Inmold eddy-current sensor detects core shiftInmold eddy-current sensor detects core shift

Sometimes, the best things in life come unexpectedly. For Ken Gabrys, mold engineering manager at Sage Products, stumbling across a magazine article was the best thing that could have happened to him and the 25-year-old molding company for which he works.

Sage specializes in molding disposable medical products, such as containers for holding used syringes and other sharp instruments. To guard against puncture, these polypropylene containers require stringent wall thickness uniformity. To ensure such uniformity, employees at the 500,000-sq-ft Crystal Lakes, IL plant routinely had to manually check the parts by cutting them open to inspect wall thickness with micrometers and snap gauges.

The molding process was constantly being taken off-line to conduct tests on core shift—the culprit of inconsistency in wall thickness uniformity. But a little more than four years ago Gabrys was browsing through a magazine and found an article about a sensor used to measure wall thickness in the manufacture of aluminum cans.

“A light bulb went off in my head,” Gabrys remembers. “They could do that online, in real time . . . . If I could just get that to happen in our molding process somehow, my life would be so much easier.”

He called the sensor manufacturer, Kaman Instrumentation, a 30-year specialist in noncontact position measurement based in Colorado Springs, CO. Kaman had yet to enter the molding industry. Gabrys knew he was facing major hurdles. After all, there’s a reason such sensing had never been achieved before. The required sensor would have to withstand the extreme pressures and temperatures involved in injection molding and would have to send a signal that was not influenced or blocked by the plastic flow. After some investigation, Kaman decided to give it a try and ultimately developed a custom inmold, flush-mount sensor for one of Sage’s deep, narrow container molds.

Simply put, the sensor measures the distance from the sensor in the cavity wall to the core of the mold (see Figure 1). If the core shifts even slightly, the sensor alerts the machine technician that a faulty part is being produced. It accurately measures the .060- to .070-inch wall thickness of Sage’s 5-qt to 3-gal containers molded on Husky and Van Dorn machines, says Gabrys. It can detect thinner walls as well. Sage also uses the sensors to assess how certain pressures and temperatures affect the core. Tools that took days to set up properly can now routinely be set up in a matter of minutes.

Today, Sage has the sensor system installed on five molds and two of its 37 presses (with plans for more in the future). Sage also recently integrated the sensor systems into its Syscon-PlantStar data acquisition system.

Overcoming Obstacles
To get to this point, however, Sage and Kaman first had to overcome several development obstacles. Withstanding the harsh molding environment of 20,000 psi and 200F was the first challenge. But since Kaman Instrumentation, a division of Kaman Corp., has manufactured sensors for detecting vibration and pressure in nuclear reactors, creating this sensor seemed like a walk in the park. A ceramic face provides protection from the molding environment.

To obtain a reading unaffected by the plastic flow, Kaman uses eddy-current technology. The 2-inch long, .75-inch-diameter sensor, which was installed by Sage’s toolmaker, houses the sensor coil. An a-c current is sent to the sensor coil, which creates an oscillating electromagnetic field. Placing the coil a nominal distance from an electrically conductive target—the mold core—induces a current flow on the surface and within the target. This current, called an eddy current because of its circular pattern, produces a secondary magnetic field that opposes and reduces the intensity of the original field, creating a coupling effect. The strength of the electromagnetic coupling between the sensor and target depends on the gap between them.

Kaman also makes a digital signal conditioner, called KµDA (Figure 2), that’s attached via an interconnecting coaxial cable. The unit senses impedance variation as the gap changes and translates it into a displacement signal. That information is sent to Sage’s PlantStar data acquisition system through RS232 communication, which alerts the technician of any out-of-spec conditions. Since the plastic inside the cavity is a nonconductive material, the sensor obtains accurate, unaffected readings. The only problem Sage encountered was delamination of the ceramic face on one sensor, which Kaman replaced with an improved design.

R&D Pays Off
According to Dan Spohn, applications engineer at Kaman, each complete system costs less than $5000 (the dual channel KµDA and power supply cost about $3320, while each sensor costs about $820). Gabrys says Sage has spent $40,000 to $50,000 since it began integrating the sensor systems into the molds, presses, and data acquisition system more than four years ago, but not without results, he says. “The benefits far exceed the expense.” The sensor has paid for itself if it stops one bad part on one mold from running unnoticed for a few hours, he adds.

Spohn says there are a few inmold pressure sensors and mold movement sensors on the market, but nothing like the KµDA system. He says Kaman has similar inmold systems in the works for such applications as sensing the position of metallic parts in insert molding. But he hopes and believes the use of inmold sensors to detect core shift will grow—as does Gabrys, the man who dared to try it first.

“It’s a great technology now,” he says. “It seems so simple, especially in our line of work. Core shift from a processing standpoint is important to anybody making parts, especially if you have a thin-wall, narrow, or deep part.”