Injection moulding lowers optics costs

Edited by the Engineeringtalk editorial team Apr 23, 2008

New injection moulding technology will enable the mass production of lightweight, high-quality plastic optics at a fraction of the cost of the machined glass alternative.

For the first time, an injection moulding process, optimised by Fosta-Tek Optics has matched the accuracy and quality of diamond turning in the production of precision military optics.

This 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 plastic".

"The idea, of course, is to reduce part weight and unit cost, but in order to do this with injection moulding you really need to precisely control the surface contours".

"So you're dealing with a lot".

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

"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 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 3/8in.

Specifications called for lens form accuracy of less than 5um PV (peak to valley), four waves per surface, surface roughness of less than 10nm RMS (root mean square) measured with laser interferometry and total transmitted wavefront distortion of under 14 fringes.

The key to achieving precise process control, according to LeBlanc, is to maintain precise control over mould temperature and pressure.

"We knew going in we would have to use injection compression 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 moulding 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 moulding techniques that arise from the settings used with a toggle machine.

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 mould 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, the company 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".

Optimisation also required precise heat control on the tool itself.

Fosta-Tek isolated the mould 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 moulds are heated to a very high-temperature and maintained very precisely.

As a result of its experiments, Fosta-Tek found that other moulding parameters, such as injection speed and pressure, were not so critical in maintaining a quality optical surface.

"These other factors may come into play as we tweak the process to improve productivity down the road, 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 moulding environment" Leblanc said.

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

"We think that's a milestone for the process".