[badhabit]

Hi Thomas - How are you going to pressurise the sample to 8bar? do you have an autoclave? 8 bar is 80 tonne per sqm.. or do you have a big pressure pot? or a big steel tube I suppose.... I don't think eliminating internal bubbles will make a measurable difference? but you will find out... You can compress bubbles but then you trap micro bubbles at pressure in the concrete and they break out eventually. In aerospace laminates they are compacted at 6-8bar but the air goes into the resin as a saturated gas. Then as the part is cured at 120C it desorbs out, plus there is a vacuum system to remove gases....So you will need a plastic bag around the concrete part with a breather layer then 7 bar applied to the outside of the bag to pressurise the part....???? Regards Peter

at 8bar a 2mm bubble will be compressed to a 1mm bubble so is this a difference? will be interesting to find out...

I was going to build a chamber for this. Some Ø150mm steel tube with flanges on the ends and then compressed air. And before the internet police arrest me, yes, i know it is "a bomb", but heavy duty seamless structural pipe(like 8-10mm wall) and with a good penetrating weld on the flanges it'll be just fine...

I'm not convinced it is going to do a whole lot to the EM, but maybe the final strength. I did some test-casting and my EG was somewhat "brittle" in small sections. Not sure if "brittle" is the correct term though...but it seemed like the test-pieces broke "through" all the trapped air-bubbles and i wanted to see if i could get rid of them somehow...this is one of those samples:
Attachment 468324
...as you can see, a ton of small bubbles....

I read somewhere, i can't recall where(was it a qoute from you, Peter?) that "someone" raised the pressure to above 6bars and the gas dissolved into the epoxy eliminating the bubbles. Like you describe above. This was actually the main reason why i wanted to try to cure it under as high pressure as i could...to try to dissolve the gas into the resin to avoid the bubbles...if i just reduce the bubble size it will not have any significant effect....but if it does dissolve, do i then need to put it under high temperature(like 70c i used for post-cure) vacuum afterwards to pull out the gas from the cured epoxy afterwards? I would use the same pressure-champer for this. It would be easy to wrap the cylinder in heating wire and isolation material to heat up the sample during the vacuum process...

/Thomas

[peteeng]

Hi Thomas - I think the process is backwards. If you understand vacuum processes I'd do it this way. 1) dry stack the mould and the mould needs to be vacuum tight or you need to put a vacuum bag around the outside of the mould 2) pull the dry stack down to below the local vapour pressure level. Vacuum soak for at least an hour to remove all the water moisture in the stack c) Then have a suitable tubing system that allows epoxy to be drawn into the evacuated mould. Since there is less than 1% air in the mould the resulting "infusion" of epoxy will be very low void. I've done this sort of thing with many materials and even 50ft boats. Infusion requires light moulds as its vacuum not pressure.... There are many infusion grade epoxies around to choose from...

I have done this with 100% alox (65% by volume) and the flexural stiffness was <20GPa. I've done it with sand, aluminium powder, steel fibres, fibreglass, carbon fibre, timber. In fact if you use a long gel time epoxy there is enough room (at least 30-40% void) in a dry stack that if you just pour infusion resin on top it will seep down to the bottom quite quickly. In this way you get no air in the structure due to the mixing as long as the stack is dry. Thats the advantage of the vacuum soak, removing moisture from the stack.

Speaking of strength my testing of fibreglass laminates for vessel survey took a significant step forward once we started vacuum soaking the dry stack vs wetting out the laminate then vacuuming it to spec. Water and epoxy are not friends. Water inhibits the coupling of the epoxy to the substrate. We tripled the strength of the laminate by vacuum soaking.... The amount of water in rocks must be huge unless your in a desert at 40% RH.... or someplace in africa with 17% RH.... Peter

[badhabit]

I think the process is backwards. If you understand vacuum processes I'd do it this way. 1) dry stack the mould and the mould needs to be vacuum tight or you need to put a vacuum bag around the outside of the mould 2) pull the dry stack down to below the local vapour pressure level. Vacuum soak for at least an hour to remove all the water moisture in the stack c) Then have a suitable tubing system that allows epoxy to be drawn into the evacuated mould. Since there is less than 1% air in the mould the resulting "infusion" of epoxy will be very low void. I've done this sort of thing with many materials and even 50ft boats. Infusion requires light moulds as its vacuum not pressure.... There are many infusion grade epoxies around to choose from...

I have done this with 100% alox (65% by volume) and the flexural stiffness was <20GPa. I've done it with sand, aluminium powder, steel fibres, fibreglass, carbon fibre, timber. In fact if you use a long gel time epoxy there is enough room (at least 30-40% void) in a dry stack that if you just pour infusion resin on top it will seep down to the bottom quite quickly. In this way you get no air in the structure due to the mixing as long as the stack is dry. Thats the advantage of the vacuum soak, removing moisture from the stack.

Speaking of strength my testing of fibreglass laminates for vessel survey took a significant step forward once we started vacuum soaking the dry stack vs wetting out the laminate then vacuuming it to spec. Water and epoxy are not friends. Water inhibits the coupling of the epoxy to the substrate. We tripled the strength of the laminate by vacuum soaking.... The amount of water in rocks must be huge unless your in a desert at 40% RH.... or someplace in africa with 17% RH....

