Printing is one of those inventions that have revolutionized our world and is the most important fundamental practices in our society. Nothing is more essential to civilization intellectually or commercially, than printing. Printing is widely used in our society to pass on information and to decorate objects. Printing is a process for reproducing text and images, typically with ink on paper using a printing press. It is often carried out as a large scale industrial process, and is an essential part of publishing and transaction printing. From newspapers, books, the covers of magazines, of books, dictionaries, clothes, down to the different labels, printing is an art that pervades our entire day. Different techniques and printing equipments are employed for each printing practices. The printing process employed includes, but are not limited to, lithographic, gravure, screen, flexographic, digital, and letterpress. The demand for innovative printing practices has been on a high in recent times. The main segments of printing includes newspaper printing, sheet fed offset printing, flexographic & gravure printing, screen printing etc. There is an enormous growth being witnessed in the printing industry. The emergence of the retail revolution and growing education across the country is acting as a fuel to the growth of the printing industry. The Indian Printing Industry is well established and presently growing at 12% per annum.
Some of the fundamentals of the book are principles of noncontact printing, inks for letterpress and lithography, tests for offset printing, halftone photography, the camera department-its equipment, description of handling originals for halftone reproduction, special processing procedures in making halftones, screen printing, application of screen printing, evolution of offset printing, principle of planographic printing, sheet fed offset machines, quality control in printing, plant and machineries for the printing technology etc.
This handbook is a complete guide to modern printing techniques along with the technology used. It contains printing methods, printing techniques, testing for printing, application of different printing, details of machinery used for printing, quality control in printing, etc. It is hoped that this book will be of immense help to its readers who are new to this sector and will also find useful to entrepreneurs, existing industries, technical institution etc.
1. Principles of Contact (Impression)
The Printing System
Binding and Finishing
Inks for Letterpress and Lithography
2. Principles of Noncontact Printing
Impactless printing system for variable printing
3. Coated Grades and Commercial Printing
Coated and Commercial Papers
Coated Paper Properties and Use
4. Tests for Gravure Printing
Gravure Print Testing
5. Tests for Letterpress Printing
Uniformity for paper surface
6 Tests for Offset Printing
7 Line Photography
Making a Negative
The Process Lens and its Care
Care of Process Lenses
Types of Contrast Emulsions
Placing the Lens, Arc-lights and Ground Glass
Obtainable Illumination of the Copy Board
Focusing the Image Same Size
Organization of the Dark Room
Preparing for the Development Operation
Making the Exposure
Making a Negative from Black and White Line Copy
Developing Action and Chemicals
Effect of Time and Temperature
Reduction and Enlargement
Producing Negatives from Fine Line Copy
Making Negatives with Filter and
Dry Plate Negatives and Paper Negatives
Clearing the Negative
Producing Negatives from Screened Copy
8. Halftone Photography
Making a Halftone with a Glass Crossline Screen
Making a Halftone with Magenta Contact Screen
9. The Camera Department-its Equipment,
Facilities and Tools
The Arc Light
Types of Arc Lamps
Light from an Arc Lamp
Types of Carbon Used
Carbon Arc Characteristics
Illumination Requirements of Arc Lamps
Spectral Characteristics of Arc Light
Hints on the use of Arc Lamps
Exposure Matters and Light Integrators
The Exposure Matter
The Light Integrator
The Photographer's Tools
Safety in the Camera Department
10. Description of Handling Originals
for Halftone Reproduction
Originals for Halftone Photography
Problems Association with Various Kinds of Copy
Pastel Drawings, Crayon Drawings and Oil Paintings
Dye Transfer Prints
Grouping of Originals
Grouping of Originals by Size
Grouping of Originals by Tonal Range
11. Focusing the Camera
Ground Glass Focusing
Focusing with Calibrated Tapes
Focusing with Numerical Counters
12. Viewing the Halftone Image
Evaluating the Halftone Negative
The Highlight Area
The Middle Tone Area
The Shadow Area
Tonal Range on the Press
13. Shooting Halftones with the Glass Screen
Focusing the Screen
Screen Ruling and the Wavelength of Light
Screen Aperture to Screen Distance Ratios Setting the Screen 208
Setting the Screen with a Calibrated Wedge
Screen Setting by Visual Focusing
Screen Setting by Camera Test Exposure
Single and Multiple Exposure System
Some Observations on Various Stop Systems
Miscellaneous Exposure Information
Exposure Changes for Different Magnification
Angular Variation of Light Intensity
The Use of the f64 Stop
14. Special Processing Procedures in Making Halftones
Etching and Chemical Reduction
The Physical Action of Reduction
Special Hardener-for after Treatment
of Films and Plates
15. Special Halftone Camera Techniques
Light Sources for Screen Tints
Light Variation with Angular Distance
Development Factors Suggestions for Better Screen Tints
Dropout Negatives (Highlight Negatives)
Large Aperture Exposures
Supplementary Line Exposure
Overlay Mask Method
Preparing the Copy for a Duotone
Shooting the Primary Negative
Shooting the Secondary Negatives
Variations on a Theme
The Indirect Process of Making Halftone Images
Procedure for Shooting the Continuous-Tone Negative
Hints for the Indirect Process
16. Screen Printing
Select Correct Screen Printing Fabric
An Antistatic Stencil Mesh
Screen Printing Frames
The Manufacture of Diapositives
17. Application of Screen Printing
Screen Printing Accessories
Chemicals Used and Formulations
Common Faults in Screen Printing
Automatic Screen Printing Machine
Screen Printing on Different Surfaces
Inks for Screen Printing
18. Offset Lithography
Origin and History of Lithography
Evolution of Offset Printing
Offset Machine Construction
Pre-Make Ready and Make Ready
Setting the Machine for Operation
Origin of Planography
Principle of Planographic Printing
Direct Printing Process
Offset Printing Process
20. Materials, Tools and Equipments
Tools and Equipments
Automatic Proof Presses402
Qualities of a Good Proof
21. Sheetfed Offset Machines
Names of the machines
Sheet feeding mechanism
Functions of blowers
Functions of the blower foot
Sheet lifting and forwarding
Sheet Insertion and Transfer
Adjustment of Rollers
Different Dampening Systems
Cleaning of Dampeners
Construction of the machine
Working on the cleaning machine
Adjustment of Cylinders
Advantages of Both Principles
22 Web Offset Machines
Main Parts of Printing Unit
Ancillary Operations by Delivery Unit
23. Colour and its Reproduction
Terminology Related to Colour
Mixing and Matching of Colors
Sequence of Colours in Printing
24. Quality Control in Printing
Paper and Board
27. Creative Frees Printer
Popular Product with Powerful Appeal
Topical Information Mix
Individual Brand of Success
As Horst Brostler Explains
Flexibility in Many Spheres
Super-Wide Rotogravure Presses in Big Demand
Brownie points of gravure
New Techniques for Handling Giant Reels
Bigger Core Diameteres Needed to
Handle Higher Speeds
A Host of Optimised Details
Light Weight Guide Rollers
Process Computer Systems With Visualisation
28. Shaftless Spearheads Expansion
Economic Effciency-the Clincher
Eightfold Increase in Sales
Confidence in KBA Technology
Commissioning to a Tight Schedule
29. Plant and Machineries for the Printing Technology
30. Manufacturers & Suppliers of Raw Material
31. Indian Printing Associations
Web Offset Machines
The machine on which a web of paper or any other material is continuous feeding is printed, is called web offset printing machine. In this process, the unwinding of paper reel is a continuous feeding system from start to the delivery part of the machine. Web-fed offset printing machines were invented to overcome the problems of sheet-fed single and multi-colour printing machines. The registration and carrying of sheets from pile board to the delivery board has also complicated mechanisms.
Printing machines of such principles are designed into different units. Number of units of a machine are designed according to the different requirements of the nature of jobs to be printed. The size of the web to be fed and printed in the machine differs according to the size of the machine and jobs. Web offset machines are being used for printing books, newspapers, magazines, stationery materials etc. there is very slight difference in all the above job suited machines.
