“Develop success from failures. Discouragement and failure are two of the surest stepping stones to success.”
Surface preparation
EQUIPMENT FOR PROOFING AND PLATES MAKING
Plate making material:
The material for Litho plates consists of the metal used for the plate, the mixture of substances used for coating them, and the chemicals used for processing them.
Lithography plate are chemically maintains by the principle that grease (ink) and water generally do not mix.
Standard thickness range from 0.0055 to 0.020 inch (0.14 to 0.51mm) and sizes go up to 59X78 inch (1.5X2.0 mm).
a. 0.28 mm
b. 0.94 mm
c. 0.67 mm
1. Aluminium: commonly used today. The lightness and strength of aluminium alloys make them more suitable although pure aluminium is not strong, its alloy have great strength. It is ductile, malleable fairly soft and light weight. It is a better conductor of electricity than copper. Both aluminium and zinc are grained to increase the capacity to hold water and image. It is protect against corrosion by atmosphere.
We have already seen that while copper is hydrophobic, aluminium is hydrophilic
2. Chromium nickel: these two metals are their corrosion resistance. In order to improve the wear and tear one of the metal to electroplated on stereos, electros bimetal, Litho plates and gravure plate cylinder.
3. Copper: it used in letterpress, lithography and gravure. It provides fine line and halftone blocks, cylinder for gravure.
It high conductivity of heat and electricity is an outstanding property.
A thin coating in steel inking rollers a presses.
Equipment in platemaking department:
1. Roll coaters: simple two roll coaters, with the coating pan under the lower roller are used for coating wipe-on plates. The machine consists of two soft synthetic rubber rollers mounted one over the other. The plate is passed facedown between the rollers coating is transferred to the face of the plate.
2. Stripping table: it provides an illuminated working surface for positioning and stripping film to make a flat. A sheet of plate glass is used on top grained on the underside to diffuse light with several fluorescent lamps for even lighting with internal reflectors and diffusers.
3. Printing down frame: the vacuum printing frames holds the negative and plate in close contact during exposure. The frame consists of two metal frames. The bottom frame holds a corrugated or channelled rubber blanket with a rubber bead or gasket around its edges, the top frame contains a sheet of flawless plate glass. Smaller vacuum frames are usually hinged together on one side. The rubber blanket is connected to a vacuum pump by a flexible rubber tube.
Operating the vacuum frame: when the printing frame is open, the blanked is horizontal and the glass is raised up out of the way. The sensitized plated laid on the plate in exact position, emulsion side down. The glass frame is lowered and two frames are locked together. The vacuum pump is on and pump suck the air from between the blanket and glass, thus forcing the PS plate and negative or positive together. Vacuum pump 20 to 28 lb square inch vacuum is needed.
Making good contact: for fine image.
Testing contact: very easy way to test the contact between the plate and the film with a pen flashlight. The light is held at 45 degree angle to the glass and the area examined with a magnifying glass.
Light source: it is the radiation of light that cause the chemical reaction on a sheet of film/ plate. We are concerned with the quality and quantity of light that creates that reaction. The sun is an ideal source of white light.
a. Tungsten light: it is composed of much more red light than blue.
b. Carbon arc lamp: light is generated by passing an electric current between two carbon rods. The arc light is having between and violet light. Voltage changes can cause unstable variation in colour temperature and light output. 60 sec for warm up.
c. Mercury vapour lamp: it is operated by passing a current through a mercury gas in a quartz envelope. The emulsion is mainly blue-violet and ultra violet. 2 to 3 minutes for warm up after started.
d. Pulses zenon lamp: the lamp is formed by filling quartz tube with low pressure zenon gas. Zenon is charged and discharge at the rate of power line frequency 120 times each second. The emission is close to sun light.
e. Metal halide lamp: it is basically mercury lamp with metal additives. A metal-helide lamp gives about twice as much useful light as a conventional mercury vapour lamp, 21 / 2-3 times as much as a carbon arc lamp, and 4 times as much as a pulsed-xenon lamp.
Colour temperature: It is a measure of the sum of colour effect of the visible light emitted by any source. As the temperature increases, the colour of the body changes colour temperature is measured in degree kelvin (k) which is derived by adding in 273 to the temperature in degree centigrade.
Light integrator: A more accurate method of controlling film exposure is with a light integrator, a light integrator use a photoelectric cell. The cell is connected to a device that measures the unit of light reaching the film with the light integrator the camera operator simply dials in the desired amount of light. Any variation in light character due to voltage change in the power source that operates the camera light will be automatically be accounted for
Processing system:
Whirlers: now use PS plate and wipe-on plate, but whirlers are still needed for conventional deep-etch and bimetal plate, but the toxic bichromated coating used in whirler coating have outlawed in many areas.
The plate whirler is a machine with rotating table on which the plate should lie perfectly flat. In some whirlers the table is horizontal and vertical.
a. The horizontal whirlers distributes the coating solely by the centrifugal force.
b. The vertical whirler distributes the coating by a combination of centrifugal force and gravity.
Each whirler should have a source of water for flushing the plate just before you coat it. The whirler is actually an oversized turnable. The plate is fastened or mounted on the turnable and the sensitized coating is poured on and centrifugally distributed over the plate while it is turning, the whirler usually has a positive variable speed.
Photo composing machine/step and repeat machine: it used for exposing lithographic plates or films.it is completely automatic and this machine produces a series of exposure in register on the same plate or on successive plate. The step for the entire job can be programmed and set on a dial exact number of steps required in each row and the total number of step for the entire job.
The plate making room should be air-conditions and well ventilated to remove fumes from chemicals and evaporation of lacquer, alcohol and other solvent.
a. Hot and cold water supply: water temperature as high as 120 degree (F) (50 degree).
b. Illumination: plate coating are sensitive to ultraviolet and blue light. Yellow or fluorescent tubes suitable light.
c. Air conditioning: controlling both temperature and relative humidity. Good conditions are 75 +/- 3 degree F (24 +/- 2 degree C) and 45% +/- 5% RH.
d. Plate making sinks: stainless steel sinks are used.
Automatic plate processor: equipment for automatically processing plates has come into almost universal use. Essential in high-volume suitable automatic processor increase productivity improve consistency and reduce chemical downtime and the number of makeover from failures on press.
Working: the operator feeds the exposed plate from the section or unit of the processor, the plate enters slowly in the developing unit, there are brushes which developers the plate uniformly. After developing the plate moves to the washing unit and the plate is washed in fresh running water. The washed plate then enters the drying unit where small fans blow hot air and dries the plate then, the plate enters the gumming unit where a thin layer of gum is applied on the plate. Finally, the plate enters in the drying unit and the plate is dried. Now, the plate enters the delivery unit and the plate is ready for production.
Replenisher: Replenisher is added to the developing solution to maintain its PH (5% is added).
Flip top exposing machine:
Double sided vacuum frame marked A and B for the side wanted to be in/an used choose the side to be operational with the help of a toggle switch in the rigid hand corner of the frame first switched on MCB (minster circuit breaker) to upward position than press green button on the control panel work the machine stand by feed the timer according to your requirement.
Step 1: press program switch:
A RED LED will start glowing than press the digit (45) from the digit now will be displayed on the display section. Press enter to save the displayed digit now the channel is fixed.
Step 2: now press vacuum switch:
A RED LED will start glowing press the digit 25, 0, 50 that (25sec) are feeds are vacuum timing. Press enter to save the timing. Minimum timing for vacuum shall be 15 sec so that there are no air bubbles between the film and plate maximum time for vacuum is act uses choose.
Step 3: press exposure switch:
A RED LED will start glowing press the digit 1, 8, 0 to feed (180sec) for plate exposure press enter to save the exposure timing.
