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Author: Subject: Press Molded Optical Glass
D4RR3N
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[*] posted on 7-10-2013 at 04:36
Press Molded Optical Glass


I'm hoping someone here has some knowledge of optical glass and can give me some suggestions. Basically I am making a device which needs to refract sunlight along its length with efficiency (Optical waveguide) in the same way a length of fused quartz or PMMA acrylic does.

Fused quartz is too expensive and as the device gets hot PMMA will not work either. I might get a small batch of these glass devices press molded in China. Thats the other thing fused quartz is not commonly press molded and I believe that is because of its very high melting point (around 1600C), usually objects made from fused quartz are hand made via torch work.

<!-- bfesser_edit_tag -->[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: changed "glass" subject]

[Edited on 8.10.13 by bfesser]
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7-10-2013 at 08:47
deltaH
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[*] posted on 7-10-2013 at 10:45


@D4RR3N

Is the light path fixed or does it have to be flexible?

I assume operation at temperatures above the softening point of optical glass?

Any hint of the dimensions/lengths we're talking about here?

Can you use mirrors? What about mirrored channels/ducts filled with air?

In short, more info please...


[Edited on 7-10-2013 by deltaH]




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[*] posted on 7-10-2013 at 13:35


Quote: Originally posted by deltaH  
@D4RR3N
In short, more info please...


The device uses both total internal refraction and geometry to concentrate solar light, the device is scalable however I'm thinking of a device say 18x18", it needs to be press molded. The larger the device the more sunlight it collects and the higher the temperature reached. PMMA would probably get soft if large collectors were made from it. Glass would be better and fused quartz excellent however nobody press molds fused quartz (probably because of the temp needed).
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[*] posted on 7-10-2013 at 13:47


Is the geometry such that it is possible to realize it by grinding and polishing? The deeper the relief on an object, the harder (and more expensive) it is to fabricate it this way, but I was wondering if you had thought about it.
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[*] posted on 7-10-2013 at 15:26


Quote: Originally posted by WGTR  
Is the geometry such that it is possible to realize it by grinding and polishing? The deeper the relief on an object, the harder (and more expensive) it is to fabricate it this way, but I was wondering if you had thought about it.


You would need a massive chunk and there would be lots of waist to machine this out of a solid block.

I'm thinking of crown glass now, it might work with that
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[*] posted on 7-10-2013 at 17:12


fused quartz is too expensive?

http://www.alibaba.com/product-gs/1105126156/Milky_white_Fus...
http://www.alibaba.com/product-gs/724052076/transparent_jgs3...

[Edited on 2013-10-08 by ElectroWin]
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[*] posted on 8-10-2013 at 01:40


Quote:
I'm thinking of crown glass now, it might work with that
Yeah I don't think you need anything exotic here, from what I understand, a 'special high performance' optic is not the issue. I doubt it would even get that hot if I'm understanding your intentions correctly.



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[*] posted on 8-10-2013 at 06:45


Quote: Originally posted by ElectroWin  
fused quartz is too expensive?

http://www.alibaba.com/product-gs/1105126156/Milky_white_Fus...
http://www.alibaba.com/product-gs/724052076/transparent_jgs3...

[Edited on 2013-10-08 by ElectroWin]


Yes you can purchase it in small pieces in simple shapes (sheet, block, rod, tube) but I haven't found many who press molds it into more complex shapes, those objects that are very expensive. The material itself silicon dioxide is dirt cheap but the temperatures needed to cast it is what makes it difficult.
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[*] posted on 8-10-2013 at 07:02


Quote: Originally posted by deltaH  
Quote:
I'm thinking of crown glass now, it might work with that
Yeah I don't think you need anything exotic here, from what I understand, a 'special high performance' optic is not the issue. I doubt it would even get that hot if I'm understanding your intentions correctly.


The difference is when you focus the Suns light with a lense or parabolic mirror the focus is outside/away from the object doing the focusing of the light. In my device the focus is actually within the device itself so it will get hot as you approach the focal point. If it was made from plastic it would probably melt itself if too much energy is absorbed. I could always make them small and make lots of them that way the amount of energy each one absorbes will be limited to the temperature range of plastic...glass would probably be better though to avoid this.
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[*] posted on 8-10-2013 at 08:20


Found a material BK7, now just need to see if it is commonly press molded
http://www.interpv.net/tech/tech_view.asp?idx=109&part_c...
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[*] posted on 8-10-2013 at 13:51


Quote: Originally posted by D4RR3N  
In my device the focus is actually within the device itself so it will get hot as you approach the focal point. If it was made from plastic it would probably melt itself if too much energy is absorbed.
You're going to have serious problems with differential thermal expansion with any optical element that has a focal point inside the body of the element. The inside will expand, the outside less so, and that means a large tensile load over the entire surface.
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[*] posted on 9-10-2013 at 15:49


Quote: Originally posted by watson.fawkes  
Quote: Originally posted by D4RR3N  
In my device the focus is actually within the device itself so it will get hot as you approach the focal point. If it was made from plastic it would probably melt itself if too much energy is absorbed.
You're going to have serious problems with differential thermal expansion with any optical element that has a focal point inside the body of the element. The inside will expand, the outside less so, and that means a large tensile load over the entire surface.


There will be expansion as you approach the focal point but no different to heating the end of a test tube in a flame I would have thought, hopefully I wont get stress fractures.

Another reason why I think plastic wont work is that the Suns UV will also be intensified within the device and plastics often deteriorate with intense UV exposure.
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[*] posted on 10-10-2013 at 06:06


Quote: Originally posted by D4RR3N  
There will be expansion as you approach the focal point but no different to heating the end of a test tube in a flame I would have thought, hopefully I wont get stress fractures.
Seems quite different to me. With a test tube, you've got relatively thin-walled ware that heats up fairly uniformly across a section, and the heat problem is largely about the stress from one part to the other. With a hot spot inside the volume, you've got a hot interior putting tensile stress on the whole surface. That's a perfect recipe for crack propagation toward the interior from anywhere on the surface.
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