Chinaplas Debut for Unusual Approach To Injection Molding Thick Lenses

At Chinaplas next month, Engel Austria will present for the very first time a time- and cost-saving method of injection molding 22-mm-thick PMMA lenses for automotive headlamps. Cooling such thick parts without distortion presents a challenge, which has been overcome in the past by molding the lens in successive layers of the same material. Because injection mold cooling time increases with the square of the wall thickness, several thin layers can cool in less total time than one thick layer, Engel says. Overmolding also compensates for any sink marks in the under-layer. Optical tests reportedly have shown that the boundary between the layers has no effect on optical performance.

Engel (U.S. office in York, Pa.) takes this a step further by pairing up two injection machines—a standard and a two-component unit—and transferring parts from one to the other with a stop for external cooling in between. Cooling in the air takes longer than in the mold, but does not affect the cycle time. Engel bills this as an advance in its optimelt multi-layer lens technology. At Chinaplas, the lens base body, or thick core of the lens was molded on a 440-ton duo two-platen machine in a four-cavity mold. An Engel easix articulated six-axis robot removed the parts, or “preforms,” and placed them in an external cooling station. From there, the robot transfers four already cooled preforms to a 4 + 4 cavity mold of a second duo machine, a 550-ton two-shot model with a rotary table. There, two more PMMA layers are applied successively before the six-axis robot removes the finished lenses. The cycle time is well below 3 min, although the preforms take about 30 min to cool outside the first press.