Exposure setting are on two modes:
1. Timing mode: this can be choose by pressing the timing or intensity return on the same switch. On pressing timing mode A RED LED will start glowing and it will work as a clock in sec.
2. Photocell mode (intensity mode): if intensity LED switch is press. It will function only the photo sensor in the cabinet senses the light it will give accurate exposure.
Graining: in order to lithograph from metal plates, it is necessary that their water-receptive areas accept moisture easily this ability as a natural property because of their pores structure. Metals, however being non-porous must roughened to produce a surface which has a capillary attraction for moisture. The gained surface has several times the effective areas of original smooth plates and affords greater facility for the adhesion of water. The adhesion must be sufficient to overcome the tendency of the surface moisture to be drawn up into droplets by its high surface tension, while the grain support the surface of the form roller and blanket, the pits or valleys in the non-printing areas forming reservoirs from which the moisture cannot be forced out.
Grain of different textures are usually produced by varying the coarseness of the abrasives the size of marbles and tome. The grain is the final result of continuoused irregular stretching of the metal by abrasive particle.
The grain depth should be measured with degree of accuracy by a microscope.
It depth should be about 2 micron (0.0008 inch).
1. Mechanical graining:
a. Rotary tub graining: it is called ball graining, because it is done in a graining machine which consists of a tub with a rotary motion. Small steel marbles which are usually reject ball bearing are rotated over the surface of the plate. Water is added and then an abrasive material and roughening done.
The character of the gain is determined by:
1. The hardness of the surface of the metal.
2. It the amount of water used.
3. The weight and uniformity of the marble load.
4. The nature amount of the abrasive.
5. The speed of the grainer.
b. Sandblasting: it is used for roughing plates both for wipe-on and other platemaking process. The plates are mounted on a rotary drum and a dry abrasive is impinged on the surface at an angel to the plate at right angle to the direction of rotation of the plate. Nozzle wear can cause variations in grains with this method, and imbedded abrasive can also be a problem.
c. Dry brush graining: it is used for treating some plate prior to pre-sensitizing. This can be done with brass or steel wire brushes. (For PS plate).
d. Wet brush graining: takes a special machine in which the plates are fed on to a conveyor belt under nylon brushes and the graining is done with a mixture of pumice and water. Every with new aluminium plates and avoid rolling – mill streaks.
e. Ball brush graining: it is a combination of rotary tub and wet brush graining. In this type of graining good depth is obtained in the tub graining operation and a fine.
2. Chemical and electrochemical graining: several method of roughening plates chemically and electrochemically are in commercial use. Now used this method for PS plate. The electrochemical treatment of aluminium in a solution of hydrofluoric acid.
a. Aluminium anodizing: aluminium anodizing is a process by which a very thin uniform layer of extremely hard aluminium oxide is produced electrolytically on the grained aluminium. This anodic layer has many extremely pores, that must be sealed before the photosensitive coating is applied. Hot solution of sodium silicate are used for sealing, which make the surface hard and highly water receptive.
b. Chemical treatment: in addition to roughing the surface, chemical treatment are also needed for some processes, especially negative Diazo PS plate. The Diazo compound for sensitizing these plates, which are ink receptive when exposed, will in themselves react with metals.
When positive PS Diazo plates are made, special surface treatment are not necessary, although cleaning and usually some type of fine graining.
Coating solution:
All photosensitive lithographic metal plates have a photo emulsion surface consisting of some form of light sensitive material combined with a collation coated on a grained metal surface. A colloidian is an organic compound that forms a strong continuous layer. When mixed with the light sensitive solution and then exposed to the light the colloid becomes in-soluable and forms a strong continuous coating on the printing plate.
Ammomium bichromate combined with the egg albumin was previously used as the photo emulsion in the lithography process. Albumin has gradually been replaced by other solution until it is now very nearly obsolete. Popular coating are polyvinyl alcohol (PVA) diazo and photo polymers.
1. Egg albumin: the proteins are complex colloidal substances found in the cell of all animals and plants. Proteins contain the elements carbon hydrogen oxygen and nitrogen and certain proteins such as albumin, contain sulphur.
In its preparation the egg are broken by hand and the whites are separated from yolks and transferred to castes, a small amount of ammonia which acts as preservative.
2. Ammonium dichromate: it is manufacturer by adding ammonium hydroxide to chronic acid solution in controlled quantities. The pure product having the composition (NH4)2 CrO7 mixture of albumin glue, gum Arabic, gelatine and the like with a dichromate. When dried and exposed to liquid became in-soluable in water.
3. Water: it is used for only vehicle of solution.
4. Ammonia: ammonium hydroxide or aqua ammonia is the essential constituent of the coating solution. It is a gas (NH3) which combine to form ammonium hydroxide.
DIAZO COMPOUNDS: It form a family of man-made chemicals of which contain two linked nitrogen atom (N2). They are prepared from compounds called amines, containing (NH2) group when these amines are treated with nitrous acid (HNO3) and (HCL) acid, a diazonation reaction takes place and diazo compound is produced.
RHN2 + HNO2 + HCL ------------------------------------------ RN2Cl + 2H2O.
Plate chemistry: Many metals and alloys are technologically well known but only a few of them fulfil the quantity parameters for an offset plate.
1. Ductility of the metals or alloys.
2. Good durability and hardness.
3. Fine grain structure.
4. Thermal stability.
5. Ability for etching.
6. Low cost.
These factors reduce the number of material to zinc, copper, aluminium, magnesium and other special alloys are often used in bi-metal and tri-metal plates. The metal behaviour and surface of an offset plate are critical for a good working product. The following properties are important for offset plates.
1. Good adhesion of photopolymer coating and the ink film to the image areas.
2. Strong bonding between plate surface and lacquer during plate development and subsequent fixing.
3. Good wettability of the whole non-image area by water.
Oleophilic and hydrophic behaviour: offset plate should have special qualities in their receptivity to water and ink besides general qualities like strength, fine grain structure, resistance to temperature and sensitivity to etching. Normally offset plates are coated with special lacquer for ink receptivity. The main property is the hydrophilic behaviour to the plate combines with the rejection of ink the wettability of metal for the offset process can be investigated by measuring the contact angle.
For measuring, a loventzen and wetness contact angle measuring device is used drops of size 0.04 ml + 10% of either Castrol oil or distilled water were applied from a pipette to the polished and degreased surface of the plate. Measurement were made after 10 second. Copper is most hydrophilic metal followed by zinc and magnesium.
Chromium, aluminium, and iron or steel are metals having similar contact angles against water than against oil. These metals are hydrophilic, chromium and aluminium also show good hydrophilic properties.
Surface tension: surface tension may be defined as a force in dynes actin gat right angle to the surface of liquid. Surface tension are expressed in dynes cm. water 72.8 nitrobenzene 41.8, benzene 28.9, tolene 2834, acetic acid 27.8 carbon tetra chloride 26.8, methyle alcohol 22.6.
It is interesting that surface tension of water is much higher that of ethyl or any other organic liquid.
Tolene: it is mixture of petrol and kerosene.
Contact angle: the relative wetability of metals in lithographic process has been investigated by the measurement of contact angle. The contact angle is a measured by introducing a small drop of oil from a pipette under the metal plate immersed in water and projecting the image of this oil drop as to a surface so that its contact angle can be measured.
Contact angle
Zinc = 30 degree
Aluminium = 50 degree
Cupper = 60 degree.
CLASSIFYING LITHOGRAPHIC PLATES
Plate making demonstrate that oil and water generally do not mix. A lithographic plates must consist of two kinds of areas the printing areas, which accept ink and repel water and the non-image area which accept water and thus repel ink.
