Laser Types

Different types of computer-to-plate systems require different types of laser light for proper exposure of the plates. Described below are some of the most popular lasers used in offset printing computer-to-plate systems.

• Diode Pumped Solid State Ultraviolet Laser
  
  355 nm (Ultraviolet)
  
  A diode pumped solid state laser is a high-powered laser that can be used with inexpensive conventional plates. The laser itself is very expensive, but it is popular with lower volume printers because it lasts many years with limited use. The lower cost of conventional plates compared to visible light or thermal plates can pay for the added expense of the laser.
  
  
  
  
• Argon Ion Gas Laser
  
  364 nm (Ultraviolet)
  
  An argon ion gas laser is a high-powered laser that can be used with inexpensive conventional plates. This laser has been valued by smaller print providers because of the cost savings in plates, but it is becoming less popular because of the success of the diode pumped solid state UV laser.
  
  
• Diode Lasers
  
  395 - 420 nm (Violet - Visible Spectrum)
  
  Violet diode lasers are one of the fastest growing visible light laser technologies. They are less expensive than many other types of lasers. They are smaller and the shorter wavelength allows for a faster imaging time. Diode lasers typically have a long lifetime.
  
  
• Diode Pumped Solid State Green Laser
  
  532 nm (Green - Visible Spectrum)
  
  Diode pumped solid state green lasers are used for imaging photopolymer plates. This type of laser was often used in early platesetter systems.
  
  
• Thermal Diode Bars
  
  830 nm (Infrared)
  
  Thermal diode bars are most often used for imaging plates intended for high-volume press runs. They are also used for imaging processless plates. Thermal diode bars are inexpensive and are among the most popular lasers used for offset printing computer-to-plate technology.
  
  
• Thermal Single Stripe Diodes
  
  830 nm (Infrared)
  
  Like diode bars, thermal single stripe diodes are used for imaging plates intended for high-volume runs and also for imaging processless plates. They are inexpensive and are widely used for offset printing CTP.

GLV Technology

When GLV (Grating Light Valve) technology is used in CTP systems, it allows for the best image quality and speed in production. The technology is often used in external drum platesetters for CTP systems as well as in high definition television.

GLV technology uses Micro Electro Mechanical Systems (MEMS), which allow for microscopic structures to form on silicon chips. The structures are reflective and are constructed of several ribbon-like elements that can be moved up and down through the control of electrostatic forces. The motion of the ribbons covers only a small distance, which is equal to a fraction of the wavelength of light. The different ribbon elements are configured in a pattern that allows the structures to reflect and diffract light waves, which varies the level of light reflected from the surface of the silicon chip. In an analog system, the light is continuous, but the level of light is variable. In a digital system the light is pulsed (switched on and off). GLV is capable of handling extremely high laser power because its high reflective properties provide an efficient use of light.

Imaging optics for GLV based CTP systems provide a larger quantity of writing beams, which means that the imaging drum rotation speed can be reduced while still providing a greater degree of productivity. GLV technology is most often used in conjunction with several thermal (infrared) CTP systems.

Useful Life of CTP Lasers

The useful life of the laser in CTP imaging systems is often determined by the length of time that the laser remains activated. The light emitted by various lasers is either continuous or pulsed energy, so continuous wave laser beams have a disadvantage in that regard. The life of the laser is very important to large print suppliers that may require the imaging of hundreds or thousands of plates per day. Smaller printers may have success with continuous wave laser beams because small quantities of plates are imaged per day. Limited use of continuous wave laser systems will prolong the life of the laser.

If a laser for a CTP system, such as the diode pumped solid-state ultraviolet laser, fails or is damaged, it can be very expensive to replace. It is certainly possible that a CTP system can experience a total failure of the laser unit, but more often the laser unit fails gradually over a long period of time. Some types of lasers, especially UV units, can be reconditioned for continued use.

CTP Plate Classifications

Plates for computer-to-plate technology can be classified in several ways. They are often described by the base substrate of the plate, emulsion type, exposure effect, exposure spectrum, exposure power, and/or the property of debris creation on the plate during the imaging process.

