Printable View
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.
https://i.ebayimg.com/images/g/MG8AA...Sw/s-l1600.jpg
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. PeterQuote:
Originally Posted by Decieved
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.
Long curing time is good
You will want to post cure anyway. Heat a little to improve strength
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.
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
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.
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
Hm just to be sure i understand, you use one size aggregate like 5mm and the rest is epoxy ~30% ?
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
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.
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.
Sent from my Redmi Note 5 using Tapatalk
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
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...
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.
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.
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?
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?
Sent from my Redmi Note 5 using Tapatalk
Careful of volume vs weight percentages
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...
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.
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
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
Real test piece, a few pinpricks doesnt feel worth worrying about.
https://www.cnczone.com/forums/attac...d=468740&stc=1
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
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
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
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?Quote:
Originally Posted by Decieved
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
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
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.
Sent from my Redmi Note 5 using Tapatalk
Could you elaborate on how heating a little will improve strength? How much improvement in stiffness can I expect? What temperature do you mean? Is 50^C too much?Quote:
Originally Posted by pippin88