[lgalla]

Exactally Ger
The larger the mass the harder it is for the heat to escape.E/G the thicker the mass the more rapid the cure and heat generation.The pot or mix container will harden rapidly because of mass or volume.Thick castings heavily filled should not have excessive exotherm while unfilled raw epoxy could "smoke"in the pot if left to long.Lets "stick "with slow curing rates.
Pot life is the max mix time for a large mass of material.Once poured to a thinner cross section the cure time may be "boring"or quite a long time.Donot watch the epoxy too closely as it will slow the cure.{if you watch the kettle the water will not boil}Famous quote from my mother.
Larry

[WilliamD]

I was at Home Depot, and the had an assortment of rocks and sand. They have 1/2" marble rocks, which looked promissing. I didn't see any granite ones. Also, in the sand section, they had some really fine "silica" sand. i some this is some sort of silicone. Is marble much like granite? Should I be looking for just granite. It was 5 bucks for a 25pound bag, so pretty cheap. The sand was around 5 bucks for a 75lb bag. I'm not saying I won't try those in the tests, but I need to know if I need to keep looking for granite.

[lgalla]

William
Marble is not much like granite.Home depot builders sand is soaking wet.Playbox sand is dryer as is sandblast sand which is SiO2,good stuff.Many,many posts ago I mentioned chicken grit.Maybe you guys thought I had too many:cheers: Agriculutural feed stores have chick,chicken,turkey grit as a feed.It is usually granite chips,but may be oyster shells,ask or check the bag.Bonsai uses chicken grit in the soil.Landscape supply places may have pea gravel or river rock for your coarse aggregates.For the non farm types,birds swallow rocks to digest food as they have no teeth.
Larry

[brunog]

William,
Also look at pool filtration sand, it's usually Quartz and coarser than play sand and most important, it's dry.

Bruno

[Eson]

How hard is it to dry sand? Cant you just shove it in the owen for awhile?:rainfro:

[WilliamD]

I was thinking of just drying it out. I was talking with some coworkers, and they said landscaping companies have crushed granite by the ton, usually for between 20 to 30 bucks a ton. It might not be worth it now, but with the gantry milling machine I plan on building, I'll need around 1300 pounds worth. Just something for the future. With how much epoxy I'm going to have, I will use a bunch of different aggregates, including the granite. Is crushed glass any help? It would basically be sand also. I don't want to really add anything that would harm its ability to dampen vibrations.

[brunog]

Eson,
it's not just drying it up, play sand has been sifted to a maximum size and it has all sorts of junk in it, unless it's washed sand. Many building material stores have dry sand.

William,
to make a good quality E/G compaction is a critical point. If your agregate mix does not compact properly this is going to create voids in your E/g which will be your weak spots or will not be dense enough. That's why it is important to make small scale tests before casting large parts. The density of E/G should be around 2.4grams/cc or 1.45onces/cubic inch. If the density of the parts you make are less then the strength and the damping factor will be weaker

Bruno

[romihs]

Hello to all of you!

I spent the last few days reading through the WHOLE thread and am really excited about the idea.

I live in Slovenia, and as an "English speaker" (I do speak Slovenian), I have difficulties finding what I am looking for. So I look for international brand names available locally.

I came across a Sika product which comes complete with the Epoxy and quartz sand; SikaDur-42.
According to the data sheet, the largest sand particle is 3.2mm. I asked the sales rep here if it is composed of particles ranging between 0 and 3.2mm, and his answer was that he believes so.

The mixing ratios, according to their data sheet, is 6(A):1(B):28-35(Sand) to achieve a pour-able consistency. I am assuming that one can add more sand to 'thicken' the mixture.

One thing I do like about this, is that the sand is clean and pure, so one does not have to be concerned about impurities in the final mixture.

The only drawback is the package size.... the smallest package is 12Kg (6 liters) which would cost ~100 EUR, which I feel is a bit expensive for experimentation.

If I decide to forego the cost, I'll get a pack and experiment a little. I'll post my results.

Does anyone have any results yet?

[brunog]

Sposl,
welcome to the forum.

