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 Post subject: Re: Foam core monster Door
PostPosted: Mon Dec 22, 2008 2:35 am 
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Second question.

I've got the foam core laminated and shaped,
Will post some unexciting photo's soon

I noticed when laminating my foam core that divinycell is pretty thirsty stuff and sucks up a lot of resin, Is it necessary when doing the lay up to pre wet the divinycell to get a good lamination between the glass and the core? or will a piece of glass if wet through bond at sufficient strength.
I'm trying to keep the weight down......

As a side note to anyone building a foam core, don't laminate layers of foam, just start with the thickness you want and shape it from there. The epoxy adds a crapload of weight.

As a spur of the moment decision, I just grabbed a 4x8 sheet of 1/4 h80 before I planned out my board and figured I could bond multiple layers to increase stiffness, Well in practice you can it just adds enough weight that it almost defeats the purpose of using a foam core. I came to realization after I had cut the foam to bring it home in my car but whatever it's my first board I'm given'r a go, I know I can lose at least a pound of needless weight.

Cheers guys and thanks for your help, I will post the complete set of engineering stiffness calculations I use to determine the number of plys required to match stiffness of one of my existing boards. Someone may find these useful to help take the black art out of choosing thickness, vs number of plies, carbon vs glass.


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 Post subject: Re: Foam core monster Door
PostPosted: Mon Dec 22, 2008 3:07 am 
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Location: Malibu
Next time go for corecell.
It is closed cell and it does not drink a tone of resin.

being closed cell it is also better for vacuum bagging as the air in the closed cells
does not come out.

You can also bond two layer together with very little resin.

Cheers.


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 Post subject: Re: Foam core monster Door
PostPosted: Mon Dec 22, 2008 3:48 pm 
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Quote:
or will a piece of glass if wet through bond at sufficient strength.

Not to foam, unless you have way too much resin. Foam has high surface area,,,
Quote:
Is it necessary when doing the lay up to pre wet the divinycell to get a good lamination between the glass and the core?

Use a stiff paste of epoxy+balloons (phenolic/red, not glass/white) to fill the surface pores of the foam, before putting glass on. It's lighter than pure resin...
Otherwise you get voids or end up filling with straight resin = too heavy.


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 Post subject: Re: Foam core monster Door
PostPosted: Mon Dec 22, 2008 5:25 pm 
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I havn't tried it but I was told if you put a thin layer of filled epoxy (silicon micro boloons) on the foam it will eat less epoxy.


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 Post subject: Re: Foam core monster Door
PostPosted: Fri Jan 09, 2009 5:18 pm 
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Ok so ran the stiffness Calcs, too confusing to post however 4 x 6oz top and 3 plies bottom to obtain similar stiffness to my JL flight deck, seems higher than usual but keep in mind this is 164 x 55 board. and the deflection is a function of length^3 and I'm modelling it after a board that's already quite stiff.

In terms of filling up the divinycell, on the recommendation of an ultralight plane builder (these guys have to cover hundreds of square feet of foam, I'm using aeropoxy light which is about half the weight of traditional epoxy but will still give a good bond with the core, (not as good as traditional epoxy) but good enough that compression buckling will be the failure mode long before delamination with the core.

I appreciate all the advice but I was a little concerned about getting the mixture right with the microballoons (thick paste) and worried I might delaminate from the core.

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clear coat,
Custom vinyl top sheet
6 oz wrapped edge 0-90
6 oz E glass 45/135 wrapped
6 oz wrapped edge 0-90
6 oz between footbeds
aeropoxy light
divinycell H-80 .75 tapered to end of foot beds then tapered to .15 at the tips edges chamfered at ~30 deg to 2 mm
aeropoxy light
6 oz E glass 0-90 wrapped edge
6 oz E glass 45/135 wrapped edge
6 oz E glass 0-90 wrapped edge
Interlux performance hull paint

Pads and strap home made as shown in previous post
Handle - home made HD-EPS foam wrapped in carbon. bolt holes reinforced with "magic putty"
Fins Omnitech engineering. (I wanted to make these myself but the process seems challenging and these are too cheap to pass up)


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 Post subject: Re: Foam core monster Door
PostPosted: Fri Jan 09, 2009 11:01 pm 
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Location: rhode island
you'll be fine.

i bet that upon closer closer examination, you will see that the magic AEROPOXY is nothing more than a filled epoxy using low density filler almost as good as microballons.

Both will do a good job of wetting out the core and fill the pores with somthing having less density than straight resin.
but it's still epoxy , wetting lots of surface area to meet all of your adhesion criteria. The resulting shear and compressive strength was never in doubt, compared to the crappy numbers of the core material.

perhaps a test to confirm your procedure and materials:?
take a piece of scrap foam.
assuming you have shaped it.
sand it clean.
vacuum any loose dust and/or solvent wipe.
pick your favorite low density epoxy fill coat recipe.
spread it over the foam, confirming the surface is completely wet out.
scrape it all off, leaving only filled pores.
let this skim coat start to kick. (but not cured, just viscous and tacky)
now do your laminations.

once cured...go break it.
if you record some numbers, you can also confirm the material properties for your design calcs.

have fun,
-bill






.


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 Post subject: Re: Foam core monster Door
PostPosted: Mon Jan 12, 2009 9:02 pm 
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Well the design changed slightly....

I realized a 60-150 will put a non-symmetric bias in the board and likely warp it during curing. I was just trying to get athe closet to 45 deg out of a 50" roll using a single sheet.
So instead I went with a 45 and laid it up with a butt joint on each end to get the extra length.

The aeropoxy was some type of putty material and there was no way I was going to be able to work with this stuff at -5C in my unheated garage. Without a scale, so I coated the Divinycell in epoxy.

After smashing the edges of a test piece with 3 plies of glass against a metal corner with barely even a dent, I felt 3 plies on the edge would be sufficient and there was no need to soil the bottom sheet by folding the edges over from the top.

Here's what I laid up

0-90
0-90 between extending 4" past foot beds and 2 sq in bolt reinforcement
45-135 + 2 sq in bolt hole reinforcements
0-90
3 plies of 1/4 Dinvycell H80 laminated with epoxy and coated in epoxy (BAD planning do not repeat)
0-90 wrapped to top
45-135 wrapped to top
0-90 wrapped to top.

I did a full vacuum bag, and added rocker by placing the ends on a 1" piece of wood and weighting down the center such that the there is only small rocker at the end to prevent nosing this into waves with a HUGE flat area

The board came out AWESOME, heavy but AWESOME defelection showed up withn 1mm of where I calculated. However it weighs 10 lbs. So I'm a little concerned it won't float well. I know I could drop a pound by not laminating the Divinycell. Just shaping a block of 3/4" Likely a bit more by coating it it more intellgently. It's also on the resin rich side as I had to do everything at -5 C and the resin doesn't flow very well so you end up using a lot. My breather fabric was quite saturated once I got it warmed up so there's weight saving to be had by better layup practices. It probably wouldn't have killed me to drop a ply or 2 as I have stiffness to spare

Realistically a board this massive with this type of stiffness would probably be better suited to using a ply of carbon on the 0-90 top and bottom with a 45 of glass. top and bottom and corecell core. This could result in a board about 1/2 the weight of my current abomination. We'll see how this one rides and then try again.

Anyone out there know the weight of the real monster door?
I know it's heavy F'n piece of wood too......


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