Factors affecting corrugated carton printing

At the packaging industry seminars and exhibitions held in Chicago, various folding cartons and cartons that use different liners and paperboards were exhibited to meet a wider range of printing needs. The successful printing method is to transfer the ink to printing materials in a controlled manner. The printed contents are text, solid and halftone images. In order to obtain a stable printing effect during the printing process, it is important to determine and control the properties of the paper that affect the relationship between the ink, the printing process, and the substrate.
First of all we have to distinguish between the main differences in print performance between paper and cardboard. The paper's easy-to-feed property plays an important role in the successful completion of the printing or finishing process. The main factors affecting the paper feeding include paper flatness, tear strength, hardness, ink absorption, dimensional stability, surface strength, tension and thickness.
Printability is a property of whether paper can stably reproduce images. It includes color density, flatness of the field output, hue range, hue, and ink transfer performance. In order to achieve better printability and paper-sheeting and corrugated paperboard transportability, it is necessary to understand the relevant paper properties.
â– Surface characteristics Print quality is affected by the surface characteristics of paper, liner, and paperboard, such as its optical appearance, ink absorbency, paper permeability, structure, and thickness. The surface properties of the backing material or paperboard are closely related to many factors, including the natural characteristics of the fibers used, the processing methods in the production process, the form of composition, the coating or post-processing procedures, and the like. One of the most important factors affecting print quality is the surface structure of the paper, which usually refers to the smoothness or roughness of the surface.
The smoothness of the paper is mainly controlled by the morphology of the fibers. Finishing, coating, or polishing can be used to obtain a specific surface structure. These factors also affect the paper's absorbency and permeability, as well as the final ink adhesion.
The thickness of paper varies with the way paper equipment and raw materials are processed. The thickness is directly affected by the moisture content, the pressure on the wet end, and the degree of polishing. Having a uniform thickness in the roll direction is an important guarantee for a stable printing effect. Variations in the thickness of the paper will lead to problems such as uneven output and overspreading of dots during the printing process.
The loose density of the paper determines the ratio of air and solid components in the paper. This relationship is very important for determining what kind of paper or liner material to use.
Compressibility refers to the degree to which the thickness of the paper decreases when the surface of the paper is subjected to a certain amount of pressure vertically. Compressibility essentially refers to the strength properties that affect the transfer of ink. Compressibility is affected by factors such as hardness, density, and composition.
â–  Absorption Absorption refers to the proportion of liquid (ink) entering the paper by capillary action. It is determined by the loose density of the paper. The structure of the paper presents many tiny channels and pores, and the capillary action depends on the relationship between the surface energy of the paper and the surface tension and viscosity of the ink.
The looser paper surface is relatively open, with many large pores that can absorb ink pigments and binders. Highly matte paper may absorb only the binder. The degree of absorption of the ink in the liner or paperboard is very important and it is actually a comparison of the information transmitted between the ink and the paper. If the pigment is absorbed only on the surface of the printing material, the ink can effectively absorb light. The more ink in the liner (corrugated), the more image loss will occur. Printing inks lose their luster once they are absorbed, and sometimes even show different colors. Black loses its brightness and sometimes even appears gray. The same type of ink is printed on the surface of a coated, less absorbent material that exhibits different colors. Therefore, the ability of paper to hold ink is of great significance.
The ink absorption capacity is also expressed in terms of density. Absorption rate mainly depends on the composition of the material, filler composition, size, degree of refining, degree of matte, density and surface coating and other factors. Uncoated paper usually has a lower density than coated paper.
â–  Ink film, dust, and chemical structure The practical ink film should be thicker than the surface of the material to ensure smooth deposition of the ink. Modern photopolymerized plate systems have Shore-A hardness and can overcome uneven paper surface structures. However, the softness of the corrugated flexo plate cannot reach this hardness, which will result in excessive dot gain.
The PET material underneath the printing plate often appears to be incompatible with the printing plate, tape, and corrugated foam, which may result in serious network expansion. The softer the corrugated plates, the easier it is for the press to overcome the problem of surface roughness; however, the clarity of the printed elements will be reduced and the image elements will be compressed as if they were together.
On the other hand, the use of a thicker ink film during the printing process leads to serious dot gain. The thicker the ink film, the harder it is to dry. Extending the drying time in turn leads to lower printing speeds and increased costs. Low viscosity inks are often used in flexo printing. In this way, the binder is more flexible and can get into the coating more quickly, leaving the pigment on the surface. The image loses gloss and the pigment can be easily wiped off the surface. This situation can easily occur in the direct printing of corrugated paper using inferior backing paper. It is therefore very important to ensure that the absorbency of the liner matches the rheology of the ink.
Dust is clearly another printability problem associated with the printing process. Dust is an accessory for small fragments of paper surfaces and edges. If no ink filtration device is used, dust and dirt can collect in the ink and transfer to the plate. This phenomenon is "dirty output."
The chemical composition of paper also has a certain influence on printability. Excessive acidity in the liner can affect the drying speed of the ink. In general, the Ph value of the uncoated backing material should be 5 or 5.5 or slightly higher, and the coating material should be 7-8.5 in order to ensure the optimum drying time.
