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September 16, 1998

5 Min Read
Improving Uptime by Avoiding Failures

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Figure 1. Internal contaminants carried on the shaft of a linear potentiometer in and out of its electronics through worn seals can lead to false readings and lost production.

Bill Reinhart, owner of White Oak Plastics Inc. (Williamsburg, OH) started his custom injection molding operation about three years ago, determined to do the jobs other shops wouldn't touch. But, like any other molder, Reinhart knew he needed the proper tools. He says, "You've got to have the uptime. And when somebody calls and says they need parts, there's no question of having machine time available."

As a start-up niche molder, Reinhart could ill afford the reliability and accuracy problems caused by maintenance-prone linear pots. "We're accustomed to linear position sensors failing about as often as hoses," he admits, "but there are also the more frequent problems from misreads and intermittents due to wear and dirt."

Reinhart tried retrofitting a sealed, noncontacting magnetostrictive transducer onto one of his five molding machines, with excellent results. But that only contributed to another problem at White Oak. Business is so good, Reinhart hasn't found sufficient downtime to install them on his other four machines. "Our presses aren't down long enough to blink," he says. "It wouldn't take but a couple of hours to do it, but if we're not running all the time, we're changing molds."

As a small startup, Reinhart saw little chance for success competing against shops doing high-volume molding of commodities. Rather, he's gone after tough applications, like overmolding TPR seals on air vents, and parts with both inserts and molded-in internal and external threads. Reinhart uses tough materials, everything from 50 percent ceramic- and glass-filled nylon to 50 percent wood-filled resins, turning sawdust bound for landfills into an asset rather than a liability. His is a full-service shop, offering even consulting and art-to-part engineering services.

White Oak will perform whatever secondaries are required, from hot stamping and assembly to packaging. And Reinhart goes after tough customers, too, in automotive, consumer, light industry, and construction. "You have to have the right tools to do this kind of molding. You have to have the best," Reinhart says.

His company's average sales are around $750,000. He employs 17, working 24-5, and he's already looking at moving into a larger facility. At the 6000-sq-ft plant in Williamsburg he presently calls home, Reinhart has standardized on Cincinnati Milacron molding machines. Capacity ranges from 44 to 300 tons. Trying something different, like a new type of position sensor, is nothing new to Reinhart.

For example, White Oak has been beta testing Milacron's tiebarless Prowler molding machine, introduced last year at NPE. Reinhart's got a 90-ton Prowler on which he says he can run 70 percent of his tools, even large multicavity molds. "We're second tier. The first tier has trouble duplicating what we have done for them on the Prowler, even in their larger presses." Reinhart is old-fashioned in one regard. He wasn't too fond of the Prowler's deep purple paint job at NPE '97. Milacron changed it for him.

Reliable and Repeatable
Regular readers already know how tough molding can be on position feedback devices, but you also know that pots have to be absolutely reliable to ensure quality and repeatability. "Accuracy of a typical linear pot, perhaps ±.07 percent when it's new, can degrade right from the start if there's play in the mounting linkage," Reinhart explains. Linear pots get you 85 percent of the way there, he says, in terms of reliability. Noncontact transducers get you into the 90th percentile range. Reinhart's Balluff Micro-Pulse sensor is accurate to ±.03 percent. And it is sealed and noncontacting, so it is designed to continue to perform up to spec.

"A linear pot's mechanical contacts, or 'fingers,' can simply wear out or bounce during abrupt clamp or ejector movements, sending erratic signals to the control and shutting down the machine."

Reinhart continues: "Internal contamination is also a problem with pots. Plastic fines, oil mist, cooling water from leaking hoses, or simply WD-40 from a machine wipe-down--all these are carried on the shaft of a linear pot, moving in and out of the electronics through weak or worn seals. This contamination eventually leads to false readings, downtime, and lost production." Imprecise alignment of a pot can have a similar effect, he says, especially on long axes. If the centerlines of a pot's fixed end of the rod and the housing aren't dead on, the rod moves into the housing at an angle and eventually wears out the seals and wipers, dragging in contaminants, and producing the same erratic results.

Reinhart was experienced enough to identify a problem with his linear pots by spotting erratic readings on his injection profile. Like most molders, he trains his shop floor people to listen for squeaks. Worn pots tend to squeak, but squeaks can be false alarms--expensive false alarms. "It could be a toggle linkage that's squeaking, or parts on a mold, or an ejector pin, which could be really serious problems.

"Shop floor people may not know what's causing the squeak, and they may be afraid to flag you because they're worried you think they're just crying wolf. But even if it is just a pot seal causing the squeak, you're still looking at downtime and lost production. It's like changing a transmission seal in a car--$600, $700 in labor for a $3.50 seal. If you don't have pots that can squeak in the first place, you'll have a better idea that the squeak means there's a more serious problem. And sure, you can replace a pot if you have to, but why should you have to?"

Reinhart retrofitted his Balluff MicroPulse on a 165-ton Vista toggle, which came with a linear pot. It was his oldest molding machine--a rebuild with about 7000 hours already on it. He immediately saw the deviations in his injection profile vanish. Feedback was no longer erratic. The MicroPulse was fast enough to pace the scan time of the high-horsepower controller on the Vista that Reinhart needed for his injection profiles.

"Even if you're simply collecting data for SPC purposes," Reinhart concludes, "the data is going to be just that much more reliable. In the end, the faster the sample time, the more accurate, repeatable, and reliable the machine--even new machines." He'll prove this out, no doubt, if he ever finds the downtime to put them on his new machines, that is.

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