MANUFACTURED and shipped in the millions, moulded plastic packaging is a growing industry. Melbourne engineering group, Scope Machinery , specialises in the design and manufacture of “contact heat” thermoforming machines - machines that use heat and compressed air to produce 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.
To address the critical issues of accuracy and speed, Scope has introduced SEW servo control for two key machine functions, also improving ongoing running costs. “Servo control has enabled us to take these machines up a notch,” said Peter Bushe, Scope Machinery’s automation manager.
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,” said Bushe.
The plastic feed roll is now driven by an SEW-Eurodrive 2-Nm DY servo motor and 1.5 kW Movidrive servo controller.
“A servo system is a positioning system,” said John Gattellari, SEW-Eurodrive applications engineer. “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 success of this project, Scope approached SEW to discuss 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.
“A major limiting factor of the machine was the cutting force achievable by the press,” Bushe said. “The force affects the equivalent length of knife edge we could cut - which in turn affects the size of the tooling.”
The original 30 tonne press comprised a toggle mechanism driven by an 8 inch pneumatic cylinder. When the pneumatics were replaced by a servo-driven crank - incorporating a 24 Nm DY servo motor and 15 kW Movidrive servo controller from SEW-Eurodrive - the cutting force was increased by more than 50 percent. This permits a 50 percent 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,” added Bushe. During the heating and forming stages, the plastic film is clamped by the mould’s knife-sharp edges at half its gauge thickness (0.1 to 0.75 mm). The final cut requires the crank/toggle mechanism to move the cutting plate up against the knives - a fraction of a millimetre.
A human-machine interface (HMI) provides access to the acceleration and positioning parameters of the Movidrive servo controllers, plus has the ability to store a variety of pre-determined set-up parameters.
“Operators can adjust servo and process parameters, such as stroke and temperature, on the fly,” said Bushe. This provides great flexibility and reliability, and facilitates product changeovers.
Other benefits of the servo-driven systems include an overall reduction in energy consumption. Pneumatic systems are around 32 percent efficient; however, Bushe estimates the thermoforming machines are running at 85 to 90 percent total efficiency, with an overall speed improvement of 25 percent over the pre-servo machines.