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

    Hello all

    Putting together tools and workspace to build using epoxy granite and was wondering whats needed to vibrate bigger molds?

    I have a few pneumatic vibrators of different size that i have used years ago but only on smaller molds of about 30kg, my planned casting will weigh 100-300kg.

    Will these do or do you recommend something else? An electric motor with an offcenter weight and a vfd is pretty easy to throw together.

  2. #2
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    Re: Vibrator for epoxy granite

    Quote Originally Posted by Decieved View Post
    Hello all

    Putting together tools and workspace to build using epoxy granite and was wondering whats needed to vibrate bigger molds?

    I have a few pneumatic vibrators of different size that i have used years ago but only on smaller molds of about 30kg, my planned casting will weigh 100-300kg.

    Will these do or do you recommend something else? An electric motor with an offcenter weight and a vfd is pretty easy to throw together.
    Hi - If you use the correct epoxy you will not need to vibrate. Vibration leads to segregation which is not good. The epoxy should be very thin like used for river tables and infusion. With this sort of resin you can dry pack the mould and pour the resin in until filled. Laminating epoxies are desigbned to get thicker when agitated. They have thixotropes added for this purpose so they do not run. Peter

  3. #3
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    Re: Vibrator for epoxy granite

    Oh my dident know that, that would be great. I have a new supplier of epoxy and its actually a product for rivertables but i doubt it is thin enough to dry pack, its viscosity is 295 mPa.s. They had another product thats lower around 200mPa.s that i passed on because the curing time started to get really long.

    Maybe i will do a small test but im pretty sure i have to vibrate my mix.

  4. #4
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    Re: Vibrator for epoxy granite

    Long curing time is good

    You will want to post cure anyway. Heat a little to improve strength
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    Re: Vibrator for epoxy granite

    Yeah but we are talking weeks of curing time. I was happy when i found this epoxy because it has a pot life of 16hours way more then what i have used before. I havent decided wether im going to post cure it or not, the product comes with no information on post curing so rather then risk doing damage i may just leave it to cure in room temp.

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    Re: Vibrator for epoxy granite

    Hi Deceived - I use an epoxy with a 25C viscosity of 240mPas... It has a pot life of 75mins (100g at 25C) I think you will find <300 mPas resins will seep thru most things especially if it has a huge gel time... Having a 16hr gel time means if you heat it 10deg it will have 8hrs and it will also halve the viscosity.... heat it 20C and its 4 hrs and really, really thin...Test my numbers they are a generalisation... Peter

  7. #7
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    Re: Vibrator for epoxy granite

    Great thanks will do a test. When packing dry dont you find it that the mix separates? And have air bubbles get stuck to walls when not vibrating? thats mostly cosmetic tho.

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    Re: Vibrator for epoxy granite

    Hi Deceived - I use a single size grit vs a graded mix. To remove bubbles from surface and using a long gel time epoxy you can brush out the mould surface with epoxy before you fill (dry fill or wet fill). With thin epoxy I expect the surface finish to be very good as long as the surface is smooth and well waxed. If the surface is a bit rough then bubbles are a given. If the mix separates filling dry then it would do the same if done wet and vibrated... I typically get 65-70% solids ratio by volume using a single size grit. I'm not yet a believer in graded curve solutions getting much better then this and use the KISS approach. If dry stacked fill from one end so the air gets displaced to the other end and it fills up slowly pushing air out the other end. You can also pigment the epoxy and brush out the mould and let it get tacky then fill. (this is sometimes called a gelcoat) Most inexperienced DIY moulders do not wax enough, if you are using melamine boards for moulds they will need at least 5-6 coats of good paste wax to get really sealed and coated.. if std masking tape sticks keep waxing...cheers Peter

  9. #9
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    Re: Vibrator for epoxy granite

    Hm just to be sure i understand, you use one size aggregate like 5mm and the rest is epoxy ~30% ?

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    Re: Vibrator for epoxy granite

    Hi Deceived - 5mm is probably too big, I use 1-2mm. The image shows 3 small samples I have done to check the volume ratio of the mix. All have been dry stacked then epoxy poured in...The right hand sample is 0.4mm aluminium powder. The LHS is 2mm alox grit. The rear sample is a mixture of the two in an attempt to increase the solids ratio. You can see how it segregated. The single size mix gets about 58% solid ratio, the mix was about 62% so I don't see the value in trying for higher... prefer to use high modulus aggregate. This approach does use more epoxy in this case 18% by weight (but that is also because alox is dense) but I'm not keen on porous solids that a 10% mix would create... by the way the LH sample has some white pigment added to the resin that's why it looks a bit different... Peter
    Attached Thumbnails Attached Thumbnails alox al mix.jpg  

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    Re: Vibrator for epoxy granite

