Replace curved glass with plastic?

Discussion in 'Case Construction, Repair & Restoration' started by Simon Holt, Apr 27, 2020.

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  1. Simon Holt

    Simon Holt Registered User
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    Good afternoon everyone; I hope this finds you all well and not at all "stir-crazy"...

    Due to carelessness on my part, this wall clock took a tumble, breaking the glass:
    2018-03-18 17.18.31.jpg
    The clock has no sentimental value, and little monetary value, but I'd like to make it "whole" again – if it can be done at little cost. There's no chance that I will find a donor, and the cost of custom-formed glass would be way out of proportion to the clock's value.

    The glass was curved, but in one plane only. So I'm wondering if there's a suitable material available in sheet form that can be used. I don't care about thickness, only clarity. The mounting method would allow use of a thin material that does not need to be 'formed':
    2020-04-26 12.21.14.jpg
    Although I could make a former in case I need to form material that was otherwise too rigid.

    So what should I be looking for? Perspex? Acrylic? Acetate? Poly carbonate? I only know that they exist; I do not know their characteristics. Feel free to add to this list! Anyway – what would be best?

    Simon
     
  2. shutterbug

    shutterbug Moderator
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    Are you trying to just fill the space, or do you want it rounded? That part is going to be tricky.
     
  3. Simon Holt

    Simon Holt Registered User
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    Maybe I've made this sound more complicated than it is, shutterbug. The original glass did not have a compound curve, just a curvature in one plane, so in theory I could use a flexible material and just fix it place so that it follows the curvature of the case.

    I made two templates; one showing the shape of the material I would have to cut, and one showing the curvature:
    2020-04-27 09.49.05.jpg 2020-04-27 10.04.39.jpg
    So I'm wondering what would the best material, what thickness should it be, and would I need to form it to shape using heat before fitting it?

    Simon
     
  4. gleber

    gleber Registered User

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    I've used PETG to make some compound curved shapes. A little heat makes it pliable and when it cools, it keeps its shape. I'm not a materials guy, but it worked for what I needed.

    Tom
     
  5. Simon Holt

    Simon Holt Registered User
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    Tom, thanks for the suggestion. I've not heard of that material before, but a Google search led me to a UK firm that carries it, together with other materials. I'm sure I can find something suitable now.

    Simon
     
  6. fbicknel

    fbicknel Registered User

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    #6 fbicknel, Apr 29, 2020
    Last edited: Apr 29, 2020
    upload_2020-4-29_14-32-27.png

    Scratch all that: Unless you can find special glass that melts at lower temperatures, the melting point of normal glass is higher than that of steel... even stainless.

    Therefore, we're probably talking about a special ceramic surface to mold the glass.

    Yeah, it was a fun thought experiment.
    ----------------8<-----------------
    As I was sketching this, some ideas came to mind:
    • Better would be firebrick lined with ceramic wool to help keep the heat in.
    • Vent the back wall of the furnace so you don't bake your torch with the backblast of gasses coming back out of the furnace front
    • Somehow keep the top steel plate up off the glass until the glass melts and sags onto the bottom plate. However, everything has to heat together so there's no temperature shock.
    • Polish the plates so the surface will be smooth
    • Something would need to be placed at the foot of the glass (closest to the torch) to keep it from sliding out as it heats up
    I'm no glass blower, but it would be interesting to see if this could produced\ what you need. :)

    Also, there will be no way to cut the glass along the bottom (side closest to back of furnace) or sides after it's bent, but you could possibly trim the top (side closest to torch) if the amount of trim were appropriate. You cant shave off 1mm, but you could snap off 5-10mm with a scored edge, I think. I would think it's best to compute the correct length of the glass in advance and get it right before bending, however.
     
  7. shutterbug

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    Most glass melts at 1250° F, but becomes pliable a little under that. Not for the faint of heart :)
     
  8. gleber

    gleber Registered User

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    You can mold PETG with a hair dryer...

    Tom
     
  9. Simon Holt

    Simon Holt Registered User
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    Maybe a dragon could produce the temperatures required...;)

    That's definitely more achievable!

    One of the materials I've come across is cast acrylic sheet, with a slight green tint that claims to look just like glass. I don't know how easily it can be formed though - I'm going to contact the supplier.

    Simon
     
  10. shutterbug

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    Back in the day, I worked in an eyeglass laboratory where we made glass lenses. We tempered the end product by heating it to near melting point and then hitting it with a blast of air. I don't know if they even make glass lenses any more :)
     
  11. Snapper

    Snapper Registered User

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  12. Simon Holt

    Simon Holt Registered User
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    That's a useful link, thanks. I'll check them out.

    Simon
     

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