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The table is "home made" from out of date prepreg and 2" vented aluminum honeycomb. I have a bunch of them I made years ago - super handy but not a readily available product and expensive materials! The aluminum core is excellent for heat transmission to the tool face while staying stiff and having matching CTE to carbon parts.

It comes in varying densities - and the light stuff will dent with a finger. This will easily dent with fingernail pressure but quite stiff and brittle over larger areas.

I haven't. My concern is that it would be very hard to get the top layer of fabric to go up and over each rib gracefully without a lot of bridging. A short-fiber chopped reinforcement might work well though.

@@ExploreComposites Ah true. The bridging, at the expense of a little extra weight might add significant torsional rigidity (much like bead rolling on a flat automotive panel). Hopefully vacuum bagging will minimise the effect. Might be worth experimenting with

Thanks for the info - that makes sense. I will have to experiment with it more. I wonder if the hot-wire process itself makes for an improved bond where the melting has happened?

Depends on temperature but probably up near 800 or 1000cps at room temperature. The styrene and breaking down reinforcement (mat) binders make it feel much thinner in use than the viscosity in a cup would lead you to expect.

Not really, but I'd like to. The problem is that doing mechanical testing right takes a lot of work and some equipment I don't have. I do a little core shearing in a newer sample - #43b - but it's nothing special!

It would machine nicely with the right cutters. I have machined many similar pieces...

What about the layers do you want to calculate? Analyzing a tapered or contoured part can be a bit of work - is it the composite properties you're after?

@@ExploreComposites no, in my case it is just the fitting/shape of the part. I would like to precut the layers and with an easy shape like a cube, this would be rather easy (lots of squares). But how many? And when I have like an organic landscape shape, do I need to have a CAD model and slice it in layers? Or could a plaster model, grinded down work as well as an template? And finally my most important question: is there an upper limit for compressing fibers? Could I (theoretically) press 100 layers under a 300 tons press or at which point would the fibers break. Because I saw a video, where they tried to break a (cured) carbon fiber cube under an hydraulic press with many, many layers and they could not break the cube.

You're right, there's a lot of resin and the carbon mat does act a lot like a core. Even at 18% FVF, the carbon probably keeps the CTE low but it is hard to know how much the uniform face plies are doing the work. I'd shoot for about 1/3-1/2 the thickness in structural face plies and use the recycled mat as a core to increase thickness and stiffness of the tool shell. It can be a challenge to wet through though, so testing is important and slow filling helps. Resin is still cheaper than carbon but not by as much!

I think you're right - though I would have had to start from one edge and work across. Without any holes through, the air would only be able to escape if worked all the way across the surface to an edge.

This is interesting - the way it failed looked very much like failure of the foam, but if your penetrating epoxy could get a better (deeper) grip, maybe it would be tougher. Usually penetrating epoxies have very low viscosity and that helps make them penetrate - do you think this would help here? Thanks!

@@ExploreComposites FINNFOAM has been used to make at least 2 boats, believe deeper penetration of epoxy helped but really vacuum infusing it is the way to go, only that would cause it to deform under pressure.

Me neither Jerry, me neither! Thanks!

A good rule of thumb is "after it has hardened and cooled off and as long as it's still tacky on the surface - best not to go more than a day or two". Depends a ton on the temperature, resin type, humidity and probably lots of other stuff.

I think I was trying not to push too hard on the top skin - to avoid pushing the fiber down into each honeycomb cell. Not sure it would have happened anyway - Kevlar isn't sure stretchy! For bagged wet layup half vacuum is often plenty.

I'd love to learn more about uhmw polyethylene fiber - do you mean like Spectra or Dyneema? Do you know where I could find more information about aramid/uhmwpe hybrid fabrics?

@@ExploreComposites google this S0734743X15001773

I guess it can be spelled with one or two L's depending on where you're from. I'd say "fill-ay" for a piece of a fish and "fill-it" for an inside radius thing.

​@@ExploreComposites :p found a super cheap source of g10 0.5mm, 40 cents a slice as lifepo4 battery insulation sheets. more videos on that please if you get the time =] thanks for your time

Sure, especially if you need a really big or odd-shaped piece when G10 gets less cost effective and practical. Also look into Coosa / Penske / Airex PXc high density core. Also aluminum backing plates and just dropping core out of the laminate in that area - though you'll probably want to thicken up the 'glass too. Infusion maximizes fiber content, but a hand-laid plate would be ok too and maybe cheaper - certainly easier.

The fibers are really long and might be too hairy and clumpy to cut a decent fillet of a normal size. You can buy shorter chopped/milled carbon - 1mm or so - that might make decent fillets if mixed with silica or some other filler.

It is a hassle to buy a small amount of Rohacell - and $$$! These were from a job I worked on long ago where we used quite a bit of it.

Drilling and backfilling with filled resin works. It needs to be a sizable fill completely touching each skin. Core can be dug back with an L-shaped tool in a drill. You can also mold it in with G10 of carbon plate if location is predictable. G10 tubes work well for through fasteners as compression bushings too - sized right they can be tapped too. Assuming carbon because you say graphite filler (silica filler is good too) but aluminum inserts or tapping plates are common with glass laminates but no good long term with carbon.