Based on the technical data from Sika, the maximum thickness to be used is 60mm. If you want to cast thicker parts you must add aggregate, which is fine. if you want to bring your cost down go to: www.r-g.de, R&G make a laminate resin L1100 and hardener EPH294 that you can mix to a sand and aggregate mixture. The site contains a lot of information on epoxy resin and a lot of it is in english.
I learned a lot by browsing through: http://5128.rapidforum.com/topic=110...6&search=beton you will need to register to be able to read through the forum. I was able to understand most of the information by using Google's translation tools, the translation from German to English is pretty bad, but I was able to pull out the logic of it.

I hope this information is helpfull to you.

Bruno

[romihs]

Bruno,
Thanks.

Thanks for the info, R & G have plenty of interesting products. Their prices are reasonable too.

That German site is a goldmine! My hats off to those guys!

Sandi

[martinw]

OK, I have ordered the Epoxy, I got some quart amounts of varying ratios, so we'll see how hard the fast set, and slow set end up. Plus, being able to mix will be another test. I could also use my wife's new KitchenAid for small batches! LOL.

.

Dear William,

Speaking of kitchen gadgets....

As you know, epoxy becomes less viscous if it is heated after it has been mixed, hence the blow-torch method. (This leaves aside the issue of exotherm, which will happen shortly after).

I've had pretty good luck putting a mixed batch of epoxy in the microwave oven for 5 second blasts on high. Inspect after each blast so that you do not over-heat the mix. You will very quickly see the mix become less gloopy, and that the air bubbles rise to the top. OK, you may need a rather large microwave for a large machine build, but I think that heating the whole of the mass of glue, rather than just the surface, should be worth exploring.

When this nicely runny mix comes into contact with your aggregates, ( re-cycled gravestones, exotic black sands from distant Southern shores, , stained glass or whatever), it will cool pretty fast UNLESS you pre-heat your aggregates. As it cools, the epoxy will become more gloopy, and you will have less chance of removing air bubbles. IMHO, you should pre-heat your aggregates to stop the epoxy thickening.

Once you have a mix poured into a mould, IMHO, the methods used to expel air from concrete "designer" worktops might be adapted. The people who make the worktops use construction industry "stingers" (illustrated in a previous post), vibrate the side of the mould with a hand-held sander, and also vibrate the surface of the concrete itself with a portable sander in a strong plastic bag. I tried the last with ordinary concrete, and it is messy.

I would try a vacuum hood over the mould to extract the air from the mix. I've tried it with ordinary concrete castings, and it works really well as long as you make sure that you do not suck gloop into the pump.

One last question to all who contributed to this incredibly interesting thread.

Epoxy/granite may cure fully in a week (I do not know) . Ordinary concrete, may take decades to do the same .

The "fact" that epoxy may "cure" in a week does not necessarily mean that it exhibits superior structural properties, especially as regards "creep", ie the tendency of most plastic materials to deflect permanently under load in the long term. My guess is that epoxy granite would do the same.

Best wishes

Martin

[lgalla]

Hey Martin
E/G should fully cure in a week.A post cure or applied heat To"bake it"results in better final properties.
I don't think the E/G will be"creepy"as it is rated to be more stable than solid granite and varying temperatures have no effect.Aluinium or steel will change with temperature.
Heating reduces viscosity a lot,but I always recommend slow curing resin as gell time will be short.When I mix sand and epoxy,when the mix becomes to stiff,I put the mix container in a tub of hot water alowing more filler to be added.After pouring puddles will appear as the epoxy exotherm thins.Now I sprinkle sand to suck up the excess resin.
As mentioned in previous posts,An air hammer may be a good vibrator.
Larry

[romihs]

Has anyone thought of using Teflon (PTFE) to line the mold with?

This is just a shot in the dark, but I know that nothing likes to stick to Teflon, and I'm assuming that epoxy resin will also not stick to it.

I guess that one should still apply wax to it, to make demolding easier.

Some might ask "but how do you attach the Teflon to the mold, it's self"... I don't think it would be necessary as the E/G's mass would keep the Teflon pressed against the mold construction.

