3D Printing

 

3D Printing

1. Introduction

Recently, the application of personalized 3D Printing (3DP) in culture and art has been experiencing a gradual increase in demand. Beautiful lively items with exquisite appearance are in need of fabrication and reproduction, with high artistic and economic value, which is greatly expanding in the market of rapid prototyping.

However, the 3D Printing process has restrictions due to forming materials and equipment; the presence of monochrome objects makes it difficult to meet the demand of high-quality reproduction. These limit the further development and prosperity of the emerging 3D market with high value-added potential products in the field of culture and art.

We have the technique and configuration needed to develop a workflow from 3D digital model to rendering images and color 3D Printing can be implemented into four stages or parts.

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

·         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 a series of new issues are also presented.

·         The third part shows the modeling data representation and data transformation with a short review of the rendering pipeline for color 3D Printing.

·         This part provides specific mechanics from 3D modeling to “ready to print.” A graphics processor usually performs several complex calculations in addition to the transformation from object space to window space. This specifies how the fragment data are used to determine the final color and the final depth of each pixel.

The last part demonstrates a few instances of color objects produced with the process, showing the color reproduction and further potential applications.

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.

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:

 1. powder-based 3D Printing

2. plastic-based 3D Printing

3. paper-based 3D Printing

4. organism-based 3D Printing

5. food-based 3D Printing and

6. 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.

3. Color 3D Printing

Whether in personalized design work, advanced aerospace component manufacturing, or even in three-dimensional cells, the color and appearance issues cannot be ignored. Although in industrial design, mechanical design, or creative and cultural industries, there are not yet satisfactory color reproduction solutions or a low-cost color reproduction technology.

But 3D color printing already has a huge and significant impact on life and society. Along with the development of different three-dimensional manufacturing processes, the 3D color printing will also accumulate certain color reproduction techniques.

3D color characterization is an important tool to communicate the color reproduction. Currently, the uniform color spaces are also applied in 3D color characterization, such as the Cielo system.

Color quality has an irreplaceable effect on the surface appearance of the color objects. Color properties and stability are the two key factors that influence the product quality.

4. Stages of Development

1. colour management

during the 3D color inkjet printing, coloring is achieved by adding CMY or CMYK color adhesive to mixed transparent adhesives. The colorant in adhesive has its main role as coloring agent. 3D digital models can have a variety of coloring methods,

such as directly on the surface, or coloration is applied as the image content, in frequently used information carrier, such as TIFF JPEG file formats files. Of course, a colorful 3D digital model needs to be saved in formats that can include its color

information, such as VRML, PLY, ZPR, or other proprietary format Issues of coloring of 3D inkjet printing include 3D color reproduction, color consistency and control, color gamut range, effects of materials and processes, the surface characteristics

of 3D Printing, and color measurement and other urgent problems to solve. Another related problem is the persistence and firmness of the finished 3D model; both are becoming increasingly important in the 3D inkjet printing and other common applications such as space planning and art and design applications.

 2. Colour detection,Colour measurement and Colour control

It has been proposed for 10 years that rapid prototyping technology can produce colored objects with basic color or full spectrum of colors, such as the Z crop Company developed the 3D inkjet printing system. While the conventional color inkjet printing is being researched and developed for a long period of time, 3D inkjet technology prints different materials with the corresponding coloring processes,

which may have special problems. The overall performance is affected by the physical appearance of the object and color appearance, material’s gamut, liquidity of base material before forming, the positioning accuracy of printed surface, and the accuracy of scanned digital information. The main effect on finished surface of 3D Printing objects is caused by different printing principles, printing equipment, as well as different features and rough texture of powdered print material particles.

Different positions on the surface of printed objects and different x, y, and z location coordinates on 3D model will affect the result of colored reproduction. This is due to manufacturing method and layered approach to generate model, which causes difficulties in superimposition of layers to achieve true vertical surface. The results have curved surfaces with certain angles, so it inevitably has questions of aliasing and grinning. Of course, parts of the surface effect issues can be solved during subsequent processing.

 3.  Colour stability and Durability

3D Printing applications are being used in new areas, people discuss about a wide range of common properties, such as durability and stability of color 3D Printing objects. These issues are of more interest to art and design people, who use rapid prototyping technology to aid their work, because they publicly display their works of art, and geospatial information systems, architecture, space planning, scenic modeling, and urban design need excellent color effects.

