Re: [ATM] Epoxy mirrors (again)

["Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>]

Wayne,

A mix of my names: "Shanti."  Interesting, but please call me Shane.  ;-)

Resins designed for infusion will have low viscosities.  Here's testing of 
both UV resin and a resin that can do infusion:
http://www.dreamscopes.com/pages/projects-04/oven/10.htm

Even in thin layers the epoxy will exhibit it's own properties and 
peculiarities.  It won't exotherm nearly as much but that won't change it's 
inherent properties.

Have you been able to accurately measure the thickness from one attempt to 
another?  The consistency of your surface is extremely important.  Both 
from attempt one versus two but also within a single part.  If the range of 
thicknesses from one area (of one part) to another is within 5 um, then 
great.  But without any secondary control of its thickness (sandwiching the 
epoxy between two layers), I bet your fluctuations across the surface are 
much greater than 5um.  As you go thinner and thinner, the relative size of 
these fluctuations will only increase.  It is highly unlikely that the 
fluctuations will scale down as you reduce the surface thickness from 500um 
to 100um.  The top surface of the resin doesn't know to follow this 5:1 
reduction in thickness.

A convex meniscus will allow the low viscosity resin you are using to flow 
right off the edge of your mandrel, wouldn't it?  How would you keep it on 
there and consistent in thickness across the entire surface?

Yes, I lay up on tooling and that "mold" surface is my "optical" 
surface.  This is, as you say, how replication mirrors have been 
produced.  Like Thomas J states, the back side is much less 
important.  It's the eventual optical surface that needs to be extremely 
accurate, in every sense.

One thing that is also a huge factor, which I neglected to mention before, 
is the specific release agent that is used.  I went through months of 
testing just regarding release agents.  The current release that I use 
costs more than my high temp epoxy...  $130/gallon versus $100/gallon.  I 
tested everything from different types of release to varying how they were 
applied to the surface.  Everything is a variable.  Like I said before, 
take very accurate notes and think in a scientific method.  If you don't 
take this approach to research you won't be able to repeat a process once 
you find something that does work.

Thank you.

Sincerely,
Shane Santi - Owner
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833


At 03:19 AM 1/11/2006, you wrote:
>Hi Shanti
>
>Thanks for the comprehensive reply. Some answers, and more questions :
>
>I am aware of the CTE issue with epoxies. I have chosen what I think is a 
>unique substrate that solves that problem. The layer of epoxy is currently 
>around 500 um thick, though in the future I hope to get this down to 
>around 100 um. I figured that at that thickness, worst-case thermally 
>induced variation in the thickness of the epoxy would amount to much less 
>than the wavelength of light ( about 0.4% of 100 um ). I have found 
>epoxies with excellent wetting properties that naturally spread under the 
>influence of normal gravity and have no problem forming such thin films 
>without resorting to mechanical spreading or centrifuges. As I said - as I 
>watched the last lot of epoxy spread out, I thought I'd found "the One" 
>since it formed a completely flat surface. No ripples, no large scale 
>distortion, no warping. When I came back to it the next morning - very 
>fine ripples that looked sort of like the convection cells on the surface 
>of the Sun had formed, but the ripples were about 1 cm across, so I don't 
>think it was actual convection causing the problem - after all, the epoxy 
>was only a few hundred microns thick. There was also a noticeable 
>macroscopic sag in the surface height around the periphery of the 15 cm 
>test disk. I think this was caused by liquid epoxy continuing to migrate 
>up the walls of the container after the bulk of the epoxy had started to 
>cure, causing the volume of epoxy under the surface to decrease. This is 
>why I think if I could make the epoxy dome up, with an inverted meniscus, 
>I could stop it migrating once it had assumed it's flat shape since it 
>would reach an equilibrium point. I have yet to try this.
>
>Now for the questions :
>
>I visited your web site - very impressive ! How did you achieve the 
>quality of surface finish on your creations ? Was it by mandrel 
>replication ? I have seen others use precise moulds to create carbon-fibre 
>mirrors, so I am familiar with that process.
>
>Again - thanks for the feedback
>Wayne

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