Main types of lithographic plates:
1. Original plates: original plates are plates on which the artist has drawn an image with a greasy crayon or a special ink called tusche. The artist may also apply a mechanical dot pattern for creating various tone values. Most original plates were on stone. The modern ‘direct image’ plates are strickly speaking, original plates. Direct image plates are seldom used today. The remaining direct image plates are paper plates used mostly on duplicator.
2. Hand transfer plates: when two or more identical image were to be printed from the same plate, the artist did not draw image more, the only draw a single design from which the required no. of ink impression were duplicated on hand-transfer paper. Hand-transfer paper is coated on one side with a gummy or gelatinous layer. The duplicate transfer were the laid face down in the proper positions on a new plate, their ink image were pressed against the plate. After soaking in water, the transfer paper was then removed, leaving the inked image on the plate. This procedure was used for the first metal lithographic plates.
3. Contemporary lithographic plates: original and hand-transfer plates are practically obsolete today. They have been replaced by photo mechanical plate images from negative or positive lith film. Direct image plates are in current use, but only for special uses.
The photomechanical principle
A photolithographic plates is a metal, paper, or plastic plate that is cleaned, treated and coated with a thin film of light sensitive, ink receptive material, and then dried.
a. Negative working plate: the image areas are solubilized or depolymerized by light.
b. Positive working plates: the non-image areas are solubilized or depolymerized by light.
c. Negative film: image area are transparent and non-image area are opaque.
d. Positive film: image area are opaque and non-image area are transparent.
The main types of contemporary plates:
Lithographic plates are mainly divided into four group:
1. Surface plate (both pre-sensitized and wipe-on or consumer-coated).
2. Deep etch plates (positive working)
3. Bimetal plates and tri-metal plates
4. Direct image, photo-direct, electro static
QUALITY CONTROL AIDS
1. Densitometer: it is an instrument to measure the density of films and prints. There are two types of densitometer (a) reflection and (b) transmission densitometer for use on opaque and transparent materials. In photo electric densitometer a point light is directed to a sample of light transmitted or reflected on photoelectric cell. The output of this operates a cell on the dial the correct density is shown in this method.
2. Sensitometer: it is the subject dealing with sensitive of films. It measures light sensitive of film suitable processing sensitivity is accurately measured.
3. Exposure meter: photo electric exposure meters and exposure photometers are used to find correct exposure timing and personal judgement also helps. Allowances for certain factors are made. They have a photoelectric cell, device to limit acceptance angel of cell, a galvanometer to show readings in terms of exposure, a calculator to derive the exposure of a film of given speed we can outer-mine exposure time.
4. Grey scale: it is a total used by camera operator as an old in judging the quality of a film image with every exposure the camera operator places the grey scale of the copy-board text to the material being photographers. The specific poison does not matter but it is important that the grey scale does not cover any line detail, that it receive the same light as the copy and that it be in a position to be recorded on the film. As the film, is developed the copy image and the grey scale will become visible. As individual density steps on the scale darken and fill in during development, they serve as visual cue that indicates the stage of film development.
After development, the grey scale image on the film server as a means as judging the film and usuability, and is unacceptable to medicating the charge to correct the short coming.
5. Step wedge: it is prepared from a strip of continuous tone film of normal negative emulsion.
a. Give a series of exposures. Use light source of constant intensity.
b. Each exposed step must bear a constant ratio to the next.
c. The exposure can be doubled 01, 1, 4, 8, 16, 32, 64 and so on in seconds.
d. After exposure as started above and after development the negative all have a series of steps of wedge.
The step wedge is used in exposure along with the original particular in color separation and color correction. It is a guide to judge proper processing.
6. Star target: they are supplied by the graphic art found art foundation USA. They help in judging proper image reproduction. It is smallest in size than a dimension (American coin) it consists of pre-shaped wedge of lines converging to the center. The target is a wheel shaped design printed down into the trim areas of the sheet. It is used to detect early doubles slur or squash in printing, dot gain and dot loss. It aids in diagnosing the causes of printed defects during running as it magnifies defect very strongly. A spread, slur or double which lengthens a halftone dot by 0.0025 mm will cause a spot 0.57mm to spear in the center of the target. When dot gain occurs during the run the center of the target fills in and indicated gain. Press slur is indicated by a figure of eight appearing in the target center, doubling produces a moire pattern in the target. Target are printed in several places in the sheet, to be examined with a magnifies.
7. Registration marks: register marks are a guide to show the correct fit of successive of printed images. It helps in proper position of image to be produced in multiples trim marks are guides for folding and trimming of printed sheet. These are production aids and quality control aids to ensure at various stages such as exposure, negative making, Correction plate making and printing.
Usually the register mark consists of horizontal and are vertical intersecting line circles ellipses are sometimes added to aid registration between one color and another.
8. Dot gain: dot gain one very important consideration when determining printable dot size is dot gain. Halftone dot tend to gain in size during plate making and on press. Critical exposure and development control is needed during plate making to accurately reproduce the dot structure as the negative. Improper plate exposure or processing will produce dot sizes on the plate that are different in size from those on the negative. If larger dots are recoded as the printing plate than those on the negative larger dots will be printed and detail will be lost, dot growth during plate making art the cause of dot gain.
In offset improper ink and water balance can cause dot gain.
9. Color control: Using different proportion of CMYK ink color seen by the edge can be controlled. The process inks are printed as separate overlapping halftone screens and the proportion of each subtractive primary is controlled by the size of the halftone dot. Thus process colors are expressed in terms of the dot percentage for each of the inks for a CMYK color could be C20, M55, Y 00, K05.
10. Slur bars: these devices are printed in the trim areas of the sheet at an angle of 90, to the gripper edge. Press slur may be caused by mechanical defect or by faults in the passing of the stock through the press. Lateral slur cause the vertical lines and Circumfarential slur cause by horizontal lines of the bar to join together.
11. Light integrator: the light integrates the camera operator simply dials in the desired amount of light.
Plate reader:
It is acceptable plate measuring device made by (gretag macteth) Swizerland. This device solves one of the most difficult task in printing industry. It is quick, accurate quality control to the computer to plate process. This device meets the need of most popular offset Lito and Polyster plates with AM screening. They are applicable form dot diameter, screen rulling, to check each individual measurement during the CTP process it allows to measure and entire curve upto 100 sample than transfer it is to the host PC.
To power saving electronics and LED technology allows upto 30, 000 measurement for each set of batteries. This device offers the user a simple and easy to understand operative concept. The software data can be copied into any word processor or a spread sheet or any other software program. This device also offered and optical quality control software to store and to save the measurements.
Safety tips:
1. This device should not be used where there is a risk of explosion.
2. It may not be used in an area with strong magnetic field.
3. Used the device in temperature between 50 C and 40C and do not expose to direct sunlight.
4. It should be used only by trend experience.
5. It should be used on dry and stable measuring planes.
6. It should be protect against chemical corrosion, vapour and strong mechanical vibration.
7. It should be cleaned with slightly damp clothes shoaked deep in soft water.
Working: the screen dots as well as measurement can be transferred and can be used for staticles documenting measuring results. The IC plates target can be used to test and device for is exactness, and to catty out and upgrade it if necessary.
Steps in using plate reader:
1. Relies the measuring head by slitting the locking device to the left hand side.
2. After that is opened press reset button on the bottom side of the measuring head.
3. The screen start to appear on the display.
The device is used to measure:
1. Standard plates.
2. Lines per square centimeter.
3. Regular screen (AM).
WIPE – ON PLATES
In this process an emulsion is hand or machine coated onto a pre-grained plate immediately prior to plate exposure. The base material is either aluminum or zinc. The emulsion is generally mixes to small quantities shortly before it is to be applied. All latent emulsion are formed by mixing a dry daizo powder with a liquid base.
