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IndustryArena Forum > Mechanical Engineering > Epoxy Granite > Vibrator for epoxy granite
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  1. #21
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    Re: Vibrator for epoxy granite

    Hi Deceived - The material consists of "solids" or aggregate, resin "liquid" and air. If you do the water test you will establish the solids ratio. This can be 60-80% by volume depending on what your doing. Ratios by weight are convenient for making stuff but you have to work in volume ratios to figure out what's happening at the material level. Then convert to weight ratios for the actual mixing. So say you figure out your aggregate is 70% the solid volume of the mould. Then since your using quartz the attached calculation shows you have 13% by volume air in there if you use a 10% addition rate of epoxy. Now 13% by vol of air in that 77kg mix is 6 grams so you can't pick that up on a kitchen scale but you will see it in the mix as bubbles. To remove that amount of air you have to deep degas the mix which is difficult to do. Vibration and warmth may allow a lot to rise to the surface. Unless you do test mixes and check carefully you never pick up the air by weight... cheers Peter check the math I have been wrong before...

    what usually happens is moulds are stacked by mixing small batches and they fill batch by batch until done. if they mixed the "correct" amount they would find they have too much by about 10% volume...due to the air in there.

  2. #22
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    Re: Vibrator for epoxy granite

    Hi Deceived - I decided to add litres to the doc as these can be envisaged easier and found I had transposed two numbers in the table. So here's the corrected table. Outcome the same.... so in this 77kg mix I'd expect 4.8 litres of air or more if the target (mould) volume was achieved. Peter

  3. #23
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    Re: Vibrator for epoxy granite

    Yesterday i had time for some testing using what equipment i had. I tried vibrating a small sample, packs nicely but some air. Degassing using vacuumchamber works nicely.

    In a 1L batch there was 13cc of air in the mix.
    If one were to cast a 100 x 60 x 30cm block it would mean that i would have 4cm layer of free epoxy ontop before degassing.

    I dont see me making sturdy enough forms to allow for vibrating it enough and its to big for my vacuum chamber is there anyone around with vacuumbag experience? Would seem to me that one would suck the epoxy out before the air starts to move.

    Sent from my Redmi Note 5 using Tapatalk

  4. #24
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    Re: Vibrator for epoxy granite

    Real test piece, a few pinpricks doesnt feel worth worrying about.



  5. #25
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    Re: Vibrator for epoxy granite

    You might find some of my findings during casting usefull: https://www.cnczone.com/forums/verti...ml#post2464916

    ...and carefull with vibrations, it might ruin surface finish(it did that at some places in my casting)....

    /Thomas

  6. #26
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    Re: Vibrator for epoxy granite

    Thank you, is there something in particular you are thinking of? I usually make molds out of melamine board it realeases well but might try phenolic sheet for bigger molds if i can get a flat sheet.

    As for concrete vibrator, i have one and dipping it directly in the mix separates the fines and its very violent. Might try dipping my pneumatic ones as its gentler and adjustable.

    Sent from my Redmi Note 5 using Tapatalk

  7. #27
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    Re: Vibrator for epoxy granite

    Hi Deceived - 1) mix does not need vibrating 2) if you do use a vibrator put the vibrator to the outside of the mould not in the mix with a damper between the vib and the mould. Something like a thick rubber matt, old thong, yoga mat etc Peter

  8. #28
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    Re: Vibrator for epoxy granite

    Quote Originally Posted by Decieved View Post
    Thank you, is there something in particular you are thinking of? I usually make molds out of melamine board it realeases well but might try phenolic sheet for bigger molds if i can get a flat sheet.

    As for concrete vibrator, i have one and dipping it directly in the mix separates the fines and its very violent. Might try dipping my pneumatic ones as its gentler and adjustable.
    I'd be carefull with the vibrations. In my mix nothing seperated at all, but it scarred the moldsurface in my casting which ruins surface finish on the final casting. The concrete vibrator did nothing good in my casting, instead i'd investigate other methods of "deairing" the casting. You can also consider how important the deairing is... if your casting i large enough, maybe the trapped air doesn't affect the performance of the final casting and then it can be ignored?

    If i'm ever to do a casting again i might convert an old large gastank(or similar) to a sort of autoclave. So do the casting, move to casting into the autoclave, pull (an allowed amount of) vacuum for a periode and then apply pressure to the tank during the curing. The vacuum will help the air escape and the pressure will minimize any remaining air bubbles to a minimum. After curing the tank could be heated for the post-cure...all in one process...but quite a lot of prep-work to make the tank, just an idea from an "always all in"-kinda guy :-D...

    /Thomas

  9. #29
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    Re: Vibrator for epoxy granite

    Hi - There is little hope that air within an EG mix that is not within a few millimetres of the surface will be removed with vacuum. 1) the hydrostatic pressure keeps the bubbles intact 2) the bubbles stick to the aggregate vs rising to the surface 3) the aggregate is a maze that bubbles can't get through to the surface.

    With composite laminates in autoclaves "communication" paths have to be established in the laminate stack to allow the outgassing to get out before the prepreg stack goes liquid. If the resin goes liquid before complete outgassing and evacuation is achieved then the autoclave is ramped up to 6-8 bars to squish the bubbles into solution. A 6-8 bar chamber has to be very strong to allow this to happen. All to make a couple of EG parts? Much better off using very thin resin and allow it to degas itself as best as possible in the knowledge that a low to zero void is not obtainable in this sort of EG process. A rubber mallet tapping around the mould will release any large included "bubbles" simple as that. Peter

  10. #30
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    Re: Vibrator for epoxy granite

    Yeah the ignore it route is where im heading im not seeing the stated amount of air in my castings. Gonna vibrate it with what i have on hand and be done with it.

    I have a 200mm deep testpiece that i vacuumdegassed, havent sliced it but seemed to realease just fine.



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