Flexible Plate Making and Frequently Asked Questions

Flexo platemaking is an indispensable process for flexo printing. This process determines the quality of printing plates and plays a decisive role in printing quality. The current flexo platemaking process is generally divided into six steps: back exposure, main exposure, rinse, drying, de-bonding, and post-exposure.

Let's talk about flexo platemaking first.

First, flexographic plate making

1. Exposure unit
(1) Back exposure (see Figure 1)
The plate supports the film up, the protective film down, and the exposure in the exposure drawer. The UV-A light transmits the support film to polymerize the photosensitive resin, and the main function is to establish the printing plate base as a basis for the relief.

Back exposure time is determined by the test.

(2) The main exposure (as shown in Figure 2)
Plate supporting film is laid down in the exposure drawer downwards, the protective film is peeled off, the negative film containing the graphic and text is placed on the photosensitive resin, and the vacuum film is attached, the vacuum pump is turned on, and the film is closely attached to the photosensitive resin. The UV-A light penetrates the vacuum film and the film graphic to crosslink the photosensitive resin to form a recessive insoluble relief pattern. During this process, the density of the film and the degree of vacuuming are the key factors. The former is used to ensure that the no-image part completely blocks ultraviolet light, and the latter ensures that the film adheres closely to the photosensitive resin, reducing the residual air generated during the exposure process and affecting the plate-making effect.

The main exposure time is determined by the test.

There are the following requirements for the printing machine:

1 The exposure part should be equipped with UV-A tubes of different sizes and power. For different series and different model sizes, the light intensity in the entire light-receiving area is generally 70 to 110mW/cm2, and the received light must be uniform and the deviation cannot be more than ±3%.

2 There is a precise exposure light timer.

3 has enough vacuum adsorption capacity, generally require the generation of negative pressure above 80KPA. In this way, the film can be kept in close contact with the plate to reduce the halo phenomenon, and ozone generated in the plate making process can be eliminated in time.

4 The seal condition around the printing machine is better, to avoid ultraviolet radiation leakage to the operator.

From the perspective of use and maintenance, the following points should be noted:

The design life of a 1UV lamp is generally about 600 hours. Since the light intensity gradually decreases during use, the exposure time should gradually increase with the increase of the use time. In addition, the energy release is unstable when the new lamp is used, so it should be started after 15 to 20 minutes.
2 Use alcohol to clean the surface of the lamp once every two weeks so that the luminous efficiency of the lamp can be guaranteed.
3 The vacuum pump is cleaned and maintained every two months to ensure that a vacuum of 80 KPA is obtained.
4 Keep the room clean, prevent dust, so as not to affect the quality of printing.
5 Keep the vacuum membrane clean and avoid contact with the solvent to reduce the life and light transmission of the vacuum membrane.
The 6-draw slide rails should be kept lubricated. When the slide rails are pulled out, heavy pressure is strictly forbidden to damage the slide rails.

2. Flushing unit

After the resin plate is printed, rinse it. In the flushing process, a photosensitive resin that has not been crosslinked without ultraviolet irradiation in a printing process is dissolved in a flushing solvent, and the photosensitive resin that has been crosslinked by ultraviolet irradiation is formed into a relief to obtain a printing plate.

In order to produce high-quality flexographic printing plates, automated plate-washing equipment must have the following functions:

1 The rinsing solvent is usually a mixture of n-butanol and tetrachloroethylene, the specific gravity is about 1.418, and the flushing temperature is controlled at 20°C. Different regions have different temperatures and require that the temperature of the rinse solvent be kept constant.
2 It is possible to set the speed of plate washing and the time of washing to suit the flushing of various plates. The height of the brush can be adjusted to suit different thickness of the plate.
3 White sensor can be used to calculate the length of the plate, and the movement of the brush can be automatically controlled. The washing solution can be circulated.
4 brush hardness, not too soft so that the plate can not be washed clean, it can not be too hard to hurt the layout.
5 can automatically preheat and dry, generally at 60 °C. The plate should be taken out and it should not be provided with a bath solution.
It has a fresh potion to supplement the function to ensure that the base plate of the clean flexo plate is clean and the details of the text are clear.

