[lgalla]

Which is better?A5to10% epoxy ratio being rather dry and full of air bubbles or 10 to 20% epoxy with the possibility of using a concrete vibrator.With the higher ratio the vibrating will compact the mix and excessive resin will pool on top.Just sprinkle some sand and small aggregates to suck up the excess.With this method I will bet the aggregates will pack as close as they can.Another thought,the pooled resin could be your surface plate if desired.
If filling a structural tube is re-bar necessary?The steel is for strength, the E/G for damping.Re-baring any E/G mix may bring back resonance????
Just my .05 pense
Larry

[LeeWay]

Rebar would still add strength to it.
It would help to spread any load on the channel or extrusion over a wider area than just a filled shell would.
Think of a span or a concrete floor in a high rise. They pour these spans on top of steel floor sometimes, but without rebar, it still couldn't carry much of a load.
Fill two small channels. One with rebar, the other without. You can set it up in a shop press and the one without will fail before the reinforced one will, even though they both have a steel or aluminum shell.
The bar is helping with the distribution of stresses and spreading that stress out along the rebar.
Smooth bar would fail also before rebar would because it can't grip.
Smooth bar is used sometimes though as long as it has a hook on the end. That keeps it from slipping through just as the ribs an rebar does.

Not sure about the resonance. Rebar doesn't have to be used to gain strength either. Sometimes fiberglass is added for this purpose. Wire mesh especially in smaller pours may be all that is needed to increase the concrete's strength. Either of these would be less likely to induce resonance.

[Stepper Monkey]

Which is better?A5to10% epoxy ratio being rather dry and full of air bubbles or 10 to 20% epoxy with the possibility of using a concrete vibrator.With the higher ratio the vibrating will compact the mix and excessive resin will pool on top.

In using epoxies for aerospace they get the best of both worlds - excess epoxy is used during the initial stage, and then removed from the matrix during the vacuum compaction stage.

Vacuum compaction solves all of your concerns at once; It allows vibration to be used, causes de-bubbling, an extremely large force for physical compaction, allows the advantages of a >10% epoxy ratio during mixing for optimal wetting and cavity removal, allows a <5% final epoxy ratio for optimal strength, positive removal of excess, and smooth surfaces on ALL sides.

easy mixing/pouring
full wetting and de-bubbling
no inclusions
physical compaction
optimal strength
no pooling
all sides surfaced

All the extra equipment needed to do this in one step?

Vinyl sheet for vacuum bag
pvc pipe for the drool can

Other than it can be messy the first time you try it, what else do you need or want? It's not only cheap and easy, it is actually the pro way to do it. It may sound hokey, but it is still the way F1 cars and spaceships get built.

I don't think this needs to be as complicated as we are making it. Most are looking at large-scale mass-manufacturing techniques which are out of context here, but in looking at processes for this project, we might be better served looking at aerospace, racing, and laboratory practices.

[martinw]

In using epoxies for aerospace they get the best of both worlds - excess epoxy is used during the initial stage, and then removed from the matrix during the vacuum compaction stage......

All the extra equipment needed to do this in one step?

Vinyl sheet for vacuum bag
pvc pipe for the drool can

.

Dear Stepper Monkey,

That's a really useful post. Thank-you.

The one thing I don't understand is how you ensure that the stuff you remove is all epoxy, and not a mix of epoxy and aggregates. Is the excess epoxy allowed to "pool" at the top before you apply the vacuum?

BTW, "drool can"???? Crazy name!

Best wishes

Martin

[brunog]

Dear Stepper Monkey,

That's a really useful post. Thank-you.

The one thing I don't understand is how you ensure that the stuff you remove is all epoxy, and not a mix of epoxy and aggregates. Is the excess epoxy allowed to "pool" at the top before you apply the vacuum?

BTW, "drool can"???? Crazy name!

Best wishes

Martin

Martin,
slow curing epoxy will have time to pool at the top and "drool", however I prefer Larry's solution of adding sand to absorb excess epoxy.

best regards

Bruno

[Stepper Monkey]

Dear Stepper Monkey,

That's a really useful post. Thank-you.