Generally, web printing machines are made in one to four units. The whole machine may cover a floor length of 8 to 13 meter and may be 2.5 to 3.5 meters high. Therefore it is necessary to divide the important parts of the huge machine into some groups for detailed study.
Design of web machines
Web-fed offset printing machines are designed to print both sides of the paper at a time. For this purpose, these machines are made into one to four units. Each unit of the machine prints both side of a web in one colour.
In this way, each unit of a web-fed machine requires one reel stand to hold its reel and print it on both sides but generally, it is not so. Most of the multi unit machines have one or two extra units, Which do not need separate reel stand. The extra units are used to print another colour on any side of the web. Therefore it is obvious that one unit of web-fed machine must have two plates, two blankets and two impression cylinders. In the same way, each unit should have two inking and two dampening systems.
Besides the above main features in one unit, there is web controlling device which look like another unit. The machine is so designed that all the webs of each reel fetch first in this unit and get proper tension. Thereafter all the webs are forwarded into their printing units. After printing all the webs through different units, they reach overlapping on the kite former.
The end par of the machine is called delivery unit. Here all the webs are folded once, twice or thrice as required.
Some of the machines also have slitting, cutting, stiching and other attachments. In this way, these machines produce complete printed books or magazines at the delivery part. Some of the machines produce 8 to 16 page sections only folded and slitted. The covering and stitching operations are performed by hand or seprate machines are used.
From the above description, it is obvious that web-fed offset machines are so designed that they cover a large area on the floor. Hence the driving mechanism of such machines also has large field to study in detail. The main driving features are described as under in different groups.
Web offset machines containing three or four units require nearly 40 horse power motor to drive all the parts of the machine. The electrical arrangements to control and maintain the operations of various parts are provided in one or two large almirahs. These almirahs are fitted about two meteres away from the machine.
The almirah contains main electric switch controls the current coming from out side. Beside, this, separate electrical arrangements in the almirah are provided for driving the main motor of the machine, compressor, pumping dampening water and other functions. Various electric wires go out of almirah and are assembled into the consol (control) box. All the wires have different switches and are connected with the operating parts of the machine.
Most of the web offset printing machines have a controlling box fitted nearly 1.5 to 2 meter away from the machine. To controlling box is installed near the delivery part of the machine to easily check and control the quality of printing. All the operating and controlling connections come in it from the electrical almirah. The same are then joined with the functioning parts of the machine.
The controlling box has various switches to control the functions of the machine which are as follows:
- Lock of the machine: It has a locking point to lock and open all the functions of this box. The locking and opening of the box is done through a key like the ordinary locks.
- Operating switches: One top of the box, these are more than 20 to 40 switches to inch, show, run the machine etc. It has also a switch to stop the running machine. Besides these, the box has two switches on for reducing and another for increasing the speed of the machine. Some of the machines have only two switches for this purpose. The pressing of the switches increases of reduces the speed of the machine.
Generally, web-fed machines are made for more than 10 to 30 thousand impressions per hour and run at nearly half or three fourth of the maximum speed. Separate switches are provided for compressor motor, impression and off, reel tear prevention and water dampening. One switch is also there for reverse running of the machine. In this way, various switches are for different functions, adjustments and controls.
- Counting machine: Some of the web offset printing machines have only one counting machine in the controlling box or at delivery part. Generally, some of the controlling boxes have two counting machines along with their switches.
Only one counting machine is used at a time and another is stopped. The counting machine of the controlling box receives signals from the counter fitted at delivery part of the machine. These counting machines count the number of copies being printed. The counted numbers of the machine generally multiplied by 2 or 10. It becomes very easy to see the counter and stop the machine.
Different designs or marks are also made near all the switches do denote the function of the switch. Small web offset machines equipped with one or two units or one side printing units have small controlling boxes fitted in the body of the machine near the delivery part.
To drive the large size web offset printing machine a heavy horse power main driving motor is provided. Nearly 30 to 40 H.P. motors are used in these machines. Four to six belts are fitted on the motor pulley, which join with the main driving wheel of the machine. There is a thick shaft in the center of the driving wheel. Its other end has a gear whose teeth mesh with the gears of each unit driving shaft. Each cylinder of the printing unit revolves through the gears fitted in the driving shafts of the unit.
At each unit, the driving shaft has a helical gear to revolve the cylinders and inking rollers. All the functions of the machine are performed through these gears attached with the main driving shaft.
Such type of huge and large machines have many number of frictional parts. Obviously they need proper lubrication. Web offset printing machines are equipped with pump mechanism. On top of each unit gear box has continuous lubricating systems through pump mechanism. On top of each unit there is a small glass cap to see whether he oil flow is continuous or not.
Beside, all unit gear boxes and other frictional parts are also joined with the oil pipes. The mobil oil flows through the tubes and reaches upto the frictional parts of the machine have spartat small oil tanks. These oil tanks are filled with mobil oil when needed.
Web-fed machines have a number of small and large bearing. Each tension roller, forwarding rollers, oscillating rollers, some of the steel drums and cylinders have bearings, which require lubricating daily or after a week or two. Good quality grease should be applied in the bearing by a grease gun or by hand and oil by oil gun or by oil can. Large sized 4 units web offset printing machines requires nearly 5 liter mobil oil in each unit gear box after a week or two. Generally, the gears of the cylinders have continuous showering of mobil oil through pump circulating system.
Reel stand unit
The first and very important part of a web-fed offset printing machine is reel stand. The side walls of the stand are fitted on the floor. Different types of reel stands are named as single, double and revolving systems. Single reel stand has only one reel, double stand has two reels and revolving type of stand may have two or three at one place. Revolving reel stand has automatic reel changing device. A reel of paper is suspended in a shaft and fitted on the stand. It has a knife, gumming brush and paster arm for changing new reel automatically in slow running.
Following are the main working parts related with the unwinding and pre-infeed system of a reel.
A thick steel rod is inserted in the central hole (core) of the reel. There is one wedge shaped coupling at one side and a round plate on the other side of the shaft. One side of the shaft has a screwed part in which a round plate is inserted permanently. Two wedge shaped couplings are inserted in the shaft, one near the round plate and another at the opposite side of the reel shaft. In the opposite side, wedge shaped coupling is loosened or tightened in the shaft by an alen wrench for adjustments.
First of all, the shaft is inserted in the hole (core) of the reel. The thinner side of the opposite side coupling, is inserted in the hole of the reel and hammered a little. After that it is tightened by alen wrench. Now the round plate is turned in shaft's screw, the shaft is pulled through this and the reel is firmly tightened on the shaft between two wedge shaped couplings.
The round plate has two or three holes to insert the pin of the lever, which is used to tighten or loosen the reel in the reel shaft. The round plate acts as a puller to pull the reel in the required position. The reel is very firmly tightened or easily loosened by turning the round plate.
One more round but wide wheel is fastened at one end of the reel shaft. The brake jaw is tightened on this wheel. The brake wheel is made of smooth steel nearly 15 cm wide and 15 cm in diameter. The brake is loosened by pressing a paddle and then the reel with shaft is taken out. The brake wheel and the puller plate do not come the reel shaft, when the shaft is taken out after finishing the reel.
Through the continuous brake system, the web does not become loose when the machine is stopped. Besides that, the web is pulled towards the printing units in required tension.
Lifting and lowering the reel
The finished reel of the shaft is lowered for changing the new. The reel with the shaft is lifted-up after tightening the reel in the shaft. This function is done by rotating a large wheel. The brake wheel is set into the brake jaw and the other end of the rod is set in its place and tightened firmly.
The lock of the brake wheel is tightened with a screw and bolt by hand. After that the reel along with the shaft is lifted-up by rotating the wheel.
Centering the reel shaft
Web-fed machines have a switch board for different adjustments of the reel after fixing it on the reel stand. There are switches for shifting the reel both sides. One switch is provided for slow and one for fast running of the machine. A small motor is separately provided to perform these functions. This motor is fitted near the opposite side of reel shaft.
