Structural Units (continued)
Doctor Unit
In gravure printing, the doctor scrapes away excess ink from the printing cylinder while leaving the necessary volume of ink in the cells formed via etching or electronic engraving, which compose the continuous gradation of shadow, half-tone, and highlight sections that make up the image on the printing cylinder surface. This is an important aspect of gravure printing in accurately reproducing the tones of the original image when printing pressure transfers the ink to a plastic film or paper substrate.
The doctor is a thin, steel plate blade, the tip of which is pressed at an even pressure against the entire width of the surface of the printing cylinder. The purpose of the doctor is
to cleanly scrape away the ink on the cylinder surface as if it were being shaved by a razor blade. The doctor structure is composed of three pieces: a thin, steel plate blade; a back blade; and a doctor holder.
Doctor Blade
The doctor blade—made of a sintered hoop, typically using Swedish steel—is very similar in nature to the blade of a razor.
The hardness of the razor is considered best at about a shore hardness Hs of 60 to65° (roughly half the Brinell hardness Hb of 1,000° of the hard chrome plated surface of the printing cylinder). If the blade is too hard, the printing cylinder surface will rapidly wear, and if the blade is too soft, the doctor blade tip will wear severely, increasing the frequency
with which the doctor needs to be changed. As such, it is necessary to select an appropriate hardness. The thickness of the blade is typically around 0.15 mm.
The tip of the blade is generally given a 30° angle, but if the blade tip angle becomes obtuse or rounded, it will not cleanly remove the ink, resulting in fogging and filaments.
These will result in printing defects, and the doctor blade will need to be changed. The standard life of a doctor blade depends on the format of the printing cylinder surface, but
is roughly 20,000 printed meters.
In addition, depending on the doctor shape and position, the doctor does not affect printing register misalignment. If the doctor is adjusted during printing, however, there will be a change in the doctor pressure, which will change the ink film thickness on the printing cylinder surface. This difference will lead to register instability. In particular, with solid
printing, because the ink volume is high, adjusting the doctor will cause slip between the printing cylinder and plastic film, which will disturb registration.
The doctor is designed to scrape away the ink from the surface, but it does not necessarily remove all of the ink. Moreover, the reason it is possible to print without the doctor damaging the printing cylinder is because this ink film itself acts as a lubricant, which reduces frictional resistance between the doctor and the printing cylinder surface.
As can be seen from the above reasons, the doctor conditions include
(1) the doctor blade tip shape is uniform andunchanging,
(2) the doctor blade tip pressure is also uniform and unchanging, and
(3) the position and angle of the blade tip against the printing cylinder surface is constant and unchanging.
Back Blade
The role of the back blade is to adjust the image being printed by changing the rigidity of the doctor blade tip. When removing the ink, if the blade tip is long, it will be flexible and dull, but if it is short, it will be hard and sharp. This blade is attached using a steel plate that is 0.5 mm thicker than the doctor blade; this steel plate is shifted slightly above the doctor blade and locked together with the doctor blade and back blade in the doctor holder.
Doctor Holder
The holder is made of a set of top and bottom units, which must be very rigid. In general, 12 mm thick polished steel plates are used, but aluminum plates are also used because
these are easy to work with when replacing the blade. In the case of such aluminum holders, we must use a holder cradle that is very rigid.
In addition, the doctor blade is locked tightly in the holder along with the back blade and fixed in place so that the blade tip does not waver. The doctor blade tip must be attached so that it is perfectly horizontal and parallel to the printing cylinder.
