Preface
Due to people's different requirements for printing quality. When printing on corrugated boxes and folding cartons. Different backings, paper jams, and cardboard are often used. General high-quality printing requires the ink to be evenly and firmly attached to the surface of the substrate without any other failure. During the printing process. If you want to get consistent print results for every piece of paper, an in-depth understanding of the paper's characteristics becomes very important. The characteristics of the paper affect the relationship between the ink, the printing process, and the substrate.
The effect of paper properties on print quality
The printing characteristics that distinguish paper and paperboard are mainly paper runability and printability. The paper runability can be understood as how easy the paper is in the printing or binding process. Factors that affect the paper run-out include: smoothness, tear strength, hardness, adhesion, stability, surface strength, tensile strength, and paper thickness.
Printability refers to the ability of paper to reproduce images in a consistent and consistent manner. It includes color density, field printing uniformity, gradation range, hue and ink transfer properties.
In order to get the best printability and paper-feeding on the liner and cardboard. We must first understand the nature of the relevant paper.
In fact. The print quality is affected by factors such as the ink adsorption capacity of the paper or board surface, the paper permeability of the paper, and the thickness of the paper.
The surface properties of the backing or paperboard depend on factors such as the way in which the fiber structure is bound by the light. One of the most important factors affecting the printing quality is the paper surface structure. Generally refers to the surface smoothness or roughness.
The shape of the fiber determines the smoothness of the paper. Such as calendering, coating, coating, etc. can get a certain surface structure. These factors also affect the paper's ability to absorb and infiltrate, even when the ink is printed.
The thickness of the paper may vary depending on the way it is supplied and stored. Paper thickness is directly related to the temperature and humidity of the air, the pressure of the outside world, and the degree of calendering. The uniformity of paper thickness is important for printing. If the thickness is not the same, it will be more difficult to print. Printing unevenness and dot gain may occur.
Each type of paper has a bulk parameter. Usually 1.2, 1.5, 1.8, 2.0 and so on. For example, one kind of thickness means that it has a thickness of 120m. Take 60 grams of paper for example. Ordinary 60 g offset paper looseness is 1.2. And if the bulk of 60 g light paper is 1.5, then (1.5/].2) x 60 = 75, that is, the thickness of 60 g of light paper with a bulk of 1.5 is equal to 7 5 Gram thickness of ordinary offset paper. Then in the actual production, the same number of books or magazines. If you use lightweight paper printing will be much lighter than ordinary offset paper. On the one hand, it can reduce transportation costs, and on the other hand, it is also convenient for readers to carry.
Compressibility refers to the rate of decrease in paper thickness when a certain pressure is applied to the paper. Compressibility is actually compressive strength, which affects the transfer of ink. Compressibility is related to the composition of paper hardness and density. In general, bulky papers are more stretchable than dense papers.
Paper's ink absorption force refers to the liquid (ink) penetrating the paper under capillary action. Adsorption capacity is the ratio of the ink into the paper by capillary action. In addition it is also affected by the thickness of the paper. The structure of the paper contains a large number of small grooves and pores. Capillary action depends on the surface tension of the paper and the viscosity of the ink.
Bulk thick paper has many fine pores that can absorb ink pigments and carriers, while calender paper can only adsorb the vehicle. Due to the contrast ratio between the ink and the paper that conveys the information, the ink adsorption that is immersed in the liner or paperboard is very important. When the ink pigment is only on the surface of the substrate and has not yet been absorbed. There may be more image information lost. After the ink loses its brightness. Adsorption into the lining will become another color. If black loses its luster, it looks like gray. The same ink is printed on glossy, low-absorbent materials and the tone and color are distorted. Therefore, this depends on the paper's ability to absorb ink. The size of the ink adsorption force is mainly related to its density. Absorption rate depends on factors such as papermaking ingredients, fillers, glazing, calendering, density and surface gloss. Ordinary paper is usually less dense than coated paper.
The rougher the surface of the substrate. The thicker the ink layer is. This requires that the ink must be evenly printed. Modern photopolymer plates can overcome a certain range of uneven paper surface structure problems. Rely on its own flexibility. Corrugated flexo does not generally produce excessive dot gain.
For the printer. Although the softer flexible corrugated media can overcome the rough surface of the paper. But the clarity of the printed picture will be reduced. And some image elements may be lost. During the printing process. Thicker ink areas will have dot gain. And it's hard to dry. If you extend the drying time. This will reduce the printing speed and increase printing costs. Therefore, flexographic printing will use some low-viscosity inks that incorporate a carrier. Wherein the carrier helps the ink penetrate quickly into the substrate. The image is not shiny and the surface pigments are easily wiped. These problems often occur in the direct printing of corrugated paper using a rough liner. It is very important to compare the adsorption capacity of the liner and the rheology of the ink.
Paper dust is also a printability problem in the printing process, which is caused by fine dust that is separated from the surface and edge of the paper. If an ink filter is not used, dust particles may remain in the ink and be transferred to the plate. This phenomenon is called dirty version.
