**4. Instances of color objects with the process**

In this section, two kinds of instances of color objects created with the process are demon‐ strated, showing the color reproduction and further potential applications.

#### **4.1. UV-ink-based 3D Printing**

UV-ink-based 3D Printing follows the similar principle as the DOD (drop on drop) 3D Printing technology. A UV-LED flatbed printer is used as a 3D printer and the UV ink as the modeling material. The UV ink jetting from the printer nozzle solidifies after being irradiated by a UV lamp. Finally, the model is shaped through layer-by-layer printing. However, the difference between UV-ink-based 3D Printing and DOD printing is that the modeling process of each layer of the former is an integrated section and of the latter is a series of lines. So, because of this characteristic of UV-ink-based 3D Printing, it is more suitable for 2.5D modeling. Below are two examples of UV-ink-based 3D Printing.

#### *4.1.1. Application for oil painting reproduction*

After completion of one layer of material, the printing nozzles moves back to the origin, and platform falls to a certain height in the Z axle, such as 0.5 mm. The printing nozzles restart to form the next layer. This is called one layer forming cycle, and after certain cycles, the color

Three-dimensional color printer with solid forming has one key technology, that is, the development of coloring software modules. The software can process STL file data, form a solid layer contour, and add an internal solid modeling tool to add gradient color information. According to the requirements of colored objects, a coloring module should have two func‐

**1.** *STL file data processing function*: (a) read and parse STL files; (b) establish triangular facet

**2.** *Gradient interpolation coloring on each layer*: (a) linear interpolation coloring; (b) cosine interpolation coloring; (c) cosine G color tone interpolation coloring; (d) power interpo‐

Using UGNX software or CAD drawing software to make three-dimensional solid modeling, mapping is performed through the menu options after forming "\* .stl" file. With common 3D modeling software, in the "File" menu there are "Export" command, and it can export a "\* .stl" file, and then manually change it into ".txt" type files. To understand the file format, use

Quick slicing algorithms create a data structure of triangular with vertices and edges. After the data structure is created, software will reorder all distinct vertexes, and then topology data are completely setup. Finally, the above-mentioned interpolation functions can be used to

In this section, two kinds of instances of color objects created with the process are demon‐

UV-ink-based 3D Printing follows the similar principle as the DOD (drop on drop) 3D Printing technology. A UV-LED flatbed printer is used as a 3D printer and the UV ink as the modeling material. The UV ink jetting from the printer nozzle solidifies after being irradiated by a UV lamp. Finally, the model is shaped through layer-by-layer printing. However, the difference between UV-ink-based 3D Printing and DOD printing is that the modeling process of each layer of the former is an integrated section and of the latter is a series of lines. So, because of

topology; (c) design solid slicing algorithm and contour generation.

notepad to open "\* .txt", and then we can see data of small triangular.

strated, showing the color reproduction and further potential applications.

**4. Instances of color objects with the process**

solid is finally completed.

42 New Trends in 3D Printing

lation coloring.

show the results.

**4.1. UV-ink-based 3D Printing**

tions:

*3.8.3. Coloring module software development*

3D Printing is a novel method to reproduce oil paintings so that it can restore the stereo brushwork of the original. In this work, we use a laser scanner to detect the surface morphology of an oil painting and first build the 3D model. Second, we extract the cross section from the 3D model with a certain contour interval according to the thickness of UV-LED ink. Finally, we print white ink layer by layer for 3D shaping and print a color image onto the neutral 3D model.

We selected an oil painting (**Figure 9**) of 7.5 cm (L) × 6 cm (W) for scanning and printing. A laser 3D scanner is used to obtain point cloud data of the original oil painting (**Figure 10**). Next, we measured the thickness of the UV ink to determine the number of the printing layers.

**Figure 9.** The original oil painting.

**Figure 10.** Point cloud model of the oil painting.

This work first conducted the UV-LED printing for 1–10 layers of C, M, Y, K, and W ink separately. Then, we measured the average thickness of the paper and every color lump by a CHY-C2 thickness gauge (see **Table 1**).


**Table 1.** Average thickness of every color lump.

Obviously, the thickness of these four ink films is different. The average thicknesses of C, M, Y, K, and W ink films are 11, 13.2, 7.9, 9, and 13.7 μm. According to the subtractive color-mixing principle, most radiation light will be absorbed after color mixing. White ink possesses both highest lightness and thickest ink film, so it is considered the best modeling materials for layered printing.

