Originally Published MPMN July 2008

BREAKTHROUGHS

Molding Process Mirrors Accuracy of Diamond Turning for Microoptics

Shana Leonard

When a manufacturer of polymer-based optical components took on a project originally designed for the tight requirements of diamond-turned glass, it was confronted with a significant challenge. In order to arrive at the desired result, Fosta-Tek Optics was able to augment an injection molding process that achieved the quality and accuracy associated with diamond-turned glass optics in the production of plastic optics.

Engineered from a high-clarity polyolefin, the lenses were designed as double-sided aspheres with thicknesses ranging from 1.5 to 6 mm and diameters down to 3⁄8 in. “We knew development was going to be tough because most optic materials shrink to about 0.006 in. and, after that shrinkage was in place, we had to have the ability to hold the surfaces accurate,” says Jim LeBlanc, vice president of Fosta-Tek. “That meant we had to have minimal stress in the injection-molded part.”

To produce the lenses, Fosta-Tek concluded that injection compression molding was the best manufacturing option since it can help reduce shrinkage to a small degree and can lessen built-in mold stress. The key to success was precisely controlling surface contours, which required maintaining control over mold temperature and pressure, according to LeBlanc.

Encountering repeatability issues, the company installed a Kistler pressure measurement system inside the mold cavity in order to obtain a more-accurate read on what was going on. “We couldn’t just use a standard pressure transducer,” LeBlanc recalls. “We needed something where we could actually watch the curve of how the system was injecting and how it was cooling. We basically wanted to see if the gate was starting to freeze off so we could compress at exactly the right time.”

This approach worked. By employing pressure monitoring, the firm could observe the effect of different injection speeds and pressures in order to help determine the minimal amount of hold pressure to prevent altering the lens surface. Additionally, the company maintained precise heat control over the mold tool itself. Together, these modifications yielded a molding process able to mimic the accuracy of diamond turning.

Although initially developed for the military, the injection-molded plastic microoptics have already aroused the interest of medical OEMs. LeBlanc says that some interest derives from the potential that the optics have for improving microsurgery techniques. During these procedures, enlarged images of the procedure are projected onto a screen to supply surgeons with a better view and more awareness of what they are doing.

“The true benefit is going to be superior optics,” LeBlanc says. “In the case of some of these miniature optics ... you’re making a much clearer image. You’ll be able to see more detail.”

Fosta-Tek, LeoMinster, Ma
www.fosta-tek.com

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