When the reel centerting or shifting switches are pushed, the mechanism fitted on the opposite side of the reel shaft pushes or pulls the reel shaft. In this way, the reel shaft as well as the reel is adjusted wherever required. This mechanism is meant for adjusting the side margin in the web in order to achieve corrent registration.
After the reel is set in its proper place, one end of the web of the reel is taken out and forwarded through an idler and tension steel roller. The very important roller which exerts first tension in the web is called tension roller. The web is set under this roller. This roller continuously vibrates up and down. The web remains in required tension from beginning to the end. The tension roller adjusts the speed of the revolutions of the reel gradually from starting a new reel and upto the last.
A new reel has a large diameter and it gives greater length of web in one revolution. Gradually, the diameter of the reel reduces and it gives less length of web in one revolution. Due to this factor, the tension of web may have to be adjusted every time which is not possible. When the speed of the web changes in the running time, the problem of loosening or excess tightening of the web occurs. Swinging or vibrating type or tension roller is fitted in each web-fed machine to overcome the problem.
Checking of momentum
In older web-fed printing machines, there is a large leather belt (strap) which continuously presses nearly the half of the peripheri of the reel. This belt generally goes towards the feeding side in the similar speed when the diameter of the reel is reduced. This belt was used for giving continuous brake to the reel to maintain even tension in the web and also to control the momentum of the reel.
The function of the idler and tension roller is done through a long spring attached with the reel shaft towards the brake system. Besides the main tension roller, there are five to six other rollers to maintain the tension of web as directed by the tension roller. The web is set on all these rollers to produce even tension at slow and high speeds. Each of the roller is made of steel and revolves freely on its bearings by the friction of the moving web.
All these systems and mechanisms are made for all the reel stand units. The machines using three or four reels, have all the same arrangements for each reel stand.
Automatic reel changing
The manual reel changing process requires stoppage of the machine until the next reel is set on the stand. Therefore, valuable time on the machine is saved by changing the reel automatically.
For automatic change of the reel, a new reel is fitted in its shaft and set on another reel stand which is fixed similarly as the running reel. When the reel is about to be finished, the following operations are done by the operator.
- The new reel stand is slowly revolved towards the finishing web. The new reel and the finishing web are maintained parallel in the splicing position. Nearly 10 to 15 mm distance should be maintained between the new reel and the finishing web.
- The speed of the running web and the new reel are operated in the similar speed. Some of the machines are equipped with variable speed motor in each reel stand, which is necessary for correct splicing and controlling even tensions in both the webs. The cline system is very suitable for perfect jointing of the webs. In this system, there are bands which press the spliced part of the web in running position. In the cline system, specially prepared powder adhesive is sprayed on the splicing part of the web. The powder becomes an active adhesive after spraying a specially prepared solvent.
- In the third process, automatic gumming in the splicing part of the web is done. The gumming is done by a brush. The whole width of the splicing web is gummed automatically.
- The last and fourth process is to cut the finishing web by a knife. The cutting of the web is so set that the knife should come into action after one revolution of splicing the web. After cutting the web, its braking device stops the reel stand to avoid unwanted opening the remaining reel. Before cutting the used web, the spliced web is pressed by an arm to ensure perfect joining of both the webs.
Web control unit
Some of the machines do not have this system as a separate unit. They have larger number of forwarding and tension rollers before the web is fed into the printing unit.
Generally, such type of printing machines have crosswise or angular tension rollers and web transfer rollers. The latest web offset printing machines, which have blanket to blanket printing principle, are equipped with the system of separated web control unit.
The purpose of this unit is to create sufficient tension in the web before it enters the printing cylinder, because the web may have some looseness when it rewinds and forwards to the printing units. All the printing webs have to enter this unit through a separate set of tension cylinders. The main parts and adjustments of this unit are as follows.
Tension control unit: This is the unit through which all the webs have to enter for the removal of any type of looseness and to reduce the speed. It has two main cylinders. One is made of steel and the other is rubber covered.
The steel cylinder is driven through P.I.V. gear system. The rubber cylinder revolves on its bearings. The web enters between these two cylinders. The web forwards under the pressure of both these cylinders. Thus the speed of the web is reduced. It is necessary that the speed of the web should be reduced before it enters the printing unit.
P.I.V. gear system: The tension of all the webs is reduced or increased through this system. It has various gears of different sizes. The mechanism is assembled in a box. The different switches, adjusting points and tension denoting dials are set on the box.
The speed of main geared steel cylinder is controlled by this system. It is fitted before entering the web into the printing nits. P.I.V. gear boxes are fitted in most of the web-fed printing machines for each web. The machines using three webs for printing at a time, are equipped with three P.I.V. system separately.
The speed of the web has to be reduced before the printing units, because the web gets a higher speed after entering the printing units. Therefore, by reducing the web speed before the printing units, the tearing of the web is prevented. The adjustment of P.I.V. system depends on the tensile strength of the paper. Hence it is abvious that the speed of the P.I.V. geared steel rolled has to be set according to the tensile strength of the paper. The tensile strength of paper is determined by tests and the P.I.V. gear system is adjusted according to that.
The tension of the web is measured in the unit "Kg" per 15mm. An approximate average of maintaining of tension in the web through out the machine is given as under:
|1. ||Infeed unit tension ||20-25 kg. per 15 mm|
|2. ||Printing unit tension ||25-30 kg. per 15 mm|
|3. ||Delivery unit tension ||30-35 kg. per 15 mm|
According to the above facts, the tension in the web should be adjusted through the P.I.V. gear system to prevent the tearing of the web.
Some of the machines are equipped with even tension in all the webs. Speed of each unit is reduced or increased by a adjusting wheel. The speed adjusting wheel and round plate are fitted in the outer wall of the unit to decide the tension of the web. The round plate is marked from 0 to 80 numbers. A long needle is fitted in the center of it which shows tension per meter in the web.
A steel strip like blade is fitted along with the rubber roller. The rollers set in contact with P.I.V. geated steel roller. The distance of this blade from the rubber roller is so set that the web should slightly touch it. The purpose of this blade is to clean the dust, dirt and other unwanted materials sticking on the surface of the web. Any unwanted material in the web will reach upto the blanket and plate cylinders which may bladly affect the quality of printing.
The tension controlling unit has a photo-electric cell fitted near the P.I.V. controlled steel roller. The machine is stopped automatically if the tearing and other disturbances occur in the web.
Web infeed system
In multi-web offset printing machines, two or more webs are used for printing. If all the webs are to be printed in a single colour, the most distant web is fed into the first printing unit, the second distant web is fed into the second unit and so on. For multi-colour printing, the web enters the next unit and then reaches upto the kite former.
In the distance between the tension controlling unitand printing unit, the web moves forwd on steel forwarding roller. These rollers revolve on theirbearings through the pulling action of the web. Generally, most of these rolles are covered with steel grained strips. In some machines, these rollers have vaccum suction to stick the web along it. It also maintains the even tension all through the web upto the delivery unit.
The tension and forwarding rollers may be lifted and lowered upto some extent. The tension of web is reduced and increased by this adjustment.
The steel grained rollers pull the web and prevent the set-off or unwanted markings in the printed web. One or two grained or plain rollers are fitted there before entering the web into the printing unit. After printing, the web forwards on a number of grained rollers and reaches the kite former.
Web transfer rollers
These rollers are also called turner bars. The machines which print only one side of the web by its one unit, are generally equipped with web transfer (turner) rollers. For printing the other side of the web, it has to enter the next printing unit or the next set of printing cylinders of the same unit.
The blanket to blanket system of printing machines has straight web feeding device. It does not need the change of side of the web. The web forwards through the grained rollers set on top of the printing units or between the units.
The different types of web turner devices are given in the diagram.
Angular turner bars
In this system, the straight web is turned by right angle on the steel bars. All the bars are fixed at 45Âº angle instead of straight travel direction of the web. The side of the web is turned through this process to print the other side of the web. These angular turning bars are set after each printing unit.
