The advantages of flexo printing in the label industry are now unquestionable. Label printing companies, which used to focus on the embossing process, have purchased high-end flexo equipment in recent years and used the flexo process to print high-end labels, which has improved the printing quality of products, increased productivity, and increased the Environmental protection. The flexo printing process is used to print various labels, which reduces the loss in the production process and improves the profit level of the enterprise. Under the trend of low profit and low profit in the printing industry, while the label industry is unique, high-end labels are led by flexo printing, and flexo printing occupies a favorable situation in the label industry.
At the beginning of the domestic flexo label, only 120 to 133 line printing plates were used, because the embossing process that was dominant in the industry at that time was only at this level, and the design of foreign flexo printing only stayed at this level. Because the offset printing screen has reached 175 lines, the pressure of market competition has forced flexo to upgrade its process level from 120 to 133 line printing plates to 150 to 175 line printing plates. Today's flexo labels have generally reached this level. Even if they are produced using domestic flexo printing machines, they rarely use medium and low screen printing plates below 150 lines.
The base materials used in flexo printing range from thick to thin, from paper to plastic, from materials that are not easily deformed to those that are easily stretched and deformed by heat. Flexographic labels are from paper stickers to film stickers, and then to film substrates without backing paper, such as synthetic paper using polyolefin materials. The experience of flexo printing in the label industry is bound to expand into the composite flexible packaging film industry.
At present, the level of flexo printing in the flexible packaging industry is only the level when flexo first entered the labeling industry. The highest printing screen line does not exceed 120 to 133 lines. A batch of enterprises that launched the second wave of wide-format flexo promotion in China ten years ago, many still insisted that they are still using 120-line printing plates, printing coloring blocks and medium and thick screens, and even retreating to 100-line printing. There are also those who withdrew from the flexo market without withstanding market pressure, and resold the printing equipment imported that year to other places. In the flexible packaging industry, although the use of satellite-based flexo printing machines has led to the quality of overprinting, the improvement of the process of gravure printing in PE film has caused the cost of flexographic film printing to fall linearly, above 50μ PE film There is almost no advantage in terms of printing costs. It is also printed on the PE film that is easily stretched and deformed by heat. The gravure printing adopts a 175-line printing plate, and the flexographic printing only uses a 120-line printing plate.
It is also printing plastic film. The label industry uses a unit-type narrow-width flexo printing machine, which can print from 150 lines to 175 lines, while the flexible packaging industry uses a satellite-type wide-width flexo printing machine to only print from 120 lines to 133 lines. It is a machine door. The reason for the narrow width? Is it the reason of the machine structure? If the width of the printing door is wide, the rigidity of the plate roller is not enough, and it is easy to deform, which causes the printing dots to expand, causing blockage failures on the printed matter, and resulting in a decrease in yield. Therefore, printing companies can only use medium and low screen printing plates. However, in recent years, the carbon fiber bridge sleeves that have been gradually used on wide-format machines can effectively avoid the rigid deformation of the plate roller. Why European flexo printing equipment suppliers that have adopted this technology have so far been reluctant to use it on their own equipment What about 150 to 175 line plate printing?
The answer to the question must be found in the different configurations of the flexo printing equipment.
The flexographic printing film generally adopts the satellite-type model. The characteristic of this equipment is that the film is wrapped on the surface of a central impression cylinder with a diameter of 2m to 3m, and the wrap angle accounts for about 85% of the circumference. This design makes the printing color groups distributed around the central impression cylinder at a relatively short distance, each color group runs at a synchronous speed, and the film shows zero tension between the color groups. The printing base material (film or paper) of the satellite flexo printing machine is closely attached to the surface of the huge metal impression cylinder, and runs in synchronization with the metal impression cylinder. The metal impression cylinder not only plays the role of printing the ink on the printing substrate, but also acts as a supporting roller for the printing substrate. Therefore, even if the film becomes soft after being heated, the driving of the film is driven by the central impression cylinder, instead of being dragged by the material itself like a unit-type machine.
