**2. Current color printing processes**

3D Printing technology is a powerful technology that is leading significant changes in many areas. 3D Printing can be defined as a process creating an object by printing each sliced section layer by layer until finishing a completed object. In traditional manufacturing industries, more customized and creative functions could be provided By 3D Printing [1]. Currently, the 3D Printing is still in the research era of printing precision and speed. However, this is also the first step to exploit all advanced 3D Printing techniques, such as whole colorful printing and large-sized integrated printing. Moreover, color 3D Printing has become a hot topic in the 3D Printing field for many researchers and manufacturers. Even though the colorimetry is not necessary for all reproductions of 3D color objects, but this function is urged to provide and improve for manufacturers.

From the view of classic materials and applications, the 3D Printing process can be divided into six categories: powder-based 3D Printing, plastic-based 3D Printing, paper-based 3D Printing, organism-based 3D Printing, food-based 3D Printing, and metal-based 3D Printing. In addition, the glass-based 3D Printing is a new useful process to produce amazing glass artistic wok which only focused on its transparency and styling. Colorimetry is the core feature of color objects, showing its artistry and aesthetics, which should be reproduced accurately to meet the advanced demands. Recently, many published articles have shown various research‐ es about color 3D Printing from different areas. According to 3D Printing applications, generic color 3D Printing technologies are demonstrated with relative principles and instances in the following parts.

## **2.1. Powder-based color 3D Printing**

The first part reviews the current situation in terms of 3D Printing and focuses on color-related

At first, a quick discussion is presented about models, which are constructed out of vertices and polygons, and the impact of modeling techniques in terms of mapping color information into model itself. And some formats as examples are used to demonstrate the differences.

The second part shows the theory of color 3D Printing process, at first about the work of our research and focuses on the hardware and process to print the interpreted data into objects and transfer color information into the model. The related 2D printing applications as well as

The third part shows the modeling data representation and data transformation with a short

This part provides specific mechanics from 3D modeling to "ready to print." A graphics processor usually performs several pervertex calculations in addition to the transformation from object space to window space. This specifies how the fragment data are used to determine

The last part demonstrates a few instances of color objects produced with the process, showing

3D Printing technology is a powerful technology that is leading significant changes in many areas. 3D Printing can be defined as a process creating an object by printing each sliced section layer by layer until finishing a completed object. In traditional manufacturing industries, more customized and creative functions could be provided By 3D Printing [1]. Currently, the 3D Printing is still in the research era of printing precision and speed. However, this is also the first step to exploit all advanced 3D Printing techniques, such as whole colorful printing and large-sized integrated printing. Moreover, color 3D Printing has become a hot topic in the 3D Printing field for many researchers and manufacturers. Even though the colorimetry is not necessary for all reproductions of 3D color objects, but this function is urged to provide and

From the view of classic materials and applications, the 3D Printing process can be divided into six categories: powder-based 3D Printing, plastic-based 3D Printing, paper-based 3D Printing, organism-based 3D Printing, food-based 3D Printing, and metal-based 3D Printing. In addition, the glass-based 3D Printing is a new useful process to produce amazing glass artistic wok which only focused on its transparency and styling. Colorimetry is the core feature of color objects, showing its artistry and aesthetics, which should be reproduced accurately to meet the advanced demands. Recently, many published articles have shown various research‐ es about color 3D Printing from different areas. According to 3D Printing applications, generic color 3D Printing technologies are demonstrated with relative principles and instances in the

research and applications.

26 New Trends in 3D Printing

a series of new issues are also presented.

review of the rendering pipeline for color 3D Printing.

the color reproduction and further potential applications.

the final color and the final depth of each pixel.

**2. Current color printing processes**

improve for manufacturers.

following parts.

Powder-based color 3D Printing is widely applied into artistic work, geographic information system, and Medical buttress applications [4]. Based on powder, two 3D Printing processes can be found in manufactured instances. One is the famous 3DP process that is innovated and patented by Massachusetts Institute of Technology. The 3DP process is mainly using basic powder and color blinder combined by the inkjet printing method to build a color part. Another is the selective laser sintering process (SLS) which creates a completed model by sintering each Powder layer [3]. The big advantage of powder is its good form ability and stability. At the same time, some special powder also provides a good connection with color binder based on low temperature.

