GARLAND, TEXAS - Extra-cold air for bubble cooling and a wider-than-normal die gap are among the keys to running monolayer film with metallocene-based LLDPE at Popst Manufacturing Corp. Battenfeld Gloucester Engi-neering Co. Inc. of Gloucester, Mass., supplied three monolayer film extrusion lines. Two of them, Gloucester Contracool Extruders, are set up to run linear low density polyethylene. They have 41/2-inch-diameter extruders with Gloucester's new Low Profile Die. One of the lines has a 16-inch die. The other die measures 20 inches.
The dies help with quick changeovers, important to the Garland-based custom extruder, which has runs as small as 2,000 pounds.
The third line, used mainly for high density PE film, has a 60 millimeter, grooved feed Contracool and a 7-inch die.
Battenfeld supplied everything, including ex-truders and dies, film towers, slit sealers and the takeoff and a bag-making system, consisting of a 4180 bag machine and a 423 bag separator. Con-air Franklin of Franklin, Pa., supplied a drying and gravimetric batch blending system. The 60,000-square-foot plant has resin delivered by rail.
Because metallocene content makes it harder to push the material through the extruder, each of the LLDPE film extruders has a 250-horsepower motor.
``What normally you would have would be a 150-horse drive on it. That's an off-the-shelf application, and you're going to have a little problem pushing metallocene at 30 percent, because of the torque. It's harder to push,'' said President Fritz Popst. ``You might even see a decrease of output because of that extra push.''
According to Battenfeld, bubble flutter is a common problem when running monolayer metallocene film. But on a recent tour, the bubbles at Popst hardly moved. According to Exxon Chemical Co., metallocene LLDPE resins have a slightly lower melt strength and bubble stability than traditional LLDPE. Fritz Popst said his company has solved that problem is by using colder-than-normal air through the air rings to cool the bubble as it exits the die. The company has a 140-ton Trane chiller.
Metallocene resins also are more viscous and generate greater heat when using LLDPE screws. Because of the need for cooling, some people advocate higher cooling towers, but Popst said the cooler air-ring allows him to use standard heights.
Exxon officials recommend that air be as cool as possible without causing condensation.
Popst also thinks screw geometry must be an issue. He just
doesn't take it too seriously.
``I'll tell you there are probably 10 trillion different screw designs out there, and more coming,'' he said, laughing.
The plant has room for a fourth film line. Popst also wants to add automation. The company's output rate is so good that employees charged with folding bags and putting them into boxes sometimes have a hard time keeping up. Popst said automating folding could allow the machines to run at least 15 percent faster.
Popst Manufacturing Corp. photo