• Base Substrate: the material used to construct the plate, such as metal, polyester, or paper.
  
  
• Emulsion Type: the type of material coated onto the base substrate.
  
  
• Exposure Effect: the plate is described as either a positive working plate or negative working plate.
  
  
• Exposure Spectrum: describes the portion of the electromagnetic spectrum used for imaging CTP plates.
  
  
• Exposure Power: described as either slow emulsion or fast emulsion.
  
  
• Emulsion Debris: a CTP plate can be described as an ablation plate, controlled ablation plate, or a non-ablation plate, based on the material that may or may not be removed from the plate during the imaging process.

CTP Plate Base Substrates

There are three types of substrates used as a base for most digital plates: metal, polyester, and paper.

• Metal
  
  Plates made from metal are used for the largest print jobs because they have a much longer life than plates made from other substrates. Aluminum, which is traditionally used for metal analog plates, is also used for digital plates. The plates are grained, allowing them to hold a thin film of moisture, which keeps the non-image areas free of ink during the print run. The areas of the plate that are imaged become receptive to ink.
  
  Metal plates are coated with a variety of photosensitive emulsions, including silver halide and photopolymers, depending on the type of plate and process used. Metal plates are typically used for print runs ranging from 100,000 to 2,000,000 or more.
  
  
• Polyester
  
  Polyester or plastic plates cost less than metal plates and are a good alternative when press runs of 25,000 or less are required. Earlier versions of polyester plates produced substandard printing, which gave them a poor reputation; however, improvements in the technology continue to be made and have resulted in print quality that is substantially better. In fact, the plates are now often used for small print runs of high quality four-color process applications. Some polyester plates are rated for line screens as high as 175 lpi, which produces a good quality print job. Tonal ranges of 5% to 95% are common with most polyester plates.

  Improved technology has helped to alleviate the problem of plate stretching, which in the past was a common problem with polyester plates, especially when used for larger quantities. Plate stretching is a phenomenon that results in distortion and poor registration of the print. Improved platesetting equipment has also contributed to the success of polyester plates.

  Polyester plates are available in a variety of sensitivities, which means that they can be imaged by several different types of lasers. The plates that are most often used are suited to visible red and infrared lasers. Common emulsion coatings for polyester plates include silver halide, which is imaged with lasers, and toner-based coatings in which the image is fused onto the plate by heat.

  Polyester plates are not grained during manufacturing as are metal plates, but are instead chemically treated so that the plate surface will hold moisture during the print process. The plates come in a variety of thicknesses, usually ranging from 0.004 to 0.012 inches. The thicker plates are used for longer runs because their bulk makes them more durable.
  

• Paper
  
  Paper plates are made with a cellulose material and come in thicknesses ranging from 0.004 to 0.008 inches. Like polyester plates, paper plates are not grained, but are chemically treated during manufacturing so that the surface is receptive to a thin layer of moisture. The plates are used for short runs on smaller printing devices and they use a toner-based technology for imaging the plates.

CTP Plate Emulsion Types

The plate emulsion is the material that is coated onto the base substrate. The emulsion reacts with the wavelength of light emitted by the CTP imaging laser to produce an image on the plate (either a positive or negative image). There are many different types of emulsions used on plates depending upon the type of laser used for imaging and the type of application that will be produced. Some plates are baked after imaging in order to increase their effectiveness for very long press runs.

• Silver Halide/Silver Diffusion
  
  The emulsion on a silver halide/diffusion plate is made of photosensitive compounds that are similar to the compounds in photographic film. The earliest type of silver halide plate was made with a polyester base and was not sensitive to visible light wavelengths—a colorblind plate. They were capable of only single or spot color printing.
  
  Early metal plates were also colorblind. Subsequent types of silver halide/diffusion metal plates have been created that are sensitive to visible light wavelengths—a panchromatic plate. They are imaged with various lasers including 488 nm blue lasers, 532 nm FD-YAG lasers, and red lasers at 670 nm They have a high resolution with screen rulings of 300 lpi or more. The plates are capable of long press runs.
  
