For the first time, Fosta-Tek Optics Inc.'s injection-compression molding process has matched the accuracy and quality of diamond turning to produce precision military optics, the company said.
As a result, Fosta-Tek Optics officials said the Leominster, Mass-based company can mass- produce lightweight, high-quality plastic optics at a fraction of the cost of the alternative, machined glass.
But it wasn't easy, said Vice President James LeBlanc.
What we've been able to do is duplicate a high-quality optical surface very accurately and predictably. We're talking submicron level tolerances, LeBlanc said.
Bill Blankenship, the project's chief engineer, said the challenge was great. We're looking at a shrinkage rate of 0.006 inch on an optic that's less than an inch across, he said.
The key is to maintain precise control over mold temperature and pressure. Injection-compression molding helps reduce shrinkage and reduces some of the built-in stresses.
However, LeBlanc said Fosta-Tek Optics engineers found they needed a better idea of what was going on inside the mold cavity, after problems with repeatability.
After running experiments in its laboratory, the firm administered tests on a 60-ton Engel press fitted with a Kistler cavity-pressure measurement system. It also put thermocouples directly into the mold. Using a Zygo GPI interferometer allows technicians to detect slight changes in the surface profile, and correlate that with mold pressure and temperature.
People have been trying for years to duplicate the accuracy of diamond-turned glass, in plastic. The idea, of course, is to reduce part weight and unit cost, but in order to do this with injection molding, you really need to precisely control the surface contours, LeBlanc said in a news release. There are tooling and temperature issues, mold shrinkage, induced distortion caused by shrinkage and more.
Fosta-Tek Optics did not reveal the application, but LeBlanc said the lenses were a series of high-end sighting optics using a high-clarity polyolefin. The firm called them complex, double-sided aspheres in thicknesses ranging from 1½-6 millimeters, and diameters down to three-eighths of an inch.
Tel. 978-534-6511, fax 978-537-2168, e-mail [email protected]