51. ***What are the characteristics of blade coating?***
A: Blade coaters operate similarly to air knife coaters but use a steel blade to scrape excess paint from the paper surface and smooth the coating layer. Advantages include very flat coating surfaces unaffected by the base paper's surface state, no requirement for soft or highly fluid coatings, and suitability for high-solid-content and high-speed coating applications. Disadvantages include the risk of debris sticking to the blade edge, causing long streaks on the paper, and frequent blade wear requiring regular replacement. Additionally, the coating thickness is generally limited, and the base paper's thickness uniformity must be strictly controlled.
52. ***What is the casting coating process?***
A: Casting coating involves applying a coating to the base paper surface and drying it with hot air at an appropriate temperature. The coated paper is then tightly dried against a highly polished dryer surface to produce mirror-smooth cast-coated paper. Cast-coated printing paper exhibits superior gloss and printability compared to ordinary coated paper. This process is used to produce high-gloss coated paper (commonly known as glass-surface paper) and glass cardboard. A disadvantage is the relatively low production speed.
53. ***What issues should be considered when producing cast-coated paper?***
A: Producing cast-coated paper requires special attention to the following points:
① The casting dryer surface must be extremely smooth. Any scars or defects will directly transfer to the coating surface. Therefore, after chromium plating, detailed mirror polishing is necessary.
② There must be no bubbles between the paper and the dryer, as this can cause markings. A water-seal device is installed on the casting dryer to prevent this issue.
③ The stripping performance of the paper from the dryer must be excellent. At the end of drying, if the stripping performance is poor, the mirror-like finish on the paper may be damaged. To prevent this, stripping agents are often added to the paint, or the cast-coating dryer surface is deactivated.
54. ***Why analyze paper structure?***
A: Paper structure refers to the arrangement and distribution of elements, such as fibers and fillers, within the entire paper sheet. Raw materials and manufacturing processes determine paper structure, which in turn determines paper properties and usage. Understanding the structural characteristics of paper is crucial for comprehending its performance and enhancing its application effectiveness.
55. ***What are the structural characteristics of paper?***
A: Paper is a non-homogeneous material with a complex structure, and its characteristics can be summarized as follows:
(1) Composed of multiple elements: fibers, fillers, binders, and colors, among which fibers are the most fundamental element of paper structure. Differences in fiber raw materials and processing methods lead to variations in paper structure and properties. Fillers, binders, etc., also exhibit performance differences depending on their varieties.
(2) Anisotropy exists in the X-Y-Z direction distribution of fibers, fillers, binders, and colors, manifested in different fiber alignment directions, size distributions, and distributions of fillers, binders, colors, and air content. This structural anisotropy mainly depends on the papermaking method and equipment used.
(3) Most paper has two sides, meaning the properties of the two sides differ.
(4) Paper has a porous structure, which determines its air permeability, moisture absorption, absorbency, and compressibility. This porosity enables paper to absorb water, ink, and other liquid substances.
(5) The combination of fibers, fillers, binders, and colors determines the mechanical strength of the paper and its bonding force with other substances.
56. ***What are the two sides of paper?***
A: Paper has a front side (the wire side) and a back side (the felt side). Due to differences in the papermaking process, the properties of the two sides differ. This phenomenon is referred to as the two-sidedness of paper. Generally, paper made by a long-wire machine or a round-wire machine exhibits two-sidedness, whereas paper made by a wire machine dehydrates simultaneously on both sides, resulting in minimal differences between the two sides.
57. ***How are the two sides of paper formed?***
A: The difference between the front and back sides arises from the distribution of long fibers, fine fibers, fillers, and glue across the paper sheet. During papermaking, water moves from the front of the paper toward the wire side. The wire cloth acts as a filter during dehydration, causing some fillers and fine fibers to be lost with the water, making the reverse side rougher and looser in structure. On the front side, the paper web deposited on the wire blocks and helps retain fine fibers and filler particles, resulting in more filler and fine fibers retained on the front side. Consequently, the front side has a tighter structure and a smoother surface compared to the back side.
58. ***What are the characteristics of the two sides of paper?***
A: The difference between the two sides is most evident in smoothness, with the front side generally being smoother than the back side, sometimes differing by up to 70%. Since more filler particles, fine fibers, and glue are lost on the reverse side, the front side contains more glue than the reverse side. However, the surface strength and absorption of the front side are not as good as those of the reverse side. For double-sided coated paper, since both sides are coated and calendered, the differences between the two sides are minimal.
59. ***What impact does the two-sidedness of paper have on printing?***
A: The two-sidedness of paper significantly affects the quality of printed materials. Differences in smoothness and sizing between the two sides lead to variations in ink acceptance and absorption. When the absorption difference between the two sides is large, the ink depth on the two sides of the printed product differs, potentially causing printing failures. The front side of the paper has higher smoothness and better inking effects but lower surface strength, making it more prone to issues during printing. Conversely, the reverse side is rougher, with worse inking effects but higher surface strength, reducing the likelihood of hairing during printing. In actual printing, it has been observed that the pile of powder on the blanket surface in contact with the front side of the paper is much more severe than on the reverse side. Since the two-sidedness of printing paper is detrimental to print quality control, paper with minimal differences between the two sides should be selected for printing.
60. ***How can the two-sidedness of printing paper be overcome?***
A: With continuous advancements in papermaking technology and equipment, the differences between the two sides of paper are gradually decreasing. For example, vertical-wire paper machines are used in papermaking, and the wire part employs a method of simultaneous dehydration on both sides, making the two sides of the paper more consistent. In long-wire paper machines, an additional wire layer is added to allow partial dehydration on the front side, making the two sides more similar. Chemical retention agents can also be used to retain fillers and fine fibers on the reverse side as much as possible, reducing losses and improving the two-sidedness of the paper. Additionally, the rational use of pressing and calendering equipment can effectively improve the two-sidedness of paper.