In the package seminar and fair hold in Chicago from November 3rd to 7th 2002, folded paper boxes and cartons processed with various kinds of interleaving paper and boards are exhibited. They can satisfy a wider scope of demand. Successful printings are to transfer ink onto printing stock in a controllable way to obtain word, solid and halftone image etc. In order to achieve stable printing effects, paper performance that can affect ink, printing technology and stock shall be strictly controlled. First of all, we shall distinguish the main printing performance differences between papers. The runnability of paper plays an important role in accomplishing printing and post-press finishing process. Runnability is affected by elements including levering, tear resistance, rigidity, ink absorbency, stability of size, surface strength, tensile force and thickness. Printability is a quality of paper as for whether it can copy an image stably. It is related with color density, levering of output solid, color tone range, hue and transitivity of ink. In order to achieve satisfactory printability and runnability of interleaving paper and corrugated boards, we shall have a good understanding of relevant performances of paper.
The quality of printed maters are affected by surface property of paper, interleaving paper and boards on aspects of optical appearance, ink absorbency, paper permeability, structure, thickness etc. Surface property of interleaving paper or boards are closely related to many elements, including nature property of used fiber, processing method, structure forms, spread coating and post-finishing procedures etc. One of the most important elements influencing quality of printed maters is surface structure of paper, usually referring to the smoothness or roughness of paper surface. The smoothness of paper is generally decided by the forms of fiber. Polishing, spread coating, glazing and other post-press finishing processes can obtain a certain surface structure, meanwhile influencing the absorbency and permeability of paper, and the adhesiveness of ink in the end. Thickness of paper differs with paper, devices and raw materials, and is influenced by dampness content, pressure in the damp end and polishing extent. Uniform thickness along the scroll is crucial for stable printing effect, while change of thickness would lead to uneven output and over dot-gain. Density of paper decides the proportion of air to solid substances in paper, which is very important for selecting paper and interleaving material. Condensability refers to condense of paper after being vertically loaded on the surface. It indicates the strength of paper which influences the transfer of ink. Condensability is affected by rigidity, density and constituent etc.
Absorbency, decided by density of paper, refers to the ratio of liquid (ink) entering paper through capillary action. As paper constitutes numerous tiny passages and pores, the capillary action depends on surface strength of paper, and tensile force and viscosity of ink. Sloppy paper surface has many big pores which can absorb ink pigment and binders, while highly matted paper can only absorb ink binders. The amount of ink being absorbed into interleaving paper or board is crucial. If pigment stays on the surface of stock without being absorbed, ink can absorb lights effectively. More ink is absorbed into interleaving paper (corrugated paper), more image loss would be incurred. Once upon being absorbed, printing primary color ink would lose luster, even change into different colors. What’s more, black would lose brightness, even appear to be grey. The same kind of ink can show different colors after being applied onto spread coated material surface with small absorbency. Therefore, the capacity of paper to keep ink is important. The absorbency of ink is also related to density. Absorptivity relies on component of constituent materials and fillers, size, refining status, matte degree, density, and surface spread coating etc. Non-spread coated paper is usually not as dense as spread coated paper.
The applicable ink film shall be thicker than the tough layer of material surface to ensure uniform deposit of ink. Modern photosensitive polymerized printing plate system is comparable to the hardness of Shore-A, and is capable to overcome uneven paper surface. However, the flexibility of flexographic plate of corrugated paper can not reach this hardness, thus too much dot-gain would be generated. PET material under printing plate often fails to match the printing plate, adhesive tape and foam material of corrugated paper, which is probably to deteriorate over-dot-gain. The softer the corrugated printing plate is, the easier for press to conquer surface toughness, but meanwhile, the definition of printing elements would be brought down and appear to be compressed together. What’s more, transfer of thick ink film during printing process would also make over-dot-gain worse. Thicker ink film means harder for it to dry, while prolonging drying time would lower down printing speed and raise cost. Flexographic printing usually adopts ink with low viscosity, thus to strengthen the flexibility of binders, enabling them to quickly enter into coated layer and leave pigment on surface. When image loses luster, pigment becomes easy to be scrubbed off, which is often seen in direct corrugated paper printing with inferior interleaving paper. Therefore, it’s essential to match absorbency of interleaving paper with rheologic capacity of ink. Dust is another issue related with printability. Dust refers to subordinate stuff of fragment on paper surface or edge. Dust and oil crust would accumulate in ink without ink filtering devices, and are then transferred into printing plate, called as “dirt output”. Chemical constitution of paper also has certain influences on printability. Over-acidity in interleaving paper would affect the drying of ink. Generally speaking, the PH value of non-spread coated paper shall be 5, 5.5 or more, and for spread coated paper shall be 7~8.5. In this condition, an optimized drying duration could be guaranteed.
