Use of liners in flexo printing
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With the continuous development of flexographic printing, the quality requirements of flexographic printing products are becoming more and more strict. How to improve the quality of printed products has become the primary problem that plagues the industry.
In flexographic printing, the pad plays a very important role, but different pads (double-sided tape) have different effects on the printed product, which is briefly explained.
Main features and classification of liners
The liner is a tacky tape whose basic function is to firmly attach the flexographic plate to the plate cylinder so that no sliding occurs between them.
The pads are roughly classified into three types according to the hardness, namely, a soft pad, a neutral pad, and a hard pad.
The pad thickness should be consistent. The stable uniform thickness facilitates the control of dot gain and ensures good printing results. Generally, the thickness of the liner used for narrow flexo printing is 0.3 mm. In plate making, pad thickness is a key consideration.
Basic composition of the liner
The liner is generally composed of an intermediate substrate layer, a double-sided adhesive layer, a single-sided or double-sided protective paper, and the like. As shown in Figure 1.
1. Substrate
The substrate is the main component that determines the thickness of the liner and its properties. It can be divided into a film-based substrate and a foam-based substrate according to its material. Film-based substrates are generally vinyl or polyester and paper, which have good elasticity and uniform texture and are used for thin flexographic liners. The foam base material is generally polyethylene or polyurethane foam. The substrate determines the elasticity of the double-sided tape, the substrate elasticity is too large, and the solid printing is not ideal; the substrate is too hard, and the dot reproduction is not ideal.
2. Glue layer
The glue layer has two functions, that is, the plate is firmly adhered to the plate cylinder, and the protective paper is adhered to the surface of the substrate. The adhesive used for the adhesive layer is divided into two types: rubber and acrylic. The viscosity of the rubberized layer has a great influence on the service life of the double-sided adhesive, and the viscosity of the adhesive layer is low. When the second use, the plate is not easy to stick; the viscosity is too high. It is easy to damage the surface of the double-sided tape when changing plates.
3. Protective paper
The purpose of the protective paper is to prevent the substrate layer from being scratched, to prevent dust, to prevent sticking, and to facilitate unwinding.
Pad selection
From the point of view of compressibility, the soft pad has the highest compressibility, followed by a neutral pad, and the hard pad has the lowest compressibility.
When selecting a liner, the first two basic elements should be considered first.
1. Printing material
Flexographic printing has a wide range of substrates, from soft plastic films to rigid glass products, from metal film to corrugated board. The choice of liners must first consider the breadth of the substrate.
2. Printed pattern features
Different liners should be used for different layouts. Hard plates are best used in the field. The cable version is best to use a soft pad. If there are both solid and small lines or small dots in the printed pattern, then both should be considered, and a neutral pad should be used.
Therefore, in actual production, the gasket should be reasonably selected according to different printing conditions and process requirements.
Experiment 1 Relationship between pad hardness and solid density
As the pad hardness increases, the solid density value gradually increases. In this phenomenon, our analysis is mainly caused by the elasticity of the liner: during the printing process, the printing plate is given normal printing pressure. Under this pressure, the liner and the printing plate are deformed at the same time, but because the soft pad is more elastic, the pressure on the printing plate is relatively small, so the ink on the printing plate is not completely transferred to the printing material, resulting in The printing density is small: the hardness of the hard pad is large, and under the same conditions, the plate adhered with the hard pad is subjected to a pressure greater than that of the plate adhered with the soft pad, so the ink The transfer is better; the neutral pad is between the hard pad and the soft pad, so the density of the printed field is also between the two.
Experiment 2: Relationship between pad hardness and dot gain
The dot gain value increases as the pad hardness increases. This is mainly due to the elasticity of the pad. The soft pad is more elastic than the hard pad. Under the same pressure, the deformation of the soft pad is much larger than that of the hard pad. Therefore, the plate adhered by the soft pad has less deformation than the hard pad. The amount of deformation of the bonded plate, so when the soft pad is used, the dot deformation is small, and the dot gain value is relatively small.
In summary, we can conclude that it is better to use hard pads when printing on the ground. When printing small lines and dots, it is more appropriate to choose a soft pad, while a neutral pad can make up for the lack of soft pad and hard pad. All the pictures can be balanced. In general, neutral pads are used more widely.
In actual production, the utilization of the liner is also considered when selecting the gasket. Hard liners have a lower surface viscosity than neutral liners, while soft liners have the highest viscosity. When changing plates, the hardness of the hard pad is lower when it is reused, and the sticking fastness is poor. The soft pad has a very high viscosity. When unloading, it is easy to separate the middle layer of the pad and cannot be used again. Neutral lining The pad can be reused several times. Therefore, from a saving point of view, it is generally more affordable to use a neutral pad.