Several applications in the fields of urban planning, construction, and cultural heritage require models that are at the same time accurate, visually pleasing, and easy to study, especially if the models are used for presentation and exhibition purposes.
Rapid prototyping (RP) techniques make it possible to easily manufacture physical models based on digital 3D models, meaning that model building is no longer a time-consuming, manual process. With RP, model making becomes an interesting and accessible tool for illustration [1
]. RP techniques can be divided into additive and subtractive techniques, even though some authors only include additive techniques in RP [3
]. With additive manufacturing techniques, such as stereolithography (SLA), the model is manufactured by adding material in layers [4
]. Machining techniques, like milling, are subtractive techniques: The manufactured object is made by removing material from a block with a tool [5
]. RP is widely used in industrial design, usually for making physical models [3
]. In engineering, RP techniques have been used, e.g., in manufacturing tooling for sheet metal forming [6
]. In addition, models made with RP have been used to validate simulation results [7
] and visualize data obtained from Geographic Information Systems (GIS) [8
]. To utilize RP, a triangle mesh model of the object is required [9
]. This mesh model has to unambiguously define the internal volume of the object, so its surface has to be free of any holes. In design applications, this is created by tessellation from a CAD model. The resulting mesh model is sliced to determine the parallel horizontal cross sections of the model [9
]. Based on these cross sections, the path that the machine is using in fabrication is calculated, typically this is referred to as a toolpath.
Additive manufacturing techniques used for RP include, for example, SLA, selective laser sintering (SLS), laminated object manufacturing (LOM), 3D printing and fused deposition modeling (FDM) [4
]. In SLA, a directed laser beam is used to cure photo-curable resin, manufacturing the object in layers [4
]. In LOM, the object is made by cutting and gluing layers of sheet material [4
]. In 3D printing, a powder material is selectively bound with glue by an inkjet, forming the object in layers [4
]. Fused deposition modeling (FDM) is one of the most common additive manufacturing techniques used in RP. With FDM, a heated thermoplastic material is extruded and used to build the model in layers (Figure 1
) (e.g., see [4
]). One of the advantages of FDM technology is that the machines can be used in a regular office environment, unlike for example SLA machines, where hazardous resins are used [10
]. A disadvantage of FDM technology is that models require support structures during the manufacturing process, which have to be removed afterwards [10
], in addition it cannot be used for manufacturing objects from metal materials. Typical build layer thicknesses for FDM range from 0.05 mm to 0.25 mm, producing a surface roughness of 56.6 μm Ra to 17.9 µm Ra, depending on the angle of the surface built [10
]. The dimensional accuracy of a FDM print was found to be low compared to other RP methods, resulting in average linear dimensional accuracy of 95.3 percent [10
In recent years, there has been a major change in the market of FDM machines. There are currently several, quite affordable, FDM machines available. Many of them are based on open-source projects. This has made RP equipment more accessible than ever before. The performance of one of these affordable RP machines has been studied by Pei et al.
], who refer to these machines as Entry Level Rapid Prototyping (ELRP) machines. Typically, these machines use acrylonitrile butadiene styrene (ABS) or polylactic acid (PLA) as printing materials [12
], ABS requiring a higher extrusion temperature [12
]. Some of the models are capable of using two materials in the same print, to produce multi-colored models [12
], while many only use one material at a time [14
]. The build envelope of ELRP machines is dependent on the specific model, but typical values range from 100 mm by 100 mm by 125 mm [17
] to 230 mm by 270 mm by 200 mm [12
]. In addition to these ELRP machines being sold, there are also numerous service providers in the market offering RP as a service over the Internet (e.g., see [18
]). When discussing RP by additive manufacturing, it has to be noted that occasionally the term “3D printing” is used to refer to all additive RP technologies. In a similar way, most RP machines are sometimes called “3D printers”, even if they are based on a different technology, such as FDM.
The operating principle of Fused Deposition Modeling.
The operating principle of Fused Deposition Modeling.
There are several different techniques for acquiring three-dimensional virtual models of the physical environment, for example photogrammetry and laser scanning (LS). Both of these are used extensively in the field of geo-information. Other methods also exist: A digital model can be created by measuring the key points of the original using, for example, a tachymeter if the geometry is simple enough. Existing drawings or plans can also be used in modeling, if they are available.
Photogrammetry is the technology of deriving 3D data, characteristics, and attributes from 2D images. Fraser [20
], and Grün et al.