The flushing device is sealed and the negative pressure of the volatile gas is eliminated to ensure a good environment.
In the maintenance of the flushing department, the following matters need attention:

1 Take care to filter and remove impurities in the solvent to prevent the transmission line and the solenoid valve from being blocked. The resin solidified in the machine is often cleaned to reduce the brush's resistance to movement and the additional load on the motor.
2 Because the brush is often in contact with the solvent, its hardness will gradually increase, as well as affect the quality of the plate. So pay attention to changes in brush hardness and replace if necessary. Brush motion, plate material transmission and other continuous motion parts should be well lubricated. The volatilization gas exhaust fan is regularly cleaned to ensure its working efficiency.

3. Drying unit

In the process of rinsing, the printing plate will inhale the solvent and swell due to prolonged contact with the solvent. The drying of the hot air will promote the volatilization of the solvent in the plate to restore the original size and thickness.
1 The drying section has a constant temperature control capability. Generally, the drying temperature should be controlled at about 60°C. The temperature is too high for the plate material. Too low the solvent evaporation rate is slow and the efficiency is low.
2 Drying in the drying process, the density of the volatile solvent gas in the drying drawer gradually increases. If there is no negative pressure eliminating function, it will block the drying of the plate, so the function of exhausting gas is necessary.
3 Drawer slides should have good lubricating properties, add oil once every two days. The solvent gas exhaust fan is regularly cleaned to maintain a good working condition.

4. Detachment unit

After the printing plate is dried, the surface is still sticky, so the surface should not be in contact with other objects, and do not touch the plate with hands to avoid sticking dirt or fingerprints. At this time, ultraviolet light with a wavelength of 220 to 260 nm can be used to irradiate the plate resin, so that the surface of the plate is not viscous. If the exposure time is too long, the plate will crack and cracks may occur during storage.

According to the different equipment and size, with different power and quantity of UV-C lamp, the illumination should be uniform.

5. Post-exposure This is the finishing work of a flexographic plate, where the photosensitive resin of the plate is thoroughly polymerized under irradiation with a UV-A lamp. Post-exposure ultimately determines plate hardness, print resistance, solvent resistance, and pressure resistance. The post-exposure is generally performed in the exposure unit. Some equipment, to stick and post-exposure at the same time to improve work efficiency.
The following section analyzes common problems in flexo plate making.

Second, common fault analysis
1. Low relief, inconsistent thickness Many platemaking plants have encountered this type of failure, causing the following causes:
1 Back exposure time When the plate is over-exposed, back-light exposure forms the base material of the cured plate material. If the back exposure time is too long, a thick substrate will be formed. Since the photosensitive material will not be washed after being cured, the relief will be lower. Therefore, according to the light intensity of the exposure lamp, the back exposure time is appropriately reduced or increased. In addition, some exposure lamps start with different speeds, resulting in different levels of exposure of the photosensitive plate material, can also lead to different substrate thickness, different levels of relief.
The current technology has enabled the lamp to always maintain its warm-up state, allowing the lamp to start instantaneously.
2 The positive and negative sides of the plate are reversed Some platemaking factories will experience some low-level mistakes. When the exposure is made, the front and back sides of the plate will be reversed, resulting in lower or no embossing of the plates. Therefore, special attention should be paid when cutting the plate. A small corner of the plate is used as a mark to prevent mistakes during exposure.
3 Insufficient Wash Times Due to the short printing time of the plate, the uncured photosensitive material is not completely washed and remains on the plate, which also results in lower plate relief. Therefore, it is necessary to control the plate washing time based on experience. Many plate-making solvents in the plate-making system are mixed with tetrachloroethylene and n-butanol. Due to the wrong proportion of solvents (the correct proportion is 1.418), the printing plate washing time has a great influence. Excessive tetrachloroethylene can cause the printing plate to swell excessively. Excessive butanol can affect the plate washing time and result in longer plate washing time. Therefore, the specific gravity of the printing plate rinsing solvent is very important and should be measured frequently.
4 Plates Storage of Inappropriate Plates have been exposed to sunlight before exposure, resulting in polymerization of the photosensitive material, resulting in lower relief or lighter graphics. Therefore, the plate should be stored away from light to prevent it from being irradiated with light.
2. Loss of a good printing plate in the line deformed network should be straight, with complete outlets and neat layouts. If the lines on the plate are bent and the dots are lost, the entire plate is abolished. There are three main reasons for the above reasons:
1 Back exposure time is insufficient When the back exposure is insufficient, the base polymer layer of the printing plate is thin, and the embossment is too high, the network dots are easily lost or the lines are bent, so the length of the back exposure time will directly affect the quality of the printing plate.
2 Insufficient main exposure time When the dot diameter of the printing plate is 0.1mm or the line width is 0.1mm, if the exposure time is insufficient, the dots will be lost or the lines will be bent. Therefore, a mask treatment method should be adopted during the plate making process to extend the main exposure time of this part.
In addition, at some times the independent printing plate on the printing plate bending deformation, the line will be straightened after drying, this is a normal phenomenon, the premise is that the main exposure time must be sufficient, otherwise the deformation of the line after drying can not be restored ,
8 Washing time is long and the brush pressure is high. Due to the long washing time, the independent lines or meshes are over-expanded, and they are easily bent or washed away, resulting in the plate becoming invalid. Therefore, when flushing lines or screens, the plate washing time and brush pressure should be properly controlled.