Current interleaving paper materials adopted in post-press finishing are generally the following: •Kraft corrugated paper is used when paper has strong resistance, or when package needs to have great tear strength. Such kind of interleaver shall be applied to A, C and B type corrugated paper. •Test interleaving paper is a cheap interleaver made out of waste paper. It has two layers: one layer of thick base and one layer of outer layer with strong adhesiveness. This kind of interleaver is also mainly applied to A, C and B type corrugated paper. Decorative interleaving paper, a best choice among non-spread coated, semi-spread coated and full-spread coated interleaver. White interleaving paper offers as a proper surface which can produce the most attractive printing quality, especially for image design with multiple colors. Decorative interleaving paper is more and more applied to B, E and F type corrugated paper. In post-press technology of corrugated paper, photosensitive polymerized plate has many basic properties which are decided by nature property, base material and various environmental conditions and elements during printing process. Considerations of various interleaving paper material, paper and board quality, ink features, anilox roller parameters and device limitations, and other products used in paper product procession industry, are key for improving printing procession technology. Photosensitive polymerized plate is made up by four major parts: cross-linker, monomer, photoinitiator and additive agent. The change of these components, their property and interaction are crucial to printing performance. Components used to produce photosensitive polymerized plate have great influence on selection of water-based oil system, and improvement of such plates shall focus on issues such as transitivity and cover-up capacity of oil, resolution, stability and anti-oxidation of printing plate and relevant performances of printing (speed limitation of devices).
Development of new photosensitive polymerized plate is challenged by problems such as high halftone printing effect, high resolution dots and solid ink transfer etc. Printing companies have to adopt different photosensitive polymerized plates in different combinations of paper bags, back lining of interleaver and interleaver materials, so as to make full use of advantages of each printing plate, especially regarding the rigidity and ink transfer capacity, and to adapt to specific application and base. In order to achieve standardization of printing quality, more and more corrugated paper plants begin to use thinner photosensitive polymerized plate and two kinds of calipers: 3.94mm (0.155 foot)and 3.18mm (0.125 foot). A usual standard for printing plate is to produce smooth and dense ink transfer and small dot-gain. Another requirement is to output fine reversal film and streak elements accurately and clearly, reduce filling elements and smooth printing in difficult color gradation. The minimum tolerance shall be reached. Hardness of printing plate shall not exceed that of E or F type corrugated paper ShA (ISO), and B, C, or A type corrugated paper 34 ShA (ISO), so as to overcome surface structure problems of the boards. At the same time, printing plants need to use printing plates with high elasticity, so as to reduce requirements on interleaving paper performance, cut stoppage time and make print-pressure setting easier. Plate makers are happy to see reduction of cleaning and drying time, so as to raise output volume and production capacity. In order to enhance efficiency, they also wish to have longer exposure time to expand pre-exposure scope and minimized fluctuation influence during UVA spreading period. Shorter surface exposure time and wider main exposure scale are needed at the same time. Digital printing plates generally have different demands, and these printing systems also have to satisfy different demands to realize clearer printing effects. Printing plate and its performance are restricted by selection of specific printing plate component. Such restriction is in conflict with many requirements listed above which is beyond solution simply by changing ink ingredients and printing plate, adding interleaver or adjusting devices. Base color combination shall meet most of the demands. The scope of these elements includes many fields from printing plate components to printing production.
In order to cut the rate of plate deformation during printing process, condensable interleaver materials are adopted under transferred paper. Condensable foam material can absorb much of the additional printing pressure and improve printing quality. The condensability and memorability of foam material can affect the ink transitivity of photosensitive polymerized plate. Thickness of foam shall not exceed that of the used printing plate material, so as to avoid unstable elements of the system (may cause uneven output).
For most post-press product procession companies, quality comparable with laminated offset printing is a new benchmark to compete for better profit. Runnability strengthening of corrugated board, stoppage decrease of devices and printability improvement of interleaver surface are pushing enterprises to keep improving their board production, seeking for better paper conformity and ink transitivity, reducing dot-gain and applying higher device speed. In order to satisfy the latest quality demand of post-press corrugated paper procession, ink transfer shall be minimized to ensure smallest dot-gain, and to obtain needed density and fewest cost. With the promotion of new and high-quality corrugated board press and application of finer anilox roller，chances for using digital printing plate emerge during post-press processing, mainly being applied to printing semi-spread-coated white surface interleaving paper of E and B type corrugated paper, while digital corrugated paper printing plates being 0.112 foot and 0.125 foot thick can achieve ideal effect.