CONFIGURATIONS USED TODAY
What are the different configurations of label printing presses?
The following pages illustrate some of the more common press configurations used today whilst highlighting the advantages and disadvantages of each type.
THE SHEET-FED PRESS
The typical sheet-fed press is configured with an auto sheet feeder unit called the ‘feeder’ and a sheet stacking unit called the ‘delivery’. (See Figure 2.3). The print units are positioned between the ‘feeder’ and ‘delivery’. Each print unit prints a single color and sheet-fed presses can be configured with the desired number of print heads as required. For instance a 6-color press will print up to 6 individual colors or less if required. The number of printing units can vary between one single unit, and in the commercial print market can be as many as 10 units.
Figure 2.3 - Diagram of sheet-fed print unit
Self-adhesive labels in sheet form are usually printed on multi-color presses and over-varnish is done either with an in-line varnish unit or alternatively on one of the printing units.
The vast majority of sheet-fed presses tend to use the litho process, but sheet-fed letterpress machines are still used today and are generally single color.
To aid output and manufacturing efficiency the modern sheet-fed press can be specified with an in-line coating unit which uses the flexo process to apply overall or spot coatings of varnish to the printed sheet. Other aids include automatic plate change, closed loop color and quality control systems and inter-deck drying which enables wet-on-dry printing between each print unit.
THE PLATEN PRESS
The illustration above (Figure 2.4) shows the Platen or Clamshell type of press configuration which is one of the earliest types of printing press. The press prints by the letterpress process, using either type or polymer plates. The inking rollers move up and down over the type or plate, which is located in a ‘chase.’
Figure 2.4 - Diagram of platen press (4impression)
The substrate, which is in sheet form, is fed onto the moving section of the platen (as indicated by the arrow on Figure 2.4). The location of the substrate within the press can be either manual or automated.
The back section of the platen (the bed) holds the ‘forme’ and remains static.
With the type ‘inked’ and the substrate in position, the platen comes together bringing the substrate into direct contact with the type thereby transferring the ink to the substrate. The platen then opens and the printed substrate is removed before the cycle begins again.
Because of the heavy construction and high impressional strength of this type of Platen configuration it is used extensively for the die-cutting of self-adhesive labels, in a sheet format.
THE SEMI – ROTARY ‘STOP FEED PRESS
The semi-rotary print head differs from the full rotary press in two important areas. Firstly the printing head is positioned to print ‘across’ a ‘stationary’ web (See Figure 2.5) and not ‘down’ the web as is the case with a full rotary printing head.
Secondly the semi-rotary head does not require print cylinders of differing diameter as does a full rotary head, which requires a print cylinder diameter that is compatible with the label size being printed.
Figure 2.5 - Semi rotary stop feed label pres
The action of the web as it passes through the press is a stop – start movement with the printing cycle taking place when the web is stationary. As the web stops the print cylinder rolls across the web and transfers the image from the letterpress plate. The cylinder then rolls back to its start position and fresh ink is applied to the plate image before the web moves forward. This action is called the ‘Pull’ and the distance of the pull is set by the printing width of the plate cylinder.
This type of semi-rotary press configuration uses multiple letterpress printing heads and can also be fitted with flatbed hot foil stamping units, flatbed embossing and flatbed die-cutting units and waste rewind.
SEMI- ROTARY PRINTING
The key advantages and disadvantages of semi-rotary printing can be summarised as follows;
Does not require additional different diameter print cylinders
Good quality print from the letterpress process
Multiple embellishing and conversion facility
Competitive preparation and tooling costs
Easy press access for the operator
THE ROTARY ‘INTERMITTENT FEED’ PRESS
The Rotary Intermittent feed press also known as a ‘translator’ or ‘reciprocating’ feed, is a completely different configuration to the semi-rotary press. The press layout is the same as a full rotary web-fed press, but the difference lies with the web transportation system.
With this system the web travels through the press with a forward and backward action and the printing cycle only takes place as the web travels forward. The print unit has a ‘fixed’ cylinder repeat length and always revolves in the same direction as the forward movement of the web.
