non-solidified resin and finally “baked” in an ultraviolet ray oven.
Fig. 6. The instruments used in this project: a: on the left, the Konika Minolta Range 7; b: in the centre, the CraftBot; c: on the right, the EnvisionTEC.
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As previously stated, the products have been printed on a 1:1 scale and acquired in digital format. To eliminate the variable introduced by the various instrumental accuracies, the same triangulation laser – Range 7 – has been used, trying to maintain unchanged the conditions of acquisition that could somehow affect the accuracy of the digital model. In fact, the purpose of these texts was only the comparison between the different printers. For the same reason, the methodology of processing the acquired data was also kept unchanged as much as possible, to ensure comparability of the final results.
7. Analysis of the results In the first place, it was considered necessary to conduct a comparison between two different scans of the same original object. In this way, we were able to verify the stability of the scanning process and the subsequent data processing. The module used for the analysis of the two data sets is contained within the alignment menu of Geomagic, which uses the same ICP algorithm used for the orientation of the scans, and it allows you to display a series of basic information for analysis: the maximum distance between two comparable points in the two meshes and the average distance and the standard deviation (s.q.m.). The two data show an average distance of 0.035 mm between the surfaces; it is concordant with the instrumental precision, a result which allowed us to proceed with the subsequent analysis. The second test was performed on the pattern printed through CraftBot, maintaining a “high” print resolution, therefore a thickness of the layer of 200 micron. The pattern shows an average distance of 0.0122 mm from the original, data, which exceeds our expectations, especially considering that this type of device is designed to “consumer” applications. However, the use of stepper motors guarantees high precision in the movements along the three axes and thus a great precision of the creation of the final pattern. Major problems, however, were found during the test on the pattern printed through EnvisionTEC. First, the material of which it is formed is not acquirable via laser scanner, because the resin allows the blade of light to enter. Therefore, we had to resort to
an ultra-fine gypsum spray, specifically created for the scanning of objects with very small details. Nevertheless, the presence of gypsum could affect the success of the test, because it was about adding an additional layer to the printed object, which, being a spray, may not be distributed evenly. In light of these considerations, it was necessary to run a first comparison test between the scanning of the original vase and a second scan of the same vase covered with a layer of gypsum. The result shows an average distance of 0.064 mm between the two surfaces, which was considered insignificant. The layer of gypsum did not form such a thickness as to compromise the analysis. However, the tests carried on the EnvisionTEC pattern have led to unexpected results: the comparison between the two data shows an average distance of 0.248 mm, therefore higher than the one found for the CraftBot pattern. The first analyses indicate that both the physical copies are smaller than the original, data supported by the confrontation on the volumes of the three meshes. In light of this, a scale factor to the EnvisionTEC pattern was imposed, calculated on the basis of the relationship between its volume and that of the original, which has led to the acquisition of an average distance comparable to that of the CraftBot pattern (Fig. 7). The scale variation, being greater in the EnvisionTEC pattern, could be caused by the material used by the printer: the solidification process to which it is subjected in the ultraviolet ray oven may reduce its size. It is important to emphasize, in fact, that the scale factor identified is isotropic, and consequently it cannot be caused by a misalignment between the optical-projective system, which works on the XY plane, and the mechanical system, which governs the displacement of the plate along the Z axes. To check the reliability of these results, the same analyses were performed on a different object: the bust of Francesco II Gonzaga [13] (Fig. 8). In this case, the original object did not have the size needed in order to be printed on a 1:1 scale. However, it was considered appropriate to take as “original” the digital model already scaled, and which gave rise to the two physical copies. These copies, in fact, had been printed in the past years, when the two output
Fig. 7. The comparisons made on the three different reproductions of the vase: the CraftBot model, the EnvisinTEC one, and the scaled one.
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Fig. 8. The bust of Francesco II Gonzaga: the original bust, its digital reproduction and its two copies.
Fig. 9. The tests performed on the bust: the CraftBot model, the EnvisinTEC one, and the scaled one.
Fig. 10. A detail of the two reproductions of the bust: the copy made with the SLA technique is much more definite than the FDM one.
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devices had been subjected to other calibration processes. However, even in this case, the comparison of the FDM model shows an average distance of 0.171 mm, while for the SLA model it is greater (0.245 mm). By imposing on the latter the same isotropic scale factor calculated for the previous object, the average distance is lowered (0.187 mm). However, in this case the error appears to have been caused by the mechanical component, because it seems distributed along the Z axes (Fig. 9). Further analyses on the calibration of the printer and on the behaviour of its materials will be carried out, but what we want to highlight is the excellent quality of the final result. Both printers allow you to get to an accuracy of a few tenths of a millimeter, which is more than adequate to the needs of a field like the Cultural Heritage, where these models are used to ensure greater usability of artefacts. Moreover, with regard to an order of magnitude of difference in the cost of printers, the metric result is comparable. What is not comparable, however, is the quality of the details that are returned: the photopolymer resin printer is certainly able to achieve a greater level of detail than the FDM (Fig. 10). Another issue concerns the final output cost. Printers like the CraftBot or in general 3D desktop printers directly create physical copies in series, while those in resin, because of the cost of the material, must necessarily pass through a silicone cast and a series production of gypsum models. Subsequent testing will have to evaluate a possible decay of precision as a result of these steps.
