Abstract: This article analyzes the causes of pinholes and white spots in plastic gravure printing, and analyzes from printing materials (substrate plastic film , paper, etc.), printing plate rollers (anilox rollers), impression rollers, printing inks, etc The influencing factors set out and proposed improvement methods.
In plastic film gravure printing, especially on large areas of solid and shallow screen (high-gloss dot printing), gradient dot printing, often small dot-like holes appear on the ink layer, or small dots are lost in the high-gloss part. Pinholes, commonly known as small white spots. There are many reasons for pinholes, but the root cause is generally caused by the ink not being well transferred on the anilox roller, printing plate, and substrate (plastic film, paper, etc.). At present, pinholes can be divided into two types: chemical pinholes and mechanical pinholes. Pinholes that are randomly distributed in different parts and in different areas, different numbers, and different sizes are called "chemical pinholes." If they are at the same location, the same size and area, and the same number, they are called mechanical pinholes.
In plastic gravure printing, there are many reasons why small dots cannot be printed, mainly due to the following phenomena:
I. Printed materials (substrate plastic film, paper, etc.)
1. If there are dirt, oil, foreign matter and small holes on the surface of the substrate (crystal points, small black spots, low molecular substances, impurities), a weak interface layer is formed on the film surface, and the ink cannot be transferred onto it, which will produce Pinhole failure. Generally, only a small number of small crystal points are allowed on the surface of the substrate without affecting the printing effect.
2. Corona treatment value of film surface is low. The corona-treated surface of films such as BOPP is less than 38dyn, the corona-treated surface of films such as PET is less than 50 dyne, and OPA and other films are less than 52dyn, or the dosage of the film is more than 500PPm, and the slipperiness is large. There are many free precipitates on the surface of the slipper. Blocking the printing plate dots results in poor ink adhesion, poor leveling, and pin-eye phenomenon. The corona treatment surface of plastic film is not comprehensive, and there may be intermittent irregular pinholes in intermittent or partial non-spark treatment.
3. The film has a large static electricity, and it is easy to adsorb a large amount of dust in the air and the environment, which can cause printing pinholes and pinholes, especially high-speed equipment and wider materials.
4. Films with high hygroscopicity (such as nylon, cellophane, etc.). Moisture content exceeding 3% will cause compound drying to slow down and produce bubbles to block small dots and pinholes.
5. Too many additives in the material will float on contact with heat, and the small dots will not be printed if they are blocked.
6. The surface roughness of the film is too large (generally Ra is controlled between 0.08 and 0.16um). If the difference is too large, the printing ink cannot fill the cavity and cause blank spots or compound instability.
Influence of printing plate roller (anilox roller) and impression cylinder
The three major factors in the gravure plastic printing transfer process are the hardness of the rubber roller, the amount of pressure applied, and the flatness of the transfer material. The transfer feature is that a certain hardness rubber roller is used to suck out the ink in the cells under the pressure and transfer it to the substrate. Due to the special shape of the plate cell, the ink transfer is generally only 50%. ~ 70%, so these factors should be taken into account when making electric engraving.
1.The gravure printing plate is generally a plate engraved by electric engraving rollers composed of cells of different sizes and different shades. After using for a period of time, it has worn out, especially small dots (shallow mesh, Less than 30% of the dots) Because the dots are shallow, the dot shape is rarely complete, and it is particularly easy to wear. In this way, the amount of local ink will be reduced, or some nets will be blocked by foreign objects, and the problem of poor ink supply will also occur. It will also cause pinholes on the surface of the substrate. Therefore, when the electro-engraving is made into 1 to 3 editions, the number of dots of the electro-engraving should be appropriately increased. When making the gradient mesh, the hue of each small dot should be separated to ensure the integrity and coordination of the gradient.
2. If the surface of the printing plate is not polished or lightly corroded, or if foreign matter adheres to the surface of the impression cylinder, pinholes may also appear if the corroded parts cannot be transferred to the ink. The general surface roughness of printing plate requires Ra≤0.4nm. Secondly, trachoma exists on the surface of the printing plate, which leads to pinholes. The distribution of this type of pinholes is generally very regular and similar in shape. Therefore, the printing plate should be carefully checked before it is placed on the plate. It is best to use 800 or 1000 mesh Japanese gravure modified mesh to polish the plate several times. Use to avoid pinhole failure caused by the problem of printing plate, and also greatly reduce the production of printing bars.
3. The radial runout of the printing plate is large, resulting in the loss of small dot printing (not printing). Generally, after the plate roller is good, use a dial indicator to test the plate roller runout. The error is high quality within ± 10um, and ± (20 ~ 30) um is acceptable. If it exceeds 30um, it will cause printing quality problems. The chamfering radius and chamfered surface roughness on both sides of the plate roll also have a certain effect (improper printing pressure, irregular jumping and affecting ink transfer).
4. Imprint cylinder and adjustment. If there is dirt on the impression cylinder, if it is too soft or too hard, or if the skin is aging, the surface of the substrate will be uneven, which will cause pinholes. Generally, the hardness of the printing plastic film pressure roller is 70 ~ 80 Shore hardness, and the hardness used for paper gravure should be 80 ~ 90 Shore hardness. The thickness of the silicone part of the rubber roller is preferably 15-18mm. If it is too thick, it will not be easy to cool, and too thin will affect the printing quality.