The print length is governed by the amount of web traveling through the press and this ‘pull’ distance is controlled by the translator feed system, which operates via servo driven cylinders and nip rollers positioned at the in-feed and outfeed sections of the press.
Because the print cylinders and/or offset blanket cylinders are not fully circular the gap in these cylinders allows the web to travel freely on the backward pull of the web and the fixed print cylinder repeat length removes the need for a stockholding of different cylinder sizes as required with the ‘cassette’ type litho, screen and flexographic processes.
The modern intermittent feed press uses the very latest printing technology and is used extensively for the manufacture of very high added value self-adhesive labels on a range of paper, film and metallic substrates. Offset litho is the print process most commonly used on this type of press, usually in combination with the screen and/or flexo process. These presses use high quality digitally driven servo drives and can be configured and retro fitted as required. Touchscreen operation with job data memory minimises make-ready times and waste on repeat jobs.
ROTARY INTERMITTENT FEED PRESS
The key advantages and disadvantages of the Rotary Intermittent feed press can be summarised as follows;
Can print in a variety of formats with minimal tooling/cylinder costs.
Touchscreen operation with job data memory
Only one operator required
THE STACK PRESS
The term ‘Stack’ describes the way in which the print units are positioned onto the side frame of the press (See Figure 2.6). These print units are ‘stacked’ on top of each other with sufficient space between each unit to allow for an ink curing/drying unit to be located. The stack can be configured to provide twin stacks allowing 6 print units to be used.
Figure 2.6 - Diagram of stack press. Source- 4impression
When printing by the letterpress process each print head has its own set of inking rollers, plate cylinder and impression cylinder and because of the close proximity of the print units a short web path is achieved, reducing waste and print register errors. Other major advantages of the stack configuration are the reduction in factory space required and easy all round access for press operation and the internal engineering.
Figure 2.7 - Typical stack label press configuration. Source- Edale
The stack press configuration was instrumental in developing and increasing the manufacture of HAV (high added value) self-adhesive labels. This press configuration can produce high quality print on both paper and filmic substrates using combinations of the letterpress, flexographic and screen printing processes.
The ability to produce filmic self-adhesive labels for the increasing ‘clear on clear’ label market and the ability to also rotary screen has given this press some considerable advantages over other types of label presses.
The ‘stack press’ can also be equipped with a hot foil stamping unit and flatbed and rotary die-cutting as an in-line operation. Its short web path and easy makeready make this type of press suitable for both short or long run work, with accurate registration quickly established and maintained throughout the run.
THE STACK PRESS
The key advantages and disadvantages of the ‘stack’ press can be summarised as follows;
Short web path – less waste
Excellent web and registration control
No offset litho capability
Possible weakness with cantilever units when fitted with heavy tooling in die station.
THE CENTRAL IMPRESSION (CI) PRESS
This type of label press features a single large diameter impression cylinder with each printing head located around it (See Figure 2.8 & 2.9) and an ink drying/curing unit positioned between each print head. These presses can be configured with up to six print units.
Figure 2.8 - Diagram of CI press. Source- 4impression
Figure 2.9 - Typical CI label press. Source- KPG
One of the major advantages with the C.I. press is that the single large diameter impression cylinder is able to support the substrate throughout the printing of each successive color. The substrate remains in contact with the impression cylinder and as a result cannot move or distort, ensuring that excellent print to print register is maintained. This feature makes this press configuration excellent for the printing of very light filmic substrates.
THE CENTRAL IMPRESSION (CI) PRESS
The key advantages and disadvantages of the CI press can be summarised as follows;
Excellent print to print registration particularly on very thin filmic substrates
Suitable for both the flexo and letterpress processes
Potential for damage to the central impression cylinder
Letterpress and Flexo only
COMBINATION – CI AND STACK PRESS
One of the hybrid presses which uses both Central Impression and Stack configurations is the Ko-Pack label press. This type of press configuration is very versatile and used for the manufacturing of complex multi-layer labels, pouches, coupons and booklets
that may require the printing and converting of multiple webs, multiple folding, hot melt gluing, over-lamination and multiple die-cuts.