Besides it, some of the machines have blowing are holes in the turner bars to create cushion of air between the web and the turning bars. The air cushion minimizes the friction of the web and prevents the breaking and other troubles.
Some of the machines have slitter device after printing and before reaching the web on the kite former. By this device, the web is halfed and both the webs are turned into the same path through turner bars. The webs forward into the folding unit. This process requires two turning bars to turn both webs into correct position.
Principally, all functions of a printing unit of web-fed machines are nearly similar to the sheet-fed machines, excepting some of the parts which are designed according to the path of web direction. Some of the controls are also of different nature. Most of the web offset machines are equipped with various controlling switches fitted on the working side of each printing unit. There are switches for stop, start, run, slow, operating the dampeners, ink controlling and reverse running.
Different designs of units of web-fed offset machines may be divided into following groups for detailed study.
One side printing units
The older principle of printing units is one side printing. Such type of printing machines have been designed into 2 to 4 units. Each unit is made into separate sections but joined with each other through continuous gearing systems. It is also called in line or open system of printing units.
Each unit has one plate cylinder, one impression cylinder and one blanket cylinder as main parts. Besides it, each unit consists of one inking and dampening system as equal. Printing by magnetic inks, carton printing and moisture-set inks may be done on these machines. Continuous stationery works in perfect delivery may also be done on such type of machines.
One side of the web is printed by one unit and the web is turned through the turner bars. After changing the printed side, the web enters in to the other unit and is printed on the other side. For multi-colour printing on the same side, the web does not turn but it may be set to go ahead straight to the next unit.
Blanket to blanket printing units
Such type of printing machines are also called both side printing system of web offset printing unit. The web forwards straight into all printing units.
These machines do not have impression cylinders. Only two plate and two blanket cylinders are fitted in one unit. The web passes between two blanket cylinders and prints both sides of it in one insertion. In this principle, and extra plate cylinder can be set to print spot colour. Generally, heat-set inks are used in these machines. Inking, dampening and web forwarding systems are almost similar to the others.
Satellite system of units
It is named satellite system of printing units due to its design of path of web. The web is printed through half of the circumference of the impression cylinder.
In the next round, the web is printed through the other half of the circumference of the impression cylinder. After printing one time, the web passes through the turner bars and the side of the web is changed. After that the web enters the printing unit for printing on its other side. Such type of web-fed offset printing machines are designed with only one large impression cylinder. Two, three or four blanket cylinder are fitted around the common impression cylinder.
All the blanket cyliners are set after some distance from each other but all the them are set in contact the common impression cylinder. All the blanket cylinders are equipped with plate cylinder, inking and dampening systems. These machines have a drying cylinder which helps in the drying of ink after an impression and before turning the web.
In this way, the printed ink of one side is perfectly dried before turning. This is necessary because the wet ink of the web may create smudging or set-off due to the friction with the turning bars. Satellite system of web-fed offset printing units has a device to disconnect any unit of blanket and plate cylinder. It facilitates the requirement of selection of multi-colour or single colour jobs.
Main Parts of Printing Unit
Printing unit of web-fed offset machine consists of three main parts i.e. plate cylinder, blanket cylinder and impression cylinder. These three main parts of a printing unit are also equipped with a number of devices for different purposes. Detailed description of all the main part is given as follows.
The plate cylinder of web offset machines hold printing plate. The driving mechinism and bearers etc. are almost similar to the sheet-fed machines. The difference is in the gap of both cylinders. The web offset machine's plate cylinder has a very small gap only just to fit the plate clamps at its both edges. It has plate tension screws and tightening bolts. Cylinder gap in web machines is nearly three cm.
The gap of the plate cylinder causes white margin on the head and tail of printed pages after each impression. Both the longer edges of plate are screwed with two flat clamping of the plate is done out side the machine. After that both the clamps along with the plate are set into the cylinder gap and tightened with the bolts.
In some of the machines, the plate does not need any bar or strip. Only the edges of the plateb are bent and fixed into the cylinder gap. The plate edges are tightened and stretched as usual. A long steel strip like a blade is fitted just near the plate cylinder. This plate is a safety device to prevent the operator's hand from accident when dampening or gumming, while the machine is running.
There is a device for the plate cylinder in lateral and peripheral direction. It has two handles or levers for shifting the state of plate cylinder. Some of the machines have line marks around the projected shaft of the plate cylinder. These marks are very successful for registration purpose. The line marks are considered for shifting the cylinder into lateral and peripheral direction.
Some of the latest machines have a device to push or pull only towards the operating side of the plate cylinders. The one side tilting of the plate cylinder is made to equalize the margin of the plate edge. After correcting the edge of the plate, registration is done as usual.
Contact of plate cylinder with the blanket cylinder is made through a switch. The function of it depends on the air pressure of the compressor. This device helps the insertion of the web without its receiving any impression from the plate cylinder.
Besides it, some of the machines do not require to throw off the plate cylinder. Only inking and dampening systems are thrown-off. In this way that plate cylinders are not supplied with water and ink. The web inserted into the printing unit goes forward to the next unit without printing.
Plate edge bending
For bending both the edges of the plate, an ordinary manually operated machine is used. It is equipped with a steel table and a flat part on which the bending plate is placed. Two screws on one side are raised are used as a guide for definite margin in the plate. The crews are removed and both side pressing handles are dropped. By this, one edge of the plate is pressed and bent. Both edges of the plate are bent as required in the same way.
Generally, the printing plates for web machines does not require to be punched as in the sheet-fed machines. The excess length of the plate and the corner edge are also cut by the bending machine.
The mechanical features of the blanket cylinder are almost similar to the plate cylinder. Its gap, driving gears and bearers have this same purpose.
The blanket is cut into a definite size. The larger edges are punched and then it is placed between the gap of the clamps. Then the clamping strips are screwed on both longer edges of the blanket outside the machine. The screws are provided at 2.5 cm distance nearly. The blanket is cut slanting under all four corners to avoid creasing, when it is fitted on the blanket cylinder.
One edge of the blanket is clamped and tightened in the gap and then the machine is slowly run. When the cylinder gap comes back in position, the other edge is also clamped. In the end, the blanket clamp is tightened and tensioned by the screws and bolts. Generally, most of the bolts are tightened by "L" wrench.
Blanket to blanket web printing machines have two blanket cylinders in one unit. Both are set in contact with each other. Both the cylinders revolve through their gears. The web passes through both the blanket cylinders. In this way, both sides of the web are printed at a time. The surfaces of both the blanket cylinders receive impression by the plate cylinder. Then the impression of the blanket cylinder is transferred on both sides of the web at a time.
One side printing machines are equipped with the impression cylinders in each unit. In this way, such type of web printing machines are equipped with all three (Plate, blanket and impression) cylinders similar to the sheet-fed machines.
The web passes between the blanket and impression cylinder to receive print only one side of it by one unit. Such type of web machines have four units to print both side of the webs of two reels. The impression cylinder is a plane steel cylinder having driving gear on its one side. The cylinder revolves along with the blanket cylinder due to the meshing of this gears.
The impression cylinder may also be called pressure cylinder because its purpose is only to exert pressure on the blanket cylinder. Due to its pressure, the reverse image of the blanket cylinder made by the plate cylinder is transferred on the web. The mechanism of bearers, gears and other parts are generally similar to the sheet-fed printing machines. The impression cylinder does not need gap like the plate the blanket cylinders.
The inking system of web-fed offset printing machine is also similar to the sheet-fed machines. Ink duct, vibrator, grinders, distributors and all the plate inking rollers are similar to the similar to the sheet-fed machines in principle and function. Only the number of rollers differ from the sheet-fed machines. Web-fed machines are equipped with a greater number of rollers for all inking functions. Due to very fast speed of web-fed machines, they require fast distribution of ink.
Therefore the inking rollers, riders and oscillators are made of different diameters in web-fed machines. Even then the oscillating distance of all the oscillators differs from each other. The uneven oscillating distance causes fine and fast distribution of ink.
