Have you ever seen white space between two colors on a printed tag or label? This printing error is due to the misalignment between two color plates on the press. Flexographic presses use flexible relief plates to print directly onto the paper. Each color used in a design requires a separate plate, so if there are four colors on the design, for example, there will need to be four corresponding plates for each separation. In the example below, you will need two plates to print the completed design. Plate 1 contains a blue background, while Plate 2 contains a yellow graphic.
Due to the nature of all the moving parts that allow the press to print at high speeds, the paper will shift as it passes through the press. Most of the time, the shift is barely even measurable, just a fraction of a millimeter, but the paper does move. This movement is what results in a white gap between colors. There are two different solutions to avoid this gap. The first is overprinting the design, and the second is by trapping the design into a knockout.
overprint
Overprinting is the simplest approach, as it involves the colors being printed directly over one another. Depending on the lightness or value of the colors used in the overprint, one of the colors may become deluded and not retain its original hue or saturation. This is a common issue when overlaying a lighter color over a darker bottom color, resulting in the top layer adopting some of the bottom color's value. Notice how in the overprinting column in the example above, the color of the yellow graphic changed because of the blue showing through underneath, making it appear as though the yellow is transparent. In some cases, overprinting can be beneficial in helping to save on the cost of printing if additional colors are needed. For example, it is the overprinting of cyan, magenta, yellow, and black which allows CMYK printing to produce thousands of colors with only four plates. However, overprinting is not ideal when a design needs to maintain color integrity. When the print needs to preserve the original spot colors used in the design trapping and knockouts are used.
knockout
A knockout is a more complex approach to multi-plate printing in which a portion of the bottom plate is "knocked out" to show the color of the paper underneath. The top plate then masks this unprinted portion of the bottom plate. As shown in the example above, plate 2's shape is knocked out of plate 1, allowing the triangle graphic to fit within its negative space like a puzzle piece. The graphic's intended pure spot color can now print directly onto the white substrate, maintaining the color's integrity. By itself, a knockout will inevitably leave gaps of white space between the adjacent colors due to the shifting that occurs during the printing process. There will need to be a slight overlap in color around their borders to compensate for this misalignment.
Trapping
This overlap of color is called trapping. Trapping adds a buffer between the intersecting elements of the knockout, thus preventing any white gaps in the print. For a design to be trapped correctly, the lighter color should always bleed into the darker color for the end image of the artwork to remain the same intended size. Depending on the design, a trap can either be a choke or a spread.
chokes
A choke is a trap that bleeds the background color into the shape. In the example below, notice how the choke reduces the size of the bottom knockout so that it overlaps, or "chokes," around the edges of the design on top. Although the knockout shape is smaller, the triangle will optically appear to remain the same size due to the darker color of the blue overpowering the lighter yellow.
spreads
A spread is essentially the opposite of a choke. Where a choke brings the outer color inward, a spread extends the inside color outward. This time, the knockout shape on the blue plate remains the same size, while the triangle graphic is enlarged and "spreads" over the knockout. Even though the lighter color is now on the top plate, the size of the triangle remains unchanged as the darker color of the knockout plate holds the shape.
As you can see, color determines everything! Some designs will require overprinting, while others will require trapping. And even then, creating a choke versus a spread will depend on what colors are used in the design. Just remember, darker colors will hold their shape while lighter colors will always trap into the darker color.
Knowing some of these basic prepress terms can help guide you in preparing artwork for print and help streamline your ordering process. Not sure if your artwork will require overprints, knockouts, or trapping? No worries! Our on-site prepress graphics team will ensure that your artwork is set up to produce the best print every time!