Factors affecting the successful printing of corrugated boxes
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At the packaging industry seminars and exhibitions held in Chicago from November 3rd to 7th, 2002, a variety of folding cartons and cartons using different liners and paperboards were exhibited to meet a wider range of printing needs. The successful printing method is to transfer the ink to the substrate in a controlled manner, with text, solid and halftone images. In order to achieve a stable printing effect during the printing process, it is important to determine and control the paper properties that affect the relationship between the ink, the printing process and the substrate.
First we have to distinguish the main differences in printing performance between paper and cardboard. Paper slipperiness plays an important role in the smooth completion of the printing or finishing process. The main factors affecting paper travel include paper flatness, tear strength, hardness, ink absorbency, dimensional stability, surface strength, tension and thickness.
Printability is a property of whether paper can stably replicate images. It includes color density, flatness of the field output, tonal range, hue and ink transfer performance. In order to achieve better printability and paper-feeding properties of the liner and corrugated board, it is necessary to understand the relevant paper properties.
■Surface characteristics
The quality of printed matter 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 paper 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 the 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, usually referred to as the smoothness or roughness of the surface.
The smoothness of the paper is mainly controlled by the morphology of the fibers. Post-processing such as buffing, coating or glazing can achieve a specific surface structure. These factors also affect the absorbency and permeability of the paper, as well as the final ink adhesion.
The thickness of the paper varies with the paper machine and the way the 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 along the paper roll is an important guarantee for a stable printing effect. Variations in the thickness of the paper will cause problems such as uneven output and excessive dot enlargement during the printing process.
The bulk density of the paper determines the ratio of air to solid content in the paper. This relationship is very important in determining which paper or liner material to use.
Compressibility refers to the extent to which the thickness of the paper decreases as the surface of the paper is subjected to a certain amount of pressure. Compressibility essentially refers to strength properties that affect the delivery of ink. Compressibility is affected by factors such as hardness, density, and composition.
■Absorbability
Absorbency refers to the proportion of liquid (ink) that enters the paper by capillary action. It is determined by the bulk density of the paper. The construction of the paper presents a number of tiny channels and pores, and the capillary action relies on the relationship between the surface energy of the paper and the surface tension and viscosity of the ink.
The looser paper surface is more open and has many large pores that absorb ink pigments and binders. Paper that is highly matte may only absorb 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 only absorbed on the surface of the substrate and is not absorbed, the ink can effectively absorb the light. The more ink that is drawn into the liner (corrugated paper), the more image loss there is. Once the printed primary ink is absorbed, it loses its luster and sometimes even shows a different color.
Black loses its brightness and sometimes even grays out. The same ink is printed on the surface of the coated, less absorbent material to exhibit different colors. Therefore, the ability of paper to hold ink is of great significance.
The absorption capacity of the ink is also expressed in terms of density. The absorption rate depends mainly on the composition of the constituent materials, the composition of the filler, the size, the degree of refining, the degree of matte, the density, and the surface coating. Uncoated paper typically has a lower density than coated paper.
■Ink film, dust, chemical structure
A practical ink film should be thicker than the surface of the material to ensure a smooth deposition of the ink. Modern photopolymerization printing plates have a Shore-A hardness that overcomes uneven paper surface structures. However, the softness of the corrugated flexo printing plate cannot reach this hardness, which will result in excessive dot enlargement.
The PET material under the printing plate often appears to be improperly matched with the printing plate, tape and corrugated foam material, which may cause serious expansion of the dot. The softer the corrugated printing plate, the easier it is for the printing press to overcome the problem of rough surface; however, the sharpness of the printed elements will also decrease, and the image elements will appear to be compressed together.
On the other hand, the use of a thicker ink film during the printing process leads to a serious expansion of the dots. The thicker the ink film, the harder it is to dry. Extending the drying time leads to a reduction in printing speed and an increase in cost.
Low viscosity inks are often used in flexographic printing. In this way, the binder is more flexible and can enter the coating more quickly, leaving the pigment on the surface. The image is tarnished and the pigment can be easily wiped off the surface. This situation is very likely to occur in direct printing of corrugated paper using inferior backing paper. It is therefore important to ensure that the absorbency of the liner matches the rheology of the ink.
Dust is clearly another printability issue associated with the printing process. Dust is an accessory to small pieces of paper surface and edges. If an ink filter is not used, dust and dirt can collect in the ink and transfer to the plate. This phenomenon is "dirty output."
