Moving to new levels of speed and performance sometimes demands a radical change in machine design philosophy and approach. For Supreme Plastics Ltd. (www.supremeplastics.com), increasing the performance of its vertical form-fill-seal, horizontal flow wrapping and bag making machines from 75 to 120 pouches per min has required a fundamental change from intermittent to continuous motion operation. And a unique linear motion system using two zip attachment heads running along the same magnet track creates a continuous feed zip insertion system that keeps pace to improve throughput.
Speed and Simplicity
"Speed and simplicity are the key elements of the new machine design," says Murray Leighton, technical sales manager for Supreme Plastics. "Moving to a continuous motion process has enabled us to save time by attaching the zipper to the film while the film is moving. The new design gives us a higher stroke and a longer sealing time by attaching the zipper to the moving web."
Leighton says that, in looking at a continuous motion concept for the machine, company engineers decided they'd handle the complex task of synchronizing zipper placement and moving web in software, rather than resorting to a mechanical system. Another goal focused on keeping the HMI simple. In the final design, the only parameters the operator can change are zip length, zip position, and dwell—making the system a cinch to operate.
All other system parameters are controlled in software by the electronic controls on the machine. The system includes a servo zip feed, a matching knife cutoff, and a linear motor with a pneumatically actuated seal bar that attaches the zip to the film.
With the previous intermittent design, machines produced up to 75 pouches per min. Leighton says that when they began targeting the 150 pouches-per-minute mark, they realized a web arrestor wouldn't work. The faster the system performs, the more acceleration affects the film. That force creates vibrations and dynamic instabilities in the entire process, he says.
Zip Insertion System
The new machines use an advanced linear motor system from Baldor Electric (www.baldor.com) to create a unique zipper applicator. Two zip attachment heads run along the same magnet track, and allow the Reseal 460X machines to achieve throughput of up to 120 pouches per min.
To support continuous operation, the zip feed mechanism is driven by a rotary servo motor which is mounted on a fast-accelerating linear motor axis that tracks the speed of the plastic web and seals the zipper into place using a heating element. For high throughput, two of these attachment heads travel a pouch length apart on the same linear magnet track. The track itself is approximately a meter in length to provide great flexibility for bag lengths between 160 and 300 mm long and zip lengths between 90 and 250 mm.
In operation, the linear stage carries two carriages each with a zip feeder and heating element. The carriages sit on a home position until the controller senses the next registration mark. They then accelerate rapidly. After matching speeds and synchronizing with the web, the heating element switches on to seal the zips into place. The carriages then accelerate back to the home position at over 1G in preparation for the next application cycle.
"We specified the linear motor because it was the easiest, most reliable way to transport the carriage," says Leighton. "We could have used a servo with a ball and screw but decided that the inertias, speeds of the motor through the gearbox, and accuracies required made it a logical application for a linear motor."
Stuart Bratton, an applications engineer for Baldor, says that the concept of "carrying" the zip carrier and heating element on a single linear carriage—which mechanically couples them via a C-shaped bracket—automatically synchronizes these system elements. The C-shaped fixture also carries a rotary servomotor for feeding zip material into the carrier.
An additional benefit of this concept is its unique expansion opportunities. For the machine to go faster, it needs only an additional carriage and zip carrier, a rotary motor, and a heating element. After such an expansion, each zip head has only to process every other bag.
Leighton says that the biggest engineering challenge in implementing the new system proved to be matching the film speed of the host machine. Rollers used in the process did not provide an ability to synchronize the two machines effectively. By using a high resolution encoder directly driven by the main drive of the host machine, the designers eliminated the problem.
"We now use a 4,000 pulse per cycle encoder which is directly driven from the film drive on the host machine," says Leighton. "That provides highly accurate film speed, and an independent eye on our machine provides film position." The combination of the encoder and eye lets a zipper adhere to the film with ±0.1 mil accuracy as the film speeds by at 18 m a min.
Another challenge in designing the new machine was attaching the zip to the high-speed web and keeping it there while the unit was in motion. Supreme eventually used a very small vacuum to resolve that issue. Getting the zip to pass reliably through all of the machine rollers was a another challenge. To solve that problem, engineers relieved the rollers to allow clearance for the zip (a cross section of 3 mm × 2 mm).
The focus of mechanical changes in the new machine concentrated on taking weight out of the zip insertion system in order to put it on the carousel and move it rapidly using the linear motor. This required a major redesign of the zip application carriage to reduce the accelerations required, with a goal of taking as much inertia out of the system as possible.
Controls System Approach
The machine's control system has four axes of motion (two rotary servomotors and two carriages on the LSS linear stage) which are controlled by a Baldor NextMove BX motion controller and four FlexDrive servo drives. The controller incorporates I/O for the sensing and actuation functions of the process, such as registration mark detection and the zip position sensor. Using CANopen fieldbus communications, the motion control system links to the operator panel which allows the operator to specify the limited number of system set-up parameters.
Stuart Bratton of Baldor wrote the application software using the Mint motion language. Multi-tasking capabilities made it possible to divide the major control functions of the machine—controlling the zip feed, the linear motor, and the man-machine interface—into separate software tasks. The availability of application-level software in the form of "keywords" allows a single keyword to synchronize the linear motor heating element axis with the web material, so that the applicator automatically tracks the web.
Leighton says that the software development of the machine went smoothly, after dealing with the dynamic issues in the machine by moving to the direct drive approach on the main drives. In the future, he says they will look at continuous operation for other machines and make product improvements, such as adding slider applications. The current machine is adapted for press-to-close zippers but they are looking at putting a slider on it as well.