Magnet sticks with the mold - really

Some people have a hard time trusting it at first. "They just couldn't believe that a magnet could hold the mold up against the platen," says Craig Powell. He's the molding manager of Pyrotherm, a division of Nypro Clinton in Clinton, MA. He's talking about the 18 pairs of magnets used to hold the mold in the press - sans bolts, clamps, or any other support.

Powell is the virtual leader of Pyrotherm's Nova project, a molding operation that uses 18 custom designed 30-ton Ferromatik Milacron electric injection machines. They're spread around the country in Clinton; Burlington, NC; Atlanta; San Diego; and in Puerto Rico, molding proprietary medical parts.

What makes them custom is the fact that each press's centerline is 53 inches off the ground, leaving room underneath for dryers, loaders, temperature controllers, and other secondary equipment. The machines are designed to save space (25-sq-ft footprint) and mold parts in short runs. "The design of the project was short-run injection molding," says Powell.

That's where the magnets come in. Powell needed quick mold changeouts to accommodate runs that lasted as little as three hours. With the traditional mold clamping system, he says, mold changes took at least 30 minutes, if not longer. He opted instead for the Pressmag, the magnetic clamping system produced by O.S. Walker Co., based in Worcester, MA. Nypro bought 18 pairs of magnets, one for each electric machine. "Now," says Powell, "changeouts take about 10 minutes."

No Clamps - No Fear

The Pressmag consists of two magnetic chucks, made of a permanent magnet metal alloy called Alnico. One is mounted on the moving platen, the other on the fixed platen. The Pressmag requires no backplate adaptations or other modifications to the machine or the mold, and is compatible with most machines, says John Daly, product manager at O.S. Walker. And without clamps or bolts to secure the mold, the magnet gives molders more surface area with which to work.

For molders who struggle with power outages, have no fear. The Pressmag does not require a continuous supply of electrical current to maintain its magnetic attraction. The electropermanent magnet requires an electrical current generated by a controller to magnetize and demagnetize only. "You don't have to worry about the electricity going out on you," Powell says. Daly reports he's not received a complaint yet from a molder who had a mold slip or move in any unwanted way on the magnet.

Still, it's no wonder that some molders are slow to give all of their faith to a 90-lb magnet holding a 400-lb mold. At Pyrotherm, Powell says that was the biggest hurdle his molders had to clear. "It was a real culture change on the manufacturing floor to suspend the mold with no visible support," he says. At first his operators mounted the mold with the magnet and clamped it to the platen. Then, they removed each bolt one by one until the mold hung by itself on the magnet and, says Powell, "they could see for themselves that it wasn't going to crash to the floor."

Powell says that once the molds are installed and aligned properly on the magnets, he's not had a single problem with slippage. He says the heaviest mold he's put in a 30-ton press is 450 lb - 50 lb over the limit for the tiebars, but not a problem for the magnet. He says the controllers also have a turnkey safety switch to prevent users from accidentally demagnetizing the magnet before the time is right.

For molders contemplating a magnetic mounting system, Powell counts three drawbacks, each of which can be overcome. The first is cost. Nypro spent about $5000 per machine for the magnets. He says that while this may seem like a lot, he counts his savings in fast and efficient mold changes, which are vital to the schedules he runs. The second drawback is the fear factor, which is just a matter of seeing and believing. "A lot of people are just afraid," Powell says.

Third, if you mount the magnets in an existing machine, press daylight will be lost to the thickness of the magnets - about 4 inches in Nypro's case. "When you add magnets to the platen, you're adding thickness to the platen," he notes. Powell says this may be of concern to custom molders who rely heavily on the flexibility that maximum daylight gives them.

Daly says that if you're worried that much about losing daylight, instead of mounting a magnet on the platen, O.S. Walker can make a new platen - one that has Pressmag built right into it. That way you don't lose space between the platens. This is especially useful for molders buying new machines. Daly says he's worked with several injection molding OEMs to offer magnetized platens for new machines at the customer's request.

One final note: Remember that magnets don't work with aluminum. For that you'll have to revert to clamps.