Dear valued partners, issues like doctor blade lines, press marks (nip marks), and streaks on paper are common quality headaches that many printers encounter. These surface defects not only ruin the appearance of your printed products but also disrupt ink transfer and even affect the overall operating efficiency of your printing presses-causing unnecessary production delays and potential losses for your business. Today, I'd like to walk you through the root causes of these defects and their specific impacts on printing, so you can quickly identify and resolve issues when they arise, minimizing disruptions to your production line.
First, let's talk about doctor blade lines, also known as coating streaks. These typically occur during the coating production of coated papers, such as gloss art paper, matte coated paper, and lightweight coated (LWC) paper. As you know, a smooth and uniform coating is crucial for high-quality coated paper, and the doctor blade plays a key role in scraping off excess coating from the paper web. However, problems arise when there are hard particles or oversized pigment agglomerates in the coating, or when the doctor blade itself is worn, chipped, or nicked, or if the blade pressure across the web is uneven. Under these conditions, the blade will scratch the wet coating layer, leaving permanent marks that dry into distinct doctor blade lines.
These lines have a characteristic appearance: they run along the machine direction (MD) of the paper, appearing as thin, continuous streaks-either a single line or multiple parallel lines. In some cases, they're only visible under oblique light. The impact on printing varies by severity: shallow lines remain visible after printing, compromising the overall look of your finished products; deep lines can lead to coating damage down to the base paper, resulting in uneven ink absorption during printing and causing issues like mottled prints, which directly harm your product's reputation and market appeal.
Next, let's cover press marks / nip marks, also called roller marks. These defects form during calendering, winding, or rewinding processes-stages where paper passes between two pressure rollers. They occur when foreign objects (such as dried coating, adhesive residue, or paper dust) stick to the roller surface, the roller itself has pits, bumps, or damage, or when paper folds or wrinkles get flattened under pressure. Essentially, the roller's surface irregularities are "stamped" onto the paper, creating press marks.
Their appearance is highly variable, matching the shape of the roller defect: they can be dots, short lines, or irregular patches. A key feature is that they often repeat at regular intervals, corresponding to the roller's full rotation cycle. What's more, they may appear on both sides of the sheet at the same position, a clear sign of damage to a pair of rollers. For your printing operations, press marks cause uneven surface topography on the paper, leading to inconsistent printing pressure. This results in incomplete image transfer, color variations, and in severe cases, coating cracking and base paper exposure-all of which can ruin your printed products and lead to costly rejections.
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