Application of advanced motion control technology in printing equipment

Today, the most common presses use a flexographic printing process. Flexo printing is a rotary printing method that embosses patterns on the surface of various substrates. That is, the non-printing area of ​​the rubber plate or photosensitive resin plate is withdrawn and lowered, and the desired printing pattern is generated on the plate. The plate is connected to a rotary cylinder of different diameter to obtain various size patterns. The ink is an ink metering roller called an anilox roller with a mesh structure transferred to the plate surface. In general, inkjet devices work in conjunction with a doctor blade to supply the anilox roller with ink. The above system operation can be repeated for each printing color printer. At present, the average number of printing stations on a printing press is 10, and the web width is 6 to 136 inches.

Main types of motion in printing applications

Printing applications range from simple monochrome printing to the need to position complex multi-color printing. Most printing applications use a rotary inkjet head containing a pattern to be printed. The printing surface is usually a web directly in contact with the inkjet head. Webs are often linear and can consist of any type of material, including paper, plastic or resin films, corrugated paper, and the like. The printing ink is supplied directly to the inkjet head, and when the inkjet head comes into contact with the web, the pattern is transferred to the web.

The main types of motion in printing applications are spindle/slave axis movements—the above is where the web is printed and the inkjet head is the slave. There is a mechanical connection between the web and ink jet heads of conventional presses, considering that high-performance motion controllers can perform electronic cam profiling, as well as printing end-users to increase machine flexibility, and hope to shorten the production period and achieve rapid production. Conversion requirements usually use servos. This is especially true of inkjet heads, which also have a requirement for web spindles.

In a multi-color printing application, a plurality of ink-jet heads are included. Each ink-jet head corresponds to a color, which serves as a spindle-spindle, and the respective inkjet heads are kept in alignment with each other. It is not common in other types of motion printing applications, such as moving the inkjet head into and out of the adjustment shaft for machine maintenance and product switching.

Typical printing applications require the use of multiple motion control components, including feedback components such as encoders or resolvers, servo motors, retarders, servo amplifiers, and high-performance motion controllers. The motion control method used in printing can also be used for other rotary spindle and linear web contact paper product processing applications.

Major problems in motion control

There are several problems in achieving motion control in printing applications.

The first is that when the inkjet head is in contact with the web, it is very important that the inkjet head speed is accurately controlled. The speeds match each other, but the head speed will be higher or lower. Inaccurate speed matching can degrade print quality and may cause material damage.

The inkjet head circumference is the same as the product length, the kinematic relationship is a ratio between the two, and there is also a positional synchronization relationship (for example, they are proportional, but the slave shaft is locked by a specific spindle position, the printing occurs on the web Correct position). The circumference of the inkjet head is different from the length of the product. When the inkjet head comes into contact with the web, it will produce a speed match, the remaining length of the product, the inkjet head must be accelerated or decelerated, and the appropriate position is in contact with the web to process the next product.

In addition, when performing the motion profile, it is necessary to synchronize the speed and also to synchronize the position. In this way, the spindle/slave axis relationship can be effective from stop to all spindle speeds at full speed, minimizing rejects. The speed depends on the substrate, the label printing speed is usually 300 ~ 1000 FPM, the plastic on the central impression cylinder flexo printing (web around a large central impression cylinder, the inkjet head positioned along the circumference of the drum) speed is 1000 to 2000 FPM, while the center of the impression cylinder on the paper flexo printing speed can exceed 3000 FPM.

There are exceptions, of course, where the web and plate speeds do not match, and one of the speeds can exceed or less than 2% of the plate roll repeat length. The end customer can try to match the dimensions with actual needs as much as possible, and accept the amount of energy the machine can produce. This saves the plate cylinder/sleeve and saves material.

Most printing applications also require positioning, and the differences in positioning are significant. The optimal positioning should be within 0.0005 inches, with a central stamped cylinder flexo of 0.002 inches and a line print of 0.003 inches. Motion curves need to be dynamically adjusted to compensate for small changes in distance between registration marks on the web. This is especially the case with multi-color printing, where color registration must be maintained so that the final printed pattern is of good quality.

It is also important to design the motor/amplifier combination over the entire length of the product. At design time, the smallest and longest products may not represent the worst case. It is generally recommended that this design be done for several product lengths (eg 5 or 10) to ensure that a suitable motor/amplifier combination is suitable for the entire product range. The force and torque randomizers vary widely in size, and load inertia also varies with design. Motion control components are designed to meet 10:1 to 200:1 inertia mismatch.

How to Apply Motion Control Techniques to Improve Printing Prepress and Postpress and Quantify When Possible

The success of high-speed product positioning printing is crucial. To achieve high speed positioning, it is necessary to provide a high speed position lock input to the drive or motion controller and capture the exact spindle position when positioning marks are encountered. The difference between the front and rear positions is used to calculate the actual distance between the registration marks. That is, this distance is compared with the theoretical distance between marks and a correction amount is calculated. This correction is then substituted into the slave axis motion profile.

It is important to produce a high quality product from the time it takes to detect the registration mark to the amount of correction applied. This time depends entirely on the motion controller used for the application and can also be affected by the technology used (for example, some digital motion control networks have a lag that must be compensated for delivery, which may be unacceptable in some applications). Certain applications require the use of specialized positioning algorithms, such as averaging several or many product corrections, filtering the corrections, or requiring that some segments of the motion profile do not apply corrections. Motion controllers should also be able to properly handle web splices and missing positioning marks.

Typical Basic Configuration The 10-color press line has more than 65 closed-loop motion control axes. In larger production lines, there are usually more than 100 axes. In the past, each axis was mechanically driven. Now, the updated control system improves the positioning performance and reduces the positioning accuracy to half that of the old system. These control systems are highly flexible and can handle special tasks, providing infinitely variable repetition. Instead of the speed matching method discussed earlier, it allows the user to increase or decrease the speed of the platen roll web to change the repeat length.

These improvements also allow faster speeds, increasing the maximum speed from 1200 FPM to 2000 FPM, and the flexo printing process can achieve higher speeds. The time required for the press to switch from one task to the next reduced from an average of 4-6 hours to 30 minutes, and in some cases was shorter. The use of motion control and printing drive positioning system manipulators has a great influence on the switching time. In the application, the entire control system provides better diagnostic information and machine production time is extended.