Color separation is the process of converting an image, design, or photograph into a set of colors that can be burned on screens and printed. The screen print separation process is a crucial element in the development of a great print.
There are several types of color separations. Each is based on the style of graphic being printed along with other variables, such as the number of colors and what color substrate the graphic will be printed on. The complexity of the separation process can range from spot color separations for simple designs to simulated process color separations for high-end, photographic designs. Each technique has different behaviors on press:
Spot Color Separations
Spot color separation refers to separate solid colors that are not mixing. Spot colors are used for printing text, solid lines, and area images, but not full color photos or designs with complex gradients. Spot color separation is the most basic idea of color separation where each different block of color is on its own plate / screen.
Simulated Process Separations
Simulated process separation is the most commonly used type of separation for printing photorealistic images and t-shirt designs that contain millions of colors. This process uses elliptical halftones that layer on top of each other at the same print angles. This separation approach works for just about every design on any color t-shirt.
Index Color Separations
Index separation is a separation method that converts images using stochastic square dots of equal size instead of elliptical halftones. The end result is a bitmapped-looking image with each square pixel representing a different color in the limited color palette. The square dots sit side by side with no overlap. Although an index separation is not right for every design, it does have many advantages for printers:
- No moiré! The stochastic bitmap dots burn to mesh easily without the chance of a moiré.
- No ink buildup. Since the square dots sit side by side like a grid or puzzle, there's no heavy ink build up.
- Prints look consistent printed on any color substrate: lights, darks, and bright colors. Shirt color does not affect index separations the way it can with elliptical dots.
- Fewer variables on the press. An index separation eliminates many press variables that can arise with other types of separations.
- Since the halftone pattern is a bitmap, no Raster Image Processing (RIP) software is needed to output films.
CMYK True Process Separations
CMYK process separation mixes 4 main colors (Cyan, Magenta, Yellow, Black) to produce a full-color image. This technique is commonly used in offset printing. When screen-printing with process inks, some colors – like bright red, purple, and green – are difficult to reproduce and may be generated by additional screens in order to create a vivid print.
Unlike the other separation techniques that use opaque inks, CMYK process inks are light and transparent. For this reason CMYK separations are generally printed only on white garments. Although CMYK process prints can be reproduced on dark garments using a white underlay, the simulated process technique on darks will achieve a brighter, more consistent print. CMYK separations print with eliptical halftones and each screen angle is separated by 30 degrees. Doing this creates a rosette pattern that allows the ink to mix and create a wide spectrum of color.
Whatever process you choose, make sure your decision is informed about what will work best for the design, the t-shirt, and your press.