Unit-type models, even if they use a servo method, that is, models with no long shafts and no gearboxes, which are directly driven by high-precision servo motors, because it is impossible to establish a mathematical model for the tensile properties of the printed material under heating, so Therefore, it is impossible to reliably compensate for material deformation. The unit-type servo model can establish a mathematical model for the transmission of the rotary machine, and effectively compensate for the mechanical synchronous speed loss caused by the printing pressure applied by each color group, but this compensation does not include the thermal stretching characteristics of the material, so The result is a kind of "fuzzy" compensation, that is, for the active rollers of each color group (the plate cylinder of the gravure printing machine, the impression cylinder of the flexo printing machine), although the synchronization speed of each color group can be corrected without having to adopt the compensation algorithm of increasing diameter, However, it was found in actual utility that the diameter needs to be kept increasing. Another result is the selection of materials, only those printing substrates that have a small thermal deformation and a certain thickness and are not easily deformed. The satellite model does not need to consider the deformation characteristics of the material, and the registration accuracy is very high. The registration accuracy of the general gear transmission model can reach 0.1mm ~ 0.15mm, the registration error of the servo model is smaller, the registration error of the satellite servo models Infiniti and Vision G of the US PCMC does not exceed 0.07mm.
However, the satellite model has an important defect. The printing pressure set at the machine's adjustment stage, that is, the pressure between the plate roller and the central impression cylinder, will increase after the machine has been running for a period of time. The gap between the plate roller and the center impression cylinder will become smaller. If high screen printing plate is used for printing, small dots in the high light area will easily block the plate, causing quality problems. If the machine type model is adopted, since the structure does not use a large-diameter metal impression cylinder, the change in dot expansion is small, and there is no risk of blocking the plate. Therefore, even in Germany, which pays more attention to printing quality, only 120-133 printing plates are basically used on satellite flexo printing machines, and printing with 150-175 printing lines is rarely seen.
How is this defect of satellite flexo printing machine caused? Famous flexographic equipment manufacturers in Europe and the United States have always attached great importance to the manufacturing accuracy of the central impression cylinder. The published data shows that the radial runout error of the central impression cylinder used by some machines, or the full runout error, is only about 10μ. In order to calibrate the central impression cylinder that has exceeded the normal allowable value, the author saw the German counterparts drilled through the 300mm diameter entrance and exit for debugging 13 times in one day. It is precisely such a precise central impression cylinder that cannot effectively control the expansion of the dots during operation. What is the reason?
Three factors determine the inevitability of this defect.
One of the factors is the structure of the printing platform with a tilting angle (or radial type).
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Early satellite flexo printing machines used vertical and downward tilting (or radial) printing platforms, that is, eight color groups distributed around the central impression cylinder, the first and eighth color groups in the highest position, and The 4th and 5th color groups in the lowest position adopt the upward or downward tilt angle (generally around 15 ° -20 °), while the other 4 color groups in the middle position basically adopt the horizontal angle. The gap adjustment between the plate roller and the central impression cylinder moves along the movement direction of the adjustment screw, and the movement direction of the screw is consistent with the arrangement angle of the printing platform. In the horizontal color printing group, the screw moves in the horizontal direction, and the gravity of the plate roller itself does not affect the adjustment accuracy, but at the position of the upward angle, the screw moves from high to low, and the angle from the horizontal direction impresses the center of the cylinder Directional movement, although the screw is equipped with a locking mechanism, using hydraulic, pneumatic or mechanical locking, in high-speed movement or under certain mechanical wear conditions, the reliability of the locking mechanism on gravity is questionable. In actual use, after running the first and eighth color groups for a period of time, the careful operator will find that the printing pressure of the plate roller to the central impression cylinder will increase slightly. In order to maintain the consistency of printing quality, it is necessary to move in the exit direction in a timely manner and to reduce the pressure. On the contrary, in the 4th and 5th color groups at the lowest position, the direction of the screw is from bottom to top, and it moves at an angle to the horizontal direction. The effect of gravity will cause the plate roller and the center impression cylinder to move out, so that The printing pressure is reduced, and the operator should increase the pressure in a timely manner. The servo-type satellite flexo printing machine uses a servo motor to adjust the position of the plate roller. It is possible that the screw is no longer used, but the way of the up and down tilt angle determines that the influence of gravity cannot be avoided, and the precise adjustment of the printing pressure is hidden.