Sensitizer ‘A’: dry diazo powder
Sensitizer ‘B’: liquid base
Proportion = 1:4.
There are two technique for coating the plate by hand and with mechanical roller with hand process the liquid is applied with a damp sponge. The coating should be uniform over every portion of the plate. In the mechanical roller process it employs a dual roller device as the plate is passed between the two rollers which distribute uniform layer of emulsion. The gap between two rollers can be adjusted to get the desired thickness.
Exposure: most wipe on plate are exposed through a film negative with the use of vacuum frames and a high intensity source. The exposure time is similar to most pre-sensitized plate being approximately one minute when using a 3 K.W metal halides lamp at a distance of 2 meter (39”).
Exposure should obtain a solid step 6 on a 21 step stuffer wedge. Exposure will produce a visible image of good contrast.
Plate inking: using a pad of cotton wool apply a small quantity of inking in solution and rub down evenly over the image area.
Development: a quantity of developing solution is poured on to the plate and swabbed until all the non-exposed coating is removed from the surface.
It for used black diamond developer.
Wipe on plates are processed by two step process by pouring a desensitizer gum onto the plate and rubbed over the entire surface of the plate with a damp sponge. The gum solution serves the dual function of remaining any un-exposed emulsion and making the non-printing area water receptive. A second solution made up of primarily of lacquer is then rubbed over the entire plate.
Rinsing off: aluminum plates are washed under running water. Paper plates are washed clean with cotton wool and minimum of water.
Additions and deletions:
Addition of the plate may be made in the followed way:
Rinse off the gum and apply a 2% sulfuric acid solution for half a minutes wash off with water and dry the plate quickly.
Deletions are easily made using the special deletion fluid recommended by the supplies of the wipe-on process.
Defects of wipe-on plate:
1. Scumming: scumming is a major problem on wipe-on plates. It addition to all of the sources covered under pre-sensitized plate.
Cause 1: the coating is very thick and exposure time is not sufficient (under exposure).
Cause 2: wipe-on coating.
Cause 3: incomplete development.
Cause 4: quality problem.
Cause 5: inadequate finisher.
Remedy 1: mix the coating solution in a properly and give the appropriate exposure time.
2. Streaked and chalking image / coating:
Cause 1: the coating is not uniform.
Remedy 1: the plate has been recoated.
3. Fibres:
Cause 1: the coating solution has not been mixed properly and filtered.
Remedy 1: mix the coating solution properly and filter the coating solution.
4. Dirt in the dried coating:
Cause 1: the coated plate was kept in open atmosphere dirty atmosphere.
Remedy 1: after coating keep the plate in a dust free atmosphere.
5. Weak image :
Cause 1: the coating is very thin.
Remedy: mix the coating solution in proper quantity.
6. Image flies off:
Cause 1: the coating solution is not in properly ratio.
Remedy: mix the coating solution in proper ratio.
BIMETAL AND TRI-METAL PLATES
Bimetal plates are excellent for long runs for printing with abrasive ink, paper, board, or metal. Bimetal plates consists of two different metals, one for the image and the other for non-image area. All bimetal plates, in present use have copper or brass as the image metal. The usual non-image metals are aluminium, chromium or stainless steel. When copper and chromium are used together, they are usually electroplated as layers on a third metal. Such as aluminium mild steel or stainless steel. Such plates are often called tri-metal plates.
There are 4 methods of making bi-metal plates:
Type – 1: form an acid resistant positive image on a plate of an image metal then, electroplate a non-image metal on the remaining area finally remove the acid resist coating from the image area.
1. Coat and expose through negative.
2. Electroplate a non-image metal on clear area (image area down).
3. Develop a positive image.
4. Remove the acid resist coating from the image area.
Type -2: form a stencil or negative image by means of an acid resist coating on a plate of an non-image metal then, electroplate an image metal on the clear areas and finally remove the protective stencil.
1. Coat and expose through a positive (image area raised, non-image area down).
2. Develop a negative stencil.
3. Electroplate an image metal on clear area (image area up).
4. Remove the acid resist coating from the non-image area.
Type – 3: electroplate an image metal plate of non-image metal, form an acid resist positive image on the layer of image metal etch away the image metal from the areas except the image area. Finally remove the acid resist coating from the image area.
Image area ------------------ copper
Non-image area ---------------------------- aluminium.
1. The copper coating of the plate is cleaned by regular counter etching with a dilute acid.
2. The cleaned plate is coated with deep etch coating solution and dried.
3. The coating plate is exposed through a negative.
4. Develop the plate with deep-etch developer which removes the un-hardened coating from the non-image area.
5. The copper is removed from the non-image area by means of special etch that does not with stainless steel or aluminium under need.
6. The hardened coating protecting the image area Is removed by scrubbing under with warm water.
7. The copper image area are sensitize to ink with weak H2SO4, or ferric nitrate solution.
8. Finally, the image area is applied in a thin film of ink.
9. Electroplate an image metal on non-image metal plate.
10. Coat and expose through a negative.
11. Develop a positive image.
12. Etch through the image metal in the non-image areas.
13. Remove the acid resist from image areas.
Type – 4: electroplate a non-image metal on a plate of image metal, form stencil or negative image by means of acid resist coating on the layer of non-image metal. Etch away the non-image metal from the image area finally remove the protective stencil from the non-image area.
All four types have been tried only type – 3 and type – 4 were successful.
Type – 1 and type – 2 gave trouble because of difficulties in finding photo resist emulsion that would stand up during the electroplating operation and because of the inability to obtain uniform electrodeposit on the metal.
1. Plate treatment prior the coating.
2. Coating with deep-etch coating solution with whirler.
3. Exposing the coating plate with positive film.
4. Stopping out before development.
5. Developing the deep-etch with developer.
6. Stopping out after development, dust stops, tape marks are stopped out with deep-etch coating solution before etching.
7. Etching the chromium layer with AlCl3 (aluminium chloride solution) - 750CC, ZnCl3 – 630 gm, phosphoric acid (PLO3) – 40CC.
8. Cleaning the copper image area by washing the plate with alcohol 3-4 times.
9. Sensitizing the copper image area with ink.
10. Removing the gum stencil by scrubbing with brush in warm water.
11. Sensitizing the non-image area with gum Arabic solution.
Tri-metal:
It is so called because they are made from three metal.
1. Base metal of Zn, Fe and Al.
2. Electroplate layer of copper (image metal).
3. Chromium and electroplated (non-image area or Al).
However, only the electroplated layers of chromium and copper are affected by plate making process for all practical purpose. Tri-metal plates are same as type- 4 bi-metal plates.
DEEP-ETCH PLATES MAKING PROCESS
The plates are formed by the actual bonding of the colloid material into the plate surface.
Depth of image = 2 micron (0.00008 inch) image area is slightly recessed. It does not come in contact with the inking and foam rollers.
So, the life of plate become longer.
Deep-etch plates is made up of only two material Al and Zn.
It Is only positive working plate.
Advantages of deep-etch plates.
1. Image area is down.
2. Image area is not made of light sensitive emulsion.
Steps of deep-etch plate making:
1. Graining: It is the final result of continuous or irregular scratching of the metal surface by abrasive particles.
2. Counter etching: the purpose of counter etching is to clean and prepare the surface of the grained plate so, the It can be coated uniformly with the light sensitive deep-etch coating.
Chemicals for counter etching:-
a. For zinc: hydrochloric acid, HCL – 8CC, water – 1000CC.
b. For aluminum: phosphoric acid (H3PO4) = 32CC, water = 1000CC.
3. Deep-etch coating: it is the solution used to coat the counter etched plate. So as to produce a light sensitive film on it when exposed to an Arc lamp. This bi-chromated gum Arabic film becomes hard and insoluble in deep-etch developer.
a. Gum Arabic solution – 120CC.
b. Ammonium bichromate – 2400CC (NH4Cr2O7).
c. Ammonium hydroxide – 36CC (NH3OH).