The length of the doctor blade, which is longer than the face length of the printing cylinder, is given a reciprocating stroke in the length direction. This motion ensures that ink scraping is uniform and that wear on the printing cylinder and doctor blade is even, which is necessary to prevent local wear that causes doctor streaks and fogging. The reciprocating motion of the doctor is typically generated from the gearbox of the printing cylinder drive by mechanically converting the rotational motion of the drive into a reciprocating motion in the length direction. This reciprocating motion has a major influence on the printing quality, so movement related to the bearings must be smooth. Moreover, because the reciprocating motion length is constant, one measure for reducing the occurrence of doctor irregularity is to use a mechanism that can change the speed and stroke of the reciprocating motion irrespective of the printing cylinder circumferential speed during printing. In this
method, the printing cylinder drive system and doctor reciprocation drive system are independent, so the reciprocating motion is driven by an independent variable speed motor.
Other methods are either pneumatic or hydraulic pressure systems in which an electrical signal is used to open and close a valve.
The doctor stroke rate is typically one stroke per 10 to 20 rotations of the printing cylinder, and the stroke length is ±10 mm of the printing cylinder, but there is no accepted opinion here. The doctor contact angle is generally considered to be 45 to 65°, but currently there is a demand for a wide adjustment range that allows the doctor to be used from a flat position to an angle of nearly 90°.
Doctor pressure was conventionally applied using a weight, but recently the pressure in most systems can be adjusted using pneumatic pressure. The pressure force differs depending on the printing speed and ink viscosity, but is typically 100 to 200 N. Doctor pressure should be used at as low a level as possible, which can be achieved by increasing the precision of the printing cylinder and precision of the doctor attachment.
The doctor unit must have the following adjustment mechanisms to handle changes in the printing cylinder diameter, ink viscosity, solvent qualities, cell depth, printing cylinder finishing quality, and printing speed, which vary by printing job.
1. Height adjustment to match printing cylinder diameter
2. Forward and backward position adjustment to match printing cylinder diameter
3. Doctor angle adjustment (doctor contact angle against the printing cylinder)
4. Doctor pressure adjustment (pressure does not change even when given a reciprocating motion)
By attaching a scale that allows us to know the movement position of the above four items, we can apply numerical control and achieve printing reproducibility.
Moreover, systems have been developed that allow for more stable printing quality by allowing the above four items to be set at constant conditions against any printing cylinder diameter (doctor angle against the printing cylinder, distance of the impression cylinder point from the doctor, etc.).
MDC Doctor Blade
(constant contact area doctor blade) Doctor blades polished to an acute blade tip angle wear ever so slightly during printing, so the contact area of the initial blade tip changes and widens over time. Because these changes affect the printing results, the doctor pressure and doctor contact angle must be adjusted or the doctor changed. The MDC doctor was developed to resolve these problems, and recently use of this MDC doctor is becoming
more common.
This doctor maintains a constant contact area against the printing cylinder regardless of blade tip wear, so the doctor pressure can be held at a constant. As such, the printing
cylinder life can be increased even without changing the doctor contact angle and doctor pressure.
Other Doctor Blades
Conventionally, metal doctor blades are commonly used, but metal doctor blades wear, causing the ink transfer weight to become unstable and resulting in printing defects.
One measure for preventing changes over time was the development and testing of a ceramic doctor blade. Because the tip of this doctor blade is plated in nickel, the blade tip
combines the soft touch of nickel with the high wear resistance of ceramic. Using these two characteristics, the doctor blade life has been significantly increased, leading to the adoption of this doctor blade to reduce the rate of defects in printed matter and shorten the amount of time the machine is idle as a result of doctor blade replacement work.
In addition, plastic doctor blades formed of polyester are unlikely to cause cuts when handled, so are safe, do not rust, and greatly reduce printing cylinder wear, which increases the usage life of printing cylinders. The doctor blade usage life, however, is short and the wearing pattern is different from metal doctor blades. Here, the wear is not uniform and wear debris is released, so these are likely to cause filaments, streaks, and color variation. As such, aside from special applications (coating, flexo printing, etc.), these are rarely used.