In web-fed machines, the ink duct fountain roller revolves continuously. The vibrator touchesthe duct roller according to the required ink for printing only one impression. Automatic clearing device is fitted with each set of printing unit. It can be removed from the machine and again fitted very easily when required.
The latest web-fed offset printing machines have automatic ink control devices. The "Inkatron" and "idotron" etc. attachment are fitted of automatic control of the ink density.
Each inking unit has safety guards to avoid the web from touching the inking rollers. In speedy and latest web offset machines, the setting screws of the ink duct fountain are fitted in a half round cover. Each screw is made as in the shape of a knob. Each knob is fitted in a cut part. The side strip of the knobs marked from 1 to 10 numbers to locate for adjusting the ink flow. The blanket to blanket machines are equipped with two inking systems each set of printing cylinders. Each plate cylinder requires one complete set of inking system similar to the sheet-fed machines.
Heat-set inks need chilling device. A steel roller is provided for this purpose. Each printing unit has this roller. There is one pipe for entering the chilled water into the steel roller. Another pipe is there to drain the water. In this way, the chilled water circulates in the roller and the surface of the roller is chilled.
Generally, the chilling steel roller is of oscillating type. These are controlled similarly to the inking rollers. The chilled roller prevents the drying of ink on the inking when the machine is stopped for mark ready pruposes.
Between the printing units
Besides all the above designs of web-fed printing machines, there is a small platform, made of steel covers, to stand and to sit for working between the printing units. The operator stands on the steel cover to adjust and control the ink duct, dampening unit and preparation of plate and blanket cylinder. In one side of the printing units, the web path is made under these covers to feed one unit to other. These covers may also be opened for inserting the web from one unit to other.
Some of the machines have various set of grained steel rollers to pull and forward the web between printing units. The grained steel rollers do not create set-off due to the wet printed ink. The grained flexible steel strips and fitted on the roller which revolve on ball bearings.
The necessity of moisture on the non-image areas of the printing plate, is an obvious necessity similar to the sheet-fed offset printing machines. In web offset printing machines, the dampening system consists of a water fountain as fountain solution tray. The dampening solution remains at a constant level in the fountain. The water in the fountain comes through a pump system. In the side opposite to the operator, there are small water tanks fitted with each printing unit.
A thick rubber tube is connected between the fountain and water tank. The water (dampening solution) of the tank is pumped to the fountain by the pump system. The working side of each printing unit has a separate operating pump, its motor, switches and other mechanism. Each one side printing unit has one dampening system. Two dampening systems are fitted in both side blanket to blanket printing units.
A fountain with a roller, distributors and plate dampeners are the main parts of the dampening system. The system does not have a vibrator, instead of it, the fountain round. Both the ridges of the roller touch the distributor in turn. Such fountain roller is used as vibrator in this process.
The fountain roller of web offset machines is made of rubber cotton tape is wrapped on the whole rubber surface and sewn at both ends. The cotton tape absorbs the water and when its ridge touches the distributor, water is supplied automatically. In this way, two ridge of a fountain roller supply water to the printing plate continuously.
The dampening solution is provided in a storage tank from where it is supplied to the fountain by means of a pump system. An exit pipe in the fountain continuously drains-off the solution back to the storage tank. In this way, fresh solution is always available in the fountain and is maintained at the desired level.
To reduce and increase the quantity of dampening water, the revolution of the fountain roller are reduced or increased by two different switches. There is a small separate motor to operate all the functions of the dampening system.
Spray fountain system
The latest and very fast web offset machines have spray fountain system. It has a fountain or tray to store the dampening solution. The fountain water remains at constant level. A long round brush is fitted on the fountain steel roller. When the fountain roller revolves, the water applied on its surface, is taken by the hairs of the round brush. Hence the brush applies water on the distributing steel rollers. The fountain roller and other distributing steel rollers are chromium plated. Chromium is water receptive. Hence the train of dampening system remains in correct and continuous process.
In this process, the brush is set at slight pressure on the fountain steel roller. Therefore the hairs of the brush spray water on the fountain roller surface. The sprayed drops of water moisten the steel distributor. Various steel distributors are set in contact with the plate dampening rollers and thus the plate is damped properly.
The fountain has a cover to increase and reduce the quantity of water. The fountain cover has various adjustable strips in the fore edge. Each strip may be pushed forward and pulled back through its separate knob. The strips are pushed to reduce or stop the sprayed water from reaching the steel distributor. The excess sprayed water is stopped by the cover strips and returns back into the fountain. Each pushed strip affects only the front surface which is about one inch wide.
Some of the machines are so equipped that the hairs of this type of fountain, spray water directly to the plate of the plate of the plate cylinder but this does not work satisfactorily. General reduction and increase of water all over the fountain, is adjusted by increasing or reducing the revolutions of the brush. The speed of its revolution is controlled through a manual switch fitted near the dampening system. Some of the latest machines have automatic control switches for increasing and reducing the revolutions of the dampening brush. The switches are set in the automatic control box.
When all the webs have passed through the printing units, all of them forward on the grained steel rollers, and reach the delivery part of the machine. The advantages of grained forwarding rollers are:
- To avoid set-off in the web due to the wet printed ink.
- To help in pulling and forwarding the web at an even speed.
- To avoid air gap in the moving web which may cause creasing defect.
The next operation on the printed web is to set and dry the newly printed ink to prevent set-off.
Ink drying unit
Generally, the sheet-fed offset printing machines, do not have any extra attachment for drying the printed ink because the inks used in sheet-fed machines dry by cold-duct process. Besides it, most of the web-fed offset printing machines have a drying chamber as an extra attachment. Some machines have a device for blowing hot air on the printed web for setting and drying the ink.
The latest high speed machines have a large drying chamber, this chamber has a device to blow hot air on both sides of the printed web. For creating hot blowing air into the drying chamber, diesel or liquid petroleum gas (cooking gas) is continuously burnt into its firing box. The firing box is joined with the pipes coming from the compressor unit.
The drying chamber has various pipes joined with the firing box. These pipes have holes through which the hot air coming from the firing box blows continuously, when the drying chamber is closed, the air blowing pipes on the upper and lower side blow hot air.
The hot air blowing pipes are set about 4 mm away from both sides of the web. The hot air blowing pressure form both sides of the web should be perfectly equal. The uneven pressure may cause uneven drying of printed ink or the web to be damaged. The hot air generating box has two main pipes. Each pipe has a stop cock with a pressure measuring gauge. The quantity of hot blowing air and its pressure may be increased or decreased through the key of the stop cock.
The temperature of the heating chamber is maintained at 170Âº to 210Âº celcious. The temperature is reduced for thin ink films and increased for thick films. Consideration should also be given for the coverage of ink on paper. Excess temperature may damage the printed paper. Low temperature will not dry the printed ink sufficiently.
For operating the heat drying chamber, the control console has different switches and adjustments. Opening of the heating chamber for inserting and passing the web through it and all other controls are adjusted by the switches of the control console.
The pressure of hot air on both sides of the web is generally maintained at 10-12 kg per square cm. It may be changed according to the bursting strength of paper and the structure of fibers of the web.
After passing the printed web through the heating chamber, it is necessary to cool it. The surface of the web, structure of the fibers and the newly printed ink densit yare affected when the web is passed through the heating chamber. All these factors of the web become normal when the web is passed through the chilling unit.
There are two large cylinders for chilling the web. Both cylinders are set away from each other. One side of the web passes on contact with one cylinder and the other side with the second cylinder. There are six rubber wheels set for pressing the web against the chilling cylinder to ensure perfect contact of the web with chilling cylinders.
Droplets of water are formed on the surface of the chilling cylinders. Due to condensation of atmospheric moisture on the cold surface of the cylinder through which ice cold water is circulated. Outer surface of the cylinder is highly glazed. The chilled drops of water vapour on the glazed cylinder normalize the heated web by this process. The cold water enters the cylinder towards the operating side and drains out towards the gear (rear) side of the machine.