4. Exposing the plate: after coating, the plate is exposed in a vacuum printing down frame through a positive film.
5. First-stopping bath: the non-image area which was hardened by exposure is now coated with an acid resist lacquer.
a. Orange shellack – 250gm.
b. (Pure spirit) denatured spirit – 1000CC.
c. Methyl violet dye – 2 gms.
Polymer plate: we use denatured spirit in plate making department for developer.
6. Deep-etch developing: the unexposed coating from the image area must be dissolved off so that they can be deep-etched. A special developing solution is used for bath Al and zinc plates.
a. Calcium chloride solution (CCl4) – 1000CC
b. Lactic acid – 53CC.
7. Deep-etching: the purpose of deep-etching is to etch or dissolve of a small amount of metal from the image area of the plate after development.
Deep-etch solutions contains stronger acid suited for the particular plate metal (Al or Zn).
(For zinc):
a. Calcium chloride – 1000CC.
b. Iron per chloride (feCl3) – 25 gms (it would be in lump solid).
c. Hydrochloride acid (HCL) – 20CC (etching solution is get by mixing these three chemicals).
(For aluminum):
a. Calcium chloride solution – 1000CC
b. Iron per chloride solution – 285CC
c. Hydrochloride acid – 14CC
d. Zinc chloride (ZnCl2) 380CC
e. Cupric chloride (CuCl2 2H2O) – 27 gms.
8. Copperizing: this solution is used for aluminum plate. It deposits a thin layer of copper on the image area of Aluminum plate which improves the image holding quality and printing.
For aluminum: isopropyl alcohol – 1000CC
Cuprous chloride – 31 gms.
HCl – 32CC.
9. Nicoholizing solution: it is used for both aluminum and Zinc plate of deep-etch. This solution completely removes the black deposits from the image area of the deep-etch plates.
Cellosolve solvent – 900CC
Nitric acid (HNO3) – 100CC
10. Washing with alcohol: after the plate is deep-etch or copperizing. The next step is to wash the plate with a hydrous denatured alcohol.
Hydrous denatured alcohol – 900CC
Ansol M solox – 1000CC
Synacol (or) shellacol
This can also be called ‘second stopping out’.
11. Deep-etch lacquer: when ink is applied to the deep-etch image area, they will hold the ink easily but can be damaged, so deep-etch lacquer is applied first.
a. What is deep-etch lacquer.
It has a dye to give them color.
12. Applying deep-etch developing ink: it is a drying greasy black ink, thinned with solvents. So that it can be applied as an even film on plate. This is applied on the top of the lacquer film.
13. Plate etch: it is used to de-sensitize the non-image area of the plate to ink after the image area have been formed and the hardened deep-etch stencils have been scrubbed off from the non-image area. A thin layer of gum is applied on the non-image area.
Chemicals for de-sensitizing deep-etch plate.
(For Zn): phosphoric acid – 7.8CC
Magnesium nitrate crystals – 11.3 gms
Cellulose gum dry – 41 gms
Water – 1000CC.
Gum Arabic solution – 1000CC
Phosphoric acid - 31CC.
14. Storing under asphatom: it is used to protect the image after the developing ink Is washed off the plate. Deep etch plates are always stored under asphalt. Since the ink tends to dry and becomes hard.
Powered asphaltom – 175 gm
Lithosine – 1000CC
Lithosine: it is a mixture of pine oil – 900CC
Caster oil – 9CC
Caster gum – 18CC.
Defects of deep-etch plates:
Defect 1: plate coating is not uniform.
Cause 1: the coating solution was made with stingy gum.
Remedy 1: mix the coating solution with appropriate 720CC proportion of gum.
Cause 2: the coating solution contains air bubbles which leaves pin holes in coating.
Remedy 2: mix the coating solution properly without any air bubbles.
Cause 3: counter etching of the plate was not proper.
Remedy 3: counter etch the plate and make fresh plates.
Cause 4: the coating solution contains solid particles like undissolved dye.
Remedy 4: filter the coating solution before coating.
Defect 2: plate requires pro-longed developing.
Cause 1: coating has hardened in the image area due to dark reaction.
Remedy 1: expose the plate immediately after coating.
Cause 2: the baume of the developer is too high for the plate temperature.
Remedy: check the beaume of the developer. The plate making room, should be temperature controlled.
Defect 3: plate develops too fast.
Cause 1: the baume of the develop is too low for the plate temperature.
Remedy: check the baume of the developer. The plate making room, should be temperature controlled.
Defect 4: halftone dots are broken.
Cause 1: plate grain is too rough.
Remedy: camera positive with too much gelation.
Defect 5: excessive dirt spots that have to be polished.
Cause 1: pin holes due to air bubbles in coating.
Remedy 1: mix the coating solution without any air bubbles.
Cause 2: specks of material that fall from the ceiling of the whirler.
Remedy 2: clean the whirler top before coating.
Defect 6: the coating does not dry hard.
Cause 1: excessive moisture in the coating when the relative humidity is high (about 70%).
Remedy 1: temperature controlled your plate making department.
Defect 7: scum developes in the non-image area of the finished plate.
Cause 1: shrinkage cracks in the coating due to excessive drying.
Remedy 1: give appropriate drying time after coating.
Defect 8: overall scum on the non-image area of the plate.
Cause 1: the coating is too thin or too moist due to humidity.
Remedy 1: mix the coating solution in proper proportion.
Defect 9: scum developes in the areas of masking tapes.
Cause 1: using masking tape of inferior quality.
Remedy 1: use a masking taken of good quality.
Defect 10: plate under developed ink can’t be washed out.
Cause 1: gum has dried all over the coating.
Remedy 1: wash the plate and re-gum the plate.
Addition and removal of work:
Removal of work: as in the case of deep-etch plates, work can be removed or added to the finished plate, consult your supplier about this as he will have the most up to date technique. Small spots that show up on the finished plate can be removed by polished with a snake slip or pencil eraser.
However, a more permanent repair can be made by electroplating chromium on the copper spots.
Addition of work: broken lettering or lines and spots in solid areas can be repaired by scratching through the chromium layer with a sharp needle and applying ink.
If it is necessary to remove one image area and replace with another.
Surface treatment: there are two types of surface treatment generally, surface treatment of the plate is not necessary for Al plate but to avoid ink dot scum, braunak process is often performed on aluminum plates.
For un-grained or partially grained zinc plate cronak process is essential.
Pre-etching: al, steel, zinc.
Pre-treatment:
Braunak (Al) (ammonium bichromate HCL + water).
Cronak (Zn) (aluminium bichromate+ H2So4 + water).
Baume: a unit for specific density (41-43).
Ford cup: an aluminium made.
FLEXOGRAPHY PLATES
Flexography plates (direct printing). It is initially called as “aniline printing”.
Anilox roller is an etched roller. It has cell and ink is filled into these cells after that it is transferred on the plate.
There are two types of plates:
a. Photo polymer plates
b. Rubber plates.
Rubber plates:
A. Natural rubber: it this rubber problem found, it is image is repeated.
B. Synthetic rubber: dimensional stability.
Make rubber plate: Both rubber and photopolymer plates are used for flexography plates making. Natural and synthetic rubber plates were the first type of flexography plates which was developed and they are still in use for some application.
The actual process of producing rubber plate is not for different from the process used to produce photo engraving used in letter press process (block making). Take a sheet of zinc, metal alloy coated with light sensitive emulsion and place it in specially design vacuum printing down frame. The emulsion is not only light sensitive, it is also acid resist. A negative of the job is placed over the coated sheet and light is passed throughout the negative. When the light strikes the emulsion, the acid resist in the image area is hardened. During processing. The unhardened area in the non-image area is washed away leaving the hardened image area which will lower the non-image area leaving the image area in relief.
