Here are two saw cut samples, wetted for the photograph.
(front and back pictured)
Ok, we have some close ups. First, the entire E/G core:
Now onto the slices - first one is polished on all sides, both sides pictured. Other two are saw cut and will be pictured in next post.
Last edited by walter; 05-27-2007 at 01:25 AM.
Here are two saw cut samples, wetted for the photograph.
(front and back pictured)
Last post, all samples dry and natural looking- just in case there is someone who can identify the matrix. Any material engineers out there..?
And once again, big thanks to Leif (AkvaCNC) for sending it all the way from France, I really appreciate it!
Going back to caveman type E/G,WilliamD had sledge hammer proof E/G with 25% epoxy and a bag of sand,no vacuum,no vibration.I beleive he had thick epoxy and the mixes were like bread dough pushed into the mold by hand.I would assume apparent high strength due to better wetting and bonding with excess epoxy.
Air release additives,wetting agents exc,do not magically get rid of entrapped air,they assist in the vacuum process.
Vibratory compaction is vibratory compaction.It may release air but its main contribution is to compact the aggregates into a formation thats locks them to gether.Too much vibratory compaction is also a bad thing.The cores you have look good,not much air.Your last samples look like you discouvered epoxy foam.Sorry for being the negatory type guy,but I must post any downsides I see.Sure,with unlimited capital its do -able but I hope to see simple solutions for the home guys.
The Busch R-5 pump I have was run 8hrs a day for 10 years with no service other than changing the oil.Really good product.Opon receiving the pump I acctually read the instructions first.It stated the pump must be shipped without oil or damage may result.Apparentally the oil will get on the wrong side or the rotary vanes and may break the vanes upon startup.Simple solution is to manually turn the pump for a minute to place the oil on the right side of the vanes.
JICYAI:Just In Case You Are Interested,Ifiberglassed coated plywood with a chopper and vacuum baged it.The pump could suck the resin right through the plywood.Talk about good bonding.This was for flight case manfacturing.The compition glued ABS to plywood and cut their pricing to knock me out of the market.It worked as my price was higher.I still have the pump and tank and the thread has convinced me to dust off the equipment and do some Vacuuming.
L GALILEO THE EPOXY SURFACE PLATE IS FLAT
Formulator epotek recommends 29 inches of vacuum but says that it should be applied and released as quickly as possible.
Formulator resins-online.com in the UK recommends brief degassing at 5 mbar= 3.75 torr gage pressure = 29.77 inches Hg Vacuum.
This thread actually comes up in page 2 for epoxy degassing in Google!
Formulator PTM-W Recommends greater than 28 inches of vacuum.
PTM-W also has some high strength cast iron filled epoxies which look like they might be the kind of thing we like!!!
So it looks like 3 epoxy formulators (counting Crosslink from my last post) recommend more than 29 inches and one formulator just says it is more than 28.
Since Crosslink, Epotek and Resins-Online all recommend values very close to 29.75 inches of vacuum, I'm not so sure this was out of line as a target vacuum value, especially considering the fact that several of the vendors state that as short a degas time as possible is recommended. Less vacuum for a longer time may work but it doesn't seem reasonable to me to deliberately purchase a vacuum pump for this application which won't produce 29.75 inches Hg of vacuum.
It's still possible that an adequate mixture can be produced without vacuum using additives but I am convinced but the very strongest mixtures will still require vacuum.
It might not have any granite in it but it looks like we could just buy cast iron filled epoxy from PTM-W and be done early with higher strength and likely decent vibration damping. . . Sigh.
Conversations moved back to degassing and vacuum?
So, Why not approach it from the other direction - just don't get gas in there in the first place.
Use the standard cascade mixers to mix epoxy. As Larry mentioned before they're intended and produced to fully mix epoxy/hardener without introducing air. You should now have an airless epoxy mix.
If you're intent on using vacuum then use an economically viable 28" pump. Put the mixed airless epoxy into the pot with the aggregate UNMIXED, and then put it under 25-28" or so. THEN mix epoxy and aggregate under vacuum. Anyone see why this wouldn't produce an mostly airless E/G mix?