Due to the different gravity conditions of each printing color group, it is reasonable to say that the servo motor control program at different positions should also be different. Only in this way, the servo control program of the motor is reasonable and reliable. However, the electrical control devices of flexo equipment in Europe and America generally use several well-known brands, and in order to ensure the interchangeability of the devices of each color group, the suppliers of these brands often adopt a unified design and ignore the difference of each color group under gravity conditions. Just ask if the control components of the 8 color groups can be interchanged on the machine of the vertical printing platform. The physical conditions of the machine structure are different, but the control device is the same, which buried the hidden danger of the expansion of the printing outlet of the satellite flexo printing machine.
The second factor is the cantilevered plate roller and anilox roller air expansion mandrel fixing structure.
The cantilever structure is actually launched in response to the disadvantage that the previous coupling type inflatable sleeve technology requires a lot of preparation time when changing the version. The coupling-type plate roller sleeve is generally installed offline, and then pushed to the flexo printing machine with a trolley, and the plate roller is hoisted in place with a crane or a special manipulator, and the bearing housing cover is closed and locked. This fixed method makes the production preparation time very long. Generally, it takes 2 to 3 hours for 8 color groups to change a production order, even if a special robot is used, it takes 2 hours. A domestic device imported from Europe actually took 6 to 7 hours to install. However, this basic method has one characteristic, namely stability. Both ends of the shaft are fixed on the bearing seats on both sides, the force is equal, and the operation of the plate roller is stable.
One end of the inflatable mandrel of the cantilever structure is fixed on the driving side of the machine, and the other end is suspended, so that the plate roller and the anilox roller sleeve can enter from here. After the assembly is completed, the bearing seat is locked. The biggest advantage of this structure is that the plate is changed quickly, without any auxiliary device, an operator can replace it with bare hands, simple and fast, and short downtime. PCMC's two satellite flexo printing machines, Infiniti models replace 10 color groups, the time does not exceed 40 minutes, VisionG models replace 8 color groups, the time does not exceed 35 minutes.
However, the cantilever structure has major hidden dangers in the fixing method on the operation side. One end is fixed and the other end is suspended. Although the bearing seat is supported after the assembly is completed, the force on both ends of the plate roller core shaft is uneven, and the influence of gravity is inevitable. After a long period of operation, the plate roller core shaft is inclined toward the operating side, and the parallelism of the plate roller core shaft is affected. The equipment of the famous European brand used by the author was difficult to pull out the inflation sleeve after several years of operation. Although the current modified design has a pop-up mechanism when pulling out the sleeve, it is not so uncomfortable to pull out, but the installation accuracy of the plate roller is reduced, and it is easy to cause the defect of dot expansion in printing. .
This fixing technique also has an unnoticed flaw, that is, when the plate roller approaches the central impression cylinder, the reverse thrust of the central impression cylinder against the plate roller acts on the center of the inflation mandrel, while the plate roller The moving force of the impression cylinder often forms a moment at the position of the bearing seat with a force arm approximately equal to the radius of the plate roller. This additional torque is likely to cause unnecessary impact on the plate roller itself, increasing the instability of the plate roller.
The thickness of the shaft end of the plate roller core and the use of ball bearings, needle bearings or roller bearings, the size of the servo motor power of the plate roller, and the heat of the motor itself will affect the stability of the plate roller operation and the dynamic conditions. Bounce error. The accuracy and stability of the plate roller movement are directly related to the printing pressure of flexo printing, and to printing high-screen printing plates without blocking failures, and therefore to improving the printing quality and yield of flexographic products.
The third factor is the influence of the drying system between the color groups of the satellite flexo printing machine on the expansion of the metal body of the central impression cylinder.
The drying system of the satellite flexo printing machine is divided into two parts. One part is the drying device between the color groups arranged around the central impression cylinder and arranged immediately after each printing color group. The other part is the bridge drying device which is set independently from the position of the central impression cylinder after the last printing color set. The drying device between the color groups adopts a V-shaped nozzle, which is closely attached to the surface of the central large impression cylinder, and hot air is sprayed to dry the film or paper, and n-1 nozzles are used on the central cylinder with n printing color groups installed. At the same time as the printed matter, the surface of the central impression cylinder is also heated.