The 3DP process was licensed to Z-Corporation Company, focusing on the concept model implementation, which developed and sold the first high-definition color 3D printer named Spectrum Z510 in 2015. The colorimetry features of color objects are reproduced by inkjetting various color binders on powder layers. Three current color binders can be obtained, such as primary cyan, magenta, and yellow. The surface color of objects can be presented by the mixture of various ratios of above primary color binders. However, the black ink in the traditional inkjet printing is replaced by clear binder that is connected to most noncolor powder layers. That is, the ingredients of color binders play an important role for the reproduction of color 3D objects. For this 3D process, it easily achieves the postfinished step by using the designed infiltrates to make a better color performance. These finished methods were intro‐ duced and contrasted by Maja Stanic using two different Infiltrates in 2012 [21]. In addition, to infiltrates, the wax is also a good supplement material for the fashion designers to improve the color features of 3D printed artifacts under this process. The 3DP process is a good technology to build Buddha state with a bit heavy.

According to the current 3D printer, the SLS process can be achieved by two general ap‐ proaches: direct method and indirect method [5]. The basic powder is varied from the metal powder to plastic powder to calcium powder in diverse applications. In this case, the color features of productions of the SLS process are limited to the simple or single color. From the current device and application, this depends on the original color of basic powder. For the plastic powder, the color of the printed objects nearly consists the original color of the basic powder. For the calcium powder, the color of 3D printed products may be changed in the lightness. Moreover, it is easy to produce different primary color powders by material manufacturers. For the last, the color of mental powder object can be added to metallic gloss. The SLS process provides a useful approach to create tissue scaffolds such as Bone and teeth [8].

#### **2.2. Plastic-based color 3D Printing**

Plastic-based color 3D Printing is a cheap and fast process to provide printed color objects, which widely applied in engineering parts. Based on the plastic material or the photosensitive resin, stereo lithography appearance (SLA) and fused deposition modeling (FDM) are two representative 3D Printing technologies. This method makes the most of the three features of plastic or resin such as easy fusion, good formality, and stable irreversibility. Moreover, the plastic-based color 3D Printing had captured a big market share of 3D Printing industry. There are two factors, such as physical strength and color characteristics, to make a huge success.

The SLA process is based on the liquid resin and the ultraviolet laser; some famous research institutes show great enthusiasm into the color function. For example, America Materialise Company had used two resins (red and green) to print a medical model to distinguish the diseased tissue and normal tissue to diagnostic etiology. University of Tokyo had adapted the inkjet printer to achieve the surface colorization for each designed layer in the SLA procedure. Pusan National University had presented a colored MEMS visualization model and layer interconnection visualization of electronic packaging part by using an inner visible multicolor Prototyping process [9]. Their idea is that first the resin in the noncolor area of each layer is solidified and then the resin in the color area is replaced by a new color resin to form the color part until A completed object [7].

The FDM process is patented and advertised by Stratasys Company, which used different colored plastic silks with low melting point to form a new color object. For FDM printer, it usually contains four hot-melt nozzles to provide three primary colors and one model bracket. The Fortus 900mc printer is an amazing star product which provides super print size with nine printing substrates. In China, Tsinghua university had developed a new own FDM printer without the laser melting device which sharply decreased the price of this printer. For this process, hundreds of colors can be produced in the surface of different color objects. However, it is so difficult to make gradient color in the local area of The whole object [6].

#### **2.3. Paper-based color 3D Printing**

The paper-based color 3D Printing is a technology using regular office paper as the main substrate and adding color inks to implement Thousands of colors [10]. This process is similar to inkjet printing but extend into three dimensional formation. Generally, according to forming principle, the paper-based color 3D Printing technology also contains two processes implanted by 3D printers such as MCOR IRIS series 3D printer of Mcor-technology Company and Mimaki UJF-3042 UV printer of Mimaki Company. For these two processes, they both provide tempting color reproduction of nonlarge color objects. Currently, the MCOR IRIS series 3D printer can achieve 256 × 169 × 150 mm (for an A4 paper) and 238.5 × 175 × 149.9 mm (for a letter-sized paper). However, the Mimaki UJF-3042 UV printer can build 300 × 420 × 50 mm. In addition, their colorization principles are also different even though using the basic office paper.