The complete engraving is then moved to a molding press where a matrix material is placed on the engraving with controlled heat and pressure and a mold is prepare either molten rubber may be poured in the mold to get a flexo be pressed against the mold to form a rubber flexo plate for printing.
Defect of flexography:
Defect 1: milky, foggy or matte appearance.
Cause 1: high humidity causing excess moisture in ink.
Remedy 1: consult your ink manufacturer for proper balance of solvent specific ink.
Defect 2: visual color difference from standard.
Cause 1: inappropriate anilox roller, cell volume or cell count too low.
Remedy 1: replace anilox roller with higher line count.
Defect 3: colour spread into the applied coating.
Cause 1: improper pigment use in ink formulation.
Remedy 1: consult your ink manufacturer to reformulate using resistant pigment.
Defect 4: ink flakes comes off or can be removed.
Cause 1: incorrect ink system for substrate.
Remedy 1: ensure that the correct ink for the substrate is being used.
PHOTO POLYMER PLATES
Two types of plate: (A) flexible polymer (used flexography), (B) rigid polymer
The photo polymer plates are pre-sensitive plates which are available in larger sizes. The plate is cut t size and then the negative to be exposed is cleaned with a felt cloth with CTC (carbon tetra chloride). Then, emulsion to emulsion contact is made and exposed in a printing down frame, the light source is ultra violet light (U.V light). The exposure depends upon the image area. Generally, it is (2-5 minutes) depending upon the image and tonal gradation.
Then, the exposed plates is developed using solvent (methyl, alcohol, industrial spirit, denature spirit) or water. The developing is done on a brush developing or spray developing machine.
The brush developing machine contains a series of plastic brushes soaked in alcohol and the brushes rotate circumferentially on the exposed plate and removes the unexposed polymer on the non-image area. This machine should be clean every 15 days because mixing of monomer and solvent there.
Spray developing machine not used in plate making but this machine should be proper maintenance and pressure should be created well.
One disadvantage of machine cause plate felt and need more water.
After development, the plate is washed thoroughly in running water and dried for some time.
After drying the plate is re-exposed/post-exposed for 2 minutes in a printing down frame without vacuum and negative film.
The post exposure is done to hardened the image area to finish and non-image area thereby increasing the life of plate.
In photopolymer plates, the polymer emulsion which is light sensitive consists of small molecules called monomers when they are exposed to actinic light or UV light, the monomers chemically link and cross-link with each other to form a strong polymer. These polymer are of complex chain of monomers which are linked so strongly that they behave as a single hard and resistive molecule (image area).
Alcohol is used for developing the plate but latest development is by water.
Dinatured spirit: not purified or raw alcohol is called denatured spirit.
Rigid polymer: rigid photopolymer plates has 2 layers. The lower one consist of a steel base of 0.27mm and the top layer is the light sensitive coating which is covered by a protective plastic layer. These plates are imported from:-
1. BASF – Germany (used in currency press).
2. JET – JAPAN
3. TOYOBA – Korea
Before exposing remove the protective layer, while exposing it should be emulsion to emulsion contact. All the processes of exposing, developing post exposure is carried out under yellow safe light. The fresh plates should be stored in a darkroom.
Photopolymer plates are available in two different types- negative and positive.
Rigid plate used in dry offset, U.V coating, rotary letterpress.
The international thickness of rigid photopolymer plates available in the market are 0.58mm, 0.73mm, 0.94mm. The different thickness plates have a common steel base of standard thickness which is 0.27mm.
Steps in making rigid photopolymer plate of dry offset printing:
1. Plate cutting and plate punching (if necessary).
2. Before exposure, remove the protective layer.
3. Keep the negative film emulsion to emulsion contact.
4. Exposure time (2-5 minutes).
5. Developing by eater development or solvent development using brush development machine and spray development machine.
6. Wash the plate is running water.
7. Dry the plate.
8. Post exposure/re-exposure the plate.
9. Plate to be kept in the grill for baking (6-8 hours) at 120 – 150 degree C.
Flash point: maintaining the temperature (150 – 200 degree C (Fahrenheit).
Water washable plate: these plates largely reduce or eliminate of the problem of solvent washable plate like lower flash point, hazardous waste and irritation to eye and lungs. Water washable plates comes in two version.
1. Sheet polymer plates: sheet polymer plates are supplied in a varieties of thickness for specific application. The plates are cut to the sizes or placed on a vacuum printing down frame having ultra-violet light source. The back side of the plate is completely exposed to (U.V light) harden base of the plate. Then the plate is turned over and negative of the job is placed over the unexposed plate now when exposed U.V light image is harden, and non-image area is washing during processing after processing the plate is re-exposed to harden the image area or stop further reaction.
2. Liquid polymer: these plates are made in a special (U.V light) exposed unit. In this process clear plastic protective cover film is molted over the negative which placed emulsion side up on a exposing unit now a layer of liquid polymer evenly over the cover film and controlled the thickness of deposit as liquid polymer is deposited carriage also places a substrate sheet over the liquid polymer. The one side substrate sheet is specially coated to bound with liquid polymer and also serves a base of the plate, now exposure is made first on a substrate sheet this exposure is made to harden the base layer of liquid polymer plate. Then other side is exposed through the negative which leaves the image area harden and soft non-image area is washed away during processing, a re-exposure is made to harden an image-area.
Advantage water washable plate:
1. Fast processing time.
2. Dimensional stability.
3. Superior ink transfer.
4. Consistent print quality.
5. Natural cylinder drop-(wrap).
6. High ozone resistance.
There are two types of polymer coating:
Two fundamental different method for structured coating.
Subtractive structuring: a homogeneous coating with the polymer material is done on the surface of the substrate.
The areas of the material film which re not necessary have to be removed so that, the required structure remains.
Additives structuring: the required structure is done by taking up the polymer material directly on the substrate.
Water washable photopolymer plates are made from 70% polyvinyl alcohol (PVA) and arcyvinyl monomers which dissolve in water and can be safely flushed into the drainage system.
Steps in making flexible polymer butanol developing:
1. Back exposure: (1.5 – 2 minutes)
2. Main exposure: (12 – 13 minutes)
3. Solvent used: per cholthylene butanol (25% + 75% water).
4. Developing time: (2 – 3 minutes).
5. Image depth: (1.1mm – 1.5mm).
6. Standard thickness: (1.14mm, 1.70mm, 1mm, 2.54mm, 2.72mm, 2.84mm).
7. Shore hardness: (54-56A) to (64-66A).
8. Spectral sensitivity: 300-450nm).
Defects of photopolymer plates:
Defect 1: Lack details in the image.
Cause 1: contact between the film and the plate is not proper.
Remedy 1: check the vacuum pressure and give the vacuum.
Defect 2: washout line is too wide.
Cause 1: image under developed.
Remedy 1: give proper developing time.
Defect 3: lines will not washout deep enough.
Cause 1: image over exposed or film is not black.
Remedy 1: give proper exposure time or replace the film or plate.
Defect 4: polymer layer is cracking or lifting up.
Cause 1: the plate has been exposed to too much heat.
Remedy 1: check your heater timing for baking and give recommended temperature (150 – 220C).
Defect 5: polymer plates are culling.
Cause 1: the plates has dried or heated too much.
Remedy 1: a quick dip in warm water then lead the plate observed it for a minutes. Dry the plate and post exposure for the same time as you made original.
Flexible plates:
1. Back exposure: without film exposure.
2. Main exposure: image area hard, non-image area soft unexposed polymer removed from non-image area, image area is relief.