8. Conclusions 3D printing technologies open, in the CH sector, new possibilities of use, cataloguing and study, where the models, both virtual and physical, are the basis both for the display and analysis of the shape (also from a metrical point of view) of each artefact of artistic and historical interest. Digital fabrication cannot be limited to providing a “photorealistic” replica of the object: the printed copy must have similarity of behaviour and performance with the original on which perceptual and conceptual activities can be done in a continuous physical interaction between observer and object. 3D printing presents more flexibility: e.g. the digital representation can be edited before producing it as a physical object. It can be scaled or changed in shape or just selected portions of the object can be printed. Therefore, digital fabrication can considerably enhance the information provided by a tangible representation of an artefact opening new possibilities for study and access of CH asset. Even if the technologies described in the paper can still present some restrictions, due to materials, or instrumental precision as demonstrated in the tests, the accuracy of the reproduction is gradually reaching an excellent level of quality. Moreover, the research on thermo-polymer is in rapid and continuous evolution, so in a short time the reproductions will always move closer to the original, not only from a morphological point of view, but also from the physical one, solving the problem of compatibility of materials. Moreover, the latest 3D modelling and printing experiences have shown the need to introduce a new professionalism in support of archaeologists, architects, engineers, restorers and conservators that require the use of digital technologies related to the instrumental survey, to 3D modelling and solid printing. It is a new professionalism, which is evidently transversal to the consolidated subject areas, which traditionally and separately have dealt with these issues. In conclusion, we must rethink and reconfigure these activities that have as reference the digital universe in which we move every day.
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Review
3D printing: State of the art and future perspectives Caterina Balletti ∗ , Martina Ballarin , Francesco Guerra Dipartimento di Architettura Costruzione Conservazione, Università Iuav di Venezia, s. croce 191, 30135 Venezia, Italy
a r t i c l e
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Article history: Received 31 August 2016 Accepted 17 February 2017 Available online xxx Keywords: 3D printing Cultural heritage Preservation Explorations Accessibility Museums
a b s t r a c t In the last years, the development of 3D technologies applied to the field of Cultural Heritage (CH) has led to results of the utmost importance from the point of view of preservation, valorisation, communication and fruition of our assets. In particular, we experienced many interdisciplinary projects in which, thanks to the cooperation of different fields of research, incredible results have been obtained, through the technological collaboration of computer graphics and documentation, of industrial engineering and preservation and access of CH. This paper aims at drawing attention to the actual technologies in use for solid printing (digital fabrication) used for the realization of material copies, therefore tangible, of three-dimensional digital virtual models. Even though ulterior developments to these technologies are possibilities to be expected, the process of 3D printing has gradually gained levels of accuracy, which can nowadays be deemed as satisfying. This is even more true in the industrial field (from the manufacturing industry to the design industry), but also in other fields, such as the medical one, for example, for the realization of artificial limbs, and the CH field, which can benefit from new instruments for the restoration and preservation of cultural assets in museums. The metric characteristics of precision and accuracy of the model printed with 3D technology are the fundaments for everything concerning Geomatics, and have to be related with the same characteristics of the digital model obtained through the survey analysis. In other terms, the precision of the printed product must be evaluated in relation to the precision of the instruments used in the analysis. Thus, in the CH field there is the possibility of new systems of access, cataloguing and study, where the models, both virtual and tangible, represent the fundament of visualization and analysis of the form (also from the metric point of view) of each artefact of artistic and historical interest. © 2017 Elsevier Masson SAS. All rights reserved.
1. Research aim In this paper, we present the state of art and the potential and large spectrum of applications of fabrication technologies in the CH. Through a brief history and characterization of the most common 3D printer technologies, we try to present a review of the applications of 3D printing on CH, considering our experiences or those of other researcher. In the final part of this paper, a particular aspect of solid printing is analysed: the level of accuracy reachable in the creation of material models. This level of precision must be related to that of the instruments of analysis through which the artefacts are converted into digital format. We must not forget that the process that leads to the realisation of a material copy must go through a numeric model and that in this process there is a progressive loss of definition, both from the qualitative and quantitative points of view. The simplification operations that the digital data
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must undergo might cause a difference between the geometry of the printed object and that of the original object. 2. Introduction Digital technologies are able to offer an essential contribution to the documentation, analysis and subsequent use of the cultural assets, as they can be used in different forms and for different aims: study and research, diagnosis, repair, preservation, protection, communication-divulgation, fruition and formation of the cultural heritage. In the last years the use of electronic and Information Technologies (IT) has increased exponentially, creating new sceneries and possibilities in the field of the CH. This evolution of instruments and methods is in partnership with a diffusion of instrumental techniques for surveys; in particular, the 3D scan, which allows observation of complex geometries impossible to analyse through traditional methods. Solid printing has gained a special role in this technological development. The rapid creation of prototypes is a technique that
