Pocket board building

[bay surfer]

Why anyone would choose Xps as core vs Eps is not understood.

XPS is Waterproof vs EPS Not so much, Pink Ridged verses White crumbly.
Both are cheap, both can be sanded down and shaped fast.

[revhed]

Why anyone would choose Xps as core vs Eps is not understood.

XPS is Waterproof vs EPS Not so much, Pink Ridged verses White crumbly.
Both are cheap, both can be sanded down and shaped fast.

Interesting we, I are maybe lost in translation.
My understanding was X meaning Expanded and E meaning extruded.
That said,
https://en.wikipedia.org/wiki/Polystyre ... rene_(EPS)
I had understood all eXpanded was not water proof, open cell, and ALL extruded was closed cell therefore waterproof, this is not clear to me now!
Exactly why words and letters need understanding for accurate communication.
https://en.wikipedia.org/wiki/Polystyre ... rene_(EPS)
Most of the older, some new, cheaper cores we see are expanded pellets that are not closed cell therefore absorb water.
I need to read and study up because what I thought I knew was lacking for sure!
https://en.wikipedia.org/wiki/Styrofoam
I see that even some higher quality Extruded is not so waterproof, I was unaware.
Again thank you for bringing this to my attention.
Time to read and learn!
R H

[TomW]

EPS = expanded polystyrene. It is beads. The water gets soaked up in between the beads, and I guess into beads too.
XPS = extruded polystyrene. It doesn't have bead structure. But it will absorb water.
PU = polyurethane foam used in some surfboard blanks. It will absorb water too. But not as much.
Corecell, airex, divinycell are PU foams, closed cell and generally higher density, but you can get them from 30kg/m2 and up to like wood density. They will absorb water too, but being higher density, less so.
Most manufacturers state percentages of absorption in spec sheets if you dig.

[jaros]

XPS is used in building constuction as thermal insolation where the insolation is in contact with water (underground walls for example), so I highly doubt it absorbs any water. Old polyurethane cores do absorb it for sure.
But, there is something else wrong with the construction of the board, if water gets to the core material...

[Trent hink]

Xps has unacceptably low shear strength. It can work, but it will fail if the board has any flex. Surfboard builders who have tried it usually say it does not bond well and is prone to delamination. However, it bonds fine with epoxy. It just fails in shear right at the foam-glass interface. I learned this the hard way.

For a sandwich construction it can work fine.

Eps foam can work fine too, and in higher densities it is not so prone to absorbing water. But the kind you can buy as insulation at Cheapo Depot is around 3/4 pound per cubic foot and will suck water like crazy. For a reasonable non-sandwich construction, you really need something around 2 pounds per cubic foot.

I suggest it is best not to cheap out on your choice of core material, unless you really know what you are doing. At the very least, do not combine cheap foam with expensive reinforcements, unless you are doing a sandwich construction.

[alpaia]

Hello I remember reading a lot about xps vs eps, and disappointments with XPS. I have used it anyway. Here is what I learned, as a garage builder :
-good EPS +epoxy is the right combination everybody uses but soaks water after any leak (ding)

-XPS is a closed cell foam which does not absorb water, higher density (35kg / m3 average), stiffer, and quite even texture to shape (no beads)
-XPS has reported issues, mainly I think because it is an insulation material not necessarily produced in a way to bond with epoxy etc, so results can vary :
-the extrusion process is easier with some additives (unknown) which are added at the producer's discretion and can prevent bond with epoxy
-the closed cell contains gas bubbles that can de-gas under heat and delam the board
-the foam is dense and stiff but as mentioned earlier shear strength is not great so the foam just beneath the laminate can delam

To mitigate above limitations, receipes I used (found searching on swaylocks etc) were :
-for an epoxy board, very rough sanding (6 or 40 grit) before glassing to improve bond

Then the receipe I found, works for me, might not for you but I am happy to share :
-for a surf, I glued wood skins on the blank with PU glue (like gorilla or PUR501)
-I mean vacuum bagging for the glueing, applying 150g/m2 on the wood side only - much quicker than vacuum bagging epoxy sandwich
-then glassed on the wood after putting rails on etc
-so it is not a real sandwich (no glass below wood)
-but the wood skin - PU glue - XPS bond seems really strong as the PU glue is more elastic than epoxy so that seems to reduce shear stress.
I have used the above surf for 2 years (strong wind, chop, banged into rocks, left in heat in the car) no trouble

Hope this helps

[bay surfer]

From watching video of XPS board building and my experience, final shaping is with 60 grit sandpaper, final surface is very rough and has small tears which the epoxy gets into and should help with the sheering problem, I used wood stringers glued to the XPS with PU glue so epoxy also has the wood to adhere to which should help.

[revhed]

eXtruded poly core tips
Place in direct sun to out gas.
Cut planform, shape rails.
I use rough wood file to leave very textured surface everywhere, rougher than any sand paper, for depth.
Mastic, epoxy on the thinner viscosity microballon to aide sheer strength, again everywhere, spatula applied applying hard pressure.
Carbon layup, impregged plastic sleeve method pressed and pulled firm, black transparent everywhere.
overlap seams, vac sack , rocker table using slow cure epoxy.
6 boards done like this over 8 yeras never delam or failure.
Time consuming but using carbon shoots and standard Extruded for construction insulation is low cost.
R H

[TomW]

I know I am nit picking, but all foam will absorb water. Read the spec sheets.
But for all practical purposes PU and xps are less of