A typical press specification for the dual configuration shown in Figure 2.10 would be :-
2 unwinds stands, web cleaning with corona discharge, 12 printing positions comprising of 11 letterpress units, 3 flexo units plus 1 rotary screen unit, interdeck UV drying between all the print units, 3 plow folding units, hot melt gluing unit, over-lamination system, and finally 3 rotary die-cutting units, matrix rewind, multiple slitting, sheeting and conveyor unit and product rewind stand.
Figure 2.10 - Diagram of combination CI and stack label press
THE WEB-FED MODULAR LABEL PRESS
The modular web-fed press prints onto a continuous running web and not as a single sheet. The unprinted reel is placed on the in-feed section of the press (this is called the ‘unwind stand’) and the printed and converted reel exits on the rewind section of the press, called the ‘rewind stand’.
The web passes through each individual printing unit and any print register adjustment that is required can be carried out from a central console or at each individual print station. These adjustments can be done manually or via the press auto registration system. Figure 2.11 shows a 4-unit web-fed modular configuration, printing in the process colors of CMYK, cyan in unit 1 – yellow in unit 2 – magenta in unit 3 and black in unit 4.
Figure 2.11 - Web-fed full rotary press. Source- 4impression
The printing head is positioned on the upper part of each unit and the die-cutting and embellishing stations are positioned between the last printing unit and the rewind unit. In addition a drying/curing unit is positioned at the base of each print unit. This is called an inter-deck drying system which allows each individual color to be dry before the next color is printed. This is called wet on dry printing.
This modular type of press configuration is very versatile in a number of ways. Extra print, embellishing and conversion units can be retrofitted if required and the press layout gives the operator easy access for job changeovers and makeready.
A standard specification for this type of press would include an unwind unit, web tension control, web cleaner, corona discharge and manual or auto print register, die-cutting unit, matrix rewind and outfeed rewind unit.
The modular press can be operated as a dedicated ‘single’ print process machine or as a multi-process platform capable of total process interchangeability. However it should be noted that the majority of these modular label presses are dedicated UV flexo.
THE WEB-FED MODULAR LABEL PRESS
The key advantages and disadvantages of the web-fed modular label press can be summarised as follows;
No multi-color restrictions
No multi-process restrictions
Suitable for easy retro-fitting of ancillary equipment
Greater factory space requirement
Requires sophisticated web and registration controls
THE MODULAR MULTI-PROCESS ‘COMBINATION’ PLATFORM PRESS
One type of press configuration in particular has revolutionised the printing of self-adhesive labels. The modular multi-process ‘Combination’ platform press offers both end users and converters a range of significant benefits.
A full article has been devoted to Combination printing but a brief description of this press type is provided below.
Combination printing allows a wide variety of print processes to be utilised on a single press. Each of the various print technologies has its own advantages and disadvantages and combination printing focuses on the strengths and advantages of each printing process.
Combination printing is the method of utilising the different printing, embellishing and converting processes on a single platform, giving full interchangeability of these processes in order to optimise the graphic advantages offered by each process.
The ‘printing’ processes that are compatible with combination printing and can be used in any combination are:-
The ‘embellishing and converting’ processes that are compatible with combination work are:-
De-lamination and re-lamination
Figure 2.12 illustrates a typical layout of a modern combination press. The diagram shows a comprehensive range of print processes, embellishments and conversion equipment and the location of each unit.
Figure 2.12 - Diagram showing position of units within combination press
Combination presses do not necessarily require all of the processes named in this section.
Automatic unwind and splicing unit
In-feed tension control
Multi-process stations for cassette system for Offset litho – Flexo – Screen – Gravure - Digital
Rotary die-cutting or embossing unit
Rotary die-cutting unit
Waste matrix removal
Sheet stacker unit
Rewind unit single or auto turret rewind
Figure 2.13 - Ilustration showing base press with portfolio of interchangeable units. Source- 4impression
The combination or platform press uses a system of interchangeable cassettes. The system allows a quick and uncomplicated method of providing the printer with a number of graphic combinations which can be easily assembled on the press allowing full interchangeability between the processes.