Color is a core part of printed objects, and people expect the original effects to remain for a certain time, Duration and stability depend on several factors, such as light, humidity, temperature, and the amount of air in contact. Currently, the durability and stability study of 3D printed color focus on how to find the right way to simulate aging or change real 3D prints, which are exposing in different environment conditions. For example, different lighting, temperature, humidity, the finished color of 3D Printing,

And the behavior of color change with different impregnating agent can be simulated and predicted. Custom color strips should be developed in test researches, as well as corresponding colors and test procedures, so that people can conduct relevant research.

 4. Digital coloring method

A color model describes the displayed color effects of print. Different color models define different range of colors. The fields of their applications are different. These applications are in addition to determining the number of colors inside a model, the number of color channels, and the file size of color image information. Here we present several related concepts of color images.

Bit depth: Bit depth, also called the pixel depth or the color depth, is used to measure color information in the image to display or print. Greater bit depth means that the digital image has more colors and more accurate color representation.

Gamut: This represents the range of colors capable of displaying or printed. Lab color space has the widest color gamut, which can contain all the color types from RGB and CMYK color mode. Typically, RGB color gamut can be displayed on a computer monitor or a television screen. However, some, such as pure cyan or pure yellow and other colors, cannot be accurately displayed on digital monitors. CMYK color space has narrower gamut, containing only color types used in printing,

which can be printed with ink. When a color cannot be printed or displayed on the screen, it is called out of the gamut, that is, beyond the CMYK color gamut. Color channel: Each color image has one or more channels, and each channel can store the color information of imagery elements. Default color channels in an image depend on their color space. The CMYK image has at least four channels, each representing the color layers cyan, magenta, yellow, and black.

The RGB image has three channels, representing the color layers of red, green, and blue. Gray image: This type of image can exhibit a rich range of tone. It uses up to 256 shades of gray. Each pixel has a grayscale image luminance value from 0 (black) to 255 (white). Black and white or grayscale scanning device produces images that often appear in this gray image. If a high-quality color image is converted into a black and white image, Photoshop will display all the color information in the original image.

When a gray image is converted into an RGB image, the color values of pixels will be lower than the previous gray image values. A gray image can be converted to a Lab or a CMYK color image.

 5. Two-dimensional interpolation method

Colorful images cannot scale without interpolation methods, and commonly used color interpolation methods are bilinear interpolation, cubic convolution, and B-spline interpolation.

 6. 3D real volume rendering

A solid color has two types of applications: one is coloring the surface of 3D solid objects. However, sometimes in order to meet industry needs, products internal parts can have color too. For example, if each part in a car engine has its own color code, it will provide the product management, assembly workers, and maintenance workers great convenience. The other type is coloring the entire inner side of an object or add color to a three-dimensional entity internally which is a commonly used method of volume rendering.

The goal of drawing color information in two-dimensional surface graphics is to add color on the face, same as the concept of pixels in plane. The number of pixels per square inch defines the concept of resolution. Corresponding to surface graphics, there is volume graphics and its models. In the application of three-dimensional grid to directly draw a three-dimensional scene, each grid in space corresponds to a value of particular property, which can be measured.

Those three-dimensional grids are called voxel. Voxel can be seen as a two-dimensional pixel in three-dimensional space. It can be understood that an entity is composed of a number of small cubes or other three-dimensional primaries. An entity may be a function of an image but composed by voxels. In real three-dimensional scene visualization, the information of a three-dimensional coordinates is corresponding to attribute data set. In this real visualization,

the three-dimensional coordinates (x, y, z) are expressed as dependent arguments to space entity for object geometry modeling. In that way, an established model can not only achieve true three-dimensional visualization, but it can also be used in a three-dimensional spatial analysis.

4. Instances of colour objects with the process

two kinds of instances of color objects created with the process are demonstrated, showing the color reproduction and further potential applications.

 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.

 Application for oil painting reproduction

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.

3D printing technology brought historic evolution in every developing field. It is utilized in Geological mapping, Historic preservation, Powder application in additive manufacturing of metallic parts, Geopolymer development, Biomimetic functional nanocomposites, Art World, Walking Robots, Bio scaffolds for developing Tissue-Engineered constructs, pharmaceutical sectors, Cardio Vascular Anatomy etc.