The MCOR IRIS series 3D printer is an undoubted powerful color 3D printer that is frequently applied into creative cultural industry. This process is divided into two parts such as colori‐ zation and modeling. The first step of colorization is to print color inks within the contour area at both sides of each paper to make sure the ink fills sufficiently the inner space of the paper. When one layer is colorized, it is sent for trimming and gluing. The color of cutting cross section and the surface paper build the completed surface of the color object. For this printer, the printing speed is and the build size is also limited. To solve the first issue, JP Yuan and GX Chen proposed a speedup method for paper-based 3D color printing by developing a new fast and reliable coloration algorithm using geometric information in STL file, combining the existing different slicing methods, which can be applied in the paper-based color 3D printer based on The uniform slicing thickness [11]. To solve the second issue, JP Yuan and GX Chen presented a feasible visualization of large-size 3D color models which are beyond the maxi‐ mum print size of the newest paper-based 3D printer using 3D cutting-bonding frame (3D-CBF), and evaluated their effects of cutting angle and layout method on the printing time of The designed models [12].

The Mimaki UJF-3042 UV printer is used as the tiny 3D Printing or 2.5D printing tasks in which the height of the color object is small but it requires high resolution. This process is similar to the traditional inkjet printing using color UV inks. It adapted six UV ink boxes and nozzles such as CMYK four-color inks and two transparent inks. A major difference is that each layer can deposit on the previous layer until finishing a whole object with excellent color surface. C. Chen et al. analyzed the effect of colorization sequences of color ink and white ink on oil painting 3D reproduction and found that the color ink printed in the upper layers can form an effective smooth and Continuous curved surface [13]. H.M. Wang and G.X. Chen also studied the process applied in the Taiwan map 3D reproduction based on different substrates including office paper, and provided a printed sequence frame to retard the jagged Edge phenomenon [14, 22].

#### **2.4. Cell-based color 3D Printing**

plastic-based color 3D Printing had captured a big market share of 3D Printing industry. There are two factors, such as physical strength and color characteristics, to make a huge success. The SLA process is based on the liquid resin and the ultraviolet laser; some famous research institutes show great enthusiasm into the color function. For example, America Materialise Company had used two resins (red and green) to print a medical model to distinguish the diseased tissue and normal tissue to diagnostic etiology. University of Tokyo had adapted the inkjet printer to achieve the surface colorization for each designed layer in the SLA procedure. Pusan National University had presented a colored MEMS visualization model and layer interconnection visualization of electronic packaging part by using an inner visible multicolor Prototyping process [9]. Their idea is that first the resin in the noncolor area of each layer is solidified and then the resin in the color area is replaced by a new color resin to form the color

The FDM process is patented and advertised by Stratasys Company, which used different colored plastic silks with low melting point to form a new color object. For FDM printer, it usually contains four hot-melt nozzles to provide three primary colors and one model bracket. The Fortus 900mc printer is an amazing star product which provides super print size with nine printing substrates. In China, Tsinghua university had developed a new own FDM printer without the laser melting device which sharply decreased the price of this printer. For this process, hundreds of colors can be produced in the surface of different color objects. However,

The paper-based color 3D Printing is a technology using regular office paper as the main substrate and adding color inks to implement Thousands of colors [10]. This process is similar to inkjet printing but extend into three dimensional formation. Generally, according to forming principle, the paper-based color 3D Printing technology also contains two processes implanted by 3D printers such as MCOR IRIS series 3D printer of Mcor-technology Company and Mimaki UJF-3042 UV printer of Mimaki Company. For these two processes, they both provide tempting color reproduction of nonlarge color objects. Currently, the MCOR IRIS series 3D printer can achieve 256 × 169 × 150 mm (for an A4 paper) and 238.5 × 175 × 149.9 mm (for a letter-sized paper). However, the Mimaki UJF-3042 UV printer can build 300 × 420 × 50 mm. In addition, their colorization principles are also different even though using the basic office paper.