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allows the production of material copies of objects with complex geometries directly from the mathematical model in relatively short periods of time and often without being expensive. In recent years, this technique has experienced a very strong development thanks to the large diffusion on the market of desktop 3D printers, printers which are quite cheap and whose dimensions are reasonable. They use a FDM technology (Fuse Deposition Modelling), an additive kind of technology, as will be described later, which creates material models through the superimposition of material layers. This innovative system has allowed a rapid growth of this technology for ‘commercial’ uses, allowing less expert users to enter the world of 3D printing, creating a community of makers, comparable to the one which formed after the introduction of Arduino [1] to the general market. The material objects are realised quickly and with quite low budgets, starting from numeric models, often with complex geometries, converted in G-code, which is the programming language of the machines working with numeric control (CNC). We are also experiencing a reduction in size and cost of these printers (Desktop 3D printer), a process which contributed to the growth of this technology and favoured its commercialisation. The opening of FabLab in Italy (laboratories including machines for digital creation, able to transform ideas in prototypes and products and open to the general public) proves the development of this technology: in a study published at the beginning of 2015, Make in Italy reports in detail on the birth of FabLab and of makerspaces in our country and its growth in the last two years. There are more than 70 active labs spread throughout Italy, a community of more than 1000 FabLabs distributed throughout 78 countries according to the Fab Foundation, an organization that emerged from MIT’s Center for Bits & Atoms Fab Lab Program (http://www.fabfoundation.org). Thousands of people, along with associations, industries, museums, universities and institutions of all kind, are investing time, resources and energies in order to open laboratories including machines for digital fabrication to the public. 3D printers have demonstrated effectiveness in many other fields of application, in particular that of the Cultural Heritage. Thanks to the recent innovations in the IT technology and multimedia it is now possible to develop new forms of analysis and fruition of the Cultural Heritage, which are used along with more traditional methods. Models, first digital, then material, have introduced new possibilities of access, cataloguing and study of the cultural assets as they form the basis for both the visualization and the metric analysis of any artefact both from the artistic and historical point of view. As far as museums are concerned, for example, there is the possibility of creating identical replicas, both digital and material, of spaces and three-dimensional objects. Exhibitions and collections can take advantage from digital fabrication as the access to their information becomes customised according to the user, the content and the complexity of the given message. All this brought 3D printing to repute as one of the most important possible outputs, at the same level of more traditional digital or paper.
3. The subtraction and addition process In order to realise an object we might use two techniques radically different from each other: the subtractive, referring today to Computer Numerically Controlled (CNC) machining, and the additive, concerning the Additive Manufacturing (AM) processes [2]. AM, popularly called 3D printing, technologies today are used by makers all over the world, but its inception can be traced back in the 1980s, at which time it was called Rapid Prototyping (RP) [3]. RP was conceived as a fast and more cost-effective method for prototypes realization for product development within the industry [4].
Before describing AM main technologies, we want to fix some important dates to tell shortly how 3D printing was born: • in 1984 – Chuck Hull invented and patented a sterolithography aparatus (SLA) machine. Hull went on to co-found 3D Systems, the first organization nowadays operating in 3D printing. The STL format file was born; • in 1986 – Carl Deckard, Joe Beaman and Paul Forderhase (with other researchers) developed the ideas of Chuck Hull and filed a patent in the US for the selective Laser Sintering (SLS); • in 1988 Crump patented the Fused deposition modelling – which is printing with fuse material. This technique does not involve the use of laser or dust and uses fused plastic to spread in strata to create the object. Crump also founded Stratasys, another leading business in the field; • 1993 – was patented the Electron beam melting (EBM); • 2005 – Mcor Technologies Ltd – an Irish company – starts the Paper 3D laminated printing: a machine, which superimposes sheets of paper and prints on them. The result is an additive method, which includes the use of colours; • in 2005, thanks to the technology of the Self replicating rapid Prototyper a 3D printer which prints itself is first realised (opensource RepRap and FAB@Home projects). The RepRap Project [5] is an abbreviation Replicating Rapid Prototyper, and it aims to develop a 3D printer, which prints on its own the majority of its own components. All the products created with this project are published with open source licences; • in 2008, Bre Pettis, Adam Mayer, and Zach “Hoeken” Smith found MakerBot Industries. Digital fabrication technology is characterized by the basic physical process employed for the tangible object to be obtained. The subtraction process consists in removing the unnecessary material from a block to obtain the final object. The lathe is a tool that allows to remove the exceeding material from a block placed on a rotating platform, thanks to a series of tips of different shapes. It has been used for centuries: the first hydraulic lathe is more than 500 years old, but the Ancient Greeks and Egyptians used it as well, creating the first tools which allowed them to rotate a plate using a pivot with their feet. Modern lathes are more complicated and versatile; they use engines instead of human strength and can have quite a high level of automatism. However, they are based on the same principle with which our ancestors created the first vases regularly shaped. Milling machines are more modern machines, which allow the realization of complex products. They work very similarly to drills, but instead of creating a hole, thus using the tip of the tool, they cut laterally. The simplest milling machines work on three axes, while the most complex ones work on five or even six axes, supporting rotation around multiple axes, so that they can reach almost any part of the product. Milling machines can work with a vast range of resistant materials, from wood to metals. Lathes and milling machines are born as tools controlled manually, but in the meantime some numerically controlled machines were created, called CNC (Computer Numerical Control), and controlled by a computer which handles the passage from a 3D model realised through a CAD application (Computer-Aided Design) to the production of the object. The linking chain is a new kind of software called CAM (Computer-Aided Manufacturing), which transforms the digital model of the object in a series of commands to impart to the machine. The file format is a standard one called G-code, which includes information like N40; G82; X1500, automatically generated by specialised software. 3D printers are also CNC machines; the only difference is that instead of removing material they add it, which makes them an
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Fig. 1. FDM approach applied to the fac¸ade of S. Giorgio’s church in Venice. Supports for protruding parts are designed in the slicer software and will be removed then from the final printed model.
