Drives & controls - Drive to reduce energy pays dividends for PET bottle producer

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Rockwell Automation has worked with Proplas International to help Europe's leading supplier of rigid plastic packaging to reduce its energy costs by £71,000 a year, with a payback period estimated at just 14 months

ONE?OF Europe's leading suppliers of rigid plastic packaging is reaping the benefits of an energy saving retrofit on 13 injection-stretch-blow moulding machines. Engineering services provider Proplas International fitted 13 of the packaging specialist's PET bottle and jar moulding machines with Rockwell Automation's Allen-Bradley PowerFlex700 variable speed drives, helping to reduce energy consumption by over 27 percent.

From its base in Burnley, Lancashire, Proplas International has built a reputation for helping industries to become more efficient - something which all companies will recognise as being vital in today's increasingly competitive market place. Amongst other areas, the company has become widely known for its energy saving projects, with a particular focus in injection moulding machines of all types where Proplas is a recognised leader in saving energy.
In a recent project, Proplas fitted Allen-Bradley PowerFlex 700 drives to 13 injection-stretch-blow-moulding machines at Europe's leading supplier of rigid plastic packaging. The machines produce a range of bottles and wide-mouth jars in Polyethylene-Terephthalate (PET) for the food and drinks industry - containers that are used by a range of industry-leading manufacturers as well as by various supermarkets for their own-brand food and drink products.

The injection-stretch-blow-moulding process is used because of its capability to produce high quality containers. In the process, molten polymer first flows into the injection cavity to produce the desired preform shape. A period of conditioning at a set temperature follows, after which the preform is ready for stretching and blowing into the finished shape. The preform is transferred to the blowmould area, and the mould closes. A stretch rod is introduced to stretch the preform lengthways, whilst differential air pressure is used to blow the preform out to the shape of the mould. Finally, after a set cooling time, the mould opens and the finished container is removed. In practice, the four stages are carried out concurrently with a revolving carousel of moulds.

Proplas director Stephen Anderson comments: "The machines were being driven by hydraulic pumps, with their motors set at a constant speed that would cater for the maximum hydraulic demand - the periods in the process where the moulds are opening and closing. But this is an inefficient process, since the energy usage remains constant (and high) while the actual power demand varies."

The Proplas solution was to fit variable speed drives to the pump motors, operating at two preset speeds - a higher speed during the maximum hydraulic demand when the moulds are opening and closing, and a lower speed during the periods of reduced demand.

"We performed a number of tests with the company to demonstrate the potential, and convinced them that there could be major energy reductions and associated cost savings," said Anderson. "As a result, we were asked to retrofit a drives solution to 13 of the company's injection-stretch-blow-mould machines."

The retrofit was built around 13 Allen-Bradley PowerFlex 700 drives, with each drive connected to an Allen-Bradley Pico micro PLC to provide the simple sequencing logic.

"The available torque was a key issue in this application, because the profile is characterised by spikes in the torque demand that can cause many other drives to trip," says Anderson. "The PowerFlex drives delivered the same torque in the actual application that was promised on the datasheet, and in this case that capability allowed us to downsize to a lower power product without any fear of it tripping out or the motor stalling, helping deliver further energy savings for the customer."

For the customer, all this has meant some substantial energy savings. The power usage of each machine before the modification was 40kW, but with the help of the drives retrofit this has been reduced by 11kW. With every kilowatt equating to around £500 in energy costs, the project is estimated to be saving the customer over £5000 on every machine, with an estimated payback period of just 14 months.