3. Post exposure.
Thinner plates are more reliable than and thicker plates.
Rigid polymer:
1. Main exposure
2. Back exposure
International thickness of plates
Plate thickness =0.58mm
Plate base = 0.27mm
Polymer coating = 0.31mm
Japan komeric = 0.58mm
Mullen machine = 0.73mm, 0.94mm
GRAVURE CYLINDER MAKING
Gravure = if it is printed from a cylinder. The image area will be separate (broken).
Characteristic of gravure cylinder:
1. Printing from wrong reading (recessed image), cylinder direct to substrate.
2. Three main segment:
a. Publishing
b. Packaging
c. Specially printing currency, vinyl etc.
3. Principle application for packaging, long run, magazine, newspaper, inserts, catalogues, wallpapers, postage stamp, vinyl flooring, plastic lamination.
4. Recognition of characteristic (identification) serrated edges to text letters and solid color area
5. Relatively short make ready, time on press high color sensitivity.
6. Gravure cylinder lasts forever making repeated runs/jobs very economical.
7. Cost of making cylinder remains high, making gravure cylinder making more expensive for short run jobs.
8. The trend is towards removing chemistry from cylinder making procedure, increased used of water based ink.
9. Break through is anticipated in electronic beam engraving and photopolymer coating cylinder.
Different types of gravure cells:
CPI = cells per square inch
1 square inch = 130 – 450 square inch.
Four ways of gravure cylinder making:
1. Diffusion etch process: in this process, a special mask is prepared by exposure first by special gravure screen and then through a film positive of the printing of the image. Next, the mask is applied to a copper cylinder and it is developed on the cylinder. After developing, the mask is thick on the non-image area of the cylinder and it is thin where and image is exposed. Now, the cylinder is placed in an acid bath and the acid penetrates through the thin areas of the mask and etches away the copper.
The next step is to apply a thin layer of chromium over the entire cylinder by electroplating process.
The purpose of chrome-plating is to extend of the life of the gravure cylinder (essential due to wear and tear).
The chrome plating hardness is between (950 to 1200 vickers (HW)).
a. Exposed mask
b. Attaching the mask of the cylinder:-
c. Etching:-
2. Direct transfer: the main difference between diffusion etch and direct transfer process is the way in which the cylinder mask is exposed in this process, a light sensitive mask is applied over the cylinder, and the mask is exposed by direct light through a gravure screen a halftone positive as it moved passed the cylinder which rotates in the same speed of the positive.
The final step of developing, etching and chrome-plating are the same as in diffusion etch technique.
3 .Electromechanical process: in this process, a clean copper cylinder is mounted in a special engraving machine like a scanner used in color separation. The original copy is read by a beam of light and the information from the reflected light is stored in a computer after doing necessary correction and adjustment, it is when translated into mechanical motion to a cutter head. A special diamond shaped stylus actually cuts into the surface of copper cylinder. After cutting, the cylinder is chrome-plating.
4. Laser engraving: in this process, a series of small holes or walls is chemically etched over the entire cylinder of clean copper cylinder. Then the holes are filled with plastic materials until the cylinder again has a smooth uniform surface, like the electromechanical method. The original copy is scanned by a beam of light. This process however uses a narrow beam of laser to remove or burn the parts of mond tools to cut away the metal: the cylinder is then chrome-plated/ electroplated of the four cylinder preparation process, diffusion etch is the oldest and most widely used in Indian printing industry.
Defects of gravure cylinder making:
Engraving, chrome-plating, copper plating
1. Engraving:
Defect 1: stylus break.
Cause 1: excess pressure for engraving or old stylus.
Remedy 1: change stylus, check the stylus pressure.
Defect 2: low cell depth.
Cause 1: inappropriate stylus pressure.
Remedy 1: check the stylus pressure.
Defect 3: high cell depth.
Defect 4: sliding shoes marks.
Defect 5: smearing in.
Defect 6: un-uniform engraving depth
Cause 1: the stylus height is not proper.
Remedy 1: change the stylus.
Defect 7: reining.
2. Chrome-plating defects:
Defect 1: the shining is not proper (polishing).
Cause 1: the chrome-plating has not been polish properly.
Remedy 1: chrome-plated the chrome-plated cylinder properly.
Defect 2: micro cracks absent.
Cause 1: chrome-plating has not been done properly.
Remedy 1: do chrome-plating again and remove the first chrome-plating.
Defect 3: roughness value not O.K.
Cause 1:
Remedy 1: Re-chrome plate the plate with proper roughness.
Defect 4: milky rough deposits.
Defect 5: uneven deposits.
Defect 6: polishing bend mark.
Defect 7: bent mark.
3. Copper plating defects:
Defect 1: rough deposits.
Defect 2: treeing (patches) uneven on bath edges of the cylinder.
Cause 1: copper deposits is not even.
Remedy 1: copper plate the cylinder again.
Defect 3: uneven hardness.
Cause 1: copper deposit is not proper.
Remedy 1: redeposit the copper.
Defect 4: low/ high hardness.
Defect 5: polishing bent marks.
Defect 6: roughness not sufficient.
P.S PLATE
Pre-sensitized plates are so called because they come already coated and are ready for exposure and processing when purchased.
Pre-sensitized plastic coated plates were originated in Germany by kale and company just prior to world-war 2. The base of the plates was plastic coated paper. These early plates and those which are now used, are coated with a diazo sensitizer. Such sensitizer are not affected as much by temperature and relative humidity as bichromated coatings as long as temperatures do not exceed 120F (49C).
These plates are easily available in the market. They are two types:-
1. Negative working plate.
2. Positive working plate.
These are also available on the three different uses styles like short run, medium run, long run, plates are coated with diazo compound which are man-made chemicals.
Steps in making PS plate:
1. Cut the plate to size and mark the gripper margin or punch the plate if required.
2. Remove the protective layer.
3. Positive film may be placed emulsion to emulsion contact with the plate in a vacuum printing down frame.
4. Exposing: expose the plate with a stable light source for (3-50 minutes. It developed to a clean step 4 or 5 on a 21 step sensitized guide.
5. Developing: develop the plate with readymade PS developer.
6. Wash the plates in running water after development and dry the plate.
7. Bake the plate if necessary.
8. Gumming: gumming is done with a gum-Arabic solution or other finisher, such as AGE (asphaltum –gum – etch). And let it dry.
9. Plate is ready for production now.
Baking: it is the process in which the plates are heated in hot air in an oven. The baking temperature should be at 220C centigrade for 8-10 minutes. Two plates should be put a single frame having images outward use hand gloves while putting the plate inside the oven. Use of plate trimmer is essential. The bell alarm indicates the right temperature and time. Take out the plate and leave it in room temperature for some time.
PS-defects:
Defect 1: scummy background.
Cause 1: plate has been exposed for a long time.
Remedy 1: check the exposure time.
Defect 2: halftone images are broken.
Cause 1: the positive film used for exposing is not sharp.
Remedy 1: check your positive and replace your positive film.
Defect 3: spotted or mottled halftone.
Cause 1: this due to less contact during exposure.
Remedy 1: check your vacuum pressure.
Mottled = break like powder.
PS plates graining or plate treatment:
PS plates are available either with chemically electrochemically or mechanically produced grains. For longer runs, PS aluminum plates are usually grained and then electrochemically hardened.
After grain the surface is coated with sodium or potassium silicate or aliminum oxide (anodizing). This forms a barrier layer between metal and diazo compound to prevent harmful reaction which would otherwise reduce its shelf life.
COMPUTER TO PLATE TECHNOLOGY
It is the most advance technology used for making plates. The computer controlled direct imaging of printing plate from digital data by violet laser. The accuracy attained is more compared to conventional methods of plate making. It refers to production of printing plates for offset and flexography printing. It is more efficient, precise, saves time, money, cost of labour etc.