The pneumatics, electrical and electronic systems used on a platform press allow easy changeover of each cassette so that all the printing, embellishments and conversion processes can be carried out as a one pass in-line operation. Graphic embellishments such as embossing, hot foil stamping and lamination are also in cassette format.
Figure 2.14 illustrates the flexibility offered by a combination press. The two illustrations show the same press in different cassette configurations giving the facility to use the most suitable printing and processing method.
Figure 2.14 - Illustration showing typical unit combinations. Source- 4impression
The configuration of the combination press is identical to the modular web-fed label press and the use of servo driven motor technology and sophisticated digital control systems is now used extensively by the manufacturers of both the modular label press and the multi-process combination press.
This kind of technology gives the operator the facility to use the computerised database to save and reuse the data that has been applied to a specific job, for example, previous press settings, ink control settings, web tensions, anilox roll specification, press speed etc.
MODULAR MULTI-PROCESS ‘COMBINATION’ PLATFORM PRESS
The key advantages and disadvantages of the modular multi-process ‘Combination’ platform press can be summarised as follows;
Ability to produce very high added value labels
Higher investment levels for the press and related equipment
Higher pre-press complexity and costs
Higher skill level requirement, knowledge of different print processes requiring additional training
Possibility of additional staffing to support combination printing
Requirement for knowledge of different ink technologies
Training of sales staff to understand the benefits of combination presses and the potential to sell added-value
Limitation on press capacity and job flexibility with a single combination press.
Whatever printing press or configuration is used there is a wide range of ancillary equipment that can be utilized, some optional and others more fundamental to the efficient running of the press. Although reference to some of this equipment will have already been made earlier in this article the following section provides a useful overview of the ancillary equipment used in the printing of reel-fed labels and will highlight their benefits.
IN-FEED REEL STAND
The in-feed reel stand holds the unprinted reel and has the facility for manually joining the end of the expiring reel to the end of the new reel. This operation is called ‘web splicing’. It is essential that this spliced joint is positioned at a suitable angle to ensure that the converted web and waste matrix does not break at the point of removal. Single sided adhesive film tape is used for the splice.
2. FLYING SPLICE IN-FEED UNIT
The Flying splice unit (See Figure 2.15) allows the automatic joining together of two webs. Splicing is the procedure of attaching the leading and trailing edges of the web in the cross direction by the use of an adhesive strip. The cross cut is usually positioned at an angle to reduce the possibility of a web break whilst the press is running.
Figure 2.15 - Flying splice unit. Source- Martin Automatic
A flying splice facilitates the automatic joining together of two webs while the web is in motion and is most commonly used when reels are changed from an expiring roll to a new roll. This is done without stopping the machine, either at the unwind or the rewind ends of a web-fed press.
The key benefit of the flying splice unit is that it allows the press to continue running.
3. WEB EDGE GUIDANCE UNIT
A device on a web-fed label press that keeps the web traveling straight and true as it passes through the machine. Web guides (See Figure 2.16) play an important part in the control of the web through a press and may consist of photo-electric, UV light or ultrasonic sensors that are focused on the edge of the web.
These sensors feed data back to either an adjustable pivoting roller, which guides the web to the left or right as required, or to the in-feed reel stand which can also move to the left or right.
Figure 2.16 - Web edge guidance. Source- Erhardt+Leimer
Reaction speeds from the system should match the speed of the press to avoid any over-correction. If this is not synchronized then the web may move erratically from side to side thereby creating problems during printing and finishing operations.
4. WEB TENSION CONTROL
Web tension is the amount of pull or tension applied to the web as it passes through the press. Poor tension control will result in registration problems in the printing, embellishing and converting processes as a result of side to side web movement, substrate stretch or creasing.
