Plate imaging technology

As far as the variety of manufacturing technologies of platemaking machines is concerned, the debate about which method is the best is more and more. The controversy over whether the equipment is constructed inside or outside the drum has continued for several years, and now it has begun to join the platform-type debate. The arguments of proponents of any kind of technology make one wonder why other manufacturers are so stupid to consider using other technologies.

In fact, most of the controversy focused on marketing, not really technical issues, and they overlooked the fact that each technology has its advantages and disadvantages. As with the manufacture of imaging machines, there are many ways to make platesetters, each of which can provide some benefits to manufacturers and users.

In the imager market, the outer drum design occupies a large size market, and smaller size (8-inch and smaller size) markets are now focused on two technologies: platform type and inner drum type. In the CTP field, the division of the size format is not so clear. The obvious difference between the CTP market and the film imaging market makes the outer drum design the main technology. The main advantage of this technology is the production of thermal plates. We will discuss the reasons for this in the plate technology section.

Types of platesetters

Because everyone is familiar with the construction of the platesetter: outer drums, inner drums, and flat benches, here are the main advantages and disadvantages of each.

Inner drum type

The inner drum design is to mount the plate on the inner surface of the cylinder. Compared with film imaging, the maximum area ratio of the imaging drum to the imaging material is also different due to the way in which plates are plated and unloaded. The imaging area of ​​some devices using film can reach 270° of the circumference, whereas the imaging of devices like Agfa and Scitex is limited to about 180°. The working angle of the imaging drum of most platesetters is approximately 180° of the circumference. (Agfa's new Galileo has a working angle of 193°.)

Inner drum CTP. Gerber uses the 4-up 3030R system to demonstrate the workings of the inner drum imager. Through a window on the front of the device, it can be seen that the plate is mounted on the inner circumference of the drum. As the dual rotor motor directs the laser light onto the surface of the plate, the imaging head moves down the center of the imaging drum.
Turn the mirror. A rotation deflection mirror moves one scan line width in the direction of the drum axis every revolution. The lens refracts the laser light at an angle of 90° to the plate fixed to the inside of the drum. The laser is focused on the turret lens and projected onto the plate at the required spot size. When imaging with film, the size of these dots changes with the resolution of the imaging media. The distance between the surface of the plate and the rotating mirror remains the same, and the size and the focus of the imaging spot are guaranteed to be perpendicular to the surface of the plate material without special complicated optical devices.

Punched. Inner drum equipment can punch holes on both sides or both sides of the plate in the imaging drum.

effectiveness. Since inner drum plate imaging only acts as a part of the circumference of its rotation, it is less efficient than a platform-type device. For example, the plate only occupies half of the circumference (180°) and its efficiency is only 50%, because 50% of the time the laser is not projected outside the plate. Imaging devices use larger circumferential surfaces, such as up to 270°, for less time and efficiency.

The efficiency of the inner drum equipment depends on the rotation speed of the rotating mirror and the speed of data transmission by the raster image processor. The fastest inner drum device now has a rotating mirror speed of 24,000 rpm, which is only half the speed of the fastest imaging device. This gap will be maintained. Plates are still the limiting factor for speed increase because thermal and photopolymer plates are not sensitive enough and each pixel requires too much exposure time to increase imaging speed.

Drum type

In the outer drum structure, the plate is fixed on the outer surface of the drum. The laser exposure beam is perpendicular to the drum axis and forms an image on the outer surface of the cylinder. The beam distance is short and remains relatively constant as the imaging drum rotates. As the drum shakes as it rotates, the weight of the plate changes the drum balance, so the beam distance changes slightly. However, the system is equipped with a shock absorber to ensure a stable distance between the optical head and the plate. The imaging head moves along a path parallel to the drum axis.

Outside drum plate maker. This type of imaging engine developed by Optronics was showcased at Nexpo last year through Monotype, which sells as the PlateExpress XL in the newspaper market. It can make a double wide newspaper page or several different size plates. The imaging head moves horizontally along the layout when the drum rotates at high speed.

Multiple beams. In order to achieve reasonable speed or high speed, multiple imaging beams are used to project multiple data paths onto the plate at the same time. However, the imaging path becomes complicated. After one revolution of the imaging drum, the entire beam must be positioned in advance of the first set of gratings projected onto the plate. In other words, the entire optical head is projected on a path first and then positioned at a path outside the first path for projection.

There are two ways to handle this situation. One approach is to first image a set of digital tracks, then all the imaging heads are advanced by regrouping the distances of the track widths, and then imaging the other set of tracks. This method is very precise, but it has to stop and start so it affects speed. Another method is often referred to as a spiral mode because the imaging laser moves continuously like a spiral at the lateral width of the plate. Due to the slight warping of the plate, the method of continuously moving the image on the surface of the plate has to overcome the influence of the phase difference. To supplement this phase difference, the entire image can be fine-tuned by an electronic program. (Advantages and disadvantages of progressive imaging and helical imaging are reviewed in the review of the Dainippon Screen DT-R 3075 and 3100 imaging devices—see Vol. 25, No. 19.)