The chemical composition of the paper can affect the printability and the acidity of the lining will delay ink drying. Usually, the lining material with the PH value between 5 and 5.5 and the coated paper with the PH value between 7 and 8.5 can ensure the best drying time of the ink.
Different corrugated printing linings
In short. The common linings for postpress processing can be roughly classified into the following types.
1. Kraft lining
Kraft paper is often used in packaging products that require durability or require high tear resistance. This type of lining usually has Type A, B, and C corrugations.
2. Test liner
Test liners are effective, inexpensive linings made from waste paper. It consists of two layers: a thicker print layer and a superficially absorbent layer.
3. Decorative lining
Lightweight and coated paper linings are often used for decorative linings. Bleached outer liner paper and paperboard provide an ideal printing surface for high-quality printing, especially for color image printing.
The characteristics of the photosensitive resin plate used in corrugated paper printing are determined by the printing method, film base, and printing pressure. Careful consideration of different linings, paper and board quality, ink characteristics, metal roller specifications and machine limitations. Helps to improve the methods and equipment used for the processing of paper products.
The photosensitive resin plate contains four components: adhesives, monomers, sensitizers, and additives. The interaction of the four plays a decisive role in the printing process. When developing a new type of photosensitive resin printing plate. Ink transfer characteristics, ink coverage, plate resolution, stability, ozone resistance, and printing-related work (limitation of device speed) have become the focus of consideration.
Corrugated paper printing plate requirements
The new printing plate needs to have the ideal halftone printing effect, high resolution, clear and realistic dot and uniform ink transfer. Often these requirements are difficult to achieve. Printers typically use photopolymer plates equipped with different cushioning liners and linings. This is to take full advantage of the characteristics of the media. For example, elasticity, hardness and ink transfer ability. This makes it suitable for some special printing or a special substrate.
In order to standardize printing quality. Corrugated paper printers are gradually using two thinner photosensitive resin plates of 3.9mm and 3.1mm. The most basic requirement for printing plates is that in addition to reducing the dot gains. Also have a uniform ink transfer force. In addition, it also requires precise printing of lines and screens, no stencil printing, and high-quality printing of color tones. For E, F corrugated board. The hardness of the plate should not exceed 42 Shore (ISO): For Type B, C, A Corrugated Cardboard. Hardness should not exceed 34 Shore. The purpose is to overcome the surface structure of the cardboard. At the same time, printers need higher plate hardness to reduce loading and unloading jobs. As well as preparing for prepress by adjusting printing pressure.
Plate makers want to reduce the number of rinsings and shorten the drying time to increase production capacity. This is for longer post-exposure times, with the goal of broadening the pre-exposure latitude and at the same time minimizing the effects of UVA lamp radiation. In addition, in order to improve the exposure efficiency, the main exposure is less. Tolerance is greater. These requirements also apply to digital flexo plates. Use this type of media to get even and clear printing results.
BASF Printing Equipment Co., Ltd. spends a lot of time analyzing the requirements of all relevant printing plates in detail and developing Nyloflex FAC X photosensitive resin printing plates to meet the challenging printing quality requirements.
Although FAC X has different plate thicknesses, Chemical Trick has the same exposure time for all parts of the FAC X plate, and the FAC 635X has the same exposure characteristics as its counterparts. Longer exposure latitude and relatively shorter main exposure time are two other features of the new plate. With the successful development of Nyloflex FAC X, BAS F also improved its plate production method. Thickness tolerances of plates can be reduced using modern scientific extrusion techniques. In order to avoid the problem of network expansion.
Compression cushion
In order to avoid the deformation of the printing plate in the printing process. Compressible cushions are widely used in paper-feeding applications. The compressible foam can offset excessive printing pressure, thereby improving the print quality. The compressibility and storage of the foam material will affect the ink transfer characteristics of the front side of the photosensitive resin plate. The thickness of the foam should not exceed the thickness of the plate to avoid any factors that may cause printing instability.
With the introduction of high-quality corrugated paper presses and the use of precision screening equipment. The use of digital plates has also risen. For E and B type corrugated paper printing, 2.84mm or 3.1 8mm digital corrugated media can achieve the ideal printing effect. The latest digital versions for corrugated printing are digiflex FAC D II (32 to 34 Shore) with a thickness of 2.84 mm or 3.1 8 mm and digiflex ART II with a thickness of 1.70 mm or 2.8 4 mm. Both plates can provide a reference for dot formation during pre-exposure after mask ablation. FAC D II is an ideal printing plate for B, E and F type corrugated offset plates and light coated printing, while ART II is an ideal plate for E, F type corrugated light coated paper and coated paper EP8. The digiflex ARTII printing plate was launched on April 1, 2001, and it competes fiercely with similar offset corrugated paper printing.
When the new polymer - 1digifIexART II printing plate was formed. A concave dot is formed and is more firmly attached to the pre-cleaned layer. Due to the presence of atmospheric oxygen, the formation of dots is generally in the imaging process. Oxygen inhibits the photosensitive resin plate. Saturated photoinitiator slows down chemical reactions and affects materials