Such a thing as "macro-ballons"?

I have a question on epoxy filler.

I am at the point of a boat build where I need to glass in many bulkheads, stringers, cabinets, etc...foam/glass to foam/glass.

My plan is to thicken up epoxy with cab-o-sil then use a fillet tool to apply fillet beads, then add biax tape, then final flow coat. Yes this works great and I have done it many times, but it will require quite a bit of epoxy as I would like the fillets to be on the larger size and the cab-o-sil, as good as it is at thickening, seems to more "absorb" into epoxy than "displace" it.

My question: Is there such a thing as "macroballons"? Either purchasable from some type of marine distributor or other industrial supply house? These would be some type of small plastic spheres that are the size of, let's say for arguments sake, BBs that one used to shoot from a toy gun....maybe 2mm diameter each, give or take? Of course one would still need to add cab-o-sil to get the fillet to form into the proper shape.

I would argue that such a filler would GREATLY REDUCE the amount of epoxy needed to get a nice, solid glassed joint, and that the said joint will be nearly as strong as one just using cab-o-sil as the bias tape is the main player in transferring the loads (I think). And I guess that adding some small amount of microFIBERS to the mix would help if that were indeed a question mark.

Yes, I understand that "balloons" are usually added to increase sandibility, but in this case that is a moot point as they will be UNDER the tape. Larger spheres would make it a bit more difficult to feather the fillet edge into the next panel, yes, but the tape is thick enough to cover that small ridge.

I thought about using foam for the fillet, but I would have to buy, rip, kerf (many, many kerfs), router out with a cover bit, then attempt to spring those "premade fillets" into all the compound surface intersections, probably having to make and use battens to keep them in place while epoxy sets. Would be much easier to just mix and apply a fillet as normal, but with similar weight savings of using foam.

Ideas welcome.

 

Interesting idea, I also need to make some large fillets for a mold. There is this:
Poraver® 1-2 mm 0.23 g/cm³ - Poraver® 1-2 mm 0.23 g/cm³ https://shop1.r-g.de/en/art/210132

Hmm. How you would calculate the resulting density? I think a sphere takes 1/6 * PI = 52% of the volume of a cube. The question is how irregularly shaped spheres would structure themselves and how much percent they'd take. But maybe something like 52% of your volume would only have a density of 230g/liter. I think if you mix larger and smaller particle sizes you get a higher fill rate. If you only have a single size particle then the size of them doesn't actually matter, just their displacement density. But in the link above it's actually quite "lumpy".

PS: You double posted btw.

 

Yes, there are "marcoballoons", "marcospheres" being the preferred term, and what you suggest has been around since the mid 1970's. Check out any of the various manufacturer's websites for "syntactic foam flotation".

 

How would you feather in the edge with a 2mm sphere?

Normal micro balloons (glass or other materials) already do this quite well, the next step is to use a foam fillet to create the radius.

 

Actually, you don't want to do that with mircospheres either. To be 100% structurally sound, no sphere can be broken. Otherwise, water pore pressure causes micro then macro cracking of the epoxy base which leads to sphere to sphere progressive failure. (there are plenty of studies on this) For best results, cast to net shape or fair but don't sand. This only applies to hollow spheres used to provide volume; sawdust, cabosil, talc, pumice, all can be added to provide bulk, but spheres are needed to get the minimum volumetric densities with maximum strength.

 

When done correctly you don't need to sand it, the application technique leaves it smooth.

With 2mm spheres it becomes more difficult to leave a smooth edge, so sanding may become necessary.

 

Ok but you can sand the spheres and then fill it / seal it with epoxy again right? Or does that still lead to water pore pressure cracking somehow through diffusion?

And am I correct that you could just as well use smaller spheres for the same weight and epoxy savings?

 

Epoxy sealing the cut edge is generally recommended, though for really high pressure applications, cast to net shape is preferred. And no, the smaller the spheres, the denser and stronger the foam. The density is a function of surface area of the spheres to their enclosed volume; i.e. the packing factor (Percentage of the volume filled by the spheres) doesn't change but the smaller spheres themselves are more dense.

 

So you most likely still finish your macro with a micro to take the tape smoothly ?. Polystyrene is the obvious answer but seeing that no one has responded with it there must be a reason why not, apart from trying to herd cats..? I mean the polystyrene balls not the people..

 

The space left void by a sphere filled with air will have the same density regardless of the diameter of the sphere. Density is not a function of the surface area, but of the mass to volume ratio.

 

2 mm dia microbaloons must be something new. We used a lot of microbaloons in the AC industry but it is very fine like talc powder. I can't see the spheres. I guess you just have to choose another type/size of microbaloons.

We vary the filler and resin mix depending on the need. Like paste for filleting and very dry for contouring.

 

A commercial fillet could be made from solid foam core. One must understand the purpose for the fillet. If it is simply to provide a smooth tape transition; you could literally run core offcuts through a moulder and both lighten the fillet and reduce the epoxy used.

I don't know if this would have strength concerns, but I don't see how the core would be any weaker than surrounding core.

I had fanciful dreams of running my offcuts through a small machine. Technically, I suppose you could build a triangle shape for glass transitions. It would be nice to hear feedback from experts.

 

We use foam core and honeycomb a lot. If it is foam, we taper the edge of the foam edge using an inclined bandsaw. For honeycomb, we have to fill up the edge with microballoons mix then sand, the apply tape. Strength is not the concern. It is the prevention of stress riser, hence the taper.

 

I missed a second point which is a fillet for appearances. Those fillets would probably need a smooth flowing, sandable thixo. I use 1 epoxy, 1/2 cabosil, 3 sil32...

 

A fillet differs from a taper in that it is right angle. Its primary purpose is to make the fiber drape easily and not break causing microscopic strain. 3/16 to 1/4" radius seems to work well for most fabric. A triangular piece of foam glued to connection but you need to sand it to blend prevent stress riser. For the fillet, the rulebook says "compliant resin". Cab-o- sil and resin bog fits the bill but the more expensive microbaloons and resin works also if not mixed too "dry" to become brittle.