Today, most printing presses employ flexographic printing. Flexographic printing is a rotary printing method that prints patterns onto various substrates using raised pattern surfaces. Specifically, the desired pattern is created on the printing plate by withdrawing and lowering the non-printing areas of a rubber or photosensitive plate. The printing plate is connected to rotating cylinders of varying diameters to produce patterns of various sizes. Ink is transferred to the printing plate surface via an ink metering roller with a mesh-like structure called an anilox roller. Typically, an inkjet unit works in conjunction with a doctor blade to supply ink to the anilox roller. This system operation can be repeated on the printing press for each printing color. Currently, the average number of printing stations on a printing press is 10, and the width of the roll paper ranges from 6 to 136 inches.
The main types of motion in printing applications are diverse, ranging from simple monochrome printing to complex multicolor printing requiring precise positioning. Most printing applications use a rotating inkjet head containing the image to be printed. The printing surface is typically a roll of paper that comes into direct contact with the inkjet head. This roll of paper is usually linear and can be made of any type of material, including paper, plastic or resin film, corrugated paper, etc. Printing ink is supplied directly to the inkjet head, and the image is transferred onto the roll of paper as the inkjet head contacts it.
The primary motion type in printing applications is master/slave axis motion—the web of paper to be printed on is the master axis, and the inkjet head is the slave axis. In traditional printing presses, the web and inkjet head are mechanically connected. However, nowadays, servo mechanisms are commonly used, considering the need for electronic cam-based contouring via high-performance motion controllers, and the demands of end-users for increased machine flexibility, shorter lead times, and rapid production changeovers. This is especially true for the inkjet head, and sometimes also for the web spindle.
Multicolor printing applications involve multiple inkjet heads, each corresponding to a single color, acting as a slave axis to the main axis. The inkjet heads must be kept aligned with each other. Other types of movement are also common in printing applications, such as adjustment axes used to move inkjet heads in and out during machine maintenance and product changeovers.
Typical printing applications require a variety of motion control components, including feedback devices such as encoders or resolvers, servo motors, speed reducers, servo amplifiers, and high-performance motion controllers. The motion control methods used in printing can also be applied to other paper processing applications where the rotary driven shaft contacts a linear paper roll.
Several problems exist in motion control. There are several problems in implementing motion control in printing applications.
First and foremost, precise control of the printhead speed is crucial when the printhead contacts the roll of paper. Sometimes the speeds match perfectly, but other times the printhead speed may be slightly higher or lower. Inaccurate speed matching can not only degrade print quality but may even damage the material.
If the circumference of the printhead is the same as the product length, the motion relationship is a transmission ratio between the two, plus a positional synchronization relationship (e.g., they are proportional, but the slave axis is locked in a specific spindle position, so printing occurs in the correct position on the roll of paper). If the circumference of the printhead is different from the product length, speed matching occurs when the printhead contacts the roll of paper. For the remaining length of the product, the printhead must accelerate or decelerate to contact the roll of paper in the correct position to process the next piece of product.
Furthermore, when executing motion curves, synchronization is required not only in speed but also in position. This ensures that the spindle/slave axis relationship is effective at all spindle speeds, from stop to full speed, thereby minimizing scrap rates. Speed depends on the substrate; label printing speeds are typically 300–1000 FPM, while central impression cylinder flexographic printing (roll paper wound around a large central impression cylinder with inkjet heads positioned around the cylinder) on plastics reaches speeds of 1000–2000 FPM, and central impression cylinder flexographic printing on paper can exceed 3000 FPM.
Of course, there are exceptions. Sometimes the speeds of the roll paper and the printing plate are mismatched, with one speed exceeding or falling short of the plate roll's repeat length by 2%. Because end customers can adjust the dimensions as close as possible to their actual needs, rather than accepting the quantity the machine can produce, this saves on plate rolls/sleeves and significantly reduces material consumption.
Most printing applications also require positioning, and the differences between different positioning methods are significant. The optimal positioning should be within 0.0005 inches, with 0.002 inches for central impression cylinder flexographic printing and 0.003 inches for inline printing.
Therefore, the motion curve needs to be dynamically adjusted to compensate for minute changes in the distance between positioning marks on the roll of paper. This is especially true in multicolor printing, where precise color registration must be maintained to ensure good quality of the final printed image.
Designing the motor/amplifier combination across the entire product length range is also important. The smallest and longest products may not represent the worst-case scenario. It is generally recommended that this design be done for several product lengths (e.g., 5 or 10) to ensure a suitable motor/amplifier combination is determined for the entire product range. Force and torque vary considerably with machine dimensions, and load inertia also differs with the design. Motion control components must be designed to accommodate inertia mismatches ranging from 10:1 to 200:1.
Improving pre- and post-press processes through motion control technology, and enabling high-speed, quantitative product positioning where possible, is crucial for printing success. High-speed positioning requires providing high-speed position lock input to the drive or motion controller to capture the precise spindle position upon encountering positioning markers. The difference between the preceding and following positions is used to calculate the actual distance between the positioning markers. This distance is then compared to the theoretical distance between the markers to calculate a correction. This correction is subsequently substituted into the slave axis motion curve.
The time required from detecting a positioning mark to applying a correction is critical for producing high-quality products. This time depends entirely on the motion controller used for the application and is also affected by the technology employed (for example, some digital motion control networks have a feed lag that must be compensated for, which may be unacceptable in some applications). Some applications require specialized positioning algorithms, such as averaging corrections over several or many products, filtering corrections, or requiring that corrections not be applied to certain segments of the motion curve. The motion controller should also be able to properly handle roll splicing and missing positioning marks.
A typical basic configuration of a 10-color printing press production line has more than 65 closed-loop motion control axes. In larger production lines, this often exceeds 100 axes. Previously, each axis was mechanically driven; now, updated control systems improve positioning performance, reducing positioning accuracy to half the value of older systems. These control systems offer high flexibility to handle special tasks, providing unlimited variable repeatability. As an alternative to the speed matching methods discussed earlier, it allows users to increase or decrease the speed of the plate rollers relative to the paper roll to change the repeat length.
These improvements also enable greater speeds, increasing the maximum speed from 1200 FPM to 2000 FPM, with even higher speeds achievable in flexographic printing. The time to switch from one printing job to the next has been reduced from an average of 4–6 hours to 30 minutes, and in some cases even less. The use of robotic arms with motion control and printing drive positioning systems significantly impacts changeover times. In practice, improved diagnostic information provided by the entire control system extends machine production time.
The above article was provided by the Technical Department of Guangzhou Bowei Servo Technology Co., Ltd. and may not be copied.