The MCOR IRIS series 3D printer is an undoubted powerful color 3D printer that is frequently applied into creative cultural industry. This process is divided into two parts such as colori‐ zation and modeling. The first step of colorization is to print color inks within the contour area at both sides of each paper to make sure the ink fills sufficiently the inner space of the paper. When one layer is colorized, it is sent for trimming and gluing. The color of cutting cross section and the surface paper build the completed surface of the color object. For this printer, the printing speed is and the build size is also limited. To solve the first issue, JP Yuan and GX Chen proposed a speedup method for paper-based 3D color printing by developing a new fast and reliable coloration algorithm using geometric information in STL file, combining the existing different slicing methods, which can be applied in the paper-based color 3D printer

it is so difficult to make gradient color in the local area of The whole object [6].

part until A completed object [7].

28 New Trends in 3D Printing

**2.3. Paper-based color 3D Printing**

Cell-based color 3D Printing is popular in medical applications such as the organism printing and tissue producing. From the printing principle, the cell-based color 3D Printing is similar to the inkjet printing, whereas the basic material is not ink but various cells. The color of cell in body behaved like a natural color, and this color is stable even if it maybe changed with the oxygen concentration In regular direction [2]. For the outside soft tissue of body, the color of prostheses is very important for transplant recipients. Xiao et al. had developed a 3D color image reproduction system for additive manufacturing of facial prostheses based on the ZP510 Printer [15]. This frame provided a good instance and flowchart for the color reproduction of 3D Printing cell. C.L. Cheung et al. also introduced the development and face validation of a pediatric pyeloplasty simulator using a low-cost laparoscopic drylaboratory model developed with three-dimensional (3D) printing and silicone modeling and provided the color repro‐ duction application of kidney Based on silicone rubber[16].

#### **2.5. Food-based color 3D Printing**

Food-based color 3D Printing is a generalized definition which contains all processes that are achieved by 3D Printing in food industry. For the personalized food, the color is also an important factor to attract customer and provide appetite temptation. Actually, the snacks are available and suitable for 3D Printing. Yang et al. had used sugar and different flavor binders to fabricate complex sculptural cakes For special events [17]. Foodjet Company had used pneumatic membrane nozzle jets to deposit selected material drops onto pizza bases, biscuits, and cupcakes by the Inkjet printing process [18]. Fu Jia et al. investigated the feasibility of supply-chain-centric business models in 3D chocolate printing and provided suggestions for color requirements of commercial chocolate produced By the food 3D printer [19]. The above instances provided color surface but just simple pure color. In order to achieve more diverse and colorful eatable food, Jie Sun et al. introduced and analyzed the effect of multimaterials and multiprintheads on color food products, and provided a feasible method to optimize the reproduction of color Eatable food [20].

#### **2.6. Current researches of color 3D Printing**

3D color printing is a promising technology that will produce all the color objects in daily life; however, there are rare relevant researches now. For the different 3D color printing processes, the colorization principles are varied from the devices and software. Like the printing graphic, the color reproduction of color 3D Printing is also a key issue for manufacturers and designers. So, 3D color measurement, 3D color characterization, and 3D color quality evaluation are the three key factors to improve the color reproduction of 3D color objects.

For the 3D color measurement, Stanic et al. systematically introduced the selected measure‐ ment method based on inkjet printing parts. This method is similar to CIE guide to the color measurement of printing graphics. In color 3D Printing instances, some researches proposed their measurement devices and conditions. Stanic had adopted the GretagMacbeth (X-rite) XTH sphere spectrophotometer as the measurement for the printed powder-based color object. Xiao et al. used a Minolta CM-2600d spectrophotometer using Spectra Magic NX Color Data Software that was employed to study color measurements in CIELAB values. The illuminant was based on the CIE standard D65 to simulate skin color in daylight conditions. He et al. selected the X-rite 530 spectrophotometer for the 3D color patches printed by the Mimaki UJF-3042 UV printer. Obviously, the color measurement tool is the same as that in printing graphics. This maybe caused the unexpected color error under the curved surface.