3. Plate relief cracked

In the platemaking and printing plants, the phenomenon of cracking of the prepared plate will be encountered. What causes this? Because the resin plate is very sensitive to ozone, the plate will produce some small Cracks, poor plate storage will crack. During the plate making process, if the tack time is too long, it will cause some small cracks in the plate, making the plate unusable.

Operators in the printing plant use chemical solvents to wipe the plates. Chemical solvents have a strong corrosive effect on the resin plates, causing the plates to crack. Therefore, it is recommended that:

1 Use a special solvent for scrubbing when rubbing the plate;
2 Avoid the corona treatment device and the motor next to the storage plate to avoid ozone:
3 Plates should be kept away from light and not be exposed to sunlight because sunlight contains a lot of ultraviolet rays, and the plates will crack when they are exposed to light and cannot be used again.

4. Plate-making operation is reversed

The correct plate making process is: Back exposure - Main exposure - Process printing - Drying - Exposure after de-sticking, for a total of 6 steps, and the order cannot be reversed. In some platemaking factories, the main exposure is performed first, so that a large amount of light is transmitted, and a low base is combined. In a place where the amount of light is small, the small dots are easily lost and the wire is slightly bent. The back exposure not only forms the substrate of the printing plate, but also activates the polymerization performance of the printing plate to form a layer of denser inducing layer. This inducing layer has a uniform density and lays the foundation for the main exposure reaction. If the two sequences are reversed, the small dots are connected. Not on the plate quality.

5. The hardness of the plate is high

The high hardness of the printing plate will greatly reduce the service life of the printing plate in printing. How to reduce the hardness of the plate? Among them, the main exposure time and the post exposure time play a decisive role. After the printing plate is washed, the uncured photosensitive material is subjected to post-exposure curing, and if the post-exposure time is increased, the hardness of the printing plate will be directly increased and the service life of the printing plate will be greatly reduced. Therefore, the post-exposure time should be properly controlled at the time of plate making.

6. Layout pattern is shallow

(1) Excessive main exposure time

Due to the different exposure latitudes of plates, the method of plate making is also different. The same film has both field and dot images on it. For a deeper pattern, the best method is to use masked layered exposure. The plates produced do not have a deep pattern, and the prints are beautiful and conducive to printing. Many plate-making plant operators are afraid of trouble, the same exposure time in the field and outlets, so that the pattern of the plate is lighter.

(2) The density of the film is low

Some film making plants use a lower density film (about 3.0 or so, and the required film density is above 4.0). The main exposure time affects the degree of solidification of the material.