It is quite expensive though.....

Sandi

[lerman]

Why not just line the form with plastic sheeting? Or 'Saran wrap'? Or aluminum foil? Or a plastic garbage bag. Or a combination of the above. One of these should work for most of the surface. The issue would be those surfaces that need to be precise, for example, the surfaces that are for mounting rails.

[skippy]

when I look at the size of the machines you are talking about here (i.e. large) and think about the cost of the epoxy and the mold required to make it, it seems excessive. If I were going to make a big machine (say 3m x 2m) of this nature I would make it out of reinforced concrete as it is super easy to work with and it is very cheap. I would start by making two parrellel walls, 60cms high, 2m wide and 3m apart, using 25cm concrete blocks . Next add some 8mm construction reinforcing bar laid vertically and fill the wall with concrete and vibrate (cheap to hire) (even cheaper to borrow!). Next use two of these 18cm high prestressed concrete beams for the outside where the rails go: http://www.precesa.es/vigueta_pre_auto.gif (they cost about $20 each). Next fill in the center section with these (viguetas y bovedillas in Spanish, don't know the name in English, if I've lost you, do a search in Google images under vigueta and you'll quickly see what I mean)
http://images.google.es/imgres?imgur...tryES%26sa%3DG, next lay a lightweight (4mm) steel mesh on top and pour with concrete and vibrate thereby creating your table top and base for the linear rails without any mould being required. And if you really want to increase the strength there's a material which is basically strands of Polypropylene fibers which you put into the concrete mix and this really increases the strength. Leave it a month and pray that you never have to move it.

[martinw]

Hey Martin

I don't think the E/G will be"creepy"as it is rated to be more stable than solid granite and varying temperatures have no effect.
Larry

Dear Larry,

Thank-you for your reply.

I am really no expert on this, but would welcome anybody's thoughts. Here goes...

I suggested that E/G might creep because epoxy is a polymer. I think it is reasonable to assume that the "aggregates" (granite, sand, whatever) are pretty strong as regards compression, and that they are also quite strong in tension. IMHO, the weakness of the E/G system, like conventional reinforced concrete, lies with the "glop" that holds the aggregates (and steel in RC) together. Cement creeps. I think epoxy will too.

I do not see how a "glop" made from a plastic material could possibly bind together aggregates as strongly as a slab of solid granite, or for that matter be as dimensionally stable as regards temperature fluctuations.

OK, E/G it might be better at damping vibrations, but isn't that the same as saying it is less rigid?

I think that skippy might be right in his post. Get a good book on concrete countertop design, wait a few months for the monster to cure, and have no plans for moving house.

Best wishes,

Martin

[lgalla]

Hey Martin
Try www.granitek.co.uk/news and google search rk series centerless grinders.Point 1 micron [.000004]"machine resolution does not happen with"creepy " materials
Larry

[romihs]

Martin,

I am by far not an expert of epoxies or concrete, but from what I have read, and from the way I understand things, epoxy has almost negligible shrinkage for two reasons:
1) The epoxy cures without having to dry out, there is no solvent in epoxy which evaporates.
2) If a slow curing epoxy is used (the exothermic reaction is slow), ensuring that the epoxy does not heat up too much durring curing, the epoxy will shrink very little once it has cured and cooled down to room temp.

Again, these are just my thoughts, and for all I know, I could be wrong....

Sandi

[Big S]

From my use of composites, polyester resin shrinks badly and is very brittle when hardened, it will shrink away from the sides of the mixing container when there is a small amount left.

Epoxy, on the other hand, is very stable in shrinkage. The leftover epoxy in the mixing container shrinks very little, if at all. Another advantage with epoxies is that they can be made harder or softer by varying the cure temperature. If they are set off at a higher temperature, they end up a lot harder, but more brittle with increasing hardness (although far superior to polyester resin).

I would think that the high heat associated with the exotherm is what we are after here, to give the epoxy the most rigidity.

Shannon.

[walter]

From my use of composites, polyester resin shrinks badly and is very brittle when hardened...

Is this the resin used in polymer concretes?