Now get profile gauging with SPC and on-line flaw detection.

A new process-control system for extruded profiles and tubing adds surface flaw detection capabilities and continuous SPC monitoring to the traditional dimension-regulating functions of downstream extrusion equipment. Extrusion Services Inc. of Stow, Ohio, integrates the system with its pullers and cutters.

ESI teams the pullers with a customized photo-diode array that scans the extrudate 1000 times/sec at a resolution of 0.0002 in. The system then processes the resulting signal in two ways. Bypassing the PID loop it uses for dimensional control, the system accomplishes a high-speed search for flaws such as "sink-ins or bump-outs," according to president Don Hays. Measurements from the same photo array also run through the PID loop, which automatically triggers the puller-speed adjustments needed to maintain extrudate height and width tolerances.

If the system uncovers flawed or out-of-spec product at the puller, it signals the cutter to produce a longer part for easy identification. The system would then activate a diverter valve to physically eject the bad part from the take-off conveyor, optionally sounding an alarm.

ESI's control system also features full-time SPC charting capabilities to document any dimensional variations or flaws. "It provides a study of the entire extrusion run," says Hays, noting that the system can automatically print X-bar and R charts, as well as histogram reports.

EYE ON CUT LENGTH

Farther downstream, ESI controls part length with two more visual devices. First a movable fiber-optic scanner assembly positioned along the take-off conveyor watches for the end of the extrudate to reach a preset distance from the cutter blade. The scanner then sends a signal to the feeder on the cutter, momentarily stopping the extrudate while the cutting takes place. At the same time, a modified photo-array "takes a snapshot of the part," as Hays puts it. Based on this visual measurement of actual cut length, the system can activate a stepper motor and fine-tune the location of the scanner to compensate for deviations from that preset length. Dimensional drift revealed at the cutter would also kick the diverter valve into action and be recorded in the automatic SPC documentation.

Hays cites accuracy as the system's primary advantage over other systems. Mechanical cut-length control devices commonly are accurate to a few thousandths of an inch, he says, while the new system's visual measurements have 0.0002-in. resolution.

ESI sells the puller- and cutter-based measurement and control systems either together or separately. (CIRCLE 5)