During the unwinding of a roll on a converting or web-fed press the paper, film, foil or laminate material being used is under tension or stress as a mechanical force operates to extend, stretch or pull the web apart. This tension needs to be controlled. Too little tension makes register control more difficult; too much tension may lead to the material stretching or growing in length.
Color to color registration, size of label, position of any perforation or punching, etc. will all be affected by inadequate tension control; the more flexible the material the more sophisticated tension control becomes essential.
The degree of control that should be built into a press will involve an additional investment at the time of purchase, but should be more than repaid through increased productivity throughout the life of the press.
A typical tension control system on a press unwind incorporates sensing rollers mounted in a pivoting floating bracket balanced against the tension setting. The bracket and rollers change the floating position by pivoting in response to changes in factors such as roll diameter, running speed, acceleration of the web and friction changes within the braking system. The pivoting motion of the bracket transmits information that constantly adjusts the brake force to keep the tension in equilibrium.
Figure 2.17 - Press with correct web tension and register aligned (4impression)
Figure 2.18 shows a press running with an incorrect web tension setting and shows the print registration ‘hunting’ to establish the correct print register. This problem, if unresolved can lead to considerable amounts of waste.
Figure 2.18 - Press with inco
Thinner filmic materials require a very sensitive web tension control.
The use of servo technology enables good control of the web and the facility to run the press at faster speeds without compromising the web tension.
Servo-driven controls make web control much easier for the press operator and eliminates the need for making manual changes to the web tension during the print run.
Tension sensors monitor the changing weight and torque of the in-feed reel and the diameters of the rewind roll to maintain the right web tension. Transducer rollers automatically calculate these parameters and the unwind reel braking system will automatically adjust to prevent 'web wander'.
Some of the common causes for web tension problems are;
The general condition of the press
Changes in roll diameter during the print run
Elliptical shaped substrate reel
Slipping of the inner core
Variations in the substrate caliper (thickness) within the reel. This occurs when there is variation between the left and right halves of the reel
Good tension control
Improves lamination quality
Allows thinner film stocks to be run
Bad tension control
5. CORONA DISCHARGE
In corona treatment, the surface of film or other material to be treated is bombarded with electrons.
These leave the electrode source and are accelerated under high tension towards the passing web material. In doing this they collide with air molecules which transmit light and react in part to ozone and nitrogen oxide. When the electrons come into contact with, say, a polyethylene film, they have so much energy that they can break the bond between the carbon hydrogen or carbon-carbon.
Reactions with the corona gas take place at these free radicals, mainly towards oxidation, with the polar functional groups thus formed providing the basis for adhesion of applied printing inks, adhesives, lacquers, etc.
Ease of operation, ease of maintenance, operational reliability and simple handling are the key criteria for selecting corona equipment.
Equipment should also be easy to install within a short period of time and must include practical functions and control features.
Corona Discharge units are usually positioned prior to the point where the web enters the first print unit and close to the web cleaning unit. The purpose of the corona surface treatment is to increase the surface wettability of the substrate (generally filmic) through an electrical discharge.
The low surface energy of polymer-based substrates often leads to poor adhesion of inks, glues and coatings. To obtain good adhesion it is necessary to increase the surface energy of the substrate.
Depending on the requirement, one or two rollers can be installed above or below the web for a double-sided pre-treatment. (See Figure 2.19).
Figure 2.19 - Corona discharge unit Source- Enercon
6. WEB CLEANING UNITS
When selecting a web cleaner, whether contact or non-contact, one of the crucial factors is to make sure that the cleaner breaks the layer of air that is held by the moving web and which can hold contamination on the substrate surface.
Contamination can come from paper dust, factory dust, airborne contamination etc and will be attracted to the surface of the substrate and in turn will be picked up by the inking rollers and also the surface of the image plate causing print defects.
To break this contamination layer, different systems are used including powerful airflows, air turbulence or actual contact with the web.
Some web cleaning systems use a tacky roller, which is in contact with the web. Any debris that is picked up by this roller is transferred to a second, more adhesive roller.
