While the rest of the flexo process has become highly automated in recent years, plate mounting has to a large degree remained a fairly low tech manual process. David Atkinson looks at some of the options available.
Despite the fact that sleeve dedicated machines with servo drives, video cameras for plate positioning and computers for storing job data have been available for a number of years now, plate mounting still requires a lot of operator intervention. That said, there is a trend to greater automation of plate mounting equipment and also a demand for it at the higher end of the market, but higher levels of automation increase the capital cost of the equipment, and these days printers are not prepared to pay a premium for features that do not offer suitable benefits.
The greater the automation the harder the equipment has to work to pay for itself by means of improved efficiency and higher capacity. Those printers working round the clock with a number of presses and handling a lot of short run business, or just wishing to mount plates during the day shift, are best placed to capitalise on automated plate mounting.
Automation
The most highly automated piece of plate mounting equipment at present is AV Flexologic’s FAMM (fully automated mounting machine). This machine is said to have the capacity to mount about 75 sleeves on average, with one or two plates per colour, during an eight hour shift. Despite difficult trading conditions, the company sold eight of these machines in 2011.
In practice, an operator places the plate(s) on a feeding belt which automatically transports them into the machine. A manipulator then picks up the first plate and moves it to the mounting position where two cameras compare the actual position of the micro dots to the pre-programmed required position. The manipulator adjusts the plate until it is within the set tolerance that can be varied and pre-programmed into the machine. The pre-taped sleeve, with tape liner removed, is then raised until it comes into contact with the plate and a pressure roller then comes into position on the surface of the plate to roll it into full contact with the adhesive.
After mounting, the sleeve rotates back to its original position where it is checked for mounting accuracy. The company guarantees that the machine will mount all plates of an eight colour job with a relative accuracy of +/- 0.02 mm. The system also offers the benefit of not requiring a skilled operator so mounting plates on the night shift becomes a more viable option.
The Camis Iriflex and Irisleeve plate mounters have been developed to remove the need to manually apply double-sided tape or plates so that all plates of a job can be mounted to the same standard. The manual application of the tape is removed by means of the Taper Slide, which is a motorised tape application system that applies the tape under controlled tension. Tension is controlled by means of a reduction motor and differential unwind system, and adhesion pressure is controlled by the constant 5 kg pressure when the slide touches the sleeve/cylinder. This eliminates the possibility of calliper variation due to uneven tension during application and ensures consistent adhesion.
The need to manually apply the plate to the tape is removed by placing the plate on a plate mounting table with a slot in the middle in line with the plate cylinder. On the table the operator, aided by means of optionally motorised cameras, can move the plate into the correct position for accurate register without it coming into contact with the adhesive. Once in position the plate is automatically brought into contact with the adhesive and a roller used to fully apply it the adhesive under even pressure.
JM Heaford’s wide web 2CS flexo plate mounter is fitted with moving cameras on linear motor drives. The vision system on the machine enables selectable target formats, digital image enlargement, video image recording and other features. The machine has various options to enhance the mounting process including a rear support table for the plates, pneumatic plate and tape laydown roller, plate application carriage, tape application carriage, tape and plate cutting head and a modular proofing system.
If any problems should occur, the company has a support team that offers rapid response, and for remote trouble shooting, all machines with electronic controls can now be fitted with digital communication for remote diagnosis and rectification of any software issues.
The importance of double-sided tape
Double-sided tape plays a critical role in the process of plate mounting, in particular with regard to the speed at which a set of plates can be mounted and the ease with which the plate and tape can be demounted at the end of the print run. Air entrapment must be kept to a minimum, plate repositioning must be readily achieved without damaging the integrity of the adhesive or the tape, and on wide web presses, where it is necessary to apply more than one wrap of adhesive tape, the individual wraps need to be accurately butted together without the need to pull and stretch the tape.
By far the easiest and best way to butt together different wraps of tape is by delivering the tape to the sleeve from some form of applicator. This can be built into the mounting equipment, such as the Taper Slide on the Camis plate mounter, or it can be an additional free standing piece of equipment, such as the AV Flexologic’s Sleeve De-Mount. These applicators minimise the possibility of air entrapment and by applying the tape under constant tension any potential calliper variation as a result of uneven tension is avoided.
3M has developed a special tape liner known as E Liner. This has a cross-hatched pattern that creates a continuous and permanent network of micro-channels that allow air to flow on both side of the tape. By allowing air to flow in this manner, any air that is trapped between the tape and the sleeve or the plate and the tape can escape. The technology is said to virtually eliminate the possibility of air entrapment.
At the end of printing, the speed and ease with which the plate and tape can be demounted become issues, especially if the plate mounting equipment is used to support the sleeves during this process. If plate damage occurs as a result of creasing or delaminating of the polyester layer during plate removal, or adhesive residues need to be removed from the sleeve, this greatly adds to the cost and the amount of lost time.
Best practice should always be used for this stage of the operation to minimise potential problems. The plate should be removed from the tape before removing the tape from the sleeve. This should be done as soon as possible after the print run is completed and is achieved by carefully lifting one end of the plate so as not to delaminate it and then evenly pulling it at a low angle to remove it from the tape. Use of a separate piece of equipment, good working practices and tapes with adhesive that is properly matched to the surface of the sleeve can minimise plate damage and free up the plate mounter and manpower.
tesa has recently produced a new range of tapes called Softprint Fast and Easy to facilitate plate and tape removal after printing and to minimise the risk of plate damage and of leaving behind adhesive residues. The tapes are suitable for use with sleeves or integral steel plate cylinders and are said to significantly reduce mounting and demounting times.
Scapa supplies tapes with a lower tack level on the closed liner side for use with thinner plates and large repeat plate cylinders and tapes with a higher tack for use with thicker plates and smaller plate cylinders, when the plate has a greater tendency to flag.
Developed for easy re-positioning of plates and residue-free demounting, Lohmann’s DuploFlex range of tapes is increasingly being used in conjunction with the high resolution repro systems and the company has been actively involved in the evaluation of many of these systems with numerous flexographic industry co-suppliers, assessing the performance of its plate mounting tapes with these innovations.
Bob Drew, national sales manager, graphics products, said, ‘It is no longer possible for a single component of the printing process to bring a dramatic improvement in the print results. The entire set up must work in harmony to achieve best results.’
An alternative approach
Another approach is the use of Twinlock sleeves. These are plate mounting sleeves covered in a layer of open cell polyurethane foam topped with an outer layer of polymer adhesive. The polymer adhesive never loses its tack and the foam retains its compressibility characteristics, and as a result the sleeves can be reused time and time again.
The typical lifetime of a sleeve is in the order of three years, before it has to be recovered as a result of surface damage. With Twinlock the need to apply tape to the sleeve during mounting and the potential problems that can result is eliminated. As the adhesive is not time related, plate removal at the end of the job is no more difficult than when repositioning the plate during plate mounting. This saves both costs and time. Those printers working with a small number of fixed repeat lengths and/or a large number of job changes are likely to reap the greatest cost benefit.
In the modern press room, where costs are critical and time is at a premium, it is necessary to take a holistic approach to plate mounting. With well chosen equipment and double-sided tape, complemented by good working practices, the plate laying process can be optimised and the associated time and cost minimised. This in turn can reduce the number of plate mounters required or the number of shifts where it is necessary to mount plates.
