Composition. Typically, doctor blades are made of high quality stainless steel. More recently, alternate materials, such as plastic and ceramic are being used. The carbon steel used for doctor blades must be clean and free of any imperfections. Stainless steel blades are often used with corrosive liquids. Plastic and ceramic blades are also non-corrosive and non-abrasive.
Shape. Steel blades are approximately .004 - .015 inches thick, and from 1-3 inches wide. The thickness tolerance is +/- .0002 inches. Blade hardness is between 500 and 600 Vickers. Plastic blades are up to .060 inches thick. Steel blades have a much longer life than plastic blades.
Doctor blades are called blades because the edge that is in contact with the cylinder has been shaped (or sharpened). Sharpening can be done in-house or by the blade manufacturer. Blade edges are important because the blade must have a perfectly straight and smooth edge to rest against the printing cylinder. Any blade imperfections will cause streaks and scratches on the cylinder face.
Edges can be of several types, however the most common is a pre-sharpened beveled edge. The blade edge has a flat shoulder and a beveled (or sloped) edge. This blade is usually supplied by the manufacturer. The beveled edge blade is the most common due to its long life and minimal cylinder wear.
Application. The doctor blade must be held in perfect contact with the cylinder to achieve a clean, even wipe. The doctor blade holder is designed for this purpose. Doctor blade holders will vary with press type.
Since doctor blades are thin strips of metal, they are often used with
a back-up blade. This heavier metal strip (.015 - .020 inches thick) gives
the thinner doctor blade support when under pressure.
The back up blade and doctor blade are clamped together in the doctor blade holder. Holders can be either straight, curved or a clamping type.
Once the blade is mounted into the holder and positioned in the press, the angle at which the blade contacts the cylinder is critical. The best wiping angle is one that minimizes cylinder wear, wipes cleanly, and allows the greatest press speed.
Blade Angle. Two factors affect the blade angle. First is the angle on the blade itself. On beveled edge blades, wear may cause this angle to change. Pre-sharpened blades are used to minimize this problem by controlling the beveled edge thickness.
The second factor, the set angle, is the actual angle that results from the application of the blade to the cylinder by the blade holder. Set angles are typically between 25-35o. The set angle may change due to blade or cylinder wear and ink type. The angle is not a permanent setting. It can be altered to achieve the best wipe under the current conditions.
For example, a steep angle is used to remove more ink from the cells. This is commonly done in four color process printing when large amounts of ink are not needed. A flat angle is used when printing large solids or coatings. This leaves more ink in the cells, resulting in greater transfer.
The distance between the doctor blade and the nip is also important. This distance is called the printing zone. The distance is important to consider because the ink may dry in the cells after the doctor blade wipe but before it reaches the printing nip. Moving the doctor blade closer or farther from the nip can control this problem.
Application Pressure. The pressure applied to the blade while in contact with the cylinder is controlled and monitored. It is achieved by gravity, springs, pneumatic or hydraulic pressure. Pneumatic is the most common and provides longer cylinder life. The pressure should be as little as possible to minimize cylinder wear. Press crews should establish and maintain the pressure that provides the most consistency for each press.
While under pressure against the cylinder, the doctor blade encounters deflection. Deflection is a slight bowing of the blade caused by the cylinder motion, pressure, and blade thickness. This deflection must be accounted for when setting the blade angle.
While the blade is in contact with the cylinder, the entire blade assembly moves from side-to-side (oscillates). This movement provides even wear and helps remove any loose debris that may have collected under the blade which may cause streaks. The maximum movement should be about 1 inch and can be adjusted.
Care. Doctor blades are extremely sharp and require care in handling. Any nicks or chips in the blade edge will result in streaking or hazing. Additionally, the extremely sharp blade edge needs to be handled carefully to avoid cuts to hands or other parts of the body.
Common print problems associated with doctor blades include print distortion, slurring, screening, snowflaking, scumming (tinting or hazing), streaking and poor color control. Mechanical problems include uneven blade wear and excessive cylinder wear.
In order for the ink to be transferred from the printing cylinder to the substrate, there must be a great amount of pressure applied between the substrate and cylinder. This pressure is supplied by the impression roll. Impression roll pressure helps transfer the ink and acts as a backing to the substrate being printed.
Impression Mechanics. While the press is at rest, the impression roll is not in contact with the substrate. When the press is started, the impression roll must be lowered into contact with the substrate and cylinder. This is called a conventional, or moving rubber roll impression system. The rubber impression roll moves into contact with the cylinder, creating pressure.
The advantage of a conventional system is the simple mechanics. The disadvantage is that every time the rubber roll is moved, the web tension and register changes. This means the web must be slowed, and the color to color register will be affected. This creates waste and lowers productivity every time the roll is moved.
The impression roll is moved using either a hydraulic or pneumatic pressure system. Pneumatic systems are the most common.
Impression Pressure. There are two ways to set the impression pressure. The first is the stop or gap method. This method uses two steel blocks to limit the contact of the impression roll against the cylinder. The blocks "stop" the roll from further movement. This method does not allow an actual impression pressure measurement. The blocks stop the roller at a certain distance. No matter how much pressure is applied, it will move no further. This type of impression pressure setting is desired for lightweight papers, films and foils because it maintains a constant nip width over the entire length of the impression roll.