Doctor Blower (printing cylinder surface blower)
Aside from the areas of image or text, even though the ink on the printing cylinder surface (unpatterned sections) should be completely scarped off by the doctor, a thin ink film remains on the unpatterned sections that must not be printed, causing a thin layer of grime (fogging). If the pressure applied to the doctor blade tip is uniform and constant,
a more acute blade tip angle will allow for a greater ink scraping capability. If wear causes the contact area with the printing cylinder surface to increase, the pressure applied to the blade tip will be more widely distributed, so ink will more easily leak, causing contamination. In addition, the quality of the polishing and cleaning method used on the
printing cylinder surface can also be a cause of contamination.
Very smooth types of paper and plastic film are more likely to result in fogging. There are two possible causes for this problem: the shape and smoothness of the doctor blade tip
and the smoothness of the printing cylinder surface. When using water-based ink, it is particularly important to manage these aspects. To prevent these problems, in general, air is blown over the printing cylinder surface between the doctor and the impression cylinder to dry the ink on the unpatterned sections before printing.
Interrelationship Between the Length of the Printing
Cylinder, Impression Cylinder, Doctor, and Ink Pan The face length of the printing cylinder and impression cylinder must be longer than the width of the substrate. The face length of the printing cylinder must also be longer than the face length of the impression cylinder. The impression cylinder rubber face length, ideally, should have the same dimension as the substrate width. If, for the sake of argument, the impression cylinder rubber face length is longer than the width of the substrate, even if the doctor scarped the ink from the printing cylinder surface, because the inks is not completely removed, as mentioned above, either edge of the surface of the impression cylinder rubber where there is no substrate will slowly accumulate ink and thicken. As a result, the printing pressure will change, as will the color tones. Registration would also become unstable.
Recently, the printing cylinder surface, however, has become much smoother, so when the printing volume is low, there is an increasing number of cases where the impression cylinder rubber surface is made longer than the width of the substrate.
Because the doctor blade is given a crosswise stroke, the length of the doctor blade must be equal to the face length of the printing cylinder + printing cylinder sidelay distance + doctor stroke length + some extra length (20 mm) to prevent the doctor from sliding beyond the face length of the printing cylinder.
The inside dimension of the ink pan in the width direction should be only long enough so that the ink returns to the ink pan after the doctor stroke. If we are considering ink splatter, the widest possible ink pan is best.
Printing Cylinder Drive System
The printing unit drive system drives the unit via a V-belt connected to the main power source, which drives the printing cylinder, doctor stroke, and furnisher roller via a drive
shaft. In the past, a differential transmission gear used to adjust the phase of the printing cylinders was installed in the registration system, but if a differential transmission gear is
used for too many years, the gear will wear, causing vibration and increasing backlash. Because this is a cause of instability in register precision, these are not used often today.
The coupling unit for the printing cylinder drive uses an internal gear coupling, which eases printing cylinder exchange and registration at printing start-up. This internal gear coupling must be a system with low backlash. NonJapanese machines sometimes use a split muff coupling in place of an internal gear coupling to completely eliminate backlash, but because these types make it diffi cult to change out the printing cylinder, they are not suited to small-lots.
The printing unit gear box is the most important place in the printing machine. If the gear processing precision and assembly precision are poor, gear wear will be rapid, backlash
will increase, and registration will become unstable. Moreover, wear will be the cause of problems and may even cause the machine to stop. As such, it is necessary that the system have a simple structure and a high machine precision. Recently, it has become common for electrically synchronized motors to be located in each printing unit (shaftless
drive method).
Guide Rollers and Other Rollers
Guide rollers, located as needed between the unwinding unit and rewinding unit, are generally driven by the substrate, so they apply resistance to the substrate. From this perspective, although it is best to use fewer guide rollers, if the distance between guide rollers is too long, the substrate will sag and wrinkles will be more likely to form. As such,
there is a limit to reducing the number of rollers. Moreover, if guide rollers are not suffi ciently horizontal and parallel, this will be a cause of substrate meandering.
One cause of tension fluctuation inside the printing machine is rotational variation of the guide rollers. Guide rollers must always rotate at the same speed as the substrate and there must be no variation in this rotation. If there is rotational variation, this will cause tension fluctuation, which is a cause of registration instability.
