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January 1, 1997

6 Min Read
Imaging: The mold and beyond

Hartzell Manufacturing, based in Minneapolis/St. Paul, has a custom molding and assembly plant in St. Croix Falls, WI, a town of about 1500 people some 50 miles northeast of its headquarters. Relatively isolated as the shop is, this molder of consumer, medical, and electronic products employs about 200 people and has developed a sense of self-reliance that makes it technologically innovative and aggressive, especially for such a small company. Says Al Neumann, automation manager at Hartzell, "Everything we purchase is with a sense of being self-sustaining. We're on our own out here."

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To protect its molds from damage caused by unejected parts, Hartzell installed this vision system from Avalon. It takes a snapshot of the mold to verify that parts are correctly and fully formed, then properly ejected.

Case in point is a large telecommunications company that has Hartzell molding pager covers to stringent tolerances. The molder is judged not on bad parts per hundred or thousand, but bad parts per million. Three parts per million, actually. Because of this, Hartzell has developed a keen sensitivity to damaged molds and nonconforming assembled parts.

The front and back pager covers are molded of polycarbonate in four-cavity steel molds filled with thin walls, multiple slides, and fine details, making complete ejection extraordinarily important to protect the mold. Early in the project, Neumann says, parts frequently stuck on the mold, even after two or three ejection strokes. When the mold closed, damage caused by the stuck part forced removal and repair of the mold, at great expense in tool costs and downtime. Neumann says these molds averaged about three visits per week to the tool shop. Depending on the damage, downtime varied from two days to two weeks, often negatively affecting Hartzell's ability to deliver product on time. He adds that even when repaired, molds never performed exactly as they did before, causing added re-engineering adjustments on the press.

To solve the problem Hartzell tracked down Avalon Imaging Inc., in Boulder, CO. The molder installed Avalon's SafetyCycle imaging system on one of the 85-ton Cincinnati presses that was molding the pager parts. The system consists of a controller, an LED light source, and CCD camera.

For those who haven't seen one work, here's what happens: The light and camera are mounted on top of the press and trained on the B side of the mold. The LED light source illuminates the mold. When the mold is opened, the camera takes a snapshot of the mold, verifying all parts are present and fully formed. After the eject cycle, the camera takes a snapshot of the mold again, verifying that all parts were properly ejected or removed. If a part is detected in the mold, the imaging system's controller, which is tied to the press's controller, stops the cycle and sounds an alarm for the operator. The image captured by the camera is displayed on a press-side monitor.

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In the assembly area this vision-guided Scara robot receives pager back covers and places in each the five rubber pads that will support the printed circuit board.

According to Dan Stratton, midwestern regional sales manager at Avalon, configuration of the imaging system requires the operator to "teach" the controller. That is, he or she must give the system two "reference" images, one a picture of a mold with full and correct parts, the other a picture of the mold free of parts. When operating, the imaging system uses these references as a standard of comparison. An image that falls outside that standard stops the press and generates an alarm. This also allows an imaging system to detect flash and short shots, Stratton says.

In Hartzell's case, the focal area of the camera is variable, allowing Hartzell to focus on a part of the mold or the entire mold. The camera's sensitivity is also adjustable, depending upon material used and possible inclusion of inserts. Three years after the first Avalon system was installed, Neumann says mold damage from unejected parts now is not an issue. "I can't think of an instance where the vision system was set up correctly that we've had a problem with a mold closing on a part," he says. Neumann says it is procedure now to use vision systems on all automatic jobs. Addressing concerns about increased cycle time, he points out that imaging systems eliminate spare ejection strokes. "I would say categorically that it decreased our cycle time because we had multiple ejector strokes per cycle just to be sure the parts fell out," he says. He adds that Hartzell pays $10,000 to $15,000 per imaging unit.

Hartzell's imaging and innovation don't stop at the press, however. Neumann points out that Hartzell is a custom assembler as well as molder, and the guidelines that dictate the quality of parts off the press continue in the assembly area. It's here that the company's vision creativity first took root. The centerpiece is an array of custom vision-guided assembly robots, the first used by a non-Fortune 500 company, according to Neumann. This vision and robot system was developed by Hartzell and Adept Technology, a robot and vision system manufacturer in San Jose, CA.

In the assembly area, back covers of the pagers are delivered and nested in assembly fixtures. Here a vision-guided Scara robot places in the back cover five rubber pads to support the printed circuit board that the pager manufacturer will insert. The pads have an adhesive on one side and measure about .0625 by .0625 by .25 inch. The robot, able to "look" to a space about half the size of a postage stamp, picks up each pad and puts it in a designated space on the back cover.

After placement, the vision system uses Adept's VisionWare software to measure X and Y coordinates on the pads to verify accurate placement. Neumann says that in other applications, a picture is taken of the part and the vision system then uses "windows" placed relative to a known part feature to locate the components to be checked. The system then does a pixel count to verify that the pad is not only accurately placed, but aligned as well. Neumann says this design has cut processing time in half.

"The idea," he explains, "is to eliminate the variable of pads not being in the same place." Pads that do not pass muster are given a no-go condition and pulled by operators who remove the pads and send the cover to the back of the line.

Meanwhile, each pager front cover also goes through a similar machine vision application. Here the front covers pass beneath a vision system, again developed with Adept, that checks for the presence and accuracy of a sonically welded plastic lens, grille cloth, threaded inserts, a stainless steel pin, a rubber pad, and battery clips. Like the back cover, those that fail the test are given a red light and pulled by an operator.

Back and front covers are packaged separately and shipped to the customer, who finishes assembly with the electronic guts that form a complete pager. To date Hartzell has honored that three-parts-per-million requirement while maintaining efficiencies that allow the company to compete with overseas labor rates.

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