Detailed list of offset defects caused by offset printing materials
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Offset printing materials mainly include paper, ink, printing plates, blankets, and padding. All materials must meet the printability requirements of offset printing; otherwise, various printing defects may occur.
1. Paper factors
The physical and chemical properties and technical indicators of paper include physical properties, optical properties, printability indicators, and chemical performance indicators. Among these, the properties closely related to offset printing suitability include tightness, tensile strength, stretchability, whiteness, absorbency, smoothness, gloss, and pH value.
Paper tightness and tensile strength are critical factors. During printing, when pressure is released, the viscosity of the ink and the adhesive force generated by moisture on the blanket surface may cause fiber picking or powdering. This results in burr-like defects or solid spots on the printed matter, leading to whitening or printing defects.
The stretchability of paper is related to the fiber orientation. The expansion rate along the fiber direction is smaller than that across the fiber direction. Therefore, in offset printing, aligning the fiber direction parallel to the cylinder axis helps minimize misregistration caused by wet expansion.
Paper whiteness and smoothness are closely related to color vividness and saturation. When ink is printed on paper with high whiteness, less light is absorbed by the paper and more is reflected, allowing the original ink color to appear more accurately. Paper with low whiteness absorbs more light and reflects less, resulting in grayish printed images. Paper with poor smoothness has a rough surface and strong ink absorption, causing dull prints with increased light absorption and diffuse reflection. As a result, the visual effect is poor and colors appear dull and gray.
Highly absorbent paper absorbs a large amount of ink binder before the ink film dries, resulting in a rough ink film on the printed surface and poor print quality. In severe cases, ink bleeding may also occur.
Generally, paper has a slightly acidic pH of 5.5 or higher. When the pH value drops below 5.5, the acidity increases, which can hinder ink drying and lead to printing defects.
2. Ink factors
Ink is a plastic fluid used to reproduce images and colors. It is transferred by ink rollers onto the image areas of the printing plate, then onto the blanket, and finally onto the paper surface, where it dries and forms a solid ink film. The main ink performance indicators required for offset printing include fixation, drying speed, color strength, fineness, viscosity, fluidity, and resistance to acid, alkali, water, light, and abrasion.
With the increasing speed of offset presses and the widespread use of multicolor printing machines, fast ink fixation has become increasingly important. Multicolor presses typically operate at speeds above 8,000 sheets per hour, and each sheet takes about 0.5 seconds to transfer to the next color unit. If ink fixation is slow, ink from the previous color may transfer to the blanket of the next unit, causing color contamination and mixing defects.
Ink drying usually takes more than four hours. If ink dries too quickly, it may crust on the ink rollers, causing difficulties in printing and cleaning. If the ink film on paper dries excessively, vitrification may occur, preventing proper overprinting of subsequent colors. Conversely, if the ink does not dry sufficiently, subsequent colors cannot be printed properly, reducing production efficiency.
A higher ink coloring rate is preferable, ideally close to 100%, although some inks only achieve 80–90%. To achieve sufficient color saturation, operators may increase the ink film thickness. However, excessively thick ink layers can cause set-off on the reverse side of the printed sheet and blur dots in dark areas.
Typically, ink particle size is about 7.5 μm. For example, in a 200-line screen with a dot area of 1,613 μm², approximately 68 pigment particles can be accommodated, which exceeds the minimum requirement of 15 particles. However, in actual printing, some domestic inks have poor resistance to polymerization. During machine operation, friction between ink rollers increases ink viscosity, and paper dust and fibers mix into the ink, causing particle agglomeration. This reduces image sharpness and tonal gradation.
3. Printing plate factors
Offset printing plates have largely transitioned to PS plates, but many factories still use protein plates, PVA plates, multilayer metal plates, and, in recent years, zinc oxide paper-based plates. These plates differ in plate-making processes. PS plates are the simplest, while PVA and multilayer metal plates involve more complex processes.
The developing, etching, and coating processes of PVA plates are mainly controlled manually, making print quality difficult to control. As a result, color reproduction often fails to meet original design requirements. Additionally, the thickness of zinc-coated plates is often determined by experience, leading to inconsistent quality. Excessive grinding can reduce plate thickness from 0.55 mm to 0.45 mm. Operators then compensate by adjusting plate padding based on experience, which can alter the plate cylinder radius and cause image misregistration.
Multilayer metal plates offer high durability and are suitable for printing invoices, tax forms, thin books, and large-volume publications with consistent quality. However, during printing, paper dust can abrade the plate surface, damaging the chrome layer and exposing the copper layer beneath. This leads to ink adhesion problems that are difficult to resolve. Removing chrome damage requires chemical treatment and re-chroming, which is time-consuming and costly.
Zinc oxide paper-based plates are low-cost and simple to produce, but they are highly sensitive to environmental temperature and humidity, electrostatic plate-making voltage, base paper moisture content, ink sensitivity, and toner conductivity. Small deviations in these factors can lead to defects such as image whitening, background scumming, and a high rate of defective plates.