Such machines are equipped with separate air conditioning plant. The air conditioning plant is maintained by the engineering department, huge and large steel pipe are carried up to the different chilling units of the web offset printing machine. The steel pipes are joined with all the main pipes. Stop cocks are fitted at various places to increase and reduce the chilling capacity. Normally the chilling unit is maintained at a temperature of 15 to 15.5ÂºC.
Silicon applicating unit
This unit of the machine is fitted just after the chilling rolls. There are two fountains each fitted with a steel roller similar to the water (dampening) fountain. The fountain is filled automatically through a pump device.
Nearly half of the steel roller is always immersed in the silicone solution. Both the sides of the web come into contact with both the steel rollers of the silicone fountain. In this way a layer of silicone solution is applied on both sides of the printed web. The surface of web and its fibers are changed due to the heat and chill treatment. The layer of silicone makes the web surface glazed. The web surface on both side looks as polished. This unit is mostly used in web offset machines which are specially made for printing magazines and periodicals.
The silicone solution is available in plastic cans. This solution is of milky white colour. The solution supplied by the manufacturer is mixed with water in the ratio of 1 : 1. The proportion of water depends on the surface of the web. Rough surface of paper requires less dilution with water while smooth surface requires less silicone.
When all the webs have been subjected to the processes described earlier, to the folding unit. This unit is also called the finishing unit of the machine because the web is converted into the form of book through this unit. This unit has more complicated mechanisms and is equipped with different attachments. Various operations of folding unit may be classified as under for detailed study.
After printing and post treatments, each web is forwarded on separate steel cylinders and rollers. One or more plane steel cylinders are fitted in some of the machines. Some machines have grained cylinders. The forwarding cylinders and rollers pull the webs from the printing units and forward them towards the folding unit.
P.I.V. gear mechanism
It is a mechanism which controls the tension of the web before it is fed into the folding unit. Here the web needs greater pulling tension. Therefore this gear mechanism increases the speed of the web by increasing the speed of any one cylinder. The driving gear of this cylinder is driven by the P.I.V. gear mechanism. The function of this mechanism is similar to that which is fitted before the web enters the printing unit. It is interesting to know that the speed of the web does not remain the same through out the machine. The principle of controlling the tension of the web through out the machine is that, the speed of the web remains higher at the reel shaft, slower before entering the printing units, higher after printing units and again slower before entering the folding unit.
The higher working speed of web forwarding rollers and cylinders is derived through the main driving gears of the machine. The P.I.V. gear machanism reduces the web speed. By this mechanism, the required tension of the web is controlled in this way described as above.
Some of the latest web machines have electronic controlling box to adjust the P.I.V. gear system.
Generally, this part is called kite former. However, the design of this part is not the same as it is named. It is of the shape of a triangle and it forms the first fold in the web. It has two thick steel pipes in V shap. In the upper open part, there is a steel forwarding roller. Three guide wheels are set on this roller. All the overlapped webs forward on the steel roller. The guide wheels are set on the non-image areas of the printed web. They should not be set on the image areas to avoid smudging of the newly printed ink.
Both the pipes of the 'V' shaped part, have holes for sucking air. The web remains fully in contact with the thick pipes and ensures the correct first half fold of the web. When the web reached the bottom pointed edge of the V part, it is converted into first fold.
The "V" shapped thick pipes of first folder (kite former) are covered with a steel sheet, on which the web slides easily. Some of the machines have half or quarter or three fourth steel covering on the former. Some of the machine do not have any steel covering on this former (folder).
Adjustment of the former
Generally, the first former does not need any adjustment in normal routine jobs. It only needs adjustment when the width of the web is increased or reduced. When the size of the web is changed, a large screwed rod is tightened or loosened. The bottom edge of this part is pulled back or pushed forward by loosening or tightening the screwed rod. This adjustment is needed to set the correct half fold of the web and easy entering into the next folding cylinders and rollers.
There is a steel strip nearly 2.5 cm wide. It is set in front of center of white space in the printed web. The center indicator and the central guide wheel, set on the upper forwarding steel roller are adjusted in an alignment. Such adjustment ensures the correct first half fold of the printed web.
Some of the old web offset printing machines are only equipped with these rollers. These rollers are also called draw rollers. One or two set of two rollers are there in the folding unit. The pulling rollers are wrapped with a grained steel strip. Due to it the web is pulled and makes the first fold very correctly. The grained surface of the rollers pulls the web in pressure but it does not affect the undried printed ink on the web. Pulling rollers are meant to ensure correct feeding of the first folded web between then next folding cylinders.
When all the webs have been printed on both sides and passed through the first former, they have to enter the second folding unit. Generally, the complete folding unit is made glass covered and various safety guards are fitted there.
The web now enters two rollers. These rollers are covered with grained steel strip. The grained rollers pull the web and make the second half fold. Newspaper printing machines have only two folding units but the magazines and book printing machines are equipped with more than two folding devices (units).
For third folding, the twice folded web turns towards the left direction and enters a set of two small cylinders. These cylinders are covered with grained steel strips in some machines. But others have plain (grainless) cylinders. The control of margin and pulling action in the folded web is made through the rubber wheels in plain cylinders. These wheels are two or four in number and set with pressure on the folded web.
The principle obtaining third fold through the cylinders, is known as buckle folding system. Besides this, some of the machines are also equipped with knife folding systems, which has been described in the fourth folder unit.
Each type of web printing machines has a cylinder for cutting the folded web into the size of a book. There are one or two cutting blades fitted in the length of the cylinder. The cutting cylinder has one blade which cuts one section in its one revolution. The cylinder having two blades, cuts two sections in its one revolution.
In front of the cutting cylinders, each machine has another cylinder parallel to it. This cylinder has a wooden strip along its length which remains at the back of the web and supports the cutting blade of the cutting cylinder. Due to the wooden strip, the edge of the blade does not get damaged.
Some machines do not have cutting blades. Instead of that there is a stretch system of cutting. When the web remains at heavy tension. The projected edge of the cylinder cuts the web sharply.
After this, all the folded sections are dropped in overlapped state on the continuous moving tapes. The sections are collected by hand in some of the machines and placed on the table. Generally, two to four persons are needed at delivery part of the machine for collecting, jogging and placing the printed sections.
Section counter machine is also fitted at the folding part, through which the number of printed copies becomes known. Slow speed machines have single section counter but the high speed machines have two section counters. For example, if the counter shows the number 000100, it means that 200 copies have been printed.
In latest web machines, the counter fitted at the folding part, through which the number of printed copies becomes known. Slow speed machines have single section counter but the high speed machines have two section counters.
Fourth fold required only in book printing but very rarely. Generally, in the machines having fourth folding unit, it is not utilized regularly. There fore the fourth folding unit is disconnected from the third folding unit. The fourth attachment is connected to the third folding unit by revolving a wheel through hands. The cut sections are forwarded on the tapes and guided by various wheels.
The section strikes a stopper and at the same time, a knife presses it exactly in the center of the section. The blade of the knife pushes the section between two steel rollers and the fourth fold is made. There are also front and side lays (stoppers) to adjust the center and side margins of the section.
After fourth folding, all the sections are dropped one by one within the steel fingers of the section separator. The separator remains in revolving state and it turns the sections on the moving belts one by one. Here also, the delivery of printed sections is done in overlapping position.
In old machines, the delivery men used to collect the sections in lots by hands. For this purpose, about 3 to 4 persons were employed. In modern machines, there is an automatic gathering divice.
The moving and overlapped sections on forwarding tapes, are jogged from both sides. After this, all the sections forward between two tapes and collect at a table one by one. Here the springed stopper is pushed continuously by the pressure of increasing number of sections. The jogged sections are taken out by hand and sent to binding department for bounding.
Ancillary Operations by Delivery Unit
The web offset machine made for stationery work are equipped with numbering, perforating stitching and cutting devices.