Yeah, well, i've never been fully convienced about the dry-stacking idea for EG, however i don't think this is even an option with the Silimix282 ayways. Some of the aggregate is very fine, like VERY fine. It is not like sand or something like that, it is extremely finely grained powder, almost like talcum or fine wheat flour or something like that. I colors your fingers without leaving any "feelable grains" on your skin. I could doubt that i would be even able to pull a vacuum through it dry! And i have even less faith in me being able to pull epoxy through any meaningfull size of the stuff. I've done quite a lot infusion in carbon/kevlar/fiberglass setups(no boats though, only smaller setups with up to 1m pulls), but this is a whole other beast IMO...and i would likely ruin my vacuumpump or at least have to change the oil afterwards due to this very fine(and likely abrasive!) dust unless i add some sort of very fine filter to the pumpline in my catchpot...

However, i DO see why this is appealing! Vaccum=no air=no entrapment=no bubbles! But i'm not sure how practical it will be with big iregular castings, like mine for instance. It will be a nightmare to bag without leaks...maybe the mold/forms could be made more "bag'able" and the casting could be performed on a large metal-plate so you wouldn't have any need to put bags below the form(the weight would likely pierce the bag)...or maybe a mold entirely out of metal sealed with o-rings/gaskets so no bag would be needed...but that is a whole other can of worms

Companies like Rampf or Studer in Germany do these castings wet and they don't seem to be the least concerned with trapping air or bubbles. Have a look at this video:

...i see a wet mix being poured, vibrated(which i don't quite understand, that didn't work very well with my mix) and then just cured...leaving a ton of "pinholes" just like in my castings, they are clearly visible at 1:15 in the video....

Maybe if trapped air is good enough for Studer, it should be good enough for me too?

/Thomas

[peteeng]

Hi Thomas - I can't see how you cannot have air in the mix when the mix is mixed mechanically. So I think your stuck with it. I have made tooling board from microshpheres that are used in cosmetics for micro abrasion. I think the spheres are less then 10um dia. They float around in the air quite easily. They infuse into board really well. But I have gone off the idea of infusing mineral cast parts. I'll go carbon if I go that way to get E= 80GPa... Look fwd to your results. I like laminated metal now especially Aluminium as I can machine it on my router... Peter

[badhabit]

Look fwd to your results.

I was planning on making 40x40mm square billets for testing, 800mm long... does that sound reasonable or ? I think my largest aggregate is around 4mm, so i'd figure 10x in size.... i was planning on doing at cantelever setup with 200-250mm "attached" to a table and then apply downwards force(via weights) 500mm from the table and measure deflection with a fitting micrometer...?

[peteeng]

Hi Thomas - I expect 40x40x800 to be a good size. Peter

[NordicCnc]

Hello Badhabit. You have built a very nice machine, I have read pretty much the whole story.

I see you have done a substantial amount of research on epoxy granite, so you might be able to easily answer my question: I have my own homemade CNC (nowhere close to this good) which is built from aluminum extrusion. Obviously the vibration dampening in the extrusions is almost non-existent, and they ring like a bell. I have been doing some reading and found out that many fill the extrusions. I've understood it is important to not use concrete when it will be in contact with aluminum, as it is alkaline. Instead, epoxy granite (also known as mineral casting) is used to fill the extrusions. It is typically a mixture of sand and epoxy. In this case I've understood that the sand must be dry, and most importantly pH neutral, pool filter sand comes to mind.. However, I stumbled on the Silimix 282 which seems to have mix of grain size and should better for vibration dampening and even stiffness (although stiffness is not what I am looking for).

Do you think Silimix 282, mixed with epoxy, will be compatible to be in contact with Aluminum? Thank you in advance.

[badhabit]

Do you think Silimix 282, mixed with epoxy, will be compatible to be in contact with Aluminum?

Defently :-)

[peteeng]

Hi Nordic - Expanding PU foam from the hardware will do the same thing, easier and cheaper.... Peter

[NordicCnc]

Defently :-)

Thanks, I'll put in an order and start testing!

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[NordicCnc]

Hi Nordic - Expanding PU foam from the hardware will do the same thing, easier and cheaper.... Peter

Thanks, I was not aware of that alternative. For sure attractibe to moving parts which are sensitive to weight. But for fixed parts (like the frame and table), a quick Google search and ChatGPT query told me that both options give good vibration dampening, while epoxy granite is probably superior.

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[peteeng]

Hi Nord - In one thread I've done extensive vibration analysis on construction extrusions. What came out is this:
1) Hollow sections suffer from acoustic noises as they are organ pipes. These noises do not affect the machine tool only your ears. So plug the ends and that organ pipe effect goes away. This is the same for open ended steel sections.
2) The many thin free edges around construction extrusions are the main culprit for vibrations. Especially at the section ends where its free in two directions. So an end cap that contacts these ends and damps them is a good strategy or a screw that constrains the free edges...
3) I deal with a company that makes this sort of machine and they have filled their extrusions with a few things and have not found that it affects the cut... so they stopped filling but did include end caps....Peter

explain why EG would be superior to PU in that application?

[badhabit]

I had some spare phenolic plywood laying around, just enough to make two forms for 40x40x800mm test bars:
Attachment 504088
...i'll cast some billets with my EG very soon and post the results here...