additive technology, allowing three-dimensional objects to be created by adding material to material layer by layer (AM). The flux followed in the additive process is analogue to the subtractive one: it begins with a numeric model in 3D and then a G-code is generated. After the model is ‘sliced’ in many thin layers with a specialised software called slicer, the G-code commands allow the machine to build the object superimposing the single strata, reaching levels of complexity which are impossible to replicate with other technologies: for example, with a milling machine it is impossible to realise an empty sphere. There are various techniques used in 3D printing; here we describe some of them briefly. The most widely used technique among printers, especially for domestic ones is the Fused Filament Fabrication type (FFF), which was born from another previous technique Fused Deposition Modeling (FDM) developed by Scott Crump at the end of the 1980s and commercialised in the 1990s by Stratasys [6]. Both techniques are based on the stratification of liquid plastic materials. Casting is based on additive principle, so that each strata once deposited is covered by the following one, forming superimpositions which can create roughness, depending on the material, on the casting dimension, on the nozzle and on the casting precision. The functioning is quite easy: a nozzle is heated in order to melt the material (filament) and can be moved on three axes, horizontally (axes X and Y) and vertically (axis Z), from an numerically controlled engine, commanded by a CAM software. The plastic filament (solid) is unrolled from the reel in which it has been put, in order to pass through a heated area with a high temperature. From here, thanks to the push of a pinion linked to the stepper engine, the liquid plastic is expelled from the nozzle, where the flux can be started and stopped. The framework which supports the structure of the plate (where the material to form the 3D object is collected) and of the engines must be controlled on a control sheet with a driver for the so called stepper motors. These engines are used to obtain increased velocity and precision and because they guarantee an elevated torque motor and a very satisfying twisting movement. There are different parameters that cooperate in creating the quality of the finished product. First of all is the kind of material used: PolyLactic Acid (PLA, which is a vegetable, biodegradable plastic), or ABS (Acrylonitrile butadiene styrene, a petrol derived from plastic, useful to realise mechanic objects thanks to its high resistance), and the section of the filament, obtaining resolutions ranging from 100 to 300 micron. It is also possible to use printers with more than one nozzle, which allow the use of different materials and colours. The FDM technique also requires the creation of a support structure (Fig. 1) during the creation of the object, especially for the empty spaces or projections. These are supports projected directly
from the software of the printer. At the end of the production, it will be necessary to remove all the additional supports generated and not included in the original drawing. The printers with one nozzle create supports with the same material of the final object, which implies that the supports must be removed once the print is complete, meaning that this removal might not be precise and sometimes it might damage the quality of the final product. The printers with two nozzles on the other hand might create the additional supports with filaments made of different material from the principal object, which means they are not fused and thus more easily removable, even manually. Some materials used to create the supports can also be made out of soluble materials, which would dissolve in water or other liquid substances. In this case, the removal of the support would be ‘chemical’. The result of a FDM print is not comparable to the quality level obtained with other industrial techniques involving laser, but the value for money is surely closer to the expectation of the public. Another industrial technique is Selective Laser Sintering (SLS): it is a technique which uses a laser as a sintering source of plastic material (polycarbonate, nylon, ABS), ceramic dust or a metallic one covered with thermoplastic resins which mingle with each other if heated (laser beam). The laser is conducted in the points of space defined by a 3D model in order to create a solid structure through sintering. Different from the process of fusion, the thermic process of sintering is intended for the use of dusts. The final product (Fig. 2) is completely smooth without the conventional roughness (though minimal) of the FDM technique. The main advantages of these techniques are: • the possibility to use different materials; • support structures are not necessary to create pieces with undercuts as the very dusts are deposited in strata and are not sintered to work as a support. The un-sintered dusts are to be removed at the end of the production process, though they are partly reusable. Sterolythography (SLA) is a printing technique that uses a tank filled with a particular liquid resin able to polymerize if exposed to light. A laser is projected through a system of mirrors in order to complete a scan of the surface. A piston lowers the tank with the liquid resin for the following scans. The scanning process repeats itself until the three-dimensional object is completed (Fig. 3). At the end, the object is taken from the liquid resin and exposed to ultraviolet light to complete the polymerization process. As for the FDM technique, a limited set of materials can be used and it requires the removal of supports from the finished product. Laminated Object Manufacturing (LOM): this technology uses a millworking system to obtain thin layers of material, later selected
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Fig. 2. The hypothetical reconstruction of Guggenheim Museum in Venice (Ca’ Venier dei Leoni). The tactile model was printed with a SLS technology with a high level of decorative details.
and united. The material is formed by special sheets of paper, cut according to the need and glued one on top of the other on the previous sheet. Dimensions are quite extended, according to the product volume. The support is formed by exceeding paper and requires an ulterior passage in order to remove the unnecessary material with tools for woodcarving. In Scopigno et al. [7] authors summarize, as can be seen in Fig. 1, the fabrication technologies that we described and they present a brief qualitative evaluation, with which we agree just considering our personal direct experience. They compare the different techniques in terms of material costs, use, resolution and geometrical accuracy especially considering the CH application (Fig. 4). From personal experience and from those described in essays, it is clear that for the Cultural Heritage, these technologies have reached a good level of quality, but one has to choose the technology
which is more apt for the needed type of application, as also explained in the examples that follow.
