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Packaging machine OEM converts from pneumatics to servo controllers

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MELBOURNE engineering group, Scope Machinery, specialises in the design and manufacture of ‘contact heat’ thermoforming machines which use heat and compressed air to produce moulded plastic packaging ‘trays’.

Cycling around once every four seconds, rigid plastic film is fed onto a heated platen, blow-formed into a custom-designed mould, and cleared.

To address the critical issues of accuracy and speed, the company introduced SEW-Eurodrive servo control for two key machine functions.

The first servo application was indexing for the plastic feed roll, which involves drawing the film through the machine to a precise length. Previously, it incorporated a standard induction motor and frequency inverter, with the feed length monitored by an encoder; however, accuracy depended on speed.

“Even at optimum tuning, it was only accurate to a couple of millimetres,” Scope Machinery’s automation manager Peter Bushe said.

The plastic feed roll is now driven by a 2Nm DY servo motor and 1.5kW Movidrive servo controller which is said to have improved accuracy, throughput, and ongoing running costs.

SEW-Eurodrive applications engineer, John Gattellari explained a servo system is a positioning system. “It stops bang-on where you tell it to. It means that we can set it to feed forward to within one micron of where we want it. There’s zero wastage.”

Following the project’s success, Scope approached SEW to apply 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 knife edges sever the moulded parts from the film. The parts are then released from the mould, and transported away.

Bushe said the cutting force of the press was a major limiting factor of the machine. “The force affects the equivalent length of knife edge we could cut which in turn affects the size of the tooling,” he said.

The original 30t press comprised a toggle mechanism driven by an 8” pneumatic cylinder.

Replacing the pneumatics with a servo-driven crank incorporating a 24Nm DY servo motor and 15kW Movidrive servo controller, is said to have increased the cutting force by more than 50%.

This permits a 50% increase in equivalent knife-length, meaning more parts can be moulded per cycle.

“Servo control also provides much greater speed and precision control of the stroke,” Bushe said.

During heating and forming, the plastic film is clamped by the mould’s knife-sharp edges at half its gauge thickness (0.1 to 0.75mm).

The final cut requires the crank/toggle mechanism to move the cutting plate a fraction of a millimetre up against the knives.

A human-machine interface (HMI) provides access to the acceleration and positioning parameters of the servo controllers. It can also store a variety of pre-determined set-up parameters.

“Operators can adjust servo and process parameters, such as stroke and temperature, on the fly,” Bushe said. This is said to provide great flexibility and reliability, and facilitate product changeovers.

The servo-driven systems are also said to reduce energy consumption. Bushe estimates the thermoforming machines run at 85 to 90% efficiency, compared with pneumatic systems which operate at around 32% efficiency.

An overall speed improvement of 25% over the pre-servo machines has also been achieved.

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