The second method is the pressure equalization system. In this case the impression roller is able to float, or move, to achieve the correct impression pressure. This floating impression system can automatically compensate for uneven web thickness during printing. Even though the roller may move, this system maintains constant pressure across the cylinder width.
Creation of Nip. When the impression system
is loaded, a "nip" is created between the two cylinders. The nip area (flat)
is the area of actual contact between the two circular parts (roller and
cylinder). The nip area extends the length of the roller and has a variable
Controlled nip width is desirable for paper, film and foil. It allows for uniform pressure across the entire width of the web.
Desirable nip widths are shown below:
TABLE: NIP WIDTHS FOR DIFFERENT SUBSTRATES
|SUBSTRATE||NIP WIDTH [inch]|
|Films and Foils||1/4|
|Heavyweight Papers and Lightweight Board||1/2|
|Boards (20 pts. +)||3/4|
The nip width is a function of physical roller characteristics and impression pressure. Physical characteristics include the type of covering, temperature, hardness (durometer), thickness and diameter.
Nip width must be measured and checked each time a new cylinder is loaded. Nip width can be checked by a variety of methods. One is the grease and paper method. Grease, mixed with carbon black, is spread on the rubber impression roller. Paper is inserted between the impression roll and the cylinder, and the impression roll is lowered. When the two contact, the grease leaves a mark on the paper that can be measured.
Other methods include impression tape, embossed (textured) foil, and film. These methods are more convenient than paper and grease and are just as effective.
Impression Roll Construction. Roller base
construction is similar to cylinder base designs. Most are made of durable
steel cores with a soft covering. The steel core, or base, must be statically
and dynamically balanced like a cylinder base.
Roll balancing is necessary to minimize wear, control web runnability, reduce vibration and heat buildup, and to assure proper contact with the cylinder while the press is running. Any distortion of the roll will affect printability and may cause damage to the roll.
Impression Roll Material. The soft covering that surrounds the steel core can be made of several rubber types. The type of rubber will depend on the solvents that are used. Most rolls are constructed of natural rubber, neoprene, and Buna N (synthetic rubber). Natural rubber rolls are recommended for alcohol and water type inks. Buna N rolls are suggested for inks that contain alcohol, toluene and normal butyl acetate.
The hardness (or durometer) of the roll is also very important. Hardness is measured on a "Shore A" scale. Soft rubber rolls are about 50 durometer and hard rolls about 90 durometer. The desired durometer of a roll is dependent on the substrate to be printed. The following chart shows preferred roll durometer for a variety of substrates.
Table: Roll Durometer for Different Substrates
|Substrate||Shore A Durometer|
Proper impression roll durometer is important. Too soft a roll can lead to deformation (distortion), web handling and rubber deterioration problems. Not using the correct durometer rolls reduces quality and productivity. The durometer of the roll should be checked periodically because it will change with age and use.
Deflection. All rollers will deflect. Deflection means that the roll will bend and curve. This is a result of its own weight and the pressure that is applied on it. Since the ends of the roll are secured in the bearings, the middle of the roll will bend. Deflection is more apparent in wider presses (over 60").
A technique used to reduce deflection requires that a bevel be cut into the rubber at the end of the impression roll. This end cut is used to minimize swelling at the ends of the roll, which then reduces deflection.
An appropriate nip area, or flat, should have a rectangular shape. When the roll is deflected, this shape takes on an hourglass shape (wide at ends and thinner in the middle). Since this deflection will affect the print quality, back up rollers are used to minimize this problem. Back up rollers provide support over the entire roll so deflection will not occur. Problems associated with deflection include excessive heat build up, roller breakdown, tension problems and register trouble.
When printing, the main purpose of the impression roll is to force the substrate into contact with the printing cylinder. This contact should be enough to allow the ink to transfer from the recessed cells.
Sometimes the ink does not transfer to the substrate, causing a condition known as skipped dot or snowflaking. There are several reasons the ink does not transfer. The most important is the roughness of the substrate surface. In other words, the pits or cavities in the substrate surface are larger than the individual cylinder cells and they simply do not come into contact with each other. Additional reasons include the shape of the cell, ink drying speed and viscosity, and improper impression pressure.
Operation. Electro Static Assist (ESA)
was developed by the Gravure Association of America to help eliminate skipped
dots. ESA is a process that uses electric charges to force the ink into
contact with the substrate. This is accomplished by developing an electric
field, either positive or negative, between the impression roll and the
print cylinder. By charging the impression roll and grounding the print
cylinder you can produce the electrostatic field.
ESA systems are manufactured by Hurletron in the United States. The Hurletron system applies a charge to the conductive rubber impression roll. This conductive rubber roll is called an applicator.
Once the electrostatic field is applied, electrostatic forces cause
the ink in the recessed cell to be drawn up out of the cell into contact
with the substrate. ESA efficiency depends on the condition and age of
the rubber impression roll, and the cleanliness of the applicator contacts.
Heat will also affect the way the ESA works. Too much heat on the roller
will reduce electrical efficiency.