The one thing I don't understand is how you ensure that the stuff you remove is all epoxy, and not a mix of epoxy and aggregates. Is the excess epoxy allowed to "pool" at the top before you apply the vacuum?

BTW, "drool can"???? Crazy name!

Best wishes

Martin

It doesn't pool, it is actively displaced. You use a relatively close-fitting "pusher" block on top of the aggregate, and small holes in the ends of the form itself. You essentially enclose the whole thing in a bag. When you draw vacuum on the bag, unlike in a rigid chamber you get two things to happen (other than not needing a giant rigid chamber!) - first the aggregate is compacted evenly at almost 1 bar, or almost a ton a square foot, by the pressure differential. Secondly, this squeezes out all of the excess epoxy rather the same as if the block was being pushed down by a hydraulic press, which can also be used for additional longer-term clamping if needed so you can turn the pump off and not lose holding force.

The excess epoxy needs to go somewhere, so to keep it from getting into your vacuum pump you use a "drool can", essentially a short chunk of PVC pipe with two end caps and hose fittings. The vacuum pump hose attaches to a top-mounted hose barb, the hoses from the form to side-mounted barbs.
This allows any excess liquid to pool in the bottom of the can while vacuum draws out through the top, this saving your pump and all but the short chunks of hose to the form.

This is how even thin, curvy items get made as well as thick ones - the bag then just goes around two blocks with mating shapes with the epoxy filled matrix in between. The vacuum "clamps" the two blocks together and squeezes out excess epoxy.
This makes for thinner, lighter, stronger, more consistent thickness race car and aircraft panels than old-fashioned hand layment.

[LeeWay]

If you use a vacuum bag, you need to take care what type plastic or vinyl the bag is. The Epoxy will heat up pretty good.
For smaller pours, they have plastic they use in ovens that would work.

The pour could also be done similar to molding something. With sprue's. Granted there would be some cleanup, but if poured from the bottom, the top should have a pristine shiny surface when complete. Most all the air bubbles and excess epoxy would wind up in the sprue's I would think. Some of these same issues are conquered when thermo forming plastics. They generally do it in a mold under pressure with irregularities in the pour being at the pour point.

[ger21]

If you use a vacuum bag, you need to take care what type plastic or vinyl the bag is. The Epoxy will heat up pretty good.

Someone who made samples earlier in the thread said it didn't get hot, just warm (Zumba?). If it was just epoxy, then yes, it would get very hot, but it's only 10-20% epoxy, so the aggregates keep it cooler.

[LeeWay]

Someone who made samples earlier in the thread said it didn't get hot, just warm (Zumba?). If it was just epoxy, then yes, it would get very hot, but it's only 10-20% epoxy, so the aggregates keep it cooler.

Thanks Gerry.
You are probably right, but when it is surrounded by a bag, the heat formed wouldn't have anywhere to escape. That is unless it's pulled out through the vacuum. I know the one guys thread I have seen spoke of it getting pretty hot if you poured too much at one time. I imagine his mixture is far more epoxy than we are talking here with this formula though. Has anyone nailed down the thickness we are talking yet? Will thicker slabs have to be done in stages?
Just really curious on this, because in the near future, I will be filling some 3060 80/20 extrusions with something. The intent right now is hydraulic anchoring cement with small rebar and #8 shot, but if we get some formula's for this figured out in the mean time, I may go that route.

[martinw]

Dear Stepper Monkey,

Thanks for that.

I'm being stupid, but I still do not understand why the "drool" is epoxy and not a mix of epoxy and aggregate.

If you are laying up carbon fibre and epoxy, or glassfibre and polyester resin,
I can see how (if you compact the mix) you are effectively "wringing" a wet sponge. In other words, if the fibres are all joined up, they will not migrate with the resin. With E/G, the aggregates, especially the smaller ones, would be mixed up with the epoxy being squeezed out would'nt they?

Sorry if I'm being dumb.

Best wishes

Martin