5.Printing pressure: If the printing pressure is improper and unbalanced, the substrate and the printing plate cannot form a good contact, and the substrate cannot be transferred and inked locally, thereby forming pinholes, especially in the field and gradient printing. The flatness of the film is particularly important. When printing plastic film, the hardness of the impression rubber roller should be slightly higher, but the pressure should not be too large, otherwise the film will wrinkle when passing through the embossed part. Generally, the pressure applied to the rubber pressing roller is not more than 15Mpa.
Third, the impact of ink
To understand inks, you must first distinguish between viscosity and tack. Viscosity is the most important property of ink fluidity. The size of the viscosity is mainly determined by the composition of the ink itself. It is a measure of the ability of the fluid molecules to interfere with the relative movement of the molecules. It is the internal resistance of the molecules, that is, the resistance of the fluid flow. If the viscosity of the ink binder is large, the viscosity of the ink is also large. And tack can be defined as the stickiness of a substance. Viscosity is a relative value. In the printing press, from the ink roller to the printing plate, and from the printing plate to the substrate, each time the ink layer is peeled, the viscosity of the ink is the ability to hinder the peeling of the ink layer. The greater the viscosity of the ink, the greater the viscosity. In high-speed printing, the viscosity of the ink must be lower than the critical value, otherwise the ink cannot transfer and adhere to the substrate.
1) If the ink has poor wettability and poor leveling, and cannot wet the substrate as required, the transferred ink layer will shrink and pinholes will also occur. A surface tension regulator of about 1.5% can be appropriately added to the ink to improve the wettability and leveling of the ink.
2) In the printing process, due to improper ink allocation, a continuous ink layer cannot be formed on the surface of printing plates and substrates, and pinhole failure is easy to occur. The ink viscosity is too small, the hiding power is poor, and pinholes are easy to appear after drying. At this time, the real solvent and the pseudo solvent for the ink should be distinguished, and the ink viscosity and the ink dryness should be adjusted in a timely manner.
3) The ink has poor performance such as leveling. In the printing process, the ink spreads unevenly or there are air bubbles, resulting in poor ink transfer and pinholes. In this case, the leveling property of the ink can be improved, the doctor blade can be adjusted to make the thickness of the ink layer appropriate, and an appropriate amount of defoamer can be added to the ink tank.
4) The drying speed of the ink is proportional to the printing speed. If the ink dries too fast or wind blows directly on the plate roller, the amount of ink transfer in the anilox roller cavity will be reduced and uneven, which will cause pinholes.
5) If the ink does not use the recirculation patrol system during production, the ink may be condensed, or the air content of the oil film nozzle is large. Once air enters, pores and bubbles are generated, and pinholes will appear after drying.
6) The particle size of the ink also affects the reproduction of small dots. Therefore, the fineness of the ink should be ≦ 15um.
7) Adaptability adjustment of ink: including adjustment of viscosity and color density, which can be adjusted by automatic viscosity adjuster. In the ink color density, do not use solvent to dilute, and use a special diluent to adjust, so as not to be caused by the dilution of ink particles Small dot printing is missing.
8) After printing for a period of time, the ink will generate many reactive groups, which will be combined with the chromium ions of the gravure cylinder. When these combinations appear in the shallow mesh part of the gradient plate, their binding force will hinder the transfer of the ink , Resulting in loss of blockage, flower dots, and small dot printing.
Fourth, other influences
1) Improper adjustment of the scraper leads to a small amount of inking and uneven ink supply, especially when printing on solid ground and small halftone dots. Generally, the gravure plastic printing uses a blade hardness between 500 ~ 600 Vickers hardness, a blade pressure of 30 ~ 35N, and a blade angle of about 45 degrees is more suitable.
2) The ambient temperature and humidity are very important during printing. If the ambient temperature is low, the humidity is high, and the viscosity of the ink is large, the leveling property will be poor, which will cause pinhole failure. In winter or summer, the climate is dry, and the fast-drying solvent volatilizes easily, which can easily cause the small dots to be blocked, and the ink cannot be printed and the small dots are lost.
3) The effect of the evaporation rate of the solvent on the printing ink film.
The solvent evaporates rapidly, and the ink fails to level sufficiently to form many pinholes, while small water droplets in the air condense on the ink film to cause blooming, or the residual solvent is squeezed out from the inside to form bubbles (burst holes). Generally, the temperature in the printing shop is 23 ℃ ± 5 ℃, and the difference between dry and humidity is 65% ± 5%. It is more suitable. You can also add some slow-drying agents to improve the leveling of the ink. Please do not add benzene solvents and pay attention to residual solvents The amount can only be 3mg / m2 to meet the basic requirements. At the same time, it is best to install a static eliminator and a substrate cleaner on the printing machine to prevent dust and debris from remaining on the surface of the substrate.
4) Shallow net transfer
When the irregular shallow mesh transfer failure occurs, the dots are irregular in shape, so the dot blocking phenomenon caused by the floating of additives can be ruled out. The missing dots of this phenomenon are round white dots, and they will bloom in full. Therefore, it is recommended to test whether the corona uniformity of this batch of materials is greater than 52 dyne (using BOPA as an example). In addition, check whether there are impurities in the ink, dirt on the guide rollers and pressure rollers, and electrostatic fields. Whether there is a segmental longitudinal thickness phenomenon, which cannot be detected on the thickness gauge, and can only be observed by parallel extension of 2 ~ 3m or by the tension of the printing machine. In addition, the humidity in the air is relatively large, and the nylon membrane is a material with high moisture absorption. If water droplets are adsorbed on the membrane surface, small dots may be lost.