The benefit of this technology is that it is relatively easy to install and gives good results.
7. ANTI-STATIC CONTROL UNITS
Static electricity is the result of an imbalance between negative and positive charges on the surface of the substrate, and can be a problem when processing plastics substrates.
The static charge builds up on the surface and needs to be discharged by a combination of an electrical charge and ionized air.
Anti-Static units (See Figure 2.20) are used to remove the static charge that, in the case of web-fed presses, is generated by the movement of the web as it comes into contact with the web path rollers.
Figure 2.20 - Anti-static unit
This issue can be particularly troublesome with filmic substrates with static electricity generated whenever two surfaces are in contact and are then separated, thus causing a dragging or holding effect. This problem can also affect sheet-fed applications.
Static causes the sheets to be electrically attracted because of the opposing charges on each sheet. One sheet has a positive charge and the facing side has a negative. The effects of static electricity can be felt and heard and will often be seen as a large spark as the static is discharge or earthed.
8. INTER-DECK DRYING UNITS
The modern self-adhesive press is fitted with inter-deck drying heads allowing each individually printed color to be cured/dried before printing the next color. This is called printing wet-on-dry and eliminates the problems which can be encountered when printing wet ink onto a wet ink.
The drying or curing head is positioned so that the printed substrate is exposed to the drying head for the correct dwell time to ensure that the ink is fully cured/dried (See Figure 2.21).
Figure 2.21 - Location of inter-deck drying unit. Source- 4impression
There are three types of drying systems used in the self-adhesive label industry: Infra-Red, which uses IR lamps to create heat; Hot Air, usually created by IR but with the addition of blown and extracted air; and UV, that uses ultra violet lamps to cure inks formulated to cure/dry when exposed to ultra violet light.
9. DIE-CUTTING UNIT
The die-cutting unit is designed to hold a rotary die-cutting tool (See Figure 2.22) in a stable position, at the required pressure to perform the cutting action. This unit has to be of substantial construction and be capable of applying a consistent and accurate pressure throughout the job run.
Figure 2.22 - Rotary die and anvil roller
The facility to make very fine adjustment is very important and modern units are fitted with pressure measurement equipment (See Figure 2.23) which allows the operator to monitor the pressure being applied to the die and avoid any overheating of the die ‘bearers’ and subsequent damage to the die. It is important that the die bearers are adequately lubricated throughout the print run.
Figure 2.23 - Diagram of die-cut unit
Self-adhesive label presses can also be fitted with a flatbed die-cutting unit. This type of profile cutting unit uses a less expensive ruled die tool.
The flatbed unit works on a platen principle with the cutting die located in the top platen and the cutting plate located on the bottom platen. The substrate moves between the two platens using a reciprocating stop - start action; when the web is in the stationary position the platens come together under pressure and the cutting action takes place.
10. REGISTER CONTROL SYSTEM
The registration control system on the press is responsible for maintaining the accurate positioning of each color being printed. The system has to be capable of dot to dot accuracy between each print unit, print to embellishing and print to die-cutting /conversion, throughout the print run.
Register may be manually or electronically controlled and the system has to be able to allow four adjustments of the print cylinders/unit whilst the press is running, i.e. side movement left – right and circumferential advance and retard.
Manual systems are totally dependent on the operator to make all the necessary adjustments, but with automatic register control, the system will read the position on each printed color and automatically make any adjustments that may be required.
11. MATRIX REMOVAL UNIT
The removal of the waste matrix can be carried out as an in-line operation on the press or as an off-line operation.
There are two methods of matrix waste collection, it can be rewound onto a waste winder system as in Figure 2.24 or extracted through a vacuum waste unit.
Figure 2.24 - Matrix removal unit
Some label shapes can prove very difficult during the matrix removal process and careful consideration needs to be given to the job layout to ensure that the removal of the matrix is trouble free and allows consistent running speeds.
A number of presses are equipped with a driven roller placed just above the matrix take off point. The pull speed of this roller can be adjusted to increase or decrease the tension applied to the matrix allowing the operator more control of the matrix removal process.