The center embossing cylinder of the satellite flexo printing machine generally adopts the assembly process. Under the surface steel plate is the interlayer with water flowing inside. The water is constant temperature, generally controlled at 30 ℃ ± 1 ℃. The temperature of the central large drum is not to prevent the film from deforming as some misleading said. In fact, the water temperature of 30 ℃ can not make the film temperature rise to the softening temperature of the film, but this is only a by-product of the constant temperature of the roller, and its real purpose is to maintain The accuracy of the central impression cylinder does not deform the cylinder due to the expansion of the metal body. There is a key in the maintenance of the center impression cylinder, that is, the power supply of the main machine of the printing press and the power supply of the temperature control device of the center cylinder must be separated. The host machine is powered off, but the thermostat cannot be powered off, and it must be ensured that the center drum is always under the protection of a constant temperature. Otherwise, the accuracy data measured by the center drum under static conditions will not be guaranteed.
The expansion of the large-diameter metal central impression cylinder under heating conditions has a rule that every time the temperature rises by 1 ° C, the diameter will expand by 12nm for every 1mm in the diameter direction. Reached nearly 36μm. This data exceeds about 1/2 of the thickness of the plastic sealing tape. In the actual printing process, if the temperature rise is not 1 ℃, but 10 ℃ or more, then the radial runout error of the center cylinder measured under static conditions is only around 10μm, while under dynamic conditions, the error will be doubled Expand or more. The expansion of the metal body of the central impression cylinder causes expansion of the printing dots. This is as long as a thin layer of plastic sealing tape is adhered to the surface of the film or paper to be printed during normal printing. The thickness of this single-sided tape is usually 60 μm At this time, the printing pressure is not readjusted, which means that the gap between the plate roller and the central impression cylinder is increased by about 60 μm compared with normal conditions. Observe the size of the dots on the tape and compare it with the dots on the non-stick tape, we can understand what this means. [next]
The central platen cylinder is controlled by constant temperature water, and the diameter of the inlet and outlet pipes is about 2 to 3 inches. A certain water pressure and flow rate can balance the temperature rise of some central cylinder surfaces. However, to accurately control the temperature rise, it is still difficult to analyze from the current printing machine manufacturing technology. The author once measured a flexo printing machine of a famous European brand. At the initial start-up, the semiconductor spot thermometer showed that the surface temperature of the drum was 30 ° C. After 2 hours of operation, the temperature was increased to more than 40 ° C. After the drum surface temperature rises to a certain temperature, the dynamics will be balanced and will not continue to rise. It is this temperature rise of more than 10 degrees that has indeed had a fatal effect on the expansion of the dots caused by the control of the printing pressure of the satellite flexo printing machine.
The above three major reasons have caused the regret that the wide-format satellite flexo printing machine can't use the 150-175 line printing plate like the unit-type flexo printing machine, but can only print large color blocks and thick screen lines of 120-133 lines. But why can the US flexible packaging market use 150-line printed composite film flexible packaging products?
The famous American flexo printing equipment manufacturer PCMC has their uniqueness in solving these problems.
1. A horizontal printing platform is used on the central impression cylinder.
This structure ensures that the gravity conditions of all printing color groups are the same. When the same electrical control software is used, the control devices correspond to the actual conditions one by one, completely matching, and the reliability of the control is improved to a high level. However, the horizontal platform also has a defect that the repetitive perimeter of printing under the condition of the central impression cylinder of the same diameter will be slightly smaller than that of the vertical tilting scheme. This is because the vertical tilting structure has a larger open space and can accommodate larger diameter Large plate roller. Therefore, in order to achieve the same printing repetition circumference, the horizontal structure needs to enlarge the diameter of the central impression cylinder, which will increase the initial investment cost of the equipment. Under the premise of ensuring the quality of printing equipment, this design of equipment may be more expensive, but the superiority of cost performance is very obvious. Some well-known European and American flexo printing equipment manufacturers no longer produce models with up-down tilt structure, including equipment introduced by Taiwan manufacturers that have adopted horizontal structure solutions. This is the worldwide trend.
