Motion Integrator Solves Push-Pull Problem

Evaluation of one problem reveals the cause of another.

By Staff -- Control Engineering, 12/15/2005

Polyair manufactures protective packaging products for many industries, including pharmaceutical, automotive, electronics, catalog and mail order, publishing, and cosmetics. For years it used converting equipment to produce air-cell lined envelopes, but outdated controls and component failure were causing frequent shutdowns.

On the advice of its drives-and-controls supplier, Bosch Rexroth, Polyair asked motion-control system integrator Custom Control Solutions (CCS) to design a retrofit solution that would update the equipment's antiquated controls. The goal was to reduce downtime and scrap.

The converting system combines bubble wrap and recycled paper, or polyethylene, and converts it to padded envelopes, such as those sold in office product stores or at the post office. It assembles two rolls of plastic film to make air-filled cells; laminates the air-filled cells to paper; and then folds, glues, and cuts the material to produce the final product.

CCS evaluated the system's performance, identifying several control and communication corrections. It was during this evaluation that CCS' president, Dave Stuber, discovered a subtle PLC-programming error. It was responsible for the frequent shutdowns and component failure, motivating Polyair's decision to take the converting system out of production.

According to Stuber, the webs of recycled paper need to match precisely with the webs of bubble wrap. However, in Polyair's case, system heat-sealing bars that join to seal and form the envelope's sides and bottom were so out of sync with the web motion that they often were still closed when the webs resumed movement. Thus, the servomotor was pulling material through the system while the brake near the material feed station was still engaged. CCS revealed the servomotor problem to be a PLC logic-timing error in the feed cycle, which caused momentary overload of the feed motor during each machine cycle.

Stuber and his team then conducted a detailed study of the existing program logic and machine timing. They identified and corrected the problem with a retrofit of the drives and controls package using Rexroth servomotors, servo drives, and a motion controller. One servomotor is responsible for pulling the web materials through the converting process from the roll stand through laminating and cutting stations and out the discharge end. The other servomotor actuates the cutting action that separates finished envelopes from the continuous rolls.

Besides installing the new controls, CCS created a comprehensive communications program, including modifications to the original PLC. CCS also designed a control panel containing the new drives and motion controller and a new operator console with a color touch-screen interface. The old operator interface and job-recipe storage program contained over 100 screens, which made operator training cumbersome. CCS reduced the number of screens to five, yet still satisfied all functions required for the operator to run the machine. Polyair operators found the new system much easier to understand and operate.

"Since CCS completed the drive and control retrofit on the converting system, Polyair has had significantly greater uptime, which, at an operating cost of $1,000 per hour, quickly adds up," Stuber says. He also notes that the retrofit caused virtually no disruption to normal production, and the newly corrected and coordinated timing (among the web feed, heat sealing bars, and brakes) means far less scrap, saving as much as 2,000 linear-feet of bubble wrap and paper daily.

Likewise, incorporation of fewer operator interface screens has streamlined operator training, which is now achieved in about half the time. Polyair also saves up to 20 minutes every time it changes from one envelope size to another due to the simplified design and consolidation of recipe screens.