In plastic gravure printing, there are many reasons for the failure of printing small dots, mainly manifested in the following phenomena:
I. Caused by Printing Materials (Printing Materials such as Plastic Films, Paper, etc.):
If there are dirt, oil stains, foreign objects, and small holes on the surface of the printing material (such as crystals, small black spots, low molecular substances, impurities), a weak interface layer will be formed on the film surface, and the ink cannot be transferred onto it, resulting in pinhole faults. Generally, a small amount of small crystals that do not affect the printing effect is allowed on the surface of the printing material.
The low corona treatment value of the film surface. For films such as BOPP, if the corona treatment surface is below 38 dyn, for PET below 50 dyn, and for films like OPA below 52 dyn, or if the additive amount in the film exceeds 500 ppm, causing excessive slipperiness, accumulation of free substances on the surface of the slip agent, clogging the plate dots, resulting in poor ink adhesion, and poor leveling, pinholes may occur. Incomplete or intermittently treated corona on plastic film surfaces can also lead to irregular pinhole phenomena.
The film generates large amounts of static electricity, easily attracting air and dust from the environment, causing ink to be blocked and resulting in pinholes. This is particularly important to consider in high-speed equipment and wide-format materials.
Films with high moisture absorption (such as nylon, glass paper, etc.) with a moisture content exceeding 3% can cause slow drying during lamination, resulting in bubble blockage in small dots and pinholes.
Excessive additives in materials may cause floating when heated, clogging small dots, and resulting in unprinted small dots.
Excessive roughness of the film surface (usually controlled between Ra 0.08~0.16um). If the difference is too large, the ink cannot fill the concave holes, resulting in blank spots or poor adhesion in the composite.
II. Effects of Printing Plate Rollers (Anilox Rollers) and Impression Cylinders:
In the gravure printing process, the three major elements are the hardness of the rubber roller, the pressure applied, and the flatness of the transfer material. The gravure characteristic is to use a roller with a certain hardness to suck the ink out of the mesh holes under the action of pressure and transfer it to the substrate. Due to the special shape of the mesh holes on the plate, the ink transfer is generally only 50% to 70%, so these factors should be considered when making the electric engraving plate.
After using the gravure plate for a period of time, it will wear out, especially for small dots (shallow dots, dots below 3%). Due to the shallowness of the mesh holes, the dot shape is rarely complete and is prone to wear. This will reduce the local ink amount or block some holes with foreign objects, causing poor ink supply and resulting in pinhole faults. Therefore, when making 1~3% plates, the engraving dot number should be appropriately increased. When making gradient plates, the tones of each small dot should be separated to ensure the completeness and coordination of the gradient.
If the plate surface is not polished or has slight corrosion or foreign objects attached to the surface of the impression cylinder, it may lead to the inability of the ink to transfer to the substrate, resulting in pinholes. The surface roughness of the plate should generally be Ra ≤ 0.4nm. In addition, if there are sand eyes on the plate surface, it may lead to the appearance of pinholes. This type of pinhole distribution is generally regular and the shapes are similar. Therefore, the plate should be carefully checked before printing. It is recommended to use an 800-mesh or 1000-mesh Japanese gravure polishing cloth to polish the plate several times before use to avoid problems with the plate causing pinholes and reduce the occurrence of printing bars.
Large radial runout of the printing plate causes poor transfer of small dots (cannot be printed). After finishing the plate roller, use a dial indicator to test the runout of the plate roller. An error within ±10μm is considered excellent, an error within ±(20~30)μm is acceptable, and an error exceeding 30μm may cause printing quality problems. The radius and surface roughness of the chamfers on both sides of the plate roller also have some influence. If the chamfer is not practical, irregular jumps and ink transfer problems may occur.
The impression cylinder and adjustment. If there is dirt on the impression cylinder, it is too soft or too hard, or it is aging and peeling, it will cause unevenness on the surface of the substrate and cause pinholes. Generally, the hardness of the rubber pressure roller for printing plastic films is suitable for 7080 Shore hardness, and for gravure printing on paper, it should be 8090 Shore hardness. The thickness of the silicone part of the rubber roller should be 15~18mm. If it is too thick, it is not easy to cool, and if it is too thin, it will affect the printing quality.
Printing pressure: If the printing pressure is improper, unbalanced, and the contact between the substrate and the plate is not good, local ink transfer is affected, resulting in pinholes. The evenness of the entire printing pressure is particularly important, especially when printing plastic films, the hardness of the rubber pressure roller should be slightly higher, but the pressure should not be too high, otherwise the film will wrinkle when passing through the printing pressure part. The pressure applied to the rubber pressure roller is generally not greater than 15Mpa.
III. Effects of Ink:
To understand ink, it is important to distinguish between viscosity and tackiness. Viscosity is the most important property in the flowability of ink. The size of viscosity is mainly determined by the components of the ink itself and is a measure of the resistance of fluid molecules to their relative movement caused by the interaction between fluid molecules. It is the impedance within the molecule itself, which is the resistance to the flow of fluid. For example, if the viscosity of the ink binder is high, the viscosity of the ink is also high. Viscosity can also be defined as the adhesiveness of a substance. Viscosity is a relative value. In the printing machine, from the ink roller to the plate, from the plate to the substrate, every transfer of the ink layer will undergo a stripping process, and the tackiness of the ink is the ability to hinder the stripping of the ink layer. The larger the viscosity of the ink, the greater the tackiness. In high-speed printing, the tackiness of the ink must be lower than a critical value, otherwise the ink cannot be transferred and adhered to the substrate.
If the ink has poor wetting, poor leveling, and cannot wet the substrate as required, resulting in partial ink layer shrinkage, pinholes may occur. Adding about 1.5% of a surface tension adjuster to the ink can improve the wetting and leveling of the ink.
Improper ink formulation during printing may result in the inability to form a continuous ink layer on the surface of the plate and substrate, leading to pinhole faults