Linear motor adds zip to resealable food packaging.

* dual-forcer motor

* software driven

Supreme Plastics, a European manufacturer of resealable flexible packaging, supplies slider and zipper systems for high volume use on form-fill seal machinery. The company's latest zipper applicator for continuous vertical form-fill-seal and horizontal flow-wrapping equipment achieves throughputs of up to 120 pouches/minute.

The machine applies 7 mm zippers across a continuous plastic web, which eliminates "leakers" at critical points because the zipper closure does not form part of a side weld. The zip feed mechanism is driven by a rotary servo motor, mounted on a fast-accelerating linear motor axis that track the speed of the web. Then the zipper is sealed into place using a heating element. To achieve the high throughput, two of these attachment heads travel--a pouch length apart on the same linear magnet track. The track is lm long to offer flexibility in pouch size.

The linear stage is based on a cog-free brushless motor for smooth operation. It carries two forcers, each with a zip feeder and heating element, which sit at home position until the controller senses the next registration mark. The forcers then accelerate rapidly and, when synchronized with the web, the heating element switches on to seal the zips into place. The forcers then accelerate back to the home position at over I G to await the next application cycle.

The machine's four axes of motion (two rotary BSM servomotors and two forcers on the LSS linear stage) are controlled by a NextMove BX motion controller and four FlexDrive servo drives. This controller also incorporates the I/O required for the various sensing and actuation functions associated with the process, such as registration mark detection and zip position sensor. Using CANopen fieldbus communications, the motion control system links to an operator panel which allows the operator to define zip length, pouch size, etc.

All the motion, I/O and human-machine interface system components, as well as the application software written in Mint motion language, were provided by Baldor Electric Co. Development time was greatly reduced by means of Mint's built-in multi-tasking operating system. This feature was used to divide the major control functions of the machine--zip feed, linear motor and man-machine interface--into separate tasks. In addition, the Mint language features "keywords" to speed software creation. For example, a single keyword allowed engineers to correlate the linear motor heating element axis to the web material, so that the applicator automatically tracks the web.

Stuart Bratton, the Baldor applications engineer who developed the motion control element, notes, "The use of a multiple forcer arrangement on a single linear motor platen means that this machine has very simple mechanics, greatly aiding reliability and maintainability. It also provides an architecture that can be easily upgraded if higher speeds are required. Another zip attach head may be added simply by lengthening the track and running another forcer element, which would increase throughput with minimal impact on the mechanical system."

Baldor Electric Co, Fort Smith, AR. Circle 146, or www.rsleads.com/508df-146