  Because the coating on the plates contains silver, which is a toxic heavy metal, any portion of the coating removed during processing must be handled as hazardous waste. The plates also tend to be more expensive than other types of plates because of the silver content.
  
  
• Photopolymer
  
  Photopolymer coatings contain no silver. The emulsion contains light sensitive plastic and is exposed by Argon Ion lasers at 488 nm or double frequency YAG lasers at 532 nm. The plates are rated for up to 200 lpi and are capable of press runs of up to 250,000, however print runs of 1,000,000 or more can be achieved if the plates are put through a baking process after they are developed.
  
  An advantage of photopolymer plates is that they are manufactured without hazardous ingredients, so they can be processed with aqueous solutions and are much safer to work with. There is no silver residue and the processing chemicals have fewer restrictions for proper disposal.
  
  
• Hybrid
  
  A hybrid plate has a metal base with two different photosensitive coatings. The top coating is made with silver halide, which can be exposed optically or digitally. The bottom coating is a conventional photopolymer, which is sensitive to ultraviolet light. The top emulsion layer is exposed and the plate is processed to remove the unexposed portion of the top coating. The exposed portions of the top coat act as a mask for the bottom coat. The plate is exposed to ultraviolet light, which reacts with the unmasked portions of the bottom coat. The plate is then processed, which removes the mask and the exposed portions of the bottom emulsion. The bottom coat is the layer that is actually used for the printing process. Some of the resolution can be lost because of the dual exposure technique, but the plates are still rated for a 200-lpi resolution and can handle press runs of over 1,000,000 if post-baked.
  
  A hybrid plate has two separate types of waste products to dispose of: silver and photopolymer. The silver is a toxic heavy metal that is under strict government regulations for proper disposal. The photopolymer residue is safer to work with and has fewer restrictions for proper disposal. Hyrbrid plates for CTP have become increasingly less popular and are now rarely used.
  
  
• Thermal
  
  Plates with thermal emulsions use the infrared portion of the electromagnetic spectrum for imaging the plates (with lasers). The infrared wavelengths, which are beyond the visible spectrum, produce heat. Thermal plates are coated with special polymers that react to the heat of a specific wavelength within the infrared band. Heat from the infrared laser beam must reach a required level in order for the reaction to occur. Heat below the required level will not affect the plate.
  
  One benefit of using thermal plates is that they can be handled in visible light because they are sensitive only to infrared light. Another benefit is the "either or" aspect of the exposure. The plate can be imaged only if the heat produced by the laser is great enough, otherwise nothing happens. The plates also provide a slightly sharper dot than the dot generated by a visible light system.
  
  Two types of thermal plates that are available are ablation plates and cross-linking polymer plates.
  
  
  • Ablation Plates
    
    Ablation plates are those in which thermal lasers actually ablate (remove) areas of the emulsion while the plate is being imaged. After exposure, a residue is left on the plate on the ablated areas, which must be washed away.
    
    
  • Cross-linking Polymer Plates
    
    Cross-linking polymer plates contain emulsions that consist of two resins that are cross-linked when the plates are exposed to the proper wavelengths of infrared light.
  
  
  The prices of thermal plates continue to fall, which makes CTP much more attractive to many print providers that have not already converted to CTP.
  

The chart below shows the typical print run lengths that can be expected when using different types of plates for CTP systems. The actual run lengths could vary according to the brand or manufacturer of the plate.

Exposure Effect

Depending on the type of plate and the type of CTP system used, CTP plates are either positive working or negative working, meaning that either the unexposed areas of the plate become the image areas or the exposed areas become the image areas.

• Positive Working Plates
  
  When a positive working plate is exposed, the CTP imaging system creates non-image areas on the plate. During the exposure process, the laser loosens the emulsion in these areas, which is then washed from the plate, revealing the metal base beneath. The metal base of the plate attracts moisture (printing press fountain solution) during the printing process and the unexposed areas of emulsion, which are the image areas, attract the ink.
  