4. Applications in the contest of CH 3D printing is leading a revolution across many sectors and a wide range of applications, including those that until a few years ago would not have even been considered appropriate for this technology: industrial production, design, health. In the medical field some important results have been achieved: engineers and physicians are able to develop 3D-printable prosthetics that are fully customized to the wearer; e.g. the German para-cycling champion Denise Schindler worked with Autodesk to become the first to compete with a fully 3D-printed prosthetic leg at the Rio
Fig. 3. SLA printing techniques: the reproduction of the bust of Francesco II Gonzaga, in the City Museum of Mantova, is taken from the liquid resin once completed and exposed to ultraviolet light to complete the polymerization process.
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Fig. 4. The comparison done by Scopigno et al. (2015) of the different techniques in terms of material costs, use, resolution and geometrical accuracy especially considering the CH application.
2016 Paralympic Games [8]. Or today complex surgical procedures can be pre-planned using 3D printed organs [9–12]. Special attention should be given to the realization reducing times and costs of CH all types of reproductions: archaeological finds, sculptures, architectural elements, paintings and artworks in general. Replicas can be used in many ways: • for study and research; • to set up alternative museum exhibitions, such as tactile museum tours for the blind and visually impaired; • for restoration, re-creating missing portions of an object; • to organize workshops with schools; • for museum merchandising (e.g. producing, with cheap reproduction technologies, accurate small-scale replicas of the artworks conserved in a museum or in a CH institution). The realization of replicas usually is through the use of rubber moulds over the original artworks, which were then used for the subsequent production of gypsum or resin copies. This process requires a direct contact with the object, sometimes risking compromising its preservation. Now, thanks to the 3D surveying methods, from laser scanning to photogrammetry [13–23], digital models are realized with a high accuracy, and 3D printing gives the possibility to obtain true copies of the original, ensuring safety of the artwork. A special consideration should be addressed on the materials to be utilized in the final use of replicas, as in the particular case of archaeological finds: these objects are characterized by a complex geometry and a high level of details, that must be preserved in order to be explored. In this case, the FDM printing is not suitable because the final result can be damaged by the inability to adequately remove the supports necessary to print the most protruding parts of the model, affecting the “tactile” quality of the replica. These difficulties are compounded by the desire to create copies that are true to the original not only in form but also in materials, to create a tactile experience as complete as possible. For this it is preferred the use of SLS technique with marble powders, which make the physical model more mimetic to the original. Moreover, the visual accuracy of the reproduction can have a key role in many CH applications. Today few 3D printing devices are able to produce coloured replicas; therefore, their colouring has to be made manually to obtain good quality results [24]. Therefore, any improvement in the colour reproduction features of available technology could be really beneficial for CH applications.
Digital fabrication of tangible 3D replicas can replace any CH artwork which has to be removed from its original position, e.g. in a museum if the original object has to be restored or lent to a temporary exhibition; or the replacement can be permanent, for example to protect the originals from further degradation caused [25]. Since preservation involves keeping statues or architectural elements in their existing state and preventing them from decaying further and reducing the damage caused by the flow of water, chemical agents and different types of pests and micro-organisms, digital fabrication demonstrates that it has many potentiality at this level of intervention. Preservation suggests moving these works from their original position to more protect environments, such as exhibition rooms or museum warehouses. But, just to revive the original concept or legibility of the historical site, 3d printed copies are made. In [26] authors describe the potentialities and criticalities of prototyping systems in a replica of the statue of a Prophet, originally located beside one of the entry to the Florence Cathedral. For this purpose, different technologies and materials were tested. Two solid model of the whole statue, at 1:1 scale, were made: a first one in polystyrene and a second one in marble, evaluating their accuracy, in terms of in geometric details, and resistance. 3D printing technologies can contribute also to CH restoration methodologies reproducing the missing components of a statue or other artwork such as for the Ebe statue of Canova [27], where prototyping allows to study possible static strains and stress applied on supporting critical points, or the restoration of the Madonna of Pietranico, a terracotta statue fragmented in several pieces due to the earthquake in Abruzzo [28]. A reconstructive hypothesis of the statue was built by working on the digital models of the scanned parts; then the reassembly of the pieces was helped by the use of 3D printed innovative supporting structures. The structural properties (e.g. minimal visual impact, resistance to vibrations and transportation hazards) were considered while designing a proper holding structure. Within the scope of regenerating geometries, that cannot be easily generated via traditional techniques, the Sala di Cristoforo Sorte, a room of the Palazzo Ducale of Mantua, is a good example: the room had a very damaged double crown moulding of the sixteenth century, decorated with Ionic and lesbian kyma, with numerous and large gaps. The reconstruction [29] has focused on the integration of those missing elements alongside the traditional restoration methods with reverse engineering and rapid prototyping technologies. Two different approaches were tested using a FFF printer with PLA filament: the first, printing the missing parts and apply them within the gaps; the second, creating and printing casts of the
