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The next level of purity

Upgrading the magnets in its granulators allowed one molder to keep all trace of metal out of its machines and its parts.

IMM Staff

February 17, 2009

5 Min Read
The next level of purity

Eriez grates have made Omni's molding process run more smoothly, with less downtime due to plugged nozzles and gates that needed cleaning.

Upgrading the magnets in its granulators allowed one molder to keep all trace of metal out of its machines and its parts.

In Omni Plastics Inc.’s Erie, PA 44,000-ft2 advanced manufacturing facility, the company produces highly complex, close-tolerance thermoplastic components used by medical, pharmaceutical, diagnostic, and electronic OEMs. As expected, the raw material for these demanding parts must be free of all impurities, but even virgin resin has trace amounts of metal contamination. To ensure that nothing sitting in the press’s hopper plugged the nozzle or marred the molded product, Omni consulted a magnetic, vibratory, and inspection system supplier in the same town – Eriez Mfg. Co.

The 50 million parts Omni molds each year in its white room include products used to make devices for medical procedures – vessel sealing, electro surgery, RF ablation, RF pain management, and auto suture – as well as blood-testing tubes and closures, and test trays for medical specimen collection. Others are used in manufacturing medical scalpels and razors, which involves molding stainless steel fasteners and blades into the components. In the unlikely event these fasteners are run through its granulator, Omni wanted to remove this metal from the regrind. Also, the company occasionally uses its regrind from rejects for some nonmedical and nonpharmaceutical customers.

So in late 2007, the company turned to Eriez for help reaching the next level of product purity. Following Eriez’s review of Omni’s hopper magnets, which included ceramic magnetic grates with a pull strength of 3-7 oz and some older Rare Earth grates that tested to 70 oz of pull strength with a ¼-inch mild steel ball, the molder installed six high-temperature (250°F) Xtreme Rare Earth, 6-inch-diameter Model P grates. These grates offered a minimum of 84 oz of pull strength with a ¼-inch mild steel ball and work with polymers that have high drying temperatures.

“Eriez came in and tested all our magnets,” says Omni process engineer Don Bambarger. “They let us know which ones were effective for our application and which weren’t. Some of them were OK, but most of them wouldn’t pull an ounce. We ended up replacing most of them.” Bambarger explains that Eriez equipment is now all over Omni’s plant. “We’ve got Eriez magnets decontaminating the lines on 12 of our 19 machines,” he says.

The grates are positioned in raw material hoppers that feed Omni’s injection machines. In this position, they prevent ferrous and small, weakly magnetic contamination, such as hardened stainless steel, from damaging injection screws and barrels and plugging small nozzles and mold valve gates, which can cause costly machine downtime and repair. The grates also prevent fine, magnetically susceptible particles from contaminating Omni’s molded parts.

One of the products Omni makes is a plastic float for a blood centrifuge. It uses virgin resin that is pulverized to powder – and that process contaminates the raw materials. “You’d be surprised at how much metal we pull out of the plastic,” says Bambarger.

Overall, he says the Eriez grates have made their process run more smoothly, with less downtime due to plugged nozzles and gates that needed cleaning. “Before we made the upgrade to Eriez, it could be a real pain at times,” said Bambarger.

For Omni, a company whose reputation for precision and purity is essential to its continuing success, the improvements are significant. Bambarger commented, “We’re glad that we asked Eriez in to help us. We’re a lot farther ahead than we were before.”

 What’s behind rare earth magnets?

Improved rare earth permanent magnets produce high-intensity magnetic circuits, operating energy free. Magnetic circuits designed with rare earth magnets now generate a magnetic force an order of magnitude greater than conventional ferrite (ceramic) magnets.

Rare earth magnetic materials are otherwise known as Lanthanides. These metals range from atomic number 57-71 on the periodic table of elements. While rare earth materials have been known for a long time, it’s only within the last two decades that their use has been economically feasible.

Rare earth (RE) magnets offer solutions to many very fine and/or weakly magnetic iron contamination problems. Their magnetic strengths fall in the medium intensity range – 4000-11,000 gauss. Previously, this strength was available only through high-intensity electromagnets, which are bulky and expensive to purchase and operate.

Higher magnetic strength coupled with flexibility in use makes RE magnets especially suited for the removal of very fine iron that is difficult to attract and hold because of its small mass, and removal of weakly magnetic contaminants, such as iron oxide or rust, which do not respond well to conventional ferrite magnets.

RE magnets from Eriez are available in various sizes and dimensions, depending on the application. The Model P Grate selected by Omni Plastics consists of a series of parallel magnetic tubes, each consisting of nonmagnetic stainless steel tubing encapsulating a magnetic circuit. The tubes are spaced and held in position by nonmagnetic stainless steel strips.

Parallel with the magnetic portion and centered between them are stainless steel grid angles, which act as a material baffle to prevent excessive speeds, and direct all the material onto the face of the magnets where iron contamination is trapped and held.

The Model P grates are placed in the hopper, the ends of the magnetic tubes resting against the sides of the hopper. In cases where the ends of the hopper are unusually steep, small strips may be attached to give the grate better support. Maintenance of the Model P grates involves periodically lifting the unit out of the hopper and wiping it clean of ferrous accumulation. [email protected]

About Omni

Omni’s complete range of services includes product design and development; form, fit, and function analysis; mold flow analysis; cooling and packing analysis; stereolithography; rapid prototyping; mold design consultation, construction, evaluation, and debugging; raw material testing; and injection and insert molding.

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