], for example, have studied 3D modeling based on terrestrial images. Studies related to 3D modeling based on image sequences have been done, e.g., Bethmann et al.
], and Cornelis et al.
Static terrestrial laser scanning (TLS) can produce dense point clouds (up to hundreds of thousands of pts/m2
from a range of 10 m). A specific TLS instrument has been demonstrated to reach an accuracy of 8 mm (in 92% of tests, 4 mm) or better in ranges of 10 m to 50 m [24
]. By combining several scans, a larger point cloud can be created for the target [25
]. TLS makes it possible to survey built environments with great accuracy, great precision, and high spatial data intensity [26
]. Studies by, for example, Arayici [26
], Buckley et al.
], Pu [28
], and Pu and Vosselman [29
], present approaches on how to use 3D modeling with TLS.
Mobile laser scanning (MLS) is an emerging technique, in which a laser scanning system is mounted on a vehicle. The vehicle might be, for example, a car or a boat and the measurements are made while the platform is moving. Global Navigation Satellite System (GNSS) and Inertia Measurement Units (IMU) are used for positioning and orienting the scanner. There have been several recent studies on MLS systems and their accuracy, as well as on environmental modeling done with MLS [30
]. A relative accuracy of 10 mm or less has been reached with the Lynx Mobile Mapper system [32
], making it possible to survey large areas with a very high detail level.
Point clouds obtained using laser scanning techniques are typically very large in size and contain up to hundreds of millions of points. Processing them requires computational resources and can also be rather time consuming [33
]. Typical problems encountered when working with point clouds include gaps in the data caused by obstructions in the target and environment and varying point density in the data set (Figure 2
The operating principle of Terrestrial Laser Scanning.
The operating principle of Terrestrial Laser Scanning.
By combining 3D digitizing techniques with RP, physical models of the environment and artifacts can easily be made. The use of RP techniques also has the benefit that copies of the same model can easily be made [34
]. These models have been used, for example, as demonstration and reconstruction models in archaeology [4
], for making replicas of objects for museum purposes or preserving original artifacts [4
], and as tactile models that make it possible for visually impaired users to study various targets [4
]. These models can be made in either real size or to scale [4
]. RP techniques can also be used to produce more sophisticated presentation models. With the use of color, more data can be included in the physical models. In a similar manner, three-dimensional legends can be added to help interpret the model [39
]. The physical model can also be made from other data than just the three-dimensional geometry of a physical original; this approach has been presented by Rase [8
] for visualizing data from geo-information systems.
Mesh models created by triangulating point clouds can also be used for RP. Some editing of the models is usually required to achieve a hole-free mesh model from 3D digitizing. The editing process can include stitching the scanned surfaces together, fixing of individual triangulation errors, filling holes in surface, and possibly splitting the model to be manufactured in pieces [4
]. This editing can be performed with the Geomagic Studio software, for example [5
]. This strategy was employed by Tucci et al.
] to create sculpture replicas. In their case, the mesh model generation and editing processes were performed using Geomagic Studio software. Models created with this method [5
] reflect the original, 3D digitized targets quite closely, and they also include any possible surface errors, like dents, scratches, or other geometric defects from them if no additional mesh editing is performed.
ELRP machines have generated a certain amount of interest, and they increase the utilization of RP. In this study, we will test whether one of these ELRP machines based on FDM technology can be used to visualize the complex models produced by TLS. Based on the existing research, we can assume that this is possible with professional hardware [4
], but not much research exists on the use of ELRP machines. Using the point clouds acquired with TLS, we will create the three-dimensional mesh models by triangulation. After this, we edit the mesh models to attain hole free models that can be used in RP. In addition, we aim to gain practical experience with using an ELRP machine. To compare the workflow and the quality of the results, we present two cases. In the first case, we utilize an older, professional-quality RP machine and in the second case we use a new ELRP machine. In both cases, TLS is used to acquire the models to be manufactured.
5. Discussion and Conclusions
Recent developments in terms of the affordability of FDM machines have changed the field of RP. With the emergence of ELRP machines, RP has become more available to users than ever before. In this paper, we looked at how RP technology is currently being used, namely FDM, to manufacture physical models from three-dimensional data sets obtained with TLS.
We tested RP with two different laser-scanned data sets and two different RP machines, both based on FDM technology. A traditional, professional-quality RP machine was used in the first case. In the second case, the model was manufactured with an ELRP machine.