  
• Negative Working Plates
  
  When a negative working plate is exposed, the process works in the opposite manner. The areas of the emulsion exposed by the lasers are the areas that attract the ink during the printing process and the unexposed areas become the non-image areas, which attract moisture (printing press fountain solution) and prevent the non-image area from attracting ink.

There are many types of positive working and negative working CTP plates available. Choosing a plate depends upon the type of imaging system used and the types of photosensitive coatings that are compatible with a particular CTP system.

Exposure Spectrum

The exposure spectrum describes the portion of the electromagnetic spectrum used for imaging CTP plates, such as infrared for thermal plates, visible light for light sensitive plates, and ultraviolet for UV-sensitive plates.

Three of the most popular technologies for CTP imaging are thermal lasers (infrared), violet lasers (visible light), and DPSS Ultraviolet lasers.

• Thermal imaging heads generate heat because of the infrared wavelengths that are necessary for imaging. The heat from the imaging lasers changes the chemistry of thermal plates, which allows the image to appear. Compared to other types of plate technologies, thermal plates require more energy to produce an image on the plate, but the thermal technology allows for plates that are chemical-free and some plates that are process-free.
  
  
• Violet imaging uses visible light to change the plate surface chemistry to create an image on the plate. Unlike thermal plates, heat is not produced and is not necessary for imaging.
  
  
• Some CTP Ultraviolet imagers allow for imaging on conventional plates. Although the laser is expensive, the system allows printers to continue using many of the conventional plates that have been successful for them in the past.

Various types of plate emulsions require different wavelengths of light for proper imaging. The table below describes some of the wavelengths that are necessary for proper imaging of specific emulsion types.

Exposure Power

CTP plates can be classified into two groups based on the exposure power required for proper imaging: slow emulsion and fast emulsion.

• Slow Emulsion
  
  Slow emulsion plates require large quantities of energy for proper exposure. An example of a slow emulsion plate is a thermal plate, which come in many varieties. The number of plates that can be imaged per hour on one machine is less than other CTP technologies because of the increased imaging time.
  
  
• Fast Emulsion
  
  Fast emulsion plates require much less energy for exposure than slow emulsion plates. Conventional offset plates imaged with UV lasers have a fast emulsion and require less energy than thermal plates. Many more fast emulsion plates can be imaged per hour than slow emulsion plates.

Emulsion Debris

When certain types of CTP plates are imaged, material is removed as a result of the imaging process and debris is created on the surface of the plate. There are other types of CTP plates, in which no material is removed from the plate and no debris is formed. There are three categories under which CTP plates can be classified according to the level of debris formation: ablation plates, controlled ablation plates, and non-ablation plates.

• Ablation Plates
  
  When ablation plates are imaged, a residue is formed on the plate from the areas of the plate emulsion that are ablated, or removed, during imaging. The residue must be washed from the surface of the plate before it can be used at the press for printing. Ablation plates are the dominant CTP plate type.
  
  
• Controlled Ablation Plates
  
  During the imaging process, the laser loosens areas of the plate emulsion, but rather than washing it off manually, the fountain solution at the press further loosens the unwanted material from the plate. The debris is carried away by the ink and is then deposited onto the paper as the first few impressions are printed at the press. This technique is faster than having to clean the plates manually before printing. This system helps to eliminate the accumulation of debris from the lens of the laser and from the press.
  
  
• Non-Ablation Plates
  
  When non-ablation plates are imaged, there is no residue left on the plate. Many of the plates are constructed of traditional grained aluminum and require less energy for imaging than ablative plates. There are also no environmental concerns because there is no ablated emulsion material that requires specific methods of disposal.

Processless CTP Plates

Processless CTP plates have several advantages over chemically processed plates. Processless CTP plate systems require a lower initial investment, the plates are daylight safe, and because the system requires fewer steps than chemically processed CTP plate systems, processless plates are less labor intensive. Less equipment is necessary because there is no need for large processing units. There are no chemicals needed for development of the plates so there are no environmental issues concerning proper disposal of chemicals, (which can be expensive). All of these advantages add up to a significant long term cost savings; a process that is environmentally friendly; and a system that is easier, safer, and less time consuming for people to use. However, it is worth noting that as CTP chemistry continues to evolve and improvements in chemically processed CTP plates are made, the chemicals are becoming safer to use and are becoming less of a burden on the environment.

