How much epoxy additive?

Discussion in 'Techniques' started by rocketcharlie, Jul 18, 2019.

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  1. Jul 18, 2019 #1

    rocketcharlie

    rocketcharlie

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    On the suggestion of Jim Jarvis I have been using laminating epoxy and adding 1/16" milled fiberglass for strength. I am wondering how much I should be adding. Can a person diminish the strength by adding too much?
     
  2. Jul 18, 2019 #2

    OverTheTop

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    The glass is what gives fiberglass its strength. The rest of the matrix (epoxy) is mechanically weaker. Add as much chopped glass as you want/can tolerate without the joint ending up looking like a porcupine.
     
  3. Jul 18, 2019 #3

    cwbullet

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    It really depends on what you are using it to do. For laminating, I use zero additives unless I want to color it. If I am making fillets, I add enough to make it spreadable and less runny but as #OvertheTop said: enough but "without the joint ending up looking like a porcupine".

    You may find that chopped kevlar or chopper carbon fiber has less of a tendency to do that.
     
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  4. Jul 18, 2019 #4

    blackjack2564

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    I add enough to make consistency of peanut butter, so it holds the shape when making fillets.
    None when laminating fiberglass cloth.

    Think of it as re-bar in concrete.
     
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  5. Jul 18, 2019 #5

    MClark

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    Fiberglass will make it hard to sand
    Use Kevlar and you will hate yourself, becomes fuzz.

    M
     
  6. Jul 18, 2019 #6

    manixFan

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    When I use filler like either of those I'll add a secondary 'cosmetic' fillet (about an 1/8") of epoxy heavily filled with phenolic or glass beads and silica thickener to make a very shapable/sandable surface.

    I learned that the hard way after trying to sand kevlar filled fillets.


    Tony
     
  7. Jul 18, 2019 #7

    manixFan

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    If I have a 3D structure with randomly oriented fibers, especially in bulk, many of them will be aligned to the applied stress, the same way a composite layup is done with the cloth rotated at different angles. Isn't a layup a 3D structure? I can see a single layer of fiber considered 2D, but once you add multiple layers you've created a 3D structure that can only be described in if you know how the depth of the layup is made.

    I'm just trying to better understand how composite structures work. I make fillets both filled and unfilled, and fortunately neither method has failed me (yet). I'm not big on tip-to-tip for my MD diameter rockets as I haven't had just fillets fail me yet, even at plus Mach on a several 38mm and 54mm rockets. (Mach 2.4 on the 54mm size.) So I am concerned with making the strongest possible fillets.

    A quick web search about glass strands shows that nearly every maker of epoxy claims that adding it to epoxy increases tensile and compressive strength. Here's a common claim: "Chopped glass strands ... can be incorporated into structural filleting putties to improve tensile strength"

    It would be interesting to do some destructive testing to see what effect the different additives have on fin fillets.


    Tony
     
  8. Jul 19, 2019 #8

    G_T

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    You're not likely to manage aligning the little chopped fibers. You can somewhat align fibers by dragging a probe through the wet mix, but that would have to be perpendicular to the joint and therefore rather hard to achieve a fillet shape afterwards!

    If your fillet is small, and your rocket is not high performance, really it is just an aero fairing more than anything else. So any thickening agent that helps you get it smooth without making it too brittle is fine. In the past I've just gone ahead and used JB Weld - which is an epoxy with a fair bit of Calcium Carbonate (chalk dust) and powdered Iron added. For that matter, West has additives. I may not think much of West Systems epoxy for aerospace applications, but their fillers are quite nice.

    https://www.westsystem.com/the-105-system/fillers/

    Peruse the list and see if anything looks useful to you.

    Gerald
     
  9. Aug 23, 2019 at 11:29 PM #9

    timbucktoo

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    Gerald's info on glassing has been moved here and made a sticky.
     

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