In some platesetters, the imaging software handles the automatic phase compensation to ensure that the starting position of each grating in the spiral mode is at an exact distance from the edge of the media. If there is no compensation procedure, the plate image will be slightly skewed with the edges of the media.

Disadvantages. Outer drum technology has both advantages and disadvantages.

The disadvantages are mainly these:

Beam intensity. Multiple beams require complex optical structures to ensure the stability of each beam intensity. (Since the thermal plate can't overexpose, as long as the energy of each beam exceeds the energy required for imaging the plate, this aspect is less important for the thermal plate.) A slight misalignment of the intensity may produce bands and other artifacts. With single-beam light and inner drum systems, this problem does not occur.

Punched. Unlike internal drums in which the plate can be punched in the drum chamber, perforating the plate on an external drum device can destroy its balance. If you want to punch, one is to play in the loading process, one is to unload the tympanum from the plate and play after the plate.

Speed ​​problem. The speed of the laser drum can create some serious problems. In a device with a limited laser beam, high-speed rotation requires a complicated plate fixing and stabilizing technique because the plate material is very high-speed in rotation. This situation requires the use of multiple fixtures when fixing large format media.

advantage. Thermal imaging. The real advantage of outer drum equipment is the use of thermal plates.

Multifunctional CTP equipment. Using the same imaging engine in addition to plate making, it also has film imaging and proofing capabilities. When using a thermal plate, this function is limited in external drum equipment because the exposure time required for the proof material is long.

Platform type

The plate type plate making machine is simpler than the drum plate making machine. The platform type is relatively simple in terms of automatic and manual loading and unloading of plates and installation of punching systems.

Platform imaging. The plate is loaded from the right, laser imaged in the middle, and the plate processor from the left.

Single beam system. Most of the platform systems use a single mirror system deflection laser to sweep the surface of the plate in a straight line. This deflection system can be a polyhedron, a single mirror galvanometer or a holographic deflector. The plate fixed on the platform moves in the direction perpendicular to the scanning direction, one scan line distance at a time. The platform moves continuously and the scan line sweeps the plate in a one-pixel increment without breaking. The laser beam is focused through a special lens to ensure that the spot area and shape of the entire scan line are consistent. This is necessary because the distance between the lens and the surface of the plate is changed in the horizontal width of the scan. This also limits the length of the scan line. The longest scan line for a flatbed system is a horizontal 24”. It is also possible to image beyond this wide range, using two laser heads and stitching the two data components together.

The platform-based system is fast imaging, with the fastest automatic loading and unloading technology. However, due to scan width limitations, they are mainly suitable for commercial printing in newspapers or small and medium formats.

Multiple imaging system. While maintaining the simplicity of the media, the platform technology can also be applied to other methods to increase the plate area: Barco Graphics connects three or five imaging units to form a single scan line, allowing multiple scans of single scan lines of optical and digital Stitching is possible.

The ICG uses a multi-beam recording head mounted on an XY plotter that moves the optical head laterally across the surface of the plate, writing 512 beams per channel.

The Basys system operates in a similar manner as the ICG, but its optical head writes a series of connection matrices in the media imaging position in a continuous iteration mode.

to sum up

As everyone knows, the debate about the technology of the platesetter is like a religious war. But overall, there are several aspects that are very clear. First, external drum technology has advantages in thermography. Second, because of the ease of plate handling, high laser efficiency, and other factors, the platform type has a clear speed advantage.

In terms of quality, these technologies have no obvious advantage. The inner drum system has always claimed its own quality advantage, in part because early CTP users applied monochromatic and low-resolution work, and external drum equipment can easily meet these requirements. But as the market matures, the controversy continues and the platesetter is required to complete the work originally done by the film. This area is the long-term exclusive possession of the internal drum film imaging system. The reason is that they have excellent fixation of the imaging medium during the imaging process. The precise control of the laser has no traces of “splicing” of the data, and it is not necessary to compensate the phase difference. From this point of view, newspaper printing focuses on speed and low resolution, so the use of external drum-type and flat-top technology, high-end commercial printing is more in-house drum technology.

Laser type

There are many different media technologies in the CTP area. In addition to ink jet technology, all CTP plates are sensitive to a particular wavelength of laser light. There are six types of lasers used for plate making. Here are:
Argon-ion laser. A blue visible laser with a frequency of 488 nm.
Helium-neon laser. A green visible laser with a frequency of 633 nm.

(Low power) laser diodes. A red visible laser with a frequency of 670 nm.

(High power) laser diodes. An infrared laser with a frequency of 830 nm for thermal processing. The frequency of the infrared laser diode is between 780nm and 900nm.

Nd (yttrium) YAG (yttrium aluminum garnet) laser. A high-power infrared laser with a frequency of 1064 nm for thermal processing.