Lower volume printers have much to gain from using processless CTP plates. Although the plates are more expensive than chemically processed CTP plates, there is a smaller initial investment. An expensive processing machine does not have to be maintained and a constant supply of chemicals does not have to be purchased. At the press, using processless plates usually requires only some adjustments in balancing the fountain solution. The print quality with processless plates is often equal to that of chemically processed CTP plates. Processless plates are ideal for smaller print providers that often produce jobs of fewer than 100,000 impressions.

Ink-Jet Plates

Plates for computer-to-plate applications can also be created with ink-jet technology. A proprietary fluid is sprayed onto a metal base to create the image from a digital record. Creating the image from digital files allows the imaging to occur without the use of lasers or any other type of exposure. The plates require no processing or post-baking and can be used immediately after the ink-jet imaging is complete. Another advantage is that the platesetters for ink-jet systems are less expensive than laser or thermal equipment. Although the quality of items printed with ink-jet systems is very good, it still cannot match the quality achieved by laser or thermal technology. This may be the only drawback of the ink-jet plate.

"No Plate Required" CTP Technology

Offset printing systems have been developed that not only eliminate film from the workflow, but also eliminate the plate as well. These systems are known as computer-to-press offset printing systems. Some of the systems use lasers and a thermal ribbon to create images on removable cylinders.

Each of the cylinders (one for each process color) is imaged in one step with a direct thermal transfer process. A thermal material, which becomes the image area, is transferred to the cylinder. A wetting agent keeps the non-image areas of the cylinder clean during the print run. A short run of 500 impressions can be produced immediately after imaging is complete. When a longer print run is required, the thermal material must be baked onto the cylinder, but this additional step requires only 2 minutes. A resolution of 3200 dpi can be achieved with this system. When the printing is complete, the thermal imaging material is automatically cleaned from the cylinder, which makes the cylinder ready for reimaging for the next job. The cylinders can be reimaged several hundred times before they require replacement. The entire process of imaging and cleaning requires less than 12 minutes.

Production Steps for Selected CTP Plates

• Thermal Pre-Baked Plates
  
  Imaging - Pre-Baking - Developing - Post-Baking - Finishing - Printing
  
  
• Visible Light Plates
  
  Imaging - Developing - Washing - Finishing - Printing
  
  
• Thermal No-Bake Chemically Processed Plates
  
  Imaging - Developing - Finishing - Printing
  
  
• Thermal Chemical-Free Plates
  
  Imaging - Washing - Printing
  
  
• Thermal Process-Free Plates
  
  Imaging - Printing

There are many different types of platesetters used for offset printing CTP systems and there are many manufacturers of the systems, but all of the platesetters are one of three basic designs: flatbed, internal drum, and external drum. Each of the three has advantages over the others and each one is accurate in its imaging capabilities.

Flatbed Platesetters

As the name implies, plates that are imaged on a flatbed platesetter lie on a flat surface. The flatbed system allows for easy loading of the plate. Most flatbed platesetters use a single imaging beam, which is deflected onto the plate surface with a rotating mirror.

• Moveable Flatbed Systems
  
  With some systems, the flatbed moves down slightly as each strip of the image is being written onto the plate by an imaging beam. The mirror remains centered over the plate, but pivots left and right as it writes the full width of the line or strip.

Moveable Flatbed Platesetter

• Stationary Flatbed Systems
  
  There are other flatbed systems in which the flatbed remains stationary. The imaging beam mirror, which remains centered over the plate, advances along the plate to write each line. Like the moving plate bed version, the mirror pivots left and right in order to write the full length of the line.

