Color can be an important design element. In addition to matching a customer’s expectation of what a design will look like, color also highlights important features or helps show aspects of a design that need further examination.
Several 3D printing companies offer machines that print in color. The terms used to describe how the machines handle color vary. You will encounter terms such as “multi-color,” “simultaneous color,” and “full color,” among others.
If you are looking into 3D Systems 3D printers, for example, you will encounter color-choice, basic color, and full spectrum color. According to the company, color-choice printers print one to three colors at a time, depending on the loaded material. Basic-color printers print a few dozen colors together in one part. And full-spectrum color printers print 500,000 to 6 million colors in a single part.
Color 3D printing technology has reached the point where it’s important to understand the methods used to deliver fine, distinct colors. Some can print color pixel by pixel, while others print color in large blocks or shells.
A brand of 3D printers that 3D Systems refers to as “color-choice” printers depend on the color of the material loaded into the printer. These printers are primarily extrusion based, or Fused Filament Fabrication (FFF) type 3D printers, and targeted at consumer/prosumer users. Some of these printers are also viewed as “multi-color.” Basically this technology works like a computerized hot-glue gun that “draws” lines of molten plastic to create a part.
Colorants are added to the plastic filament fed into the extruder. To reproduce multiple colors, you either need multiple nozzles to extrude different colored plastics, or you need to mix colored filaments together in a single nozzle. While gradients and block colors are possible in different regions of the model, a photo-realistic part is not possible because of the nature of the FFF technology.
“Drop-on-demand” inkjet type 3D printers can be viewed as basic or full-spectrum color printers. The quality of the color print is determined by three main factors: the resolution of the printer, the number of primary colors and the processing capability per channel, and the printer’s capability of dithering or half-toning.
The print head selectively “jets” drops of ink onto a substrate (often paper or powder). Through the use of multiple print heads or multiple channels in one print head, you can jet multiple colors selectively and simultaneously.
A print head with high resolution can print more dots per inch (DPI). A higher resolution enables “crisper” colors and more accurate color placement. Resolution is typically given per axis of motion. In 3D printers, resolution can vary for each axis.
ColorJet Printing (CJP) involves two major components – core and binder. The core material is spread in thin layers over the build platform with a roller. After each layer is spread, color binder is selectively jetted from inkjet print heads over the core layer, which causes the core to solidify. By using at least three, and sometimes four channels of color, CJP does not limit the colors that can be created to one section of the rainbow. Colors can be placed anywhere on the model and can be matched. Ranging from 6 bits per voxel (BPV) to 40 BVP, ColorJet Printers can print more than 6 million discrete colors than the human eye can see.
3D Systems’ ColorJet printers can print photo-realistic 3D models with the use of half-toning and variable drop per voxel technology. This is possible by using Cyan, Magenta, Yellow, and in some printers, Black binders to print onto a white powder.
In the bits
The term voxel is used to help denote “bit” depth. A “bit” is the smallest unit of data and it either on or off, 1 or 0, black or white, or ink or no ink. The number of bits in each voxel indicates the number of printable tones. Typical digital images contain 8 bits per channel, 2⁸, or 256 discrete tones. When more than one color is involved, the total number of printable colors is the product of the number of printable tone of each color. You will often hear the term “bits per voxel,” or BPV.
BPV = (# bits x # channels)
The number of colors available per voxel is therefore:
Possible colors per voxel = 2BPV
The following table illustrates how the BPV relates to number of colors:
Color variations
A 3D printer with the ability to dither, or halftone, color information can handle minor gradations of color, enabling more color choices. Gradations are formed by changing the size and pattern of ink drop on the paper.
Halftone printing lets you print patterns of color such that the different color drops appear to be a single combined color when viewed from a sufficient distance. Halftoning delivers more photorealistic models.
Thus, a higher resolution printer can print more dots per inch (DPI). A printer that can process more bits per voxel can print more colors. A printer with the ability to dither or half-tone, as well as variably drop different amounts of colors into each voxel, enables photo-realistic parts.
Leslie Langnau
llangnau@wtwhmedia.com
Source: 3D Systems
