The scope of servo control

While many consumers might consider the brand of biscuits or batteries they've just bought, few would wonder at the source of their packaging. Yet both the biscuit tray and blister pack are critical components of such pre-packaged items in the shopping trolley. Manufactured and shipped in the millions, moulded plastic packaging is now almost a product in its own right - and the industry is growing fast.

Melbourne engineering group, Scope Machinery specialises in the design and manufacture of 'contact heat' thermoforming ma-chines: machines that use heat and compressed air to produce the now-indispensable moulded plastic packaging trays. Cycling once every four seconds or so, rigid plastic film is fed onto a heated platen, blow-formed into a custom-designed mould, and cleared.

The number one challenge is getting the machine to run accurately and as fast as possible. The raw material costs of plastics such as PVC and PET are dependent on oil prices. It's an industry with shrinking margins, and the packaging manufacturers depend on maximum possible throughput.

In-feed indexing

The feed system, which involves drawing the film through the machine to a precise length for each cycle, previously incorporated a standard induction motor with frequency inverter. The feed length was monitored by an encoder mounted on the gearbox, however the accuracy depended on the speed, with acceptable accuracy achieved at a given 'optimum' speed. But even at the optimum tuning, it wasn't really that spot-on. It was accurate to perhaps a couple of millimetres.

After a few technology evolutions, the plastic feed roll is now driven by an SEW-Eurodrive 2 Nm DY servomotor and 1.5 kW Movidrive servo controller. A servo system is a positioning system which stops exactly where it is meant to, there's no worrying about slow-down distances. This means that it can be set to feed forward to within one micron of where it is required. There's zero wastage, which is a significant improvement considering the previous situation of two millimetres of waste for every four-second cycle.

The servo-driven feed system resulted in significant speed and accuracy improvements, but more was to come. The company also wanted to look at applying servo technology to the thermoforming machine's most critical component, the pneumatic press/cutting mechanism.

The press is responsible for moving the heated platen (or cutting plate) up and down against the mould's knife-sharp edges. During forming, the film is trapped between the cutting plate and the edges of the mould; once forming is complete, the mould's knife edges sever the moulded parts from the film.

The parts are then released from the mould and transported away. Thus the press provides the critical clamping, cutting and clearance of virgin film and formed product.

On the knife's edge

A major limiting factor of the machine was the cutting force achievable by the press. The force affects the equivalent length of knife edge we could cut with, which in turn affects the size of the tooling.

The original 30 tonne press comprised a toggle mechanism driven by an 8" pneumatic cylinder. When the pneumatics were replaced by a servo-driven crank incorporating a 24 Nm DY servomotor and 15 kW Movidrive servo controller, the cutting force was increased by more than 50%. This permits a 50% increase in equivalent knife-length (trimming each moulded part), meaning more parts can be moulded per cycle. The servo-driven machines can now cut more than seven equivalent metres of PET, which is the hardest thermoforming polymer to cut.

Servo control also provides much greater speed and precision control of the stroke. Once the position of the mould has been mechanically adjusted relative to the cutting plate, it can be fine-tuned using the servo. This is illustrated by the accuracy required during clamping and cutting of the plastic film. During the heating and forming stages, the plastic film is clamped by the mould's knife-sharp edges at half its gauge thickness, where the gauge thickness is typically 0.1 to 0.75 millimetres. The final cut requires the crank/toggle mechanism to move the cutting plate up against the knives, so essentially the maximum work done by the press involves a movement of a fraction of a millimetre.

Both Movidrive servo controllers communicate with the thermoforming machine's onboard automation system via the DeviceNet open device-level communications network. A human-machine interface mounted on a swing-arm provides access to the acceleration and positioning parameters of the drive, plus has the ability to store a variety of predetermined set-up parameters. The automation system allows the operators to adjust servo and process parameters, such as stroke and temperature, on the fly. This provides great flexibility and reliability and minimises the time taken in setting up, which makes product changeovers quick and easy.

Other benefits of the servo-driven systems include an overall reduction in energy consumption. Unlike pneumatic systems, which are around 32% efficient, servomotors in isolation can approach 100% efficiency. The thermoforming machines are estimated to be running at 85 to 90% total efficiency, with an overall speed improvement of 25% over the pre-servo machines.

Special projects

Scope is planning to update to the locally assembled CM servomotors in conjunction with the Movidrive 'B' intelligent application inverter and servo controller. The CM servomotors offer improved functionality and performance, and with more torque per motor size they are less costly on a per Nm basis. The Movidrive 'B' servo controller offers a host of advantages for Scope, such as a category three safety stop feature, faster scan rate, interchangeable memory card, and simplified control circuitry.

Machines under development by Scope's 'special projects' department include a new machine specially designed to bond fine membranes to the ends of wine corks, an innovative solution developed by Melbourne company ProCork for the elimination of cork taint.

The corks are loaded onto a rotating turntable, around which are located the various stations for bonding the membranes to cork-ends under heat and pressure. Speed and accuracy are critical, as the films need to be applied and adhered to the dead centre of each upended cork at a rate of 15,000 per hour. Scope uses servo control here for the indexing and rotation of the turntable.

Movidrive 'B' is being used as the servo controller for the turntable application, and in particular the pre-programmed 'Modulo' application module. The Modulo function of the drive is used to self-correct the indexing with every rotation. Even with non-integer gear ratios, Movidrive 'B' Modulo ensures that the zero point stays zero for every rotation.

Whether focusing on thermoforming machines or the growing area of special engineering projects, Scope Machinery recognises the merit of bringing in the drive supplier at the design/concept stage when it comes to servo applications.

More and more modern processes depend on the art of precision positioning, particularly as margins get squeezed and customer expectations grow. Consumers may remain unaware of the origin of the blister packaging found on supermarket shelves, but to those who count in the industry, the scope of servo control may be a deciding factor.