An injection molding process, optimised by Fosta-Tek Optics, has matched the accuracy and quality of diamond turning in the production of precision military optics

 

The result, according to company officials, will enable the mass production of lightweight, high-quality plastic optics at a fraction of the cost of the machined glass alternative.

 

But achieving that level of quality was not easy, said Fosta-Tek Vice President, Jim LeBlanc.

 

"People have been trying for years to duplicate the accuracy of diamond-turned glass in plastics.

 

"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.

 

"So you're dealing with a lot.

 

"There are tooling and temperature issues, mold shrinkage, induced distortion caused by shrinkage, and more.

 

"What we've been able to do is duplicate a high-quality optical surface very accurately and predictably.

 

"We're talking sub-micron level tolerances." LeBlanc said the nature of the application could not be revealed, but that the lenses in question were a series of high-end sighting system optics made using a high-clarity polyolefin.

 

These were complex, double-sided aspheres ranging from 1.5 to 6mm in thickness and in diameters down to 3/8 inch.

 

Specifications called for lens form accuracy of <5um PV (Peak to Valley), 4 waves per surface, surface roughness of <10nm RMS (Root Mean Square) measured with laser interferometry, and total transmitted wavefront distortion of under 14 fringes.

 

"To appreciate the level of difficulty," said Bill Blankenship, chief engineer on the project, "we're looking at a shrinkage rate of 0.006/inch on an optic that's less than an inch across.

 

"So we have to be very accurate with our shrinkage prediction and make sure our process is exactly repeatable down to microns." The key to achieving this level of process control, said LeBlanc, is to maintain precise and intimate control over mold temperature and pressure.

 

"We knew going in we would have to use injection compression," he said, "because we had tried straight tooling in the past and couldn't duplicate the surface very well at all.

 

"Compression helps reduce shrinkage a little bit and also reduces some of the built-in molding stress.

 

"However, when we started using injection compression, we found we could duplicate the surface, but it was difficult to repeat.

 

"We would get intermittently good parts, so we felt we had to get a better sense of what was going on inside the cavity." This method is also more precise than traditional compression molding techniques that arise from the settings used with a toggle machine.

 

Proceeding methodically, Fosta-Tek set up a vibration-free metrology lab, equipped with a ZYGO GPI interferometer, and designed a series of experiments to run with a two-cavity mold on a fairly new 60-ton Engle machine with an EC 100 control system and a power pack for injection compression.

 

To get a true inside picture of the process, they installed a cavity pressure measurement system from Kistler and fed the signals back to the machine's control system.

 

"I had previous experience with pressure transducers that were installed inside the cavity," LeBlanc said.

 

"The pressure sensors told us when to initiate the compression cycle, and by watching the curves, we could actually see when some of the slides on the injection compression were coming up.

 

"We could see when it was compressing properly, and we could match the curves.

 

"Using this set-up, we were able to understand what was happening with different injection speeds, different injection pressures, two-stage versus single stage.

 

"What we were trying to find was the minimal amount of hold pressure, so that we would not induce any frozen type stresses that would increase birefringence and change the optical surface." Optimization also required precise heat control on the tool itself.

 

Fosta-Tek isolated the mold from the machine so the machine didn't act as a heat sink.

 

"Then we put thermocouples directly in the tool so we were not just depending on a hot oil unit," said LeBlanc.

 

The molds are heated to a very high temperature and maintained very precisely.

 

Blankenship added: "Using the interferometer we were able to detect slight changes in the surface profile and could correlate that with pressure or temperature of the mold.

 

"We've found that a very small change in either profile will affect our surface.

 

"So, we'd check the part and if we were not in spec, then we would go back to the machine, adjust our pressure or temperature either up or down and take some shots, measure them again, and depending on whether we got better or worse, we'd know which direction to adjust our parameters." As a result of their experiments, Fosta-Tek found that other molding parameters, such as injection speed and pressure, were not so critical in maintaining a quality optical surface.

 

Their process is also kept in check with 100% part inspection, but that too, may change.

 

"These other factors may come into play as we tweak the process to improve productivity down the road," said LeBlanc, "but the important thing right now is that we can make parts that could previously only be made with diamond turning, accurately and repeatably in a production injection molding environment.

 

"Even at cycle times of six minutes, the impact on production rates is enormous.