    Hi Deceived - I'll take two steps back. The particle size does not matter if the particles are similar shape. They will achieve a similar packing ratio in a relatively same space. By this I mean that in a very large container they will achieve the same volume ratio. The larger the aggregate size the less they pack at the mould surface so if you used a 10mm aggregate in a 20mm gap they would not pack well. That's why I go for a small aggregate size like 1-2mm so that the surface effect (or surface volume loss) is minimised. Where I live basalt (blue metal) is available in 3mm 5mm 10mm and 20mm. To check your volume ratio get a 500ml container or 1 litre and fill with water to check volume. Then dry pack the container and weigh the dry pack and you also then know its volume. You can calculate its bulk specific gravity if you like. Then add water to fill and weigh the addition. Now you know the volume ratio to fill with epoxy. You can play with mixes if you like doing the same thing. This will tell you if your mixes are improving the volume fractions or not. Peter

    in regard to mineral stiffness - sand (quartz) is less stiff then basalt. Sand is about 60-70GPa (can be 100 depends on what you read, pure quartz crystals are tested at 100GPa ) and basalt is about 70-80GPa but general basalt is about same as sand as its mostly Sio2 same as sand. Problem is when you buy these materials they can't tell you what its modulus is.

  12. #12
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    Re: Vibrator for epoxy granite

    Thats some segregation.

    I have used crushed rock and quartz earlier ranging from 1-8mm in size <12% epoxy by weight. Yielded good castings but was long ago so i cant really remember if or how much i vibrated the mold.

    Those aggregates arent available to me now so i made a new recipe that looks to need 8% epoxy, thats with 2.7g/cm3 density aggregates.

    I will make a couple of small deep testpieces 1 drypacked, 1 overvibrated and one normal.


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    Re: Vibrator for epoxy granite

    Good morning Deceived - Your figures bring up the usual discussion about porosity. I personally don't want porosity as it dilutes the modulus. Looking at your 8% epoxy by weight addition means if you mix with no porosity/air then you achieve 84% solid packing in the aggregate. Although theoretically possible if the particles are spheres its unlikely in practice. You do need to do some water fill tests to determine the solids ratio. Then you need to do a mix and check how much air you introduce in reality if you mechanically mix the mix. It can be considerable. I have done this and degassed in vacuum chamber and it surprised me how much air is in there.... I cast prism blocks so I can measure its size and calculate its actual density. This gives me a reality check on the paper values. Keep us up to date with the figures its all good info.. Peter

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    Re: Vibrator for epoxy granite

    Peter, epoxy modulus is 3.5 from memory?

    Does more epoxy really increase stiffness much?

    Epoxy is almost the entire cost.

    Blue metal (basalt) is $100 a cubic metre or so.

    So use double the epoxy to get rid of porosity = nearly double $$$

    Or use minimum epoxy and increase section size...
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    Re: Vibrator for epoxy granite

    Happy Easter Pippin - Yes EP about E=3.5GPa. Its not about more epoxy its about less air. If you want to lower cost then organise the shape better or include designed voids vs having 10% air distributed through the material... if you want to reduce cost further then UHPC (concrete) is half the cost of epoxy... Its not a simple equation, all depends on what you are aiming at... Peter

    addition - a good strategy? - Use Basalt as its stiffer then sand. Say 5mm or 3mm aggregate. Do a packing test and then add sand until you get max packing factor. This will be your cheapest/stiffest EG solution. Then if you want to use less EP then "correct" so be it and ignore the porosity. Then test and see what stiffness you get or move onto casting your part in bliss.

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    Re: Vibrator for epoxy granite

    The water fill test you speak of i have already done, its how i find my mixes. How do commercial casters avoid or get rid of porosity?

    Still a bit to cold around here to make samples but in a few weeks i will have a go at it.

  17. #17
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    Re: Vibrator for epoxy granite

    Hi Deceived - Videos of mixing large amounts of EG show they either mix at "correct" ratio or if dry they do not worry about porosity.. For large castings they add enough resin to allow the mix to just flow. Rampf show sections of their stuff being tested and it shows minimal porosity. What solid ratios (packing factors) are you getting on your mixes?

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    Re: Vibrator for epoxy granite

    Ok i have only seen premixing and vibrating done.

    My old recipe used 12% epoxy by weight and the new one i want to try ~8% aint the remainder the solid ratio?

    I havent measured any of my castings to calculate how much air they contain.

    I was thinking if you have to up your epoxy to 30% to replace the air shouldn't a low epoxy casting have so much air that its visible in a crossection?

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  19. #19
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    Re: Vibrator for epoxy granite

    Careful of volume vs weight percentages
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    Re: Vibrator for epoxy granite

    Depending on where you live, you might be able to find premixed stuff. I bought Silimix282 here in europe. I'm mixing it with "R&G ResinL/GL2". I'm going to use a "concrete rod vibrator" to vibrate the mix... in all the tests i've done with this material i have never seen any segregation at all, so i'm not worried about that...but i guess i depends on your materials...

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