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Thread: creating custom shaped mirrors?

  1. #1
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    Default creating custom shaped mirrors?

    In one of my previous topicss I was talking about using curved mirrors for a project. I want to have complete control of the shape of the mirror and it can't be just an ordinary concave or convex mirror. Since I'll need to make probably a few dozen shapes during the prototyping period I need to make more than one so processing a glass to a shape I want and metallizing each is not really accessible or in any way an easy option.

    1) So far in theory the easiest option and fastest for prototyping I could think of would probably be vacuum forming a metallized ABS or other plastic sheet.
    DIY vacuum forming machines are easy to make:
    https://youtu.be/Gx66mS7U2vY?t=6m41s

    There might be three issues with this approach:
    1) unsourceability of metallized plastic sheets. In fact I cant find anything as of the time of writing this.
    2) Damage to the aluminum metal film when reshaping the plastic
    3) Poor (<80%) reflectance of such metallized plastics

    If someone has any knowledge or experience about the above three concerns please let me know.

    2) Another option might be metallizing a plastic yourself after thermoforming it. I couldn't find much about DIY metallizing machines. If there is I'd love to know.

    3) Finally third option I talked about in the previous topic but couldn't find any info on as well: thermaforming relatively thick mylar sheets. Mylar seems like a good relfective material and doesn't need to be metallized itself.

    Previously I was also thinking of using a vacuum for forming a thin mylar sheet to the shape I want and applying a curable epoxy/glue/etc. while the vacuum is on to keep the mylar sheet in the desired shape after the vacuum is turned off, but I couldn't figure out how this could be done without the vacuum blowing the glue away or getting between the mylar film and a plastic shape this way.

    So what do you think, are any of these actually a good idea worth the time invested? Maybe there are other options which come to your mind?

  2. #2
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    I used to use chemical silvering to put a reflective coating on holograms embossed into mylar. Got the kit from Peacock Labs. Same process astronomers used to use to silver mirrors. First part is a stannous chloride solution to tin the surface. Two other components (caustic and silver nitrate) are diluted, then mixed together and immediately poured over the mylar, coating out silver as it's poured (or sprayed). Problem then is you need to dispose of the stannous chloride (nasty stuff), and the caustic solution. The latter is typically neutralized with acid and put down the drain.

    Might also look for a glass slumping class, and a used kiln.

  3. #3
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    Glass and some plastics are shaped in the University shop with a wet diamond wheel, a cheap tiny table top belt sander with a glassblowing belt, oven slumping, and if need be, pitch lapping like you would make an amateur telescope with.

    It does not take much to make a diamond disk glass saw, basically go to a machine tool supply place and get a half inch slit saw arbor and some bearing blocks, plus a small slow motor and a source of dripping water on the blade. Diamond blades of decent size are about nine dollars each.
    ~
    The 75$ one inch wide belt sander from Harbor freight is not bad, and the glass sanding belts come from Wale Apparatus.
    ~
    One of my friends uses the Ebay glass cutting diamond bandsaw and swears by it..
    http://www.ebay.com/bhp/glass-band-saw
    There is a 199$ small one out there some place.
    ~
    The astronomy glass polishing supplies are all over the web.
    ~
    Classical iron wheel lapidary machines probably show up cheap once their buyers, who dream of cutting/polishing/faceting gem stones, find out how hard it is to make precise pretty rocks at home from anything but glass and quartz.
    ~
    There are evap services for telescope makers who will put enhanced aluminum down on your glass, but have one whole run ready, and help them figure out how to mount it on the planetary in their coating chamber.. Glass only not plastic, in those chambers.
    ~
    Lapidary and Bead Making suppliers should have you covered on doing this cheap.
    Steve
    Last edited by mixedgas; 09-23-2017 at 04:37.
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  4. #4
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    Www.shapeways.com. Design it up and have the print it in aluminum. You can the coat it or polish it yourself. If the shape isn't crazy maybe even put Mylar on the surface. You can also have it made in copper and plate Nickle then silver plate sand clear coat. Anyway that place rocks and the cost isn't horrible. The can even do silver with rhodium coating but now the cost goes up. I say 3d print to copper and plate as needed to coat with silver or rhodium.

    What are you building?

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    Oh forgot. You can also make it from plaster and shape it. You can the put Mylar on that. Did that for a little solar cooker but I was not worried about distortion. I found metalized plastic sheet search plastic mirror sheet. Even Home Depot sells it

  6. #6
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    Appreciate all the input
    mixedgas shows how cheap shaping glass can be but like I said it isn't going to be just an ordinary concave or convex mirror so it's not so simple as rotating glass and grinding/ploishing it. I should have probably posted an illustration of the shape of the mirror to show how difficult it would be exactly, my bad.
    https://i.imgur.com/Iktw4oK.png

    How do you put mylar on stuff, kecked?