In molding process use vacuum to assure packing, As someone mentioned before bag it down to ~28" to provide a compacted airless mix, then bag again and pressure the external bag 2bar.
You'll need a very strong mold.....
The bigger problem is still getting complete wet coverage, a voidless cast and reasonable distribution of aggregates. You can run a 'wet' mix to assure full coverage and void filling and remove the excess under bagging in the mold.
Distribution is more tricky but the commercial samples Walter recieved seem be be rather sparse in aggregate fill and what there is is randomly spread so distribution might not be an issue in practice.
Any more discussion on how you're actually going to use the material, machine designs ?
Thanks Andrew, that makes sense.
Here are some of the uses for home brewed E/G, these are only examples, please don't pick on them. The main advantage is that you would be able to build a machine without any welding or metal working tools. Today, there is no such possibility- hobbyist has to use MDF of expensive aluminum, and both are pretty weak materials- not suitable for any real machine. There are no choices really! E/G is also weak but you can use more of it- just buy another bag of sand/gravel and dump it into the plywood mold. How hard is that? At $0.80 per lb is a no brainer really. I wouldn't use steel tubing even if you pay me to - I think it's crap and only the big manufacturers can make it work in machine tools. I hear there's one guy here who bought a ton of steel tubing, was welding it for 2 years and now he's able to take light cuts in aluminum. Laughable if you ask me.
I think Andrew's suggestion of using a static mixer to mix the epoxy and then adding it to the pot and mixing it with the aggregate under vacuum is a good suggestion in practice. My main concern is that most of the commerical metering pumps with manifolds and cascade mixers on the end for epoxy that I've seen are almost as expensive as a surplus vacuum pump. Grebeard's suggestion of DWV pipe for the vacuum chamber is ingenious.
See http://www.michaelengineering.com/ for example costs on static mixing epoxy dispensers.
The 1975 paper by gamski said that setting under pressure was not effective in removing air voids but that it helped maintain good aggregate distribution and produced stronger samples. It's my opinion that bagged setting under pressure will do better for keeping air out of the cast and good aggregate distribution than vibratory compaction unless thixotropes are used to help maintain the spatial aggregate distribution and they require vibration.
I'm getting the impression that a carefully poured mixture that isn't too viscous and was properly deaired will be able to be poured into a mold and work ok though vacuum bagging and pressurizing ought to work better.
The biggest problem with air that I see is that it doesn't just sit around forming voids in the matrix. The air is concentrated in pockets around the aggregate effectively reducing the mix to pure epoxy for strength and modulus.
I agree with Andrew that the commercial samples Walter got are a bit sparse. I recall walter saying that they were from a machine base which I'd guess probably didn't need the high test stuff. To me, they appear to have only a few large voids around the edges which would indicate they were made from vacuum deaired epoxy poured into a mold at atmospheric pressure.
The distribution of aggregate uniformly throughout the sample ought to be random. When it isn't: that's when there's a problem. The Machine Design article way back written by folks from accures castings indicates that accures uses accurately graded aggregates to ensure the proper size distribution.
Finally, I'll have to politely disagree with Walter about steel tube generally being bad. While light gage steel tube, the stuff you'd voluntarily cut with a hacksaw, is going to be tough sledding for machine building, heavy wall stuff or I beam is likely infinitely easier to get to perform well than E/G. Most of this could be put together by bolted joints with epoxy grout in them so they stay square (this is the E/G thread).
I'd guess that with an oxy-acetylene welding cutting setup, an angle grinder, a hack saw, a drill with lots of sharp metal drilling bits, a big square, epoxy grout, and some stuff like 80/20 channel for the precision parts one could build a very nice machine. I'd also go out on a limb and state that such a brute force approach might be less difficult and less expensive than E/G. The secret is that none of the metalwork is precision work: it's just holding the precision parts up. I'd also suspect that the tools to make the metal one will ultimately be cheaper and more generally applicable than the tools for E/G.
