optical tooling for plastic parts

[Stella]

I am currently developing optical quality parts in polycarbonate for a bespoke application.<br>
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The part in question is a dome - used to house a camera. The 1st off tooling samples in optical grade polycarbonate exhibited macroscopic optical distortion as have the samples following some tooling rework and high level polishing. The tool material is Nak80 and the surface finish specification is SME1. <br>
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I am keen to find out what are the main points to be aware of when producing optical grade plastic parts - you know the general dos and donts.<br>
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Also, how much bearing the machining process and the process control of the tooling manufacture have to bear - how important for examples is the tool cutting machine resolution in relation to the cutter path - similarly the programme. Is it necessary to ensure exact compensation for tool wear during the tool machining process to a level beyond that which is utilised for other parts. Is this an extremely specialised field? Any help, pointers, or names of experts in the field would be useful - I am also very willing to help people who may be starting out on this route and give them the benefity of the problems I have experienced - i.e. here are some donts.<br>
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Any help appreciated.<br>
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Stella

 

[kvolt]

For having stumbled upon your post through the optical systems engineering forum, my reply mostly has to do with plastics and optics. Your problem seems to lie with the difficulties in molding optical quality components for which I have little experience.<br>
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The general attractiveness with plastic optical components, besides the possibility of injection molding, is the possibility of (quickly) producing odd shaped lenses or mirror substrates through diamond turning, and light weight.<br>
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The general drawback not surprisingly has to do with the material itself, with regard to structural and thermal characteristics. I have had to forego antireflection coating on an aspheric lens, about 30mm diameter and pretty thick, for reasons of residual stresses for example. This is the scope the problem can take.<br>
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Your application is a housing and you would do well to pay attention to the temperature exrtremes at which you hope your equipment to operate: the effect of the mounting hardware will make itself apparent through analysis, prototype testing or otherwise. On the scale of the difficulties you describe, this falls in the low priority though, maybe already covered.<br>
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Your concern about machining tool wear rate seems a bit displaced given that the tool shape does not seem to be verified against optical requirements. Replication or performance the piece will not be properly evaluated if the shape of the master copy is not established.<br>
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Your description of an optical macroscopic defect compensated for by tooling change and finishing upgrade doesn't establish whether poor quality is visually detected or detected through the camera system. I did not trace back the &quot;bespoke application&quot;. <br>
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A visually detected defect would implicate poor replication of the tool through surface imperfections and will normally disqualify use on the camera system. Visually detected defects point to problems in the entire production process: tool shape and cleanliness, material choice and purity, flow rate/pattern, molding environment, temperature anywhere, solidification distortions, anything the crusty old hand has an opinion on... You might still get away with a lot by just trying the thing: surface imperfections are a concern close to an optical focal plane mostly.<br>
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Given the dimensions you might be tempted, but a visual inspection of thickness will not be sufficient by putting the dome against your eyebrow. A &quot;macroscopic&quot; distortion through the camera system will relate to anything beyond the basic lens design. Design and production quality problems might be difficult to distinguish. Definitive measurement is no doubt elusive.<br>
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With the dimensions you outline of a 100mm diameter hemisphere with 5mm wall thickness, I am torn between hoping the camera system outer lens is scaled to match closely the dome radius - risky - or sized very, very, small in comparison to the dome radius. Does the camera move within the dome? Is the intent and design of the dome to be the equivalent of something of a &quot;flat plate&quot;? Has this been checked through analysis or written off as housing detail? Every surface radius counts in an optical design.<br>
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In the belated hope that this points you to fewer questions than solutions, or at least better qualified questions,<br>
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kvolt<br>