â–  Liner (Corrugated Paper) Materials In general, the backing materials used in postpress processing are generally classified into the following types:
● Kraft paper Corrugated paper is often used where surface papers are highly resistant or when the packaging needs to have strong tear resistance. This type of liner is usually applied to Type A, C, and B type corrugated paper.
● Test liners are an effective, inexpensive liner made from recycled paper. It consists of two layers: a thicker base and a more absorbent outer layer. This liner is also mainly used in A, C and B type corrugated paper.
● Decorative liner. There is an alternative between non-coated, semi-coated, and fully coated liners, typically decorative decorative liners. The white backing paper provides the product with a suitable surface for the most attractive print quality, especially for image designs with multiple colors. Decorative liners are increasingly used in B, E, and F type corrugated paper.
â–  Photopolymerization Plates for Postprinting In the corrugated postprinting process, photopolymerizable plates have many basic properties. These properties depend on the nature of the printing process, the substrate materials, and the external conditions and factors in the printing process.
Factors such as the quality of various backing materials, paper and board quality, ink characteristics, anilox roller parameters, and equipment limitations, as well as other products used in the paper product processing industry, occupy the concept and improvement process of the printing process technology. Very important position.
Photopolymerized printing plates consist of four major parts: crosslinkers, monomers, photoinitiators and additives. The changes and choices of these ingredients, as well as their performance and interrelationships, play a crucial role in the printing performance. The composition used to produce the photopolymerization printing plate has a great influence on the choice of the water-based ink system. The main issues that should be considered when developing improved photopolymer printing plates are the transferability and hiding of the ink, resolution of the printing plate, stability, oxidation resistance, and printing-related properties (limitation of device speed).
â–  Requirements for printing plates The development of new photopolymerization printing plates is challenged by issues such as high screen printing, high resolution dots, and ink delivery in the field. In the end, printing companies had to use different photosensitive polymer plates in different paper bags, liner backings, and backing material combinations, resulting in a variety of different combinations. This is to make the best use of the special printing plate's characteristic advantages, especially hardness and ink transfer ability, to suit a specific application and specific substrate.
In order to standardize print quality, corrugated paper printing plants are increasingly using thinner photopolymer plates and two standard calipers: 3.94mm (0.155 inch) and 3.18mm (0.125 inch). The most commonly used standard for printing plates is to produce smooth dense ink transfer and low dot gain.
Another requirement is the precise and clear output of fine reversal films and line elements, reduced filling elements, and smooth printing of harder levels. The minimum tolerance on the specification must be met. The hardness of the plate must not exceed that of type E or F corrugated board 42 ShA (ISO) and type B, C or A type of corrugated board 34 ShA (ISO) to overcome the surface structure problems of the board. At the same time, the printing plant needs a higher flexibility of the printing plate to reduce the requirements for the performance of the lining paper, reduce the downtime, and facilitate the setting of printing pressure.
Plate makers all want to shorten the cleaning time and reduce the drying time to increase output and production capacity. It is desirable to have a longer back exposure time to broaden the range of the pre-exposure while minimizing the effects of fluctuations during UVA emission. In addition, shorter surface exposure times and longer main exposure ranges are also needed to improve efficiency. Digital printing plates usually have different needs, and these printing systems also have to meet different needs in order to achieve a clearer printing effect.
Due to the limitations of the choice of specific printing plate ingredients, the printing plate and its performance have certain limitations. These limitations are in conflict with many of the requirements mentioned above. The needs of these conflicts can often not be solved simply by means of ink composition, changing plates, padding or adjusting equipment. The combination of primary colors often must be converted to meet most needs. The range of these elements includes aspects from plate composition to application to print production.
â–  Compressible liners In order to reduce the overall degree of plate deformation during the printing process, compressible liner materials are commonly used under the transferred paper. Compressible foams absorb most of the extra printing pressure and improve print quality.
The compressibility and memory properties of the foam material affect the ink transfer performance of the above photopolymerizable printing plate. The thickness of the foam should not be greater than the thickness of the plate material used to prevent any possible instability of the system (which may result in non-uniform printed output).
â–  The quality of prepress quality in post-print applications can be comparable to the quality of offset laminations, which has become the benchmark for most post-print paper processing companies to compete for new, more profitable businesses. The increasing paper-carrying capacity of corrugated board, the continuous reduction of equipment downtime, and the improvement of the printability of the liner surface all stimulate printing companies to continuously improve the production of paperboard, seek more reliable paper consistency, better ink transferability, and reduce Outreach features, and the use of higher device speeds.
In order to meet the latest quality requirements in corrugated post-printing applications, the ink should deliver the least amount, ensure minimal dot gain, and achieve the required density, with minimal material cost.
With the introduction of new high-quality corrugated board presses and the application of finer anilox systems, there is an opportunity to use digitally printed plates in the post-printing process, mainly for the semi-coating of printed E and B type corrugated papers. White surface liner. Digital corrugated plates of 0.112 inch and 0.125 inch thickness can provide satisfactory results.