There are three types:
1. Flat bed
2. Internal drum
3. External drum.
Advantage of CTP:
a. Human error is reduced.
b. Manual film stripping and plate making is eliminated.
c. Storing digital image is easy.
d. Can require large amount of physical space.
e. Film can get scratched and damaged. Even in storage but digital image will always remain constant, clear and sharp.
f. Print quality will improve.
g. It will require the potential dot gain allow dense ink to be used on the press (dense ink can be used, dot will not gain).
h. Due to temperature and humidity changes. The film may shrink or stretch causing registration problem. CTP eliminates these problem.
i. Copy changes are easier on CTP than by film.
j. Time saving and faster.
k. Reduce material usage (film, space, Astronol sheet).
Reduce labour expenditure.
Finally improves quality better looking finished product.
1. Flatbed CTP: this CTP offers the simplest handling of plates and pin registration system. Imaging methods involved complex optics lens (complicated). A rotating mirror scans across the width of the plate with a laser scan upto 22 inches. A laser scan mover perpendicular to the plate and as the laser beam scans across the flatbed surface at a very high speed and the imaging is possible. Flatbed CTS system for newspaper are compatible with the width constraint “eight” “up” and “sixteen” “up”.
Plate imaging requires more complicated flat-bed designs. These CTP use multiple imaging heads or mechanical scanning methods.
Companies who made flat-bed CTP:-
1. BARCO GRAPHICS.
2. BASYS PRINT
3. CORTON
4. KRAUSE
Advantage:
a. Plates are easier to handle.
b. Laser is only short distance from the plate.
c. It is only suitable for image size upto 22 inch image can be distorted if the image size is more than 22 inches.
2. Internal drum: this CTP mounts the plate on the inside surface of a partial cylinder usually open at one or more ends. Physical loading (manual) and unloading of plate is complicated especially for registration. This CTP has simple optics and no drum run up time before imaging (not required time for production starting, works instant).
A spinning deflection mirror deflect the laser perpendicular to the axis of the plate. The distance from the focusing lens to the plate surface is constant. The mirror assembly moves one bit position per revolution.
Models of internal drum (company name):
a. AGFA
b. GERBER
c. DAINIPPON
d. KRAUSE
e. HEIDELBERG
Advantages:
a. Plate exposure is roughly the same size as the film.
b. A single beam laser address the plate bed remain constant keeping the plate firm (plate remains constant, only laser moves).
Disadvantage:
a. Proper imaging not happen when laser moves regularly.
3. External drum: the plate is mounted on the outside of the rotating cylinder almost like a plate being mounted on press. Plate hanging is complex, pin registration system are limited. The laser beam is perpendicular to the axis of the cylinder and it focus on the surface of the plate. The optical path remains constant as the drum rotates. The exposure had move on a track parallel to the cylinder axis. Multiple exposure beam can be used to speed-up imaging. As the laser had advances each beam writes on the plates. The external drum optics are simple but slow rotation speed makes multiple beam design essential for speed.
Workflow of external drum CTP:
1. The pdf file is ripped through the herliquin (a software) and it is converted into four separations in 1 bit TIFF format.
2. 1 bit TIFF file is input into the work flow and the registration marks are fixed and the machine information are incorporated automatically.
3. The plate is then exposed and then send to color-man, Geo-man server by CTP server. These values are used for ink pre-setting on both machine.
4. In the exposing unit, the top cover paper is removed by paper removal device and then the plate is lifted with the help of suckers and placed on the exposing bed.
5. The plate passes through the violet laser head which exposes the image on the plate. The exposing unit has a polygon which guides the laser uniformly throughout the plate.
6. It then goes to the pre-heat unit from where the plate goes into the pre-wash, the plate goes into the developing section, after developing the plate is washed by a water jet to remove the non-image area finally, the plate passes through from oxidation, after gumming, the plate is passed through the drying section.
7. The plate then moves into the bending is punching unit. It has two cameras on the head and foot sides of the plate. This system checks the registration marks on the plate and adjust the plate accordingly. After this, punching and bending operation is performed the plate is ready for production.
Advantages of CTP:
1. High run length – (2 lakhs -3,50,000 impressions).
2. Robust (very heavy).
3. Environment friendly (free from silver plates which cause pollution due to chemicals).
4. Plate supplier (Kodak, Fuji, AGFA.
5. Slightly faster than thermal plates.
6. Plates and chemistry can be exchanged.
Disadvantages:
1. The plates are highly sensitive due to back reflection from laser changes of more dot gain increased.
2. Inconsistency in processing (at developing stage).
3. Needs pre-heat.
4. Dark room conditions.
LATEST TRENDS IN PLATE MAKING
Sleeve technology: the used of sleeve greasy increases the economic efficiency of preparation and handling of the gravure printing cylinders. The base cylinder and the sleeve consists of a ground stainless steel body. Conventional base cylinder different used for manufacturing base cylinder different printing length covered by 1 base cylinder. The aluminum sleeve has high resistance to solvent and ink. No foam or fiber glass re-enforced plastic material is used during manufacturing. The connection between the base cylinder and the sleeve is achieved by mechanical friction without the use of compressed air protection against chemicals between the sleeve and the base cylinder is achieved with an O-ring at each end of the cylinder.
The required sleeve with appropriate metal coating are stored and are available for laser engraving within on day. The sleeve can be used for almost every type or gravure process. High resistance to distortion and heat. Heat also makes the sleeve suitable for anilox applications. Sleeve reduce capital investment and increase productivity. The low weight of the aluminum base which also reduces the cost of storage, transportation and handling. Sleeve are easy to clean and have good resistance to mechanical wear and tear which increases the life span of the gravure sleeves. The integrated bowing compensation of the base cylinder ensures faultless printing image. The sleeve system can be integrated into the existing manufacturing process of gravure printing process. The engraving of sleeves as well as chrome-plating is performed in the same way in conventional gravure cylinder preparation.
The hollow cylinder design extends the range of gelanium sleeves which is mostly used in packaging industries.
Sleeve mounter: the mounter and dismounting of sleeves is performed without compressed with the sleeve mounter. This equipment can mount a wide range of different printing and embossing sleeves quickly and easily. The sleeves are mounted automatically.
Direct laser engraving: this technology has been used since 2001. In this a pulled laser the type and images are directly engraved into the surface by vaporizing the material without any wear. Halftone area of any size and rulling from 60 to 400 lines/cm and the engraving rate is 7,00,00 cells per second.
1. Contact free digital engraving.
2. Improved re-productivity.
3. Flexible shape and screen dots.
4. Broad halftone ranges.
5. Variable engraving in shadow and solids as well as combinations.
6. High ink volume.
7. Good ink trapping and ink transfer properties.
8. Different cell geometry.
9. High engraving resolution at high ink density.
10. Reducing missing out.
11. Reduced ink consumption.
12. Use of inferior quality substrates.
13. Good moisturing properties.
14. Higher productivity.
15. Shorter passing time.
Characteristic of sleeve technology:
1. Specially developed for gravure printing.
2. Pure water surface with metal functioning layer.
3. Different printing length with the same base cylinder.
4. Simple storage and handling.
5. Low weight due to aluminum base.
6. Easy cleaning.
7. Long life span.
8. Integrated bowing for faultless printed image.
9. Integrated sealing of lids.
10. Sleeve change on or off printing press.
11. Base cylinder remains with the customer.
12. Combination of printing and converting sleeve with some press.
13. Cost reduction due to automated process.
14. Lower capital investment due to moderate construction, handling and storage.
15. Low cost of transportation.
16. According to the application, the cost of purchase is about 20-70% less than solid cylinder.
17. Cylinder preparation is done by direct laser engraving.