Replace curved glass with plastic?

Discussion in 'Clock 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
     
  13. FatrCat

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    Greetings, Simon-
    Were you able to resolve your curved glass needs? If not and still interested in a possible alternative I've done custom / unique acrylic thermal forming prototype work for many years and would be happy to guide you through a process by which you can create what you need relatively simply and inexpensively right from your home kitchen. David
     
  14. Simon Holt

    Simon Holt Registered User
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    Hi David. Thanks for getting in touch. No - I haven't found the solution yet, so any guidance you can offer would be much appreciated!
    Simon
     
  15. FatrCat

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    Simon,

    Going by your photos, I'm assuming the graphed mat used is marked off in 1 centimeter increments. If different from this the feasibility of the following idea could change.
    The following outlines a method to form a replacement lens using acrylic sheet, e.g. Lucite, Plexiglas, with a thickness ranging between 1/8 to 1/4 inch. Forming is acheived using heat, which can be applied either by heating the entire form in a household oven, or by use of a 'heat gun' worked slowly over the area that needs to be arced.

    In looking at the drawing note that to begin, you'd want to cut 3-4 forming braces (1) which match the intended curve, and 2 end boards (3 & 4). The assembled frame should be a bit wider and longer at each end than your finished glazing, to accommodate clamping strips at each end. The end pieces (3 & 4) are cut so that #3 is taller, creating a square stop, and #4 with a slight angle. This is done to create a square starting point (3), and an end attachment point (4) for the metal forming bed, which is to be made from a non-stick 'cookie sheet', (shown in dark green-blue) removing the raised sides of the sheet, but leaving the raised ends. The cookie sheet should be attached to the frame at the top, then held taut as it is attached at the bottom to #4 so as to maintain a smooth surface.

    The acrylic "blank" to be formed is held at the top edge by a wood lathe which pinches the acrylic down to the metal sheet, and then screws are used to just barely pierce through the lathe to create 'teeth' which hold the acrylic and prevent it slipping during forming. (detail "A")

    Along each side, as drawn, you'll see a tension assembly consisting of a spring attached to a wire which draws around a pin and then loops to hold each end of a small dowel rod (detail "B") which spans across the top of the acrylic. This apparatus could be omitted if forming with a heat gun, however it does ensure that the arc is formed with even tension and 'automates' bending, allowing you more freedom of hands. The spring tension is minimal, and should be attached so that the springs remain only slightly in tension once the acrylic has formed down to the metal sheet surface.

    HEATING PROCESS
    If by oven- pre-drill the top lathe screw attachment points on the form and have the bottom lathe attached to the acrylic in preparation. Also, be sure any protective surface sheeting has been removed from the acrylic. Pre-heat oven and the form to 350°F. With gloves, remove the form and put the acrylic in place, screwing down the top lathe and putting the tension dowel in place, then place form back in the oven, leaving the door open to the 'broil' stop. The time needed will vary depending on acrylic thickness, between approx. 4 to 8 minutes. After 2 minutes, reduce heat to 300°F and monitor closely. As the acrylic begins to bow downward turn off heat and open door fully, removing form just as the acrylic fully drops. Use a small fan to cool for 10-15 minutes before removing the acrylic. NOTE: Be sure to set your oven on 'Bake' so that no upper element (if electric) comes on, as this can cause the acrylic to blister.

    If by heat gun- keeping the heat gun 8-12 inches away from the acrylic, heat the entire curve area plus 2-3 inches above the start of the curve, continuously moving the gun across the surface. You can begin by aiming the gun beneath the acrylic to heat the underside and the metal sheet a little, but can mostly be done from the top. Once the sheet begins to bow, work the gun side to side from the beginning of the arc, slowly down the curve until sheet has evenly dropped to finish level, then cool as above. Be sure to keep spring tension on dowel or otherwise keep downward pressure during cooling to prevent acrylic from returning upward.

    arc-acrylic.jpg
     
  16. Simon Holt

    Simon Holt Registered User
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    David, thanks so much for this. You've gone to a lot of trouble and I really appreciate it.

    You're right that the cutting board graph is marked in centimetres. I'll almost certainly use the heat gun method though, so the size of the oven won't be a limiting factor. I'll gather the necessary materials and let you know how it goes.

    Thanks again!

    Best regards
    Simon
     
  17. FatrCat

    FatrCat Registered User
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    Glad to be of help, Simon :)
     

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