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mould by using 3D scanning, used for reproducing the decoration avoiding any contact with the original. In [30] authors describe the Opus Digitale project: an interesting application of high resolution laser scanning survey, with photogrammetry data integration, in order to create, noninvasively and without any contact with tesserae, a digital 3D model of a portion of a Byzantine mosaic later reproduced by rapid prototyping technology. Digital technologies are combined with skilful craftsmanship, creating a production of plaster polychrome replicas of historic mosaics, faithful to the original. The goal of the project is the preservation and future dissemination of our cultural heritage: new technologies will perpetuate as long as possible both the matter and the essence of artworks. Today physical model, which was traditionally used to anticipate and see what had not yet been realized, becomes a true copy that will replace the original, becoming a monument itself. 3D printed replicas can replace a lost heritage to prevent its memory, as it was planned for the city of Palmira, in Syria, where monuments were recently destroyed. A very ambitious project plans to recreate the ruins of the archaeological site by replacing them with their physical copies made through robotics and 3D printing. Projects like this make it obvious that there is the need to assess how much the printed model corresponds to the original. As an example, in the Project Mosul [31] 3D models of artefacts, destroyed by Islamic State (Isis), are reconstructed with a pioneering crowdsourcing system. The items are 3D prints of virtual models of four ancient pieces created by a group of archaeologists by applying digital technology to simple tourist photos. Scale reproductions were displayed at a private event at the Museum of Arts and Design in Manhattan. The Million Image Database Project at the Oxford Institute [32] for Digital Archaeology distributed cameras to volunteers across the Middle East to collect 3D photos of sites. As well as creating 3D models, they recreated full-scale artefacts, sites, and architectural features using cement-based 3D printing techniques [33]. This started with a recreation of the arch from Palmyra’s Temple of Bel, placed in London in April 2016. A peculiar application is the tactile exploration supporting blind and visually impaired people (haptic exploration): the transmission
of knowledge for them occurs prevalently through touch and the use of alphabetical forms, such as the Braille alphabet. Just because some researches demonstrated that physical models have been shown to enhance learning experience among student populations as they include their sense of touch [34], 3D printed copies can be a support to allow everybody to know and explore CH with fingers, from sculptures to paintings, to bring even two-dimensional art closer to being accessible. This can be done just producing a tangible replica or integrating the surface of the printed object with other sensors to obtain a multisensory experience (Fig. 5), for example with contextual audio information, relevant to the part of the object that is being touched in a specific moment. Tooteko [35] is a smart wearable device that combines touch and hearing to help the visually impaired to visualize objects that they could not experience otherwise. It enables blind people to perceive the world through touch and hearing. Tactile models of artworks, obtained by laserscanning or photogrammetry, are transformed into speaking models by the use of NFC tags, thus allowing an interactive and independent exploration. Haptic experience heavily depend on the question of scale, but digital fabrication brings objects to a human scale, so tactile models fit in our hand and details are adapted to the size of our fingers [36]. The project Tactile Painting [37] present a computer-assisted workflow for the creation of tactile representations of paintings, suitable to be used as a learning tool in the context of guided tours in museums or galleries. Starting from high-resolution images of original paintings, data suitable for rapid prototyping machines are generated to produce the physical touch tools. CNC-milled textured reliefs render fine details, like brush strokes and texture suitable for the sense of touch, so the haptic output is quite faithful to the original paintings. Finally, 3D printing technologies can contribute also to contemporary art: for example, Marc Quinn, in collaboration with Factum Arte, realized some of his sculptures [38] on the base of real shells forms. An STL file of the found form is obtained from scanning the shell in three dimensions. Then, it is enlarged using a 3D printer and cast in aluminium, concrete, stainless steel or bronze [39].
Fig. 5. Tooteko multisensory system allows to navigate any 3D surface with finger tips and get in return an audio content that is relevant in relation to which part of the surface you are touching. Tooteko is made of three elements: a high-tech ring that detects and reads the NFC tags and communicates in wireless mode with the smart device; a tactile surface tagged with NFC sensors and an app for tablet or smartphone. During the tactile navigation of the surface, when the finger reaches a hotspot, the ring identifies the NFC tag and activates, through the app, the audio track related to that hotspot. Thus, a relevant audio content relates to each hotspot.