The chemical composition of the paper also has a certain influence on the printability. Excessive acidity in the liner can affect the drying speed of the ink. In general, the Ph value of the non-coated backing paper material should be 5 or 5.5 or slightly higher, and the coating material should be 7-8.5 to ensure optimum drying time.
■ Backing paper (corrugated paper) material
In general, the current liner materials used in post-press processing are generally divided into the following categories:
●Kraft paper corrugated paper is often used when the surface paper is highly resistant, or when the packaging needs to have strong tear resistance. This type of liner is commonly used in A, C and B corrugated paper.
• Test liner is 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 corrugated paper.
● Decorative lining paper. There is an option between non-coated, semi-coated and fully coated pads, commonly referred to as decorative backing. 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 corrugated paper.
■Photopolymerized printing plate for post printing
Photosensitive polymeric printing plates have many basic properties in the corrugated printing process, which depend on the natural characteristics of the printing process, the substrate materials, and the external conditions and factors in the printing process.
Considerations for various liner 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, in the concept and improvement of printing processing technology Very important position.
Photosensitive polymeric plates consist of four main components: crosslinkers, monomers, photoinitiators, and additives. The changes and choices of these ingredients, as well as their properties and interrelationships, play a crucial role in print performance. The ingredients used to produce photopolymerizable printing plates have a large impact on the choice of water based ink systems. Ink transferability and opacity, plate resolution, stability, oxidation resistance, and printing-related properties (limitations of device speed) are major issues that should be considered when developing improved photopolymerization plates.
■ necessary conditions for printing plates
The development of new photopolymerization plates is challenged by high-mesh printing effects, high-resolution dots, and ink delivery in the field. Finally, printing companies had to use different photopolymerizable plates in different combinations of paper bags, liner backings and liner materials, resulting in a variety of different combinations. This is to maximize the use of the specific characteristics of a particular plate, particularly hardness and ink transfer capabilities, to suit a particular application and a particular substrate.
To achieve standardization of print quality, corrugated printing plants are increasingly using thinner photopolymer plates and two standard calipers: 3.94 mm (0.155 in) and 3.18 mm (0.125 in). The most common standard for printing plates is to produce smooth, dense ink delivery and low dot gain.
Another requirement is to accurately and clearly output fine reversal film and line elements, reduce fill elements, and smooth printing of difficult color levels. The minimum tolerance on the specification must be reached. The hardness of the plate should not exceed the E or F type corrugated paper 42 ShA (ISO) and the B, C or A type corrugated paper 34 ShA (ISO) to overcome the surface structure of the paperboard. At the same time, printers need higher plate flexibility to reduce the performance requirements of the paper, reduce downtime, and facilitate printing pressure settings.
Plate makers want to reduce cleaning time and drying time to increase output and productivity. It is desirable to have a longer back exposure time to broaden the range of pre-exposure while minimizing the effects of fluctuations during UVA emissions. In addition, shorter surface exposure times and longer main exposure ranges are required to increase efficiency. Digital plates often have different needs, and these printing systems also need to meet different needs for clearer printing.
Plates and their performance have limitations due to the choice of specific plate components. These limitations conflict with many of the requirements mentioned above. The need for these conflicts can often be solved simply by ink composition, changing plates, padding or adjusting equipment. The combination of primary colors often has to be translated to meet most needs. The range of these elements ranges from plate components to applications to print production.
■Compressible liner
In order to reduce the overall extent of the deformation of the printing plate during the printing process, a compressible cushioning material is commonly used under the conveyed paper. Compressible foam absorbs most of the extra printing pressure and improves print quality.
The compressibility and memory properties of the foam affect the ink transfer properties of the above photopolymerizable printing plates. The thickness of the foam should not be greater than the thickness of the plate material used to prevent any instability that may occur in any system (which may result in uneven print output).
■Prepress quality in post-printing applications
Print quality can be comparable to the quality of offset film, which is the benchmark for most post-printing products processing companies to compete for new, more profitable businesses. The increasing paper-feeding properties of corrugated paperboard, the reduction in equipment downtime, and the improved printability of the pad surface have spurred printing companies to continuously improve paperboard production, seek more reliable paper consistency, better ink transfer, and reduce Dot expansion features and higher device speeds.
In order to meet the latest quality requirements in corrugated post-press processing applications, the ink should deliver the least amount of material, minimize dot gain, and achieve the required density to minimize material costs.
With the introduction of new high-quality corrugated board presses and the application of finer anilox roll systems, there has been an opportunity to use digital printing plates during post-press processing, primarily for semi-coating of E and B corrugated papers. White surface liner. Digital corrugated printing plates of 0.112" and 0.125" thickness provide satisfactory results.