You really have to test to be sure, but estimating the volume of perforations and flow grooves is pretty easy if you measure with a caliper and do lots of multiplication. Plain foam surfaces take up resin inversely proportional to density of the foam because high density core has smaller cells. Nothing like a few square meters of test panel to get it figured pretty well including surface flow and feed lines.

Mostly so the mold will be air tight and the vacuum bag will work. It also helps support the fairing and primer.

Thanks for the suggestion! I have been wanting to try LW-PLA for a while but hadn't considered it for a core. Next time I order filament I'll try to get some. Could probably be printed as a skinned honeycomb and just get laminated over with a light layer of fiber on each side...

@@ExploreComposites For an outside skin on aircraft or cars I don’t think the comb would be necessary. For stringers and ribs I would use 3D formed reinforcement, like half a circle to hopefully match up with some carbon tube. I haven’t drawn it on F360 yet but that’s how I’m imagining it in my head. I’m thinking carbon tube for transferring loads between 3d printed panels and along the fuselage.

Stiffer maybe, and stronger in some ways - but not as resistant to hammers! Carbon might also have long term galvanic issues with the aluminum core. But yes, carbon would be a more practical and common choice here.

@@ExploreComposites OK, thank you for your response!

This is 6oz / 200gsm e-glass.

I meant that that piece of Nomex is of a density that is 40kg per cubic meter - but yes, the little piece is light! And the reason to do both sides it to see how it comes out... which in this case isn't ideal.

that makes way more sense indeed...I actually meant to ask was why do both sides separately and not 1 on lay-up? @@ExploreComposites

@@MaartenOosterbaan I was trying to get two "finished" sides - but I only got two pin-hole filled but flat sides. With honeycomb cores and thin skins, the bag side laminate gets pushed into the holes in the honeycomb leaving a dimpled (and rough from peel ply) surface. Also resin in the "top" skin can drain out into the honeycomb cells leaving the laminate dry.

ah, ok...well better to be safe than sorry...no cheap materials...@@ExploreComposites

I am not sure I understand the exact issue - does too much resin bleed out, or is it migrating under the bag to places you don't want it (where?) leaving dry areas? A couple of ideas: 1. Perforated release film (hole patterns can sized vary by product) is necessary for limiting bleed. Laps should only be 10-15mm (1/2"). 2. Use less vacuum - half or even less can be plenty to consolidate. 3. Allow resin to start to gel (slightly) before pulling vacuum. Some wet-layup resins specify a time to first compaction or some other thing - so the resin isn't sitting uncured and very liquid for a long time under vacuum. 4. Sometimes you are best off dividing a lamination into multiple curing steps - especially if there is core or things are complicated.

@@ExploreComposites thank you very much for the detailed response. I will try your Tipps in the next laminations. Thanks again

No videos, but some general core data here: explorecomposites.com/articles/design-for-composites/basics-cored-structures/

Thanks! @@ExploreComposites

Thank you for the information about XPS - I didn't know about the PE added or consider the trapped air expansion risk. I came off as pretty down on it in this video, but only because I was trying to get more from it than it can offer. If you're building super light stuff with little risk of impact - it can be great!

Oh for sure! So much composites work is very wasteful especially as things move up the performance/weight curve. It is a fundamental problem that needs lots of attention and effort!

Yes, my arm kind of hides it but it the spiral is fit over the tube for the last foot or so. It is probably overkill here but is a good inexpensive solution when you have room for a decent sized resin/vacuum break to keep resin from just rushing into the vacuum outlet side.

You are right - it is a hydraulic flow control and says "Parker Par Trol PN600B -40BT" and "MAX 2000 PSI" which is more than vacuum is ever going to give it. I don't recall where I got it from, but it was part of something I bought used because I never purchased it by itself or even considered what it might be besides a vacuum regulator. Thank you for making me learn!

@@ExploreComposites Most excellent reply! Thanks! edit: I found one similar on eBay - Thanks for confirming that. That's one more piece of the puzzle!

Air - didn't notice that - come to think of it, I see air everywhere! Your idea about squishing between plastic and checking for uniform coverage and adding more as needed is great! I've never made anything this way before and was really just playing around to see. All my "chopped carbon" stuff has been prepreg or infused/injected so you saw me being pretty clueless here. Your advice is always welcome - thank you!

Always happy to be wrong - do you have any more info? Was it a core or a semi-structural bit of leave-in-place tooling?

I’m no expert here - use what the manufacturer suggests if you can find it. Not much out there on a Dresser DC2 though! Most (rotary vane) vacuum pump oil is ISO68 viscosity mineral oil and unless you’re using a special pump generic vacuum pump oil should work ok. You won’t likely ruin the pump with the wrong oil but it may not last as long or work as well.

I haven't found an easy source for higher density XPS, but I bet it would work pretty well even at 2-3 pound density. Compression strength and peel-resistance is probably closely correlated with density. Hoping to try some out soon!