    I've used Shapeways a lot, they are great, but waiting times can be long. I rather print with my own lower res 3d printer in plastic and polish by hand then order and wait for them to print and deliver because I'll probably go over a dozen different shapes during prototyping, refining previous one each time. The final version I'll definitely get them to print.

    Heating plastic mirror sheets and vacuum forming them on a 3d printed shape would be a very easy option though I cant find any info on the reflective properties of these sheets (specular reflectance) as well as any ABS sheets. Are you by any chance referring to plexiglass/acrylic mirrors instead of ABS? I only thought about that now and after checking vacuum forming acrylic is indeed possible. Only possible obstacle might be crackign the reflective film on the acrylic as it is heated or shapen and the reflective efficiency of the acrylic mirrors.
    I sure hope both of the above are not an issue. If mylar can be "put" on plastic as well somehow then sure that will work too. I only found one video about this with two people trying to use heatgun but they ended up burning th mylar.

    Great advice as usual.

    I'll show what I'm working on after I have the mirror. It involves a PicoP projection system and a mirror.

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    You can glue the Mylar on with most glue BUT. You have to hold its shape while the glue dries or it wrinkles and the glue must be low shrinkage. A form and stretch is superior for Mylar. You can plate on plastic. I have no idea if you can really make a mirror though. Polish metal. It's always better.

    The mirror sheet I saw is a 1/16. " thick plastic sheet they use to make mirrors in baths. You put it on glass and it sticks to it making the mirror. The back of it is not shiny. Not a first surface mirror unfortunately.

    Are you making a flight sim by chance?

  8. #8
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    Nice guess but no not a flight sim. Although I think people wanting to make one would be interested in the solution, haven't seen what they use first hand but for projected geodesic domes people use security half sphere mirrors and those aren't perfectly suited for the job although aren't too bad either.


    And that's interesting I didn't know the bath mirrors used reflective plastic instead of aluminum on glass. What's the benefit of using plastic, the protective side is less likely to penetrate humidity? Since both use glass anyway it's probably not for protection from shattered glass.


    I just had a realization: why not adhere a thin mylar sheet to a flat plastic sheet then thermoform that plastic sheet with the mylar already on it, instead of the other way round and worrying about properly stretching the mylar?
    Only question then is what adhesive to use, it has to both be strong and handle the heating temerature of thermoforming.
    Cyanocrylate becomes liquidy when heated and generates vapors.
    Epoxy is too viscous for me to apply at even thickness.
    silicone is too deformable and less adhesive.

    A mirror plastic sheet would also work and be even less work but I have a feeling it's a metallized plastic that will have the mirror layer crack and break into pieces when thermoformed, unless I'm wrong and its not metallized bur rather something else like a mylar on top of ABS as well or a very thick (1mm+) all-mylar sheet which btw I couldnt source anywhere so Im not sure if they exist.

  9. #9
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    http://plaskolite.com/coatings/coati...mirror-coating They can coat

    Ok so done some reading. Mylar is nothing more than aluminum coated plastic. You can deposit silver on abs. Caswell sells the plating kit. You can polish stainless steel to a mirror. The surface prep is everything. You can electrodeless plate nickel on plastic and polish it. It all messy and dangerous. I recommend having it all vapor coated with aluminum. They overcoat it too.

    The plastic mirror is used for weight. If you show me the kind of surface I can make better recommendations. I don't know how complex the object is. If it is generally straight and curved it should be possible to just silver coat it and then clear coat and hand polish. If it's complex you need vapor or electrodeless methods way beyond diy.

    Thermoforming sounds possible....

  10. #10
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    How does plastic mirror change the weight of a mirror if theres still glass in the front protecting the mirror film?

    check this image for mirror shape
    https://i.imgur.com/Iktw4oK.png

    Only info I could find about thermoforming mirror plastic was this where it says thermoforming mirror acrylic/plexiglass specifically is not posssible, but not sure about mylar or other plastics. I don't see the aluminum on a shiny mylar suddenly turning into dust and falling off no matter how hard a pull and bend it unlike a more rigid plexiglass with metalized aluminum film on it so it might work. Never thought how that worked, probably transparent film covering both sides of the reflective film?
    https://www.theplasticshop.co.uk/pla...hop_manual.pdf

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