My neighbor is a composite materials expert for Boeing on the international space station. He said to me the other day that if composites had been invented first that everybody would be rushing to aluminum as the miracle material for most applications. He recommended aluminum filled epoxy as used by industry for "soft" dies in forming operations for what we are doing.
In short, I like E/G for the reasons that Walter does: it's theoretically inexpensive and easily formed with wood molds into complex shapes that home shop types couldn't form without extensive metal cutting tools. On the downside however, it is apt to require relatively bizarre equipment like vacuum pumps and because E/G isn't particularly strong, it will require heavily engineered parts to avoid metal reinforcement.
If all a person wants to do is make a big CNC router, I'd think that several other cast materials besides E/G will be less work and have a more predictable good outcome. Metal dust filled epoxy and metal whisker filled epoxy like some of the stuff in the PTM-W catlog might be a lot easier if they aren't prohibitively expensive. Using E/G for a router, it will be a lot easier to reinforce with rebar where necessary and accept the theoretical few percent vibration increase than to design metal free parts. My own interest is to create an open E/G design that will work without reinforcement for machine applications but it doesn't mean I think that this is the easiest way. It is however the most interesting way and the road less traveled. It's probably also the cheapest way for a <u>mass produced</u> part.
Hope I've been playing devil's advocate here and not the devil,
A thought for those intending to use some random mix of sand/crushed granite/rocks.
If a "perfect" mix of different sizes is to make such a big difference to the ultimate strength of the E/G, there needs to be some method of checking if the mix we have in front of us is good/bad/indifferent.
Could this be a dry check of the density ?
Take a small box of known dimensions. Fill it with the largest size rocks and weigh it. This will give me the lowest "density" of my E/G( I'm ignoring the weight of the epoxy for simplicity.)
If I now pour in smaller rocks, shaking the box as I do so, till I can get no more to go in below the rim, and weigh again, this gives me an improved "density".
Now I pour in another smaller sized fill, and repeat the process.
I can, by this method, see what I can make with a mix of segregated, and known sizes of fill particles.
If I now take a "random" mix of crushed stones and pour it into the box, shaking it as I do so, the weight/density figure will give me a check on how good a particular sample is. Pushing it a bit, it might let me see if the mix could be improved with an addition of some particular size, but I'm not sure if that's worth considering. I'd need to separate the "random" mix through a set of seives to see how the size distribution was.
What I'm looking for is a simple method of checking that I'm starting with the best mix from whatever is available to me locally.
It's like doing jigsaw puzzles in the dark.
Enjoy today's problems, for tomorrow's may be worse.
You're exactly right. The only thing that I would suggest is using two measuring vessels. Fill the first with your aggregate to a known volume and then add water from another vessel until the water reaches the same mark where the top of the aggregate is. The volume you filled the aggregate vessel to minus the volume of water added divided by the volume you filled the aggregate vessel to will be the fill percentage.
This won't tell you what aggregate mixture to use but it will tell you how well you did. This assumes of course that none of the aggregate is water reactive.
New bits of info.
Some 'polymer concrete' machine base manufacturers use fluids/binders with viscosity of 1-10cP. Water is 1cP, automotive antifreeze is 20cP, Motor oil SAE 30 is 200cP. My epoxy is 600cP.
Here's another one (before you buy the aggregate):
High grade quartz contributes to great wear resistance but also lowers elastic modulus. In another words, buy stuff other than quartz to get the best Young's modulus.
The fluid they speak of in that patent is acrylic resin which I thought was rejected on this thread due to high shrinkage.
According to the nist paper brunog posted some time back, http://fire.nist.gov/bfrlpubs/build99/PDF/b99032.pdf
crushed quartz does better than natural granite on young's modulus but not quite as good as basalt. I suspect that there is a huge amount of variability depending on exact particles sizes so I have no idea what data to believe about aggregate.
My belief is that whatever material has the highest value of a parameter called fracture toughness which is particle size dependent is going to work best.