hand lay up of coracell m foam

Don't forget, the bond of foam to fg isn't very much. If you find pulling the glass and foam is hard to do with fingers - that's enough.

There are plenty of discussions in this forum to clarify the principle that foam is only there for compression, and that the good bonding is only to limit the skins inward movement, and prevent moisture ingress.

Its up to the glass/epoxy to be solid enough to handle the other forces on the hull. For that reason, all strength test of panels must be done with fg on both sides of the foam.

That said, 5mm foam with 6 oz epoxy on both sides takes a lot of pressure from fingers to bend or break. Its amazing to see.

Make your tests on longer ( but narrower like 1 foot) sections for testing - say 5 feet. Laminate up both sides in the glass thickness you think you need, support it at either end, and stand on the sucker - bounce up and down. If it holds, you are on a winner. If you are smart with the test sections, they will be of a size that you can re-use on other parts of the boat - like seats etc.

 

i second what catbuilder said, you dont have to pretreat the surface with anything when your laminating with epoxy... theres some good reading from the suppliers of structural foams in their data sheets and application guides, you can look them up on the web.

This type of Pre treatment is generally recommended (although still not absolutely nessesary) when laminating with polyester or similar resins containing large amounts of styrene and longer open times etc to prevent styrene migration into the core and weakening the foam/skin interface.

With epoxy, its not necessary as the solvents are not there to affect the foam...

 

ok so i 've got my feet wet and mixed up some epoxy,just like cat suggested the resin does not soak in much.I layed down a layer of 6oz on a test piece and the glass just sucked up the resin.i didnt put down that much or so i thought.
So another question,should i be worried that i'm starving the glass to foam connection?Maybe put more resin down after the first layer is wetted out?

sorry just used to doing this on wood,glassing the foam was like spreading resin on wood that had resin allready on it.it just weird.

thanks again for all the help.

 

The core doesn't soak up any resin. It just sits in those microscopic craters on the surface.

If your 6oz cloth is too dry, add more resin to the first coat.

I mostly do 36oz triax in a single pass the same way. Glob the resin on, stick the 34oz, then wet out from the front too, because its so thick.

With 6oz, you don't even need to wet the front out (assuming a single layer of glass). You just didn't get enough on to wet the glass out is all.

 

Did the other side with 18 oz WR to see what it was like ,very easy to wet out .letting it cure.

Another question,is there a rule that both sides have to have the same weight of glass?

 

No, strictly speaking a sandwich panel in simple bending loading should have slightly more glass on the compression skin than the tension skin because glass is stronger in tension than compression. Theres a bit more to it than that, but in general it holds true.

 

Ok thanks,i'm thinking about some of my interior parts where people may step onto or fall on.

 

Just for my own understanding, would it be more proper to say that the loaded surface should be thicker because it needs to counteract the sheer forces, rather than compression forces?

The reason I came to this question is that glass/epoxy is virtually incompressible due to its density, so it is the 'snappability' (sheer) that would seem to be the force that the 'upper' surface has to resist.

Any clarification is very welcome.

 

no, in a simple bending situation, the top and bottom skins are in tension and compression with the core in shear.

There is however another problem with sandwich panels and that is the localised point loading for which you need to deal with a skin buckling problem in 3 point bending rather than evenly distributed load between 2 points. This is generally handled by the compressive strength of the core material, its more efficient dealing with it like this rather than increasing the skin thickness.

A good read on all loadings and design criteria to satisfy an ultimate strength problem based on limiting factors and failure modes below...
ignore the honeycomb core, the theory is valid for all sandwich panels regardless of core type.
http://www.hexcel.com/Resources/Dat...eets/Honeycomb_Sandwich_Design_Technology.pdf

 

Many thanks for that link. Its a great reference.

Just to clarify my thinking, while I understand that the whole upper surface is in compression, I think that the sheer I was thinking of, is what you more accurately describe as "localised 3 point bending".

I have the image of a stilleto heel sinking though a deck - even though the deck can easily support the weight of the person.

If a stilleto heel goes through a deck, could this be accurately described as a 'sheer' failure of the top layer of fibreglass ?

 

It means your girls wearing the stiletto heels are a bit on the heavy side and you need to get different ones.

 

so long as its the girls wearing the heels cat.....

 

No, the stilletto going thru the deck is because the core is crushing beneath the very thin skin. If you increase the COMPRESSIVE STRENGTH of the core material, then it will have more resistance to the stilletto ie. use higher density foam or endgrain balsa or plywood etc. You can do the same thing by increasing the thickness of the glass skin, however its not as efficient because you need much greater thickness (and weight) to get just a small gain in this regard because the load is perpendicular to the fibres ie. out of plane.

 

More questions,what about bending the foam. can you heat it up and bend a curve into it?will it stay that shape after its cooled?Or do you need to score it and heat it up?

 

I understand the overall logic - but before the foam is crushed by the stilleto - the f'glass has to fail in shear , does it not ?

If we were talking about a large area, might there be cases where it is cheaper and lighter to increase the top surface shear strength ?


( ... say if my girlfriend is paying for the deck, and she won't lose weight, or the ridiculous shoes )