Moreover, if there is rotational variation in the guide rollers or the guide rollers do not spin, this can cause scratches on or generate electrostatic charges in plastic film. In particular, when printing a plastic film with a low Young’s modulus, lowtension printing is required, so to reduce the resistance that the guide rollers apply to the plastic film, we must use as light a structure as possible and reduce any imbalance as much as possible. We must also ensure as low a rotational resistance as possible in the bearings.
In addition, a mirror finish equal to that of the printing cylinder surface is not necessary for the guide roller surfaces, and this finish must only be good enough that the rollers do not damage standard plastic film. On the other hand, to prevent wear of the guide rollers when the substrate is hard paper, or to prevent marks and scratching with aluminum foil or ultra-thin films, it is particularly necessary to apply a surface treatment to the rollers, such as hard chrome plating.
Drying machine guide rollers are subject to heat, so the rollers themselves must have a mechanism that deals with thermal expansion. In addition, eccentric guide rollers with
eccentric bearings, twist rollers installed directly before the printing cylinder, and spiral rollers with a groove cut in the roller surface are also used.
Eccentric Rollers, Twist Rollers
When there is thickness variation in the substrate, wrinkles will form. As such, these types of rollers are used to remove such wrinkles. In addition, twist rollers are also used when the printing cylinder is defective or during one-sided pressure adjustment. Because paper and film substrates naturally enter the roller at a right angle (right angle entry rule), during acceleration, deceleration, or tension transition, such rollers will be a cause of meandering, so we should avoid these as much as possible and pay particular attention when we do use these.
Spiral Roller
These are used to release vertical wrinkles in the width direction. The grooves in the roller, which span out from the center, are designed to allow the wrinkles to escape in the rotational direction (herringbone roller).
In addition, some roller grooves are made in a twill line pattern. If the printing speed is increased, the guide roller rotational speed will also increase. As such, if air becomes
entrained between the roller and the film, the frictional resistance between these will decrease and result in slip. Because slip will become a cause of film meandering, the surface of these rollers is formed with grooves to allow the air to escape, increasing adhesion with the substrate, and thus increasing the frictional resistance. Depending on the
frictional coefficient of the film, the tension, the wrap angle, and the roller surface treatment, the frictional contact resistance will change, but there is no accepted opinion on
the shape and pitch of the grooves, so the design depends on the experience of the individual manufacturers.
Materials include steel tubes, aluminum tubes, and carbon fiber reinforced plastic tubes. Moreover, some roller surfaces are wrapped in rubber. The different types are used depending on the purpose. Aluminum tubes are generally used in the case of plastic film, and carbon fiber reinforced plastic rollers are particularly used for low-tension, highspeed printing.
Air Floating Roller
This roller uses air pressure and magnetic forces to fully float the roller above the shaft, meaning the mechanical friction is zero and there is no need for bearings. As a result,
the roller is maintenance-free.
Moreover, such rollers even spin under low-tension, so do not damage the surface of the printed surface or film surface, and are unlikely to cause meandering. Even so, these
are extremely expensive, so are only used for special cases.
Cooling Roller
Cooling rollers are installed before the subsequent printing unit to stabilize the temperature of the substrate. These rollers prevent heat build up in the rollers caused by the substrate that has been dried using heat, as well as prevent elongation/ contraction of the substrate caused by heated drying.
Like guide rollers, cooling rollers must be designed to have low rotational resistance and to rotate smoothly so that they do not apply resistance to the substrate. Moreover, water is cycled to cool the roller, so there must be no water leakage. In some cases, these rollers are actively driven to reduce rotational resistance.
Like guide rollers, most cooling rollers are made of aluminum tubes to lighten their weight. Depending on the application, the roller surface can be grooved, plated in hard chrome, coated in Teflon, or given an orange peel texture plating.
