PLC control takes screen printing to higher plane

PLCs and drives from Omron have helped Registerprint develop a novel multicolour screen-printing machine.

Owners of single-colour screen printing machines, can now upgrade to multicolour printing using a new sheet transfer and curing system.

Named Printlink and developed by Registerprint Machinery of Coulsdon, the system is so radical that it stands to change the industry forever.

Screen printing is used for printing medium to short run posters, point of sale displays, official notices and signs, PCBs, white goods, glass windscreens, gaming machines etc, but to date has largely been restricted to single-colour printing.

The multicolour machines that are available, are generally large format and can cost between GBP 400,000 and GBP 1,000,000.

Registerprint decided to develop a drying/transfer system that could use machinery purchased up to 15 years ago.

Therefore a screen printer with an existing Printmaster line could, for a much smaller outlay, add another Printmaster and a Printlink to form a multicolour print line with a registration accuracy of +/-0.5mm.

Registerprint understood the mechanics of what would be required to produce multicolour screen-printing lines from a machine that was originally designed for single colour printing, and worked with the technical distributor More Control and equipment supplier Omron Electronics to assist with the development of the sophisticated control system the new machine demanded.

The basic concept was to collect and control the sheet as it exited the first print station by means of a transfer table that held the sheet, by means of a vacuum, so its position is maintained and the first colour is dried sufficiently to have the second colour applied.

The table, with the sheet still under vacuum control, moves forward to become the infeed of the second (or subsequent) print machine.

When the next print machines gripper takes the sheet from the transfer table it is imperative that the vacuum is released at exactly the correct moment.

The main problem was one of maintaining the registration accuracy and timing between print and transfer stations of completely separate machines at the speed needed for economic printing.

A second problem was developing a control system that could be manipulated on the fly to adjust printing parameters from a master or satellite touch panel.

Registerprint's Technical Manager, Gary Tolchard assessed the problem of position control and immediately suggested that the answer lay with a particular PLC, the Omron CQM-1H, to which a dedicated position control module can be added.

A suitable package of CQM-1H and remote I/O terminals was supplied by More Control for a prototype machine and proved that it could easily do the job.

Registerprint made the decision to go for an all-Omron control system and set about the designing procedure.

The CQM-1H also provides control for the actual transfer mechanism, which is in two planes for flexibility of machine lay out.

The actual motions have to be so smooth that the sheets are not disturbed from their registration position despite the high accelerations, decelerations and speed required.

Typical working speeds are around 800-1000 impressions an hour, each with table out and return stokes of say 500mm.

The natural assumption is that to achieve these levels of motion control, a servo drive system has to be used.

But when the Registerprint designers looked at the smoothness of motion required to keep the paper sheet in position on the transfer table, it was realised that an air drive would be a better solution because of its intrinsic cushioning and its ease of maintenance in the field (an important issue as the machines are likely to be exported around the world).

The control system timing also had to ensure accuracy between the print heads and transfer station.

This however put considerable onus on the designers to develop a control system that could guarantee such accuracy of motion in an air drive: fortunately the Omron CQM-1H proved it was more than up to the task.

Sheet position sensors were built into critical points of the transfer table and connected to the CQM-1H by Omron's E3X DA fibre optic amplifiers.

The amplifiers provide a digital display of signal strength and cope with sheet colours from clear to black with ease.

An NT31 touchscreen is provided on each table's stand for local control and adjustment, and the information is duplicated to remote terminals used for overall machine management.

The system is completed with Omron power supplies and relays.

A final matter to be addressed was the operating environment, as the transfer tables would run under an ultraviolet curing system, Registerprint wanted to be sure that the control components would be able to stand up to this radiation for prolonged periods.

The machine has caused an immediate stir in the screen printing world, with interest running high.

Registerprint originally expected two to four colours to be the normal requirement, but potential users are looking into the possibility for more.

The Omron control system can handle up to 32 different colours if required, although the practicalities of the process, limit this to six or seven colours.

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