While color can be created with ink that is pre-mixed or formulated with ink mixing systems, the substrate will always challenge the true weakness in all inks -- opacity. Ink manufacturers match color based on a standard printing parameter on a white cotton substrate. This is a necessity to maintain control of a standard matching requirement, but it falls short under certain circumstances. When an ink is printed on a dark substrate, or worse, a dark polyester blend, it requires a base plate.
Color is reflective light. Light travels into the color and reflects back to our eye the base and color of the sample we are viewing. We see good color when ink is printed directly on a white cotton fabric, because the ink absorbs into the cotton creating a base that is virtually the same color as the top coat. When the same color is printed on a white ink base, the color is not absorbed and the white reflects through as well. This results in a lightening of the overprint color.
To combat this, screen printers adjust screen meshes and printing techniques to deposit more overprint color onto the under-base white. Eventually, the color blocks enough light that it appears to be the correct color when viewed by eye.
The unfortunate twist to this solution is that different color ways need custom print parameters to achieve satisfactory results. For instance; gold, oranges and pastel colors need less film coverage than blue, purple and red hues. This is generally due to pigment strength and color formulas for each mix. Once the print starts in production, the ink shears, thus changing the opacity once again and requiring the technician to adjust parameters during the run.
Shades of Gray
Taking color to its basic root, ink can be created to match a specific hue or saturation. Once a dark substrate is introduced, the brightness of the color is compromised. If we strip the hue and saturation out of a design, we wind up with a gray study of the art. Keeping this picture in mind, imagine all the color in the world, then de-saturate them to the point of no hue. What is left is; one true white, one true black, and all other colors falling into shades of gray. With this mindset, a strong argument can be made for less full-white plates for under-basing and more use of gray for most colors.
This is not to say that using full-white under-base will not yield good color results, just that this print technique will enable the press technician to hit color quicker, resulting in faster set-up and truer color throughout the run.
Printing Techniques with Gray
There are three basic techniques to create a great base for quick color approval.
Halftone white on Black substrate
This technique involves two white plates and is only exact on a black fabric. The first plate is the full image of the design. This screen is printed with a based-back white through a high mesh. Once printed on the black fabric, it will appear gray in color. The second white plate is a scaled back white plate that can either be printed directly on the first plate or after a flash. The second plate is constructed to lay white under colors that need a higher white under-base to achieve brightness. These colors will include white, pastels and bright yellows which are located in the tint side of the Pantone Book. All other colors like deep saturated tones, dull shades and mid-tone saturated colors will be printed directly on the “gray plate”.
Gray plate with White Overprint
This technique is best used on polyester or polyester-blend materials that require bleed resistance. Using the same basic plates as used in the first example, the first plate is printed with a low-bleed under-base gray ink to created the gray color and help with bleed resistance. This plate must be flashed before a second plate is applied over it. The second print will be a low-bleed white. Again, this white ink is under the brighter colors only.
Using the same plates, and printing on a dischargeable cotton fabric, the discharge base will replace the gray portion of this technique since the ink is saturating into the “natural” color of the fabric, the colors will create a true hue and the lighter colors will still need a bit of white to pull in the brightness needed to hit color quickly.
Standardization at the press
Using this technique in the art and under-base application allows the technician to standardize other aspects of the print. Once the lightness and brightness of the design is established, the last supplication (color) can be printed through similar mesh counts with similar pressure and squeegee settings. This makes for a quicker set-up and approval.