For numbering, cylindrical numbering machines are provided, through which numbers in a sequence are printed on the web wherever required. Number printing machines are fitted in the grooves of the cylinder. Number printing machines are fitted in the grooves of the cylinder. Inking system is fitted along with the cylinder. All the numbering machines are inked in each turn of the cylinder and print numbers on the web in the usual manner.
For perfotating a cylinder holds serrated steel strips at various distances. When the web enters the cylinder, the serrated teeth of steel strips perforate the web. The perforations in the paper are done for easy tearing. Receipt books etc. need this operation.
Stitching divice is the last operation of the delivery unit. Here, each book is trimmed at the edges. The sequence of trimming is fore-edge, tail and finally the head. Sharp edged blades are fitted for trimming the edges of the books.
Lastly the printed and bound books or magazines are placed on the conveying tapes in overlapping position and carried up to the dispatch department.
The cutting of the web in its longer side into one or more parts is called slitting. Some of the web printing machines are equipped with slitting device in the delivery part. Slitting wheels, having sharp edged blades are fitted in a rod. The wheels are set on the web at required distances. The sharp edge of the wheels slit the running web continuously.
The slitted parts of the printed web are again wrapped into reel. After that, the unprinted slitted reels may be used for printing another impression on any small sized machine. It is printed, it may be cut into sheets and used as inserts or plates.
Cutting of web along the width or converting the web into sheets, is called sheeting. Some of the machines are also equipped with sheeting device. The posters, plates, inserts or other advertising products need not be a regular part of the book. They have to be cut into sheets after printing.
Some web offset machines do not have sheeting (sheeter) device as an ancillary attachment Therefore, the printed web is taken out and set on a sheeter (sheeting machine) by which the web is converted into sheets.
Making by the action of light an enhoed copper surface for printing from. Shortly after the appearance of photography attempts were made to produce printing plates for intaglio by photographic means. In Vienna J. Berrers attempted to etch daguerreotypes, and elsewhere H.L. Fizeau used them in efforts to produce printing plates galvanically. After discovering the sensitivity of dichromate gelatine, for Talbot patented a process dichromate gelatine, For Talbot patented a process (1852) for producing printing plates with it, a method which may be considered the basis of all photogravure reproduction of modern times. Talbot prepared a copper plate by means of a mixture of dischormate and gelatine, and on this film a positive picture was copied. On developing, a relief was obtained through which the copper cold be etched, preferably with ferric chloride. The speed with which the etching liquid penerates the gelatine film varies according to the thickness, thus the contrasts of the image are interpreted by varying depths when etching. Talbot also found a way to improve the quality of etching by using successively several etching solutions of varying concentration, and he discovered the necessity of dividing the printing surface into a number of small ink containers which allowed the ink to be held by the plate. At first he impressed a gauze tissue on the gelatine layer before copying the picture, but later he adopt the method of covering the copper which was then heated and fused (1859).
In 1864 J. W. Swan patented a chromate gelatine process which consisted in preparing a gelatine film saturated with a pigment dye on a paper foundation whereupon the carbon tissue was sensitized in a bath of dichromate solution. Karl Klic later combined the work of Talbot ad Swan in this photogravure process described below.
Apart from etching this copper plate by means of a gelatine relief other methods have been devised for transforming a gelatine relief into a printing forme. In 1854 Paul Pretsch made such reliefs by a galvanic process, thereby laying the foundation for a reproduction method generally known is heligravure. The use of the two names photogravure and heliogravure is not always consistent, the same term often being applied to both.
As long ago as 1783 method was used for rotary intaglio printing on cloth and wallpaper where, to remove any surplus ink, a knife which moved across the forme was used : its inventor was Thomas Bel, and the knife developed into the modern doctor blade, q.v. A photogravure process had been invented in 1878 b Karl Klic, a bohemian living in Vienna, and in 1890 he combined with it his knowledge of the doctor-blade principle from the cloth print workshops. His method embodied all the stages forming the modern process: copying an image and screen on a carbon tissue, transferring the carbon layer to a cylinder surface, etching the latter through the layer, rotary printing using a thinly flowering liquid, and removing the surplus with a knife (doctor blade). In 1895 Klic founded. The Rembrandt Intaglio printing Co. in Lancaster, at the same time introducing etched cylinders instead of flat plates. Although he tried to keep his methods secret, about 1900 the inventors Ernst Rolfs of Siegburg and Eduard Merterns took out a patent, making their inventions and methods public. Under the guidance of Mertens the 1910 Easter issue of the "Gtornithrt Zeitung" was printed in a rotary letterpress machine for the text, combined with a gravure printing press, which worked at a speed of 8000 to 10,000 revs. An hour, for the illustrations. In 1906 Rolffs and merterns together with August Nefgen and other inventors in this sphere formed a company, the Deutsche Photogravure A.G., which became the nucleus of the Tiefdrucksyndikat established in 1913, a world-wide association owing practically all the patents in gravure printing.
Summary of modern photogravure: The picture (image), which is a continuous tone positive, is transferred to a copper-surfaced cylinder my means of a pigmented gelatine-coated, sensitized carbon tissue on which the type or picture to be reproduced has been exposed. The assembling of the positive for this purpose is known as planning. After exposure the tissue is laid and mounted on the copper cylinder for developing by rotation in hot water. It is varnished in any are as which are not be etched, fanned dry, and etched with ferric chloride.
Screen: Due to technical reasons fundamental to the printing process the picture etched into the copper is divided into a number of small class which hold the ink. This is done by means of a screen imposed on the carbon tissue which, unlike the half-tone screen, is not part of the design (cf. autotype ad screen reproduction) but merely serves to divided the surface of the cylinder into etch-resisting wall for the cells which will hold the ink. There must be screen walls present over all the etched surface, i.e. illustrations. Text, frames, etc. The screen was first used for photogravure by Karl Klic and his process manager Samuel Fawcett qq.v., in Lancashire. Other types of screen are now in use, including irregular dotted screens, (See screen). For practical reasons the screen is first copied on the carbon tissue, this being done in a vacuum frame, q.v., in which the screen is laid and fixed. After this the positive pictures and text, mounted on glass, are superimposed on the screened tissue. At one time it was common to copy, transfer, and etch the text fist, and then to do so with the pictures. Later a method was devised for the simultaneous copying and transferring of illustration and text matter, but they were etched separately. In modern reproduction (especially magazine work where speed is essential) attempts have been made to do the etching as well in one operation. For this the transparencies (dia positives) must be made in a special way. The text transparencies present a sharp contrast of black and white. The time required for copying must be long enough to allow the gelatine film underneath this text surface to become hardened through. At the same time, during this protracted copying the highlights of the picture must not be excessively tanned, for which reason they must have a rather pronounced density in the transparency. The etching of the darkest tones in the black text.
A way of solving the problem of a text-picture combination is offered by the Rinco process, by which the text is printed in white ink on glazed black paper and mounted, together with the picture negatives made on bromide paper, for photographing into a film positive. When this method is employed, it is necessary to maintain a very constant quality on the type pulls. For mass-produced publications, where speed is important, it is very valuable.
Planning: The positive which will be combined to make up a printed sheet are mounted on a polished glass plate. Under the plate is a carefully marked up plan of the layout to indicate its positioning. The margins outside the image area are covered with black strips which will ultimately make a 1 cm. Or 2 cm. Unexposed border on the carbon tissue (the unhardened gelatine under this border will then adhere more easily to the cylinder).
Exposure: The glass plate is put on the bed of a vacuum frame, the screened carbon tissue fitted to it, and an exposure made. Various light sources are used including are kamps, mercury vapour lamps, and fluorescent tubes. Each kind has its special merits. The amount of exposure is controlled by a photometer.