After 3D model slicing, 2D cross-sectional data can be obtained by intersecting the 3D model horizontally with a series of planes. The number of slices that we can obtain in this case is equal to the ratio of model height to white ink-film thickness. As a result, 91 layers are determined to be printed. **Figure 11** shows the result of cross sections extraction. Through contour vectorization and RIP, the neutral 3D model (**Figure 12**) and the reproduction (**Figure 13**) were printed by a UV LED printer.

**Figure 11.** Cross sections.

**Figure 12.** Neutral 3D model.

**Figure 13.** Reproduction.

**Layers Color lump 0 1 2 3 4 5 6 7 8 9 10** Average thickness (μm) C 243.9 251.7 261.6 272.6 283.2 294.4 307.1 318.2 328.5 339.2 351.8

Obviously, the thickness of these four ink films is different. The average thicknesses of C, M, Y, K, and W ink films are 11, 13.2, 7.9, 9, and 13.7 μm. According to the subtractive color-mixing principle, most radiation light will be absorbed after color mixing. White ink possesses both highest lightness and thickest ink film, so it is considered the best modeling materials for

After 3D model slicing, 2D cross-sectional data can be obtained by intersecting the 3D model horizontally with a series of planes. The number of slices that we can obtain in this case is equal to the ratio of model height to white ink-film thickness. As a result, 91 layers are determined to be printed. **Figure 11** shows the result of cross sections extraction. Through contour vectorization and RIP, the neutral 3D model (**Figure 12**) and the reproduction (**Figure 13**) were

**Table 1.** Average thickness of every color lump.

layered printing.

44 New Trends in 3D Printing

printed by a UV LED printer.

**Figure 11.** Cross sections.

**Figure 12.** Neutral 3D model.

M 243.9 255.7 268.9 282.6 296.2 309.5 322.5 335.4 348.3 361.5 374.6 Y 243.9 247.7 255.8 264.6 272.1 279.8 287.4 295.9 303.7 310.9 318.6 K 243.9 248.9 258.0 266.8 276.3 285.1 294.0 303.4 312.1 321.3 330.5 W 243.9 254.8 268.6 282.9 296.9 310.1 323.8 337.3 350.2 364.1 378.9

#### *4.1.2. Application for a topographical map of Taiwan*

3D Printing of a colored topographical map follows the same principle as printing a stereo oil painting. First, we extracted and vectorized the contours of the topographical map of Taiwan. Then, we provided different colors to various heights of contour sections according to the standard of altitude tinting. Based on the actual situation, in order to obtain the best printing effects and quality, 14 layers were tinted as shown in **Table 2**.


**Table 2.** Altitude tinting of the topographical map.

Finally, we use UV-LED printer to print the topographic relief and altitude-tint legend. The horizontal scale is 1:3,000,000, the vertical scale is 1:1,500,000, and the printing height is 2.59 mm.

#### **4.2. Paper-based 3D Printing**

Matrix 300A, a kind of paper-based 3D printer, is provided by MCOR technologies. It contains a standard Epson 310N printer supplied with the color system and a matrix printer responsible for elevation building. An A4 paper is used and the aqueous adhesives used to adhere two layers volatilize no harmful gas.

**Figure 14** shows the flowchart of printing steps. The process begins with a 3D digital model in "\*.stl", "\*.obj" or "\*.wrl" format. Each edge of the layer is printed with bar codes, which are used to read the sequence. Before the 3D building, a piece of a blank paper is affixed to the build plate, and the glue spreader will uniformly coat aqueous adhesive on the paper. Then, the colorful layers are laid on the paper tray of the matrix printer, and the top layer will be pulled by paper-feed roller and gripper onto the built plate. As shown in **Figure 14**, Step 3, the build plate will rise to the top of the build space to press the heat plate, thereby smoothing the top layer and ensuring that it will adhere firmly to the blank paper at a high temperature. The blade will cut the top layer into the contour of the corresponding 2D cross section once the build plate lowers, and the glue spreader will coat aqueous adhesive on the top layer. Similarly, the next layer is processed; the process is repeated until all the necessary layers are processed. When matrix printer stops working, a large paper cube as shown in **Figure 14**, Step 4, is obtained. After the basal and excess portion is removed, the 3D object is obtained.