2. The design patent of PCMC in cantilever type inflation mandrel fixing technology.
The characteristic of this technology is: the integral metal frame digs out a box-shaped window at the fixed end of the operating side where the cantilevered mandrel is installed to ensure the structural rigidity of the printing platform, and the bearing seat is installed inside the movable steel block. The block is driven by a high-precision ball screw. The moving position of the screw is on the center line of the mandrel of the plate roller. Linear guides are installed at the upper and lower ends of the box window to fix the mobile steel block, thus ensuring the printing platform is running at high speed. Stability.
In this design, the fixation of the mandrel is completely automatic, and the movement of the entire screw and the locking of the shaft end exclude human factors. The movable steel block is fixed to the frame by two linear guides on the upper and lower sides, and because the screw moves on the center line of the core axis of the plate roller, the thrust of the central impression cylinder to the plate roller and the driving force along the direction of the ball screw act In the center of the plate roller mandrel, only a small local pressure is formed, and because the load of the force only acts on the center of the plate roller mandrel, there is no moment on the support frame.
Due to the protection of the patent, other equipment suppliers can only use manual or semi-automatic plate roller mandrel fixing methods, or use the upper or lower single rail fixing method, which is a bit regrettable.
3. The patented technology of compressed air drying method is used to replace the traditional drying method.
The so-called eXtreme drying method is characterized by the use of 40 psi of compressed air, a 0.04 inch diameter nozzle, and an effective impact on the surface of the ink film at a flow rate of 330 m / s. This surface layer is the boundary layer of the solvent molecules volatilized by the ink during the drying process, so that the solvent molecules in the boundary layer escape a large amount, and the boundary layer is continuously updated to make the volatilization more thorough. Because the high-speed air flow is ejected from the small hole with a diameter of 1mm, the adiabatic expansion, air entrainment, and the cluster jet flow form a circular shock wave of a certain diameter at the surface of the film. The size of the shock wave area is related to the distance from the injection hole to the film. The temperature of the high-speed hot air flow drops rapidly when it reaches the surface of the film, and a negative pressure is formed at the nozzle. The temperature of the surface of the film does not increase significantly, and the temperature of the large roller as the center of the film support does not increase significantly. Therefore, even at a printing speed of 600m per minute, the ink on the surface of the film is effectively dried, but the large central impression cylinder will not be deformed by the hot air jets sprayed by the drying device between the surrounding color groups. Body expansion can be effectively controlled.
Due to patent protection, many flexo printing equipment in Europe and the United States cannot currently use this drying method. Therefore, the popular drying method cannot avoid the influence of the hot air sprayed by the drying device between the color groups on the dynamic accuracy of the central impression cylinder. The higher the heat of the device, the greater the air volume, and the greater the impact on the central impression cylinder under dynamic conditions. Satellite flexo printing machine, even the famous brands in Europe and America, choose to use 120-133 printing plates to hide, but can't use 150-printing printing plates like the label industry, and even 175-printing printing under suitable process conditions. Version, to improve the quality of flexo printing to a new height.
After solving key technologies such as high-line-number printing plates and high-line-number anilox rollers, flexo printing is already just around the corner to promote the effective experience that flexo printing has achieved in the label industry to the flexible packaging industry. "If workers want to do their best, they must first sharpen their tools." Choosing the right flexo equipment must be considered by every company that wants to try flexible packaging. Different from other printing processes, the key data such as quality level and yield in the production process of flexo printing have a great relationship with the selection of equipment, which is not something that ordinary engineering and technical personnel can do. Looking at the rise and fall of some companies that have adopted the flexo process in the domestic flexible packaging industry for more than a decade, all prove this truth. Therefore, in the decision-making process, the decision-making layer must have an understanding of equipment technology, international advanced manufacturing technology, and the characteristics of various models of different structures, and be good at finding differences through comparison to It is what we hope to avoid the regret of future wear and tear and low yield.
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