12. SHEETING UNIT
Sheeting units are located at the unwind end of the press and allow the converting of printed substrates from a reel into individual sheets, as an in-line operation.
The material, in reel form, is fed from the unwind, passes through the printing and converting stations into a sheeting station which then sheets-off the printed material into specified lengths. The cutting operation can be done with a dedicated sheeting head which is fitted with a rotating cutting blade which passes over a fixed ‘bottom’ blade to make the cut.
The speed of this type of cutting head can be adjusted, which allows differing lengths of sheet size to be cut.
Cutting can also be done using a rotary die fitted with a cutting blade. This type of sheeting tool fits into the existing die-cutting unit and runs at a fixed speed. The cut is made when the cutting blade comes into contact with the anvil roller which is an integral part of the die-cutting unit.
13. SLITTING UNIT
The function of the slitting operation is to remove the unwanted edge of the web or divide the web in the lengthwise direction, to produce two or more narrower webs. The printed reels coming off the press are then slit into even narrower reels for automatic application on to the product.
The slitting unit is generally positioned prior to the press rewind unit, but can also be positioned at the unwind end of a press to ensure accurate web widths enter the press. The slitting unit will be adjacent to a waste extraction unit which is used for the removal of unrequired web edge waste.
The cutting heads can be of a rotary or razor cut construction.
There are two types of rotary cutting actions, the crush cutter and the scissor cut. The rotary crush blade cuts against a bottom anvil roller or segment, whereas the scissor cutting head has two rotary blades that are in contact with each other and rotate to create the scissor action (See Figure 2.25). Razor units allow the fixing of sharp razor type blades which cut the substrate as it passes through the unit.
Figure 2.25 - Slitting Unit. Source- ABG
Modern slitting units are now capable of automatically positioning the top and bottom cutters, working from a digital program. These units eliminate the need for manual setting of the slitters.
14. REWIND UNIT
The outfeed rewind unit is placed at the end of the press and is usually the last process on the press. This unit can be a single shaft rewind or a twin shaft rewind and the amount of rewind torque/tension can be adjusted during the job run.
The facility to vary the torque is a very important factor particularly if heavy or lightweight stocks are being converted.
Reels can be rewound too tight or too slack and this problem can cause adhesive bleed and substrate creasing or even cause the reel to collapse whilst the press is running.
The shafts on the rewind unit can be rotated whilst the press is running allowing manual or fully automated splices to be done and thereby eliminating the need to stop the press during the job run.
15. TURRET AUTO REWIND UNIT
The auto rewind unit known as a ‘turret rewind’ can be used as an off-line or in-line facility. The purpose of a turret rewind is to produce self-adhesive labels in a finished format to a specified reel size or quantity of labels, ready for dispatch to the customer.
The strips of labels which are created after the slitting process are rewound onto individual cores. As the reels reach the specified diameter the turret revolves to a point suitable for automatic splicing to take place. As the reel reaches the correct diameter or quantity of labels the new cores, which are adhesive coated, start rotating and a striker blade moves down and cuts through the ribbons, pressing the leading edge of the ribbons onto the new set of cores. If the rewind unit is positioned ‘in-line’ on the press there is no secondary process i.e. slitting.
Q.C. checks at the rewinding stage complete the operation and the finished labels, on the correct core size, are ready for product application.
The process is just the same when done off-line.
16. TURNER BARS
The function of the turner bars is to change the direction of a moving web to allow the printing of both sides of the substrate, the web turner can also change the direction of the web to allow a side exit from the press so the web can be diverted into a secondary conversion unit.
Figure 2.26 – 2.27 shows a web traveling from the right and passing through a series of angled rollers. Some of these rollers rotate and some are static. The static rollers are air assisted allowing the web to ride on a cushion of air. The web then exits on the left exposing the reverse side of the web. This facility helps change the direction of the web and allows printing on both sides of the substrate to take place.
Figure 2.26 - 2.27 - Turner bar units. Source- MPS