Retrofit safety system developed for hand-fed platen press

In November 2009, Pilz Automation Technology and SCA Display UK, a manufacturer of point-of-display (POD) and point-of-sale (POS) products for retail customers, began working together to improve the safety of a hand-fed platen press at SCA Display's production plant in Shenstone. This plant employs around 80 staff and manufactures mostly custom-designed products made from cardboard. The factory uses a wide range of machinery, including digital and silkscreen printers, die cutters, folding and gluing machines, and platen presses. Most of the die cutting is undertaken on semi-automatic presses, but there is one hand-fed press that is used for short runs and one-off customer specials.

Ian Clews, the Facilities Manager at Shenstone, has been working with platen presses for more than 30 years and SCA is a member of the Confederation of Paper Industries, where machinery safety is high on the agenda. Clews states: "Quite frankly, the use of these machines has been a concern for many years, particularly in the dwell mode. With two fatalities in the UK in recent years, users of these machines need to review their safety systems.

"A typical hand-fed platen press closes with a tremendous force, often with the operator's body in close proximity."

HSE Safety Alerts and Inspections

In Great Britain, the Health & Safety Executive (HSE) has recently issued safety alerts to users of hand-fed platen presses. It is estimated that in the UK alone, there are between 1000 and 2000 such presses that require safety upgrades, mostly operated by companies in the printing and packaging industries. Most of the high-risk machines fall into the 1.4m to 2m-width range.

The HSE is currently considering issuing alerts to users in order to raise awareness of the potential dangers of whole-body access between the platens of these machines when operating in the 'dwell' mode. The alerts are in response to two fatalities since 2008 in the UK, where operators were crushed between the platens as they intervened during normal production. In both cases, the hand-fed machine was being used in the dwell mode and the power to the platen was not isolated before the intervention occurred, hence the safety devices fitted to the machine did not prevent the platens closing on the operator.

Clews explains: "During the setting process, waste removal, replacement of lay stops or misfeed retrieval, even the most safety-conscious operators may try to beat the dwell and climb onto the platen press bed or lean into the machine. Whilst between the fixed and moving platen beds the operator is not detected by any of the usual safety systems."

Dangers of dwell mode

According to the HSE, the latest hand-fed platen presses are much larger than their predecessors and the machine's dwell mode is often used to either give them more production flexibility or to speed up the machine's output, as this mode can often double the speed of operation. On most hand-fed platen presses, climbing onto the platen from the normal operating position in front of the machine can be done without operating the safe edge that is normally fitted to the leading edge of the lower platen. This means that if a safe system of work is not followed, including isolating the machine, the platen may then continue to cycle if it is being used in the dwell mode and the protective devices will not automatically be triggered.

Further investigation by the HSE has showed that standards of guarding and working practices on many existing, larger hand-fed platen presses need to be improved significantly. The HSE has announced that it will commence on-site inspections of hand-fed platen presses in October 2010.

In his presentation to a group of machinery safety suppliers at the HSE's Birmingham offices on 12 July 2010, Tim Small, HM Principal Inspector at the HSE, stated: "The HSE deems hand-fed platen presses as high-risk machines. Enforcement action will be taken against any UK firms that are not taking appropriate action to meet these new safety expectations."

SCA Display UK's requirements

In November 2009, SCA Display UK decided to improve the safety of its hand-fed platen press at Shenstone. Clews states: "By this time, I had already devised several concepts for upgrading the safety of the platen press, but needed to talk to a machinery safety specialist supplier such as Pilz about how the solution could be realised from a technological viewpoint. I also wanted the solution to be modular, enabling a best option for employee protection rather than a cost option. It was critical that we worked with a supplier that had in-depth knowledge of current machinery safety legislation and standards, but with the ability to then apply that knowledge to developing the most appropriate safety system for our machine."

Chris Sloan (Area Sales Engineer) and Stewart Robinson (Consultant Engineer) at Pilz have worked closely with Clews on the platen press project since early 2010. Sloan says: "Initially we visited Shenstone to assess and analyse the platen press and to discuss safety issues and potential solutions. SCA wanted a working solution based on a modular safety platform that would include all the necessary hardware and software, tailored to suit the machine."

Robinson adds: "One of Ian Clews' favoured concepts was to use an array of light beam sensors that would monitor the side and platens in order to detect any violation of the machine's danger zone whilst the press was closing. As far as we are aware, the solution we have developed is the only one of its kind in the UK, where light beam sensors are being used to monitor the area directly above the moving platen."

Configurable safety controller

The overall safety solution is based on a Pilz PNOZmulti modular safety system. The PNOZmulti is multifunctional, freely configurable and designed for monitoring safety functions such as emergency-stop switches, safety gates and light barriers, as well as performing standard (non-safety) control functions. Rather than hard wiring, safety circuits can be configured on a PC using the PNOZmulti Configurator software. This configuration can then be stored on a memory card and downloaded to the PNOZmulti base unit.

For the hand-fed press project, a PNOZmulti base unit is used, with input and output expansion modules to control all aspects of machinery safety and control. Robinson says: "We wrote a functional specification for the press, then engineered a solution based on these functional safety requirements. We supplied all the hardware, including the light beam array and safety controller, as well as writing all the software code for the application."

The hand-fed platen press at Shenstone is fitted with a number of general-purpose safety devices such as trip bars, interlocked guards, right- and left-hand emergency stops, safety mat, front safe edge, lumbar bar, safety gate key switch, and left- and right-side safety light curtains. All of these devices have the same functional safety requirements, which are to prevent all hazardous movements by ensuring that the clutch safety valve is de-energised and that the main motor is stopped, and to prevent unintended restarting. The target safety integrity level for each general safety function is SIL 3 (EN 62061), which is broadly equivalent to PLe (Performance Level e) according to EN ISO 13849-1.

Safety light beams

As well as the PNOZmulti, the hand-fed press now uses an array of Pilz PSEN op4S light beams. The light beams detect a violation of the danger zone while the press is closing; this is achieved by sequentially muting the beams as the press closes. If any beam is broken before it has been muted, the press clutch safety valve is de-energised, immediately bringing the hazardous movement to a stop. Once the final beam has been broken (press closed) the beams remain muted until the press cam switch has operated, signalling that the press is at the 'home' position. The muting points are derived from the simulation of press position carried out within the PNOZmulti control system. The application also includes checking that the press cam switch operates correctly in each press cycle.

It should be remembered that this safety system must be accompanied with robust, safe systems of work, training, regular maintenance and risk assessments.

Clews concludes: "The services provided by Pilz were exceptionally good and I cannot fault them at all. Any modifications or software updates were carried out quickly as and when required, and technical support from Pilz engineers was excellent. The working relationship was very close and has resulted in a truly collaborative solution. The project itself was completed successfully on the 16th of July and the press was handed over to our production team two weeks later."

Please email [email protected] for more information about similar projects, telephone +44 (0)1536 460766 or visit www.pilz.co.uk.