Since the imaging beam mirror on both types of systems remains centered and does not travel side to side (it pivots to project the laser beam left and right across the plate), the distance between the mirror and the plate changes as the mirror pivots. The distance the laser beam travels is shortest when the mirror is directly overhead and it is the longest when the laser is imaging the left and right edges of the plate. To ensure that the dot size and shape is not distorted across the width of the plate (due to the varying distance between the mirror and the plate), the laser is directed through a special lens that compensates for this.

The design of the flatbed unit allows for efficiency in plate handling and high production rates, but the imaging technique used with flatbed systems limits the width of the imaged area. Flatbed systems are most often used for imaging visible light plates.

Internal Drum Platesetters

The internal drum platesetter resembles the letter "C" because of its open concave design. The plate is mounted on the inside of the drum and is held in place with a vacuum that draws the plate tight against the curved surface. A single laser beam writes the digital image onto the plate. The laser beam is projected onto a spinning mirror that rotates at a very high speed. The spinning mirror deflects the laser beam onto the plate at a 90° angle. As the mirror travels down the axis of the drum, one scanned line per revolution of the mirror is written onto the plate. The concave drum remains stationary. In order to change the resolution of the image on the plate, the diameter of the laser beam is changed.

When imaging thermal plates on an internal drum platesetter, there may be limits on how fast the mirror can spin when deflecting the laser light onto the plate. This is because the slower emulsion of the thermal plates requires high energy and a long imaging duration, which means that the production rate will be slower compared to that of other types of plates.

External Drum Platesetters

External drum platesetters are designed to have the plate mounted on the outside of the drum. As the drum rotates on its axis, a laser beam head moves parallel to the drum and burns the image on the plate.

The external drum system does not require any mirrors to deflect the imaging lasers and it allows the imaging head to be placed closer to the plate surface. This is beneficial for imaging thermal plates, which require high-energy output for proper results.

Some external drum systems require counter balancing measures. When the drum spins at high speed, the weight of the plate mounted onto the drum can shift the drum out of balance because of centrifugal force. To achieve high quality results, the drum must spin accurately in order for the imaging head to maintain optimum beam intensity. A wobbly drum can change the intensity of the light striking different areas of the plate.

Some manufacturers avoid the imbalance problem by producing external platesetter systems in which the drums rotate slowly. The systems are equipped with multiple imaging heads. This type of system, however, encounters a different problem known as stitching, which can occur when multiple imaging heads are used. Stitching occurs in areas where the first imaged line of one laser abuts the last imaged line from another laser. A distinct line between the two areas may be noticeable on the printed page because of a slight overlap between the areas imaged by the different laser heads. Stitching can also occur because the dots may not be as sharp in adjacent areas because of one the slightly different deterioration rates of the various lasers used in the system. Periodic adjustments may be necessary for multiple imaging heads to maintain proper quality.

In order to change the resolution when using external drum systems, the distance between the multiple beams must be changed. Unlike internal drum systems, the diameter of the beams cannot be changed.

• CTP eliminates some of the steps that are required for conventional platemaking. The elimination of the steps required for producing and stripping film for plate production saves a considerable amount of time.
  
  
• Human error is reduced with the elimination of manual film stripping and platemaking.
  
  
• Storing digital images is easier and more efficient than storing film and film flats, which can require large amounts of physical space.
  
  
• Film can get scratched and damaged, even in storage, but digital images will always look consistent, clear, and sharp.
  
  
• Print quality is improved because the dot gain associated with imaging plates from film is eliminated. In fact, with a film workflow, dot gain may be as much as 5 or 6 percent.
  
  
• The reduction in potential dot gain allows denser inks to be used at the press, which provides improved color saturation and print quality.
  
  
• Because film substrates are affected by temperature and humidity changes, the film may stretch or shrink resulting in registration problems at the press and substandard print quality. CTP eliminates these problems because no film is involved.
  
  
• Copy changes are much easier with CTP because changes are made to a digital file, which is imaged directly onto a plate.
  
  
• Film and conventional platemaking can create large quantities of hazardous waste, such as silver and developing chemicals. The hazardous waste materials must be disposed of properly, which can be costly. CTP technology eliminates much of the hazardous waste problems.