**Figure 14.** The printing process of the Matrix 300A.

Using paper as a printing material, paper-based 3D Printing shows an excellent performance on full-color reproduction. In fact, the printer only colors the surface of object, the excess and interior portions are all blank, which contribute to low-cost and high efficiency.

#### *4.2.1. Application for historic preservation*

Cultural relic is a very important part in the development of human culture. When visitors enter the scenic spots and historical sites, it is often difficult to see the full view of the cultural relics. Because of long time, corrosion, or man-made damage, cultural relics are difficult to save. 3D scanning technology provides a good permanent preservation of cultural relics, and 3D Printing technology provides a good method to produce a copy.

Take a Buddha sculpture for instance, it describes how paper-based 3D Printing technology can do the trick in historic preservation through 3D data acquisition, processing, and printing. The first step is to use the Mephisto EX PRO 3D scanner to obtain the 3D data of a Buddha, as shown in **Figure 15**. The second step is to use the Mephisto software to process the data. It mainly includes removing outliers, smoothing, and data compression, as shown in **Figure 16**. The final step is fabrication. The paper-based 3D printer of Matrix 300A is actuated to finish the reproduction of the sculpture. Basic workflow includes coloring-carving-sizing-pressingbonding, and the physical 3D model is shown in **Figure 17**.

**Figure 15.** Scanning process.

**4.2. Paper-based 3D Printing**

46 New Trends in 3D Printing

layers volatilize no harmful gas.

**Figure 14.** The printing process of the Matrix 300A.

*4.2.1. Application for historic preservation*

Matrix 300A, a kind of paper-based 3D printer, is provided by MCOR technologies. It contains a standard Epson 310N printer supplied with the color system and a matrix printer responsible for elevation building. An A4 paper is used and the aqueous adhesives used to adhere two

**Figure 14** shows the flowchart of printing steps. The process begins with a 3D digital model in "\*.stl", "\*.obj" or "\*.wrl" format. Each edge of the layer is printed with bar codes, which are used to read the sequence. Before the 3D building, a piece of a blank paper is affixed to the build plate, and the glue spreader will uniformly coat aqueous adhesive on the paper. Then, the colorful layers are laid on the paper tray of the matrix printer, and the top layer will be pulled by paper-feed roller and gripper onto the built plate. As shown in **Figure 14**, Step 3, the build plate will rise to the top of the build space to press the heat plate, thereby smoothing the top layer and ensuring that it will adhere firmly to the blank paper at a high temperature. The blade will cut the top layer into the contour of the corresponding 2D cross section once the build plate lowers, and the glue spreader will coat aqueous adhesive on the top layer. Similarly, the next layer is processed; the process is repeated until all the necessary layers are processed. When matrix printer stops working, a large paper cube as shown in **Figure 14**, Step 4, is

obtained. After the basal and excess portion is removed, the 3D object is obtained.

Using paper as a printing material, paper-based 3D Printing shows an excellent performance on full-color reproduction. In fact, the printer only colors the surface of object, the excess and

Cultural relic is a very important part in the development of human culture. When visitors enter the scenic spots and historical sites, it is often difficult to see the full view of the cultural relics. Because of long time, corrosion, or man-made damage, cultural relics are difficult to save. 3D scanning technology provides a good permanent preservation of cultural relics, and

Take a Buddha sculpture for instance, it describes how paper-based 3D Printing technology can do the trick in historic preservation through 3D data acquisition, processing, and printing. The first step is to use the Mephisto EX PRO 3D scanner to obtain the 3D data of a Buddha, as

interior portions are all blank, which contribute to low-cost and high efficiency.

3D Printing technology provides a good method to produce a copy.

**Figure 16.** Processing photograph.

**Figure 17.** Physical 3D model.

#### *4.2.2. Application for topographic maps*

Digital elevation model (DEM) plays a vital role in engineering construction, hydrology, geological prospecting, etc. Although considering preservation and update of map data, it could not convey actual and accurate 3D feelings to people. In addition, the traditional mold is time-consuming and costly. Thereby, an ability to easily build physical topographic map is necessary. 3D Printing provides the support.

**Figure 18.** Monochrome physical 3D model.

**Figure 19.** Colorful physical 3D model.

Slice IT that supports Matrix 300A is used to slice 3D digital model into many 2D layers with equal thickness, the thickness of the A4 paper is a fixed number, 103.2 μm. The 3D solid model of DEM is sliced into 293 layers. After the printing process of Matrix, the physical 3D model is obtained, as shown in **Figures 18** and **19**.