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5. Case study: considerations about 3D printing precision The widespread use of solid printers has highlighted, on the one hand, the problem of building digital models that need to be printed and, on the other hand, that of the conformity of the copy to the original. To identify the different applications of such reproductions, it is now necessary to determine and verify metric accuracy of the digital and physical models: only in this way will the copy effectively replace the original in various fields, especially in the event that this must be preserved and its usability is limited to exposure. Physical models, in fact, are more and more frequently used on the occasions in which physical contact is necessary; for example, museum exhibits devoted to children or the visually impaired. This contribution is part of a broader research, with the purpose to optimize the path that goes from the acquisition of a point cloud of an object (i.e. vase, statue, architecture, urban area) to its representation in a scaled 3D model. In this process, one must of course take into consideration, on one hand, the accuracy of the acquisition tools (multi image photogrammetry, TOF laser scanner, triangulation laser scanner) and, on the other, that of the final output device: FDM, CNC, SLA, SLS printers, etc. to avoid diseconomies dictated by the different instrumental precision and resolution. The main purpose of this first phase of the research is to analyse the metrical characteristics of the printed model in relation to the original object. We want to focus, in particular, on the methodology used: the acquisition in digital format of an artefact has given rise to two physical models, created by two different devices. Obviously, the procedure below should be extended using other printers, supporting other materials, technologies. That is why we would like to share our data (the original scanned model) with those interested in the testing. Both models have undergone a scan process and compared with the original. Since the object of our study was the precision that can be reached through these solid-printing systems, the test was designed so as to maintain unchanged conditions for the acquisition and processing of products as much as possible. 6. Data acquisition and 3D printing The scanned object is a vase of dimensions such as to be printed on a 1:1 scale. The artefact was first acquired in digital form by means of a triangulation laser scanner, the Konica Minolta Range 7 [40], which allows reaching sub millimetrical precisions (up to 40 microns) (Fig. 6a). The tool allows us to scan small objects from a distance between 450 and 800 mm and can use two different lenses, tele and wide-angle, which guarantee the acquisition of a portion of variable reality, based on the optics used and on the distance from the object, in a range which goes from 79 × 99 to 267 × 334 mm (on the XY plane). In the case presented here, a “tele” optics was used, subjected beforehand to a special calibration process. To ensure the scanning
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of the object in its entirety, a rotating stage has been used which, through the Minolta software Range Viewer, allows obtaining a first alignment of the different scans using the ICP algorithm (Iterative Closest Point). During the acquisition phase, the objects were positioned at a distance between 600 and 700 mm. By imposing the rotary plate on a 30◦ angular step, we obtained 12 scans for each object and ensured a sufficient overlap for its orientation. The processing phase of the digital product has been carried out using the Geomagic Studio software. In this environment, the numerical model obtained was subjected to processing that has optimized the data for printing, maintaining the original geometry of the object as much as possible. First, each single scan – that the instrument returns already in the form of mesh – was cleaned and the marginal portions have been eliminated, in correspondence of which there is a degradation of precision. Second, the orientation of the scans has been refined by applying an additional ICP algorithm. As a result of these operations, it was necessary to close the surface, by completing via software the model where the data had not been acquired. The last operation carried out was a decimation of the final mesh, because the printers’ software products do not allow you to work on models consisting of a number of triangles much greater than one million. The printers used in this test phase were chosen not only because they use two different printing systems, but also because they belong to two diametrically opposed worlds. The first is a CraftBot Plus (CraftUnique) [41], a 3D desktop printer, which is very affordable and uses a FDM system, an additive manufacturing technology in which the plastic material – in this case a PLA – is melted using an extruder (Fig. 6b). This extruder is moved on two axes (X and Y) by stepper motors, which deposit the filament in different positions. The displacement along the Z-axis is guaranteed by a third motor of the same type; it affects the plate on which the material is deposited and determines the layer’s thickness. The manufacturer guarantees an accuracy of displacements of a few microns, while the layer can be of different thicknesses: from a maximum of 300 to a minimum of 100 microns. The size of the printing area is quite small: 250 × 200 × 200 mm. The second printer used is EnvisionTEC Ultra [42], which uses a SLA technique and, therefore, a system of projections of ultraviolet light sections on a volume of liquid resin (Fig. 6c). In this case, the device consists of two distinct elements: an optical-projective system determines the creation of the pattern on the XY plane, while a mechanical system moves the plate on which the pattern is formed, ensuring its three-dimensionality. The cost of the tool is significantly higher than the previous one and the resolution of the final model arrives to a maximum of 25 micron in Z and 45 micron in XY. The maximum size of the printing area remains rather limited: 264 × 164 × 203 mm. Another difference compared to the previous device consists of the fact that, after printing, the model must be immersed in a bath of alcohol to remove the residue of non-solidified resin and finally “baked” in an ultraviolet ray oven.
Fig. 6. The instruments used in this project: a: on the left, the Konika Minolta Range 7; b: in the centre, the CraftBot; c: on the right, the EnvisionTEC.
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As previously stated, the products have been printed on a 1:1 scale and acquired in digital format. To eliminate the variable introduced by the various instrumental accuracies, the same triangulation laser – Range 7 – has been used, trying to maintain unchanged the conditions of acquisition that could somehow affect the accuracy of the digital model. In fact, the purpose of these texts was only the comparison between the different printers. For the same reason, the methodology of processing the acquired data was also kept unchanged as much as possible, to ensure comparability of the final results.