The data sets were different in terms of their algorithmic and perceived complexity. The complexity of the statue data set was significantly greater with all of the estimation methods used: It had a 68% higher triangle count and its complexity indexes were only 15% and 64% of the complexity indexes for the façade segment data set, indicating a higher degree of complexity.
It was possible to manufacture the models in both cases, but in the statue model case the model had to be made in two parts. This was because the ELRP machine was unable to create removable support structures for the object during the manufacturing process, and unsupported shapes could therefore not be made. The process of splitting the model of the statue before manufacturing illustrated the trade-offs that become necessary when utilizing ELRP machines. The other alternative method would have involved making manually removable support structures from the same material as the model. In this case, the professional-quality RP machine offered a higher degree of geometric freedom during manufacturing. Evaluating the manufacturability of solid mesh models is clearly dependent on the chosen RP technology and its characteristics as well as the model’s geometry. The complexity of the model does not necessarily affect the ability to manufacture it with an RP machine. More attention had to be paid to the orientation and geometric properties of the model when manufacturing it with the ELRP machine. This highlights the need for obtaining more experience in using ELRP machines. Because it was possible to remove the support material with an ultra-sonic cleaner used in the professional-quality machine, there was less manual work in finishing the model manufactured with it. All models have visible artifacts caused by the RP manufacturing technology. More of these artifacts can be found in the models made with the ELRP machine. However, the results that we obtained with a single ELRP machine cannot be generalized to all ELRP machines, since there are several different models available. Some of them, for example, have a dual extrusion system, enabling the extrusion of removable support structures.
Looking at the accuracy of models, the data acquisition can be performed by TLS with the accuracy of a few mm. With the Leica HDS6100 the ranging accuracy is reported by manufacture to be ±2 mm. In addition to the instruments accuracy, the co-registering of point clouds affects the accuracy of the data set obtained. In our cases, the largest co-registration average overlap error of two scans was found to be 12.46 mm. In a model made to scale of 1:40, this creates an error of roughly 14 mm/40 = 0.35 mm in the final model. The deviations caused to the models by the manufacturing were similar or larger, ranging from 0.35 mm to 0.42 mm. The accuracy of the 3D digitizing instrument used in measuring models was ±0.05 mm, as reported by the manufacturer. In the 1:40 scale used, the errors caused by the TLS are comparable or smaller than the observed errors of FDM manufacturing, making TLS a well-suited method for acquiring virtual models that can be used to manufacture physical representations. The FDM machines could be used to create illustrative models, with a few centimeters accuracy in targets’ real world size.
Since a single material RP machine was used in both cases, the physical models produced were mono-colored. Colored physical models could have been made by using a different RP technique [34
], such as powder-based devices (e.g., [41
]). However, it would have been necessary to acquire texture images from the digitized targets to obtain color information. In our case, only the laser intensities were recorded.
One alternative to buying and operating ELRP machines is to subcontract. There are service providers offering RP over the Internet that use different additive manufacturing technologies [18
]. The choice between using subcontractors versus
owning and operating the machines is a difficult one. When using a service provider, all of the setup work and machine maintenance work are also subcontracted. Neither, does the client have to spend time manufacturing the model. On the other hand, the delivery times need to be taken into the account and price of the models is higher. In “one-off” models, using service providers is an interesting option. If RP becomes more commonly used as an illustration tool in the projects, the decision will be more difficult. In both of our cases, the file size of the models was too large to be easily ordered from these services.
Successfully using RP technologies to visualize complex, three-dimensional models have been studied by many authors. The technologies have been used to visualize geometrically complicated objects [4
]. In some cases, the original targets have been documented using TLS [4
]. Clearly, RP machines can be used to visualize three-dimensional models produced with TLS. However, the process of using RP machines is often not documented, making it impossible to estimate how easy it actually is to use the techniques [4
]. In addition, there is very little research on using ELRP machines in this manner; in most reported cases, professional-quality machines have been used. It was fascinating to see that a modern, affordable RP machine could achieve the same surface quality and level of detail as a traditional professional machine. New 3D-digitizing methods, such as backpack-based MLS [42
], make it possible to efficiently survey large and geometrically complicated outdoor and indoor environments. New presentation methods are needed to communicate the measured results in a simple manner. ELRP machines offer an interesting possibility for doing so in this particular field. With the price of machinery such as this being quite low already, we expect to see an increase in the use of RP-made models in a variety of purposes, including the visualization of 3D models obtained via 3D-digitizing methods.