Mounting: The carbon layer has to be transferred to the printing forme surface. This transfer must be done by damping the gelatine layer sufficiently for it to adhere, but damping must be restricted to the least possible amount in order to prevent any change in size of the tissue. Instead of by wet mounting, q.v., where the tissue is soaked in alcohol and water before mounting on the copper surface, rotary photogravure work is usually by dry mounting. For this a dry laying machine is used which enables damping to take place at the same time as transferring. The copper cylinder, or copper plate similarly fitted, is put in the machine and the margin of the carbon tissue is pasted to its surface. This may be done by means of adhesive type of by careful damping of the unexposed marginal strip of the carbon tissue (see above). On revolving the cylinder a rubber roller presses the tissue against its surface, water being slowly fed in at the point where the paper comes into contact with the cylinder. The transfer being completed, the carbon tissue adhering to the cylinder is soaked in methlylated spirit before development takes place in a bath of water at about 40ÂºC (105Âº F). The backing paper may be removed in a little while, leaving the gelatine resist in relief on the copper surface; this must be treated continually with warm water until every trace of soluble gelatine is removed. The remaining gelatine resist in relifer on the copper surface : this must be treated continually with warm water until every trace of soluble gelatine is removed. The remaining gelatine resist in relifer on the copper surface : this must be treated continually with warm water until every trace of soluble gelatine is removed. The remaining gelatine relief is soaked first with diluted, and later with concentrated spirit, being then left to dry.
Etching: Before etching is begun any part of the surface of the cylinder which is not to be etched is covered with an asphalt (bituminous) varnish. Etching is then done with ferric chloride solution of varying strengths, commencing with a full strength pelted the copper surface is quickly washed with cold water, then with diluted acid, and finally the gelatine relief is subbed of entirely with purified chalk and water. The asphalt coating can be washed off with benzo. The final stage, s a rule, is to wash the copper with a mixture of vinegar and common salt solution.
For the printing of large editions, at least, it is usual to give the copper a chromium facing, after which they cylinder (or plate) is ready for the press from which printing will be done.
Copper cylinder: Formally doctor-blade gravure work was printed from thick copper plates or cylinders. The center of such a cylinder was an iron core bearing a thick coating or galvanic deposit of copper. After printing, the surface of the cylinder was ground until any traces of etching were removed, then the copper surface was polished and used again. This continued until the reduction of the cylinder diameter caused inconvenience and it become necessary to give it its original size by re-plating. For this work gravure printing offices has special plants for plating, grinding, and polishing. The modern Ballard process considerably simplifies this by enabling the etched copper film to be stripped from the cylinder without difficulty, and another to be substituted galvanically and polished in one operation. In this way the cylinder always has the same diameter. Alternatively, under certain circumstances, detachable copper sleeves or thin copper plates covering the entire cylinder may be fitted to steel mandrels in the same way as the plate in an offset machine.
Experiments have been made for replacing the carbon tissue transfer by copying directly on a prepared plate or cylinder.
Phogravure has been known as "Copper gravure'', and not without reason since practically no material other than copper is used for the formes. The carbon tissue requires a scrupulously cleaned copper surface, which at the same time must be free from oxide stains. The surface is usually cleaned immediately before use with purified chalk and a little ammonia or potash solution as used for etching, all of which must be washed off afterwards. Sometimes the copper surface is treated with cyanide of potassium and silver nitrate solution, which gives a perfect surface for etching, but the cyanide is so poisonous that most firms avid using it. The silver plating does not affect the etching procedure.
A means of identifying photogravure work is work is provided by the fine screen network which is faintly discernible in the finished reproduction ; this is particularly so in the lack of sharpness it gives to accompanying type. (with G.U.) accompanying type.
See also Collopals, colour gravure.
Photogavure Printing Ink: Think, volatile ink which dries by evaporation of the solvent. It usually contains bitumen or resins, e.g. dammar, dammar, hartshorn, albertol or gilsonite asphalt in a solution of one or more volatile solvents such as zylol, toluol, and benzine has also been used as a solvents, but its use ink connected with risk of poisoning. Water based inks are in common use, particularly for security printing anink now used for food wrappers is made on an Arrangements for feeding in paper correspond largely to those for letter press and offset printing (see feeding, printing press, rotary press). The printing mechanism, however, is built on a different principle. In photogavure the whole forme is coloured with thinly flowing ink, the surplus then being removed by the doctor blade (see diagrams). The inking arrangement can thus often be simplified by allowing the morme to revolve in a trough of ink. Above the through is the doctor blade. As the cylnder must in most cases be removed for the transfer to it if the image and for etching, the paper course with this in view. In reel-fed presses the paper course is pressed against the forme cylinder by another cylinder, known as the impression filler, but in any case, the impression cylinder is coated with a suitable rubber. In sheet presses, the printing cylinder is supporting the sheet must have a circumference equal to or exactly double the size of the forme cylinder. The latter is more usual when working with entire copper cylinder, the forme cylinder then being inked and scraped with the blade twice for each impression. In presses for-built-in copper plates forme cylinder double the size are used to avoid any excessive bending of the plate (see below).
In reel-fed presses it is desirable to increase the speed of printing as much as possible. Gravure ink dires by evaporation and ways have been tried to accelerate this so that the paper course can be introduced quickly into subsequent printing stages, or into folders or guillotines. It was formerly usually to conduct the paper across rotating steam heated drying drums. Heating, however, was detrimental to the paper, due both to change in size and dust from the dry paper fluffing the forme. There was the additional risk of static electricity arising in the paper with a danger of sparks igniting the inflammable vaporous in the ink. Cool are drying, developed by Fuykers & Walber of Dusseld for, was more successful, especially since 1934 when it was combined with a device for the 1934 when it was combined with a device for the recovery of the solvent from the ink. In Fuyker's air dryer fresh air is blown against the moving paper course from the mouthpieces of a drum arranged like a saddle over the printing mechanism. By entrirely closing this, and then conducting the air current through a recovery apparatus, the evaporating solvent is recovered, and at the same time the often poisonous and inflammable vaporous round the press are removed. In some modern high-speed presses working with extremely volatile solvents, practically the whole press is enclosed and the inking system forms an enclosed container limited above by the forme cylinder, on one side by the doctor blade, and on the other by a sprint lid. Only the narrow strip of forme cylinder required for printing is to be found outside the enclosed container.
The doctor blade consists of a thin spring blade which rests at a certain pressure atainst ! The cylinder surface. In the knife-holder of the press the blade is strengthened by a thicker supporting blade. Many different types of doctor blade have been tried; one patent is an endless blade which wipes the cylinder, travels on, is cleaned, honed, lubricated, and comes round again to continue the operation. Different means of adjusting the blade to the cylinder have been tried, including hydraulic, but in every case the blade is set at an angle to the cylinder the ink forming a lubricating film which is imperative not to interfere with this. Modern rotary presses can be brought up to about 20,000 revs. an hour. When the printing formes consist of whole cylinders which by an earlier method are turned after each etching and occasionally given a new plating up to their original thickness, the varying diameter to the cylinder is apt to cause difficulties.
Attempts made to substitute copper plates for the cylinders are mostly confined to rotary sheet presses. Among these is the "Ideal" gravure printing press made by the schenell pressenfabrick. J.G. Mailander of Stuttgart, in which the plate cylinder is twice the size of the impression cylinder and has a knife-cleaning arrangement which free the blade from remnants of colour after each impression. The "Liti" Johnisberg Press has a plate fitted round the whole circumference of the cylinder and the joint is filled with packing over which the blade glides without losing contact with the cylinder surface. In other cases, it is usually in such press to arrange for the blade to rise and descend when passing the joint. Vomag's "Olympia" and Frankenthal-Albert's "Palatia" have arrangements by which fitted copper plates as well as whole cylinder can be used. In the latter, when doing copper-plate printing, a forme, cylinder twice the size of the whole copper cylinder normally used is fitted. This is made possible by the bearing cases revolving to 1800* and the distance between the cylinder axles being changed. The "Olympia" press has a special cascade inking system and also a drying apparatus for the printed sheets. Rotary sheet presses for plates are also made by Linotype & Machinery Ltd. Koing & Bauer have built a rotary press (the "Tiepolo") with a convenient arrangement for inserting the copper cylinder into the machine. The latter firm and Vomag are among well-known suppliers of reel-fed gravure printing presses.