7. Analysis of the results In the first place, it was considered necessary to conduct a comparison between two different scans of the same original object. In this way, we were able to verify the stability of the scanning process and the subsequent data processing. The module used for the analysis of the two data sets is contained within the alignment menu of Geomagic, which uses the same ICP algorithm used for the orientation of the scans, and it allows you to display a series of basic information for analysis: the maximum distance between two comparable points in the two meshes and the average distance and the standard deviation (s.q.m.). The two data show an average distance of 0.035 mm between the surfaces; it is concordant with the instrumental precision, a result which allowed us to proceed with the subsequent analysis. The second test was performed on the pattern printed through CraftBot, maintaining a “high” print resolution, therefore a thickness of the layer of 200 micron. The pattern shows an average distance of 0.0122 mm from the original, data, which exceeds our expectations, especially considering that this type of device is designed to “consumer” applications. However, the use of stepper motors guarantees high precision in the movements along the three axes and thus a great precision of the creation of the final pattern. Major problems, however, were found during the test on the pattern printed through EnvisionTEC. First, the material of which it is formed is not acquirable via laser scanner, because the resin allows the blade of light to enter. Therefore, we had to resort to
an ultra-fine gypsum spray, specifically created for the scanning of objects with very small details. Nevertheless, the presence of gypsum could affect the success of the test, because it was about adding an additional layer to the printed object, which, being a spray, may not be distributed evenly. In light of these considerations, it was necessary to run a first comparison test between the scanning of the original vase and a second scan of the same vase covered with a layer of gypsum. The result shows an average distance of 0.064 mm between the two surfaces, which was considered insignificant. The layer of gypsum did not form such a thickness as to compromise the analysis. However, the tests carried on the EnvisionTEC pattern have led to unexpected results: the comparison between the two data shows an average distance of 0.248 mm, therefore higher than the one found for the CraftBot pattern. The first analyses indicate that both the physical copies are smaller than the original, data supported by the confrontation on the volumes of the three meshes. In light of this, a scale factor to the EnvisionTEC pattern was imposed, calculated on the basis of the relationship between its volume and that of the original, which has led to the acquisition of an average distance comparable to that of the CraftBot pattern (Fig. 7). The scale variation, being greater in the EnvisionTEC pattern, could be caused by the material used by the printer: the solidification process to which it is subjected in the ultraviolet ray oven may reduce its size. It is important to emphasize, in fact, that the scale factor identified is isotropic, and consequently it cannot be caused by a misalignment between the optical-projective system, which works on the XY plane, and the mechanical system, which governs the displacement of the plate along the Z axes. To check the reliability of these results, the same analyses were performed on a different object: the bust of Francesco II Gonzaga [13] (Fig. 8). In this case, the original object did not have the size needed in order to be printed on a 1:1 scale. However, it was considered appropriate to take as “original” the digital model already scaled, and which gave rise to the two physical copies. These copies, in fact, had been printed in the past years, when the two output
Fig. 7. The comparisons made on the three different reproductions of the vase: the CraftBot model, the EnvisinTEC one, and the scaled one.
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Fig. 8. The bust of Francesco II Gonzaga: the original bust, its digital reproduction and its two copies.
Fig. 9. The tests performed on the bust: the CraftBot model, the EnvisinTEC one, and the scaled one.
Fig. 10. A detail of the two reproductions of the bust: the copy made with the SLA technique is much more definite than the FDM one.
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devices had been subjected to other calibration processes. However, even in this case, the comparison of the FDM model shows an average distance of 0.171 mm, while for the SLA model it is greater (0.245 mm). By imposing on the latter the same isotropic scale factor calculated for the previous object, the average distance is lowered (0.187 mm). However, in this case the error appears to have been caused by the mechanical component, because it seems distributed along the Z axes (Fig. 9). Further analyses on the calibration of the printer and on the behaviour of its materials will be carried out, but what we want to highlight is the excellent quality of the final result. Both printers allow you to get to an accuracy of a few tenths of a millimeter, which is more than adequate to the needs of a field like the Cultural Heritage, where these models are used to ensure greater usability of artefacts. Moreover, with regard to an order of magnitude of difference in the cost of printers, the metric result is comparable. What is not comparable, however, is the quality of the details that are returned: the photopolymer resin printer is certainly able to achieve a greater level of detail than the FDM (Fig. 10). Another issue concerns the final output cost. Printers like the CraftBot or in general 3D desktop printers directly create physical copies in series, while those in resin, because of the cost of the material, must necessarily pass through a silicone cast and a series production of gypsum models. Subsequent testing will have to evaluate a possible decay of precision as a result of these steps.
8. Conclusions 3D printing technologies open, in the CH sector, new possibilities of use, cataloguing and study, where the models, both virtual and physical, are the basis both for the display and analysis of the shape (also from a metrical point of view) of each artefact of artistic and historical interest. Digital fabrication cannot be limited to providing a “photorealistic” replica of the object: the printed copy must have similarity of behaviour and performance with the original on which perceptual and conceptual activities can be done in a continuous physical interaction between observer and object. 3D printing presents more flexibility: e.g. the digital representation can be edited before producing it as a physical object. It can be scaled or changed in shape or just selected portions of the object can be printed. Therefore, digital fabrication can considerably enhance the information provided by a tangible representation of an artefact opening new possibilities for study and access of CH asset. Even if the technologies described in the paper can still present some restrictions, due to materials, or instrumental precision as demonstrated in the tests, the accuracy of the reproduction is gradually reaching an excellent level of quality. Moreover, the research on thermo-polymer is in rapid and continuous evolution, so in a short time the reproductions will always move closer to the original, not only from a morphological point of view, but also from the physical one, solving the problem of compatibility of materials. Moreover, the latest 3D modelling and printing experiences have shown the need to introduce a new professionalism in support of archaeologists, architects, engineers, restorers and conservators that require the use of digital technologies related to the instrumental survey, to 3D modelling and solid printing. It is a new professionalism, which is evidently transversal to the consolidated subject areas, which traditionally and separately have dealt with these issues. In conclusion, we must rethink and reconfigure these activities that have as reference the digital universe in which we move every day.
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Please cite this article in press as: C. Balletti, et al., 3D printing: State of the art and future perspectives, Journal of Cultural Heritage (2017), http://dx.doi.org/10.1016/j.culher.2017.02.010