German injection molding machine producer Arburg GmbH + Co. KG has launched a new add-on injection unit for micromolding.
Rather than the screw-and-plunger combinations used by some rivals, the new Arburg module uses two screws. Initial plasticizing is carried out using a three-zone plasticizing screw while final metering and injection is carried out in a second servo-electric-drive 8-millimeter-diameter screw with a non-return valve.
The pre-plasticization screw has an almost conventional flight design, and is mounted piggyback-style at 45 degrees. It is used simply to melt the plastic material prior to delivery to the 8mm injection screw. The latter has purely a material-transport function.
Arburg said this technique enables extremely small parts to be molded with standard-size granulates, making the use of specially prepared micro-granulates unnecessary. Micro-granulate is usually required on micromolding machines with screw diameters below 15mm, according to Lossburg-based Arburg.
The use of a screw rather than a plunger also has a benefit in terms of quality, according to technical director Herbert Kraibuhler, as the polymer material is injected on a “first-in/first-out” basis. This means there is a reduced risk of contamination of moldings due to extended melt residence times, which can lead to black-speck development, Kraibuhler said.
Arburg demonstrated the module running on a standard 35-metric-ton all-electric Allrounder 270A Alldrive at the company's recent Technology Days open house in Lossburg.
An Arburg Multilift H robot was used to remove the 1.58mm-diameter, 0.001-gram micro-gearwheels, while a special pneumatically driven gripper system applied pneumatic force to remove the parts from an eight-cavity mold from Stamm AG in Hallau, Switzerland.
The mold and parts were the same as those run on a 15-metric-ton hydraulic-drive Allrounder 170S, equipped with a size 30 injection unit and 12mm-diameter screw, at the 2009 Technology Days. That machine also was running at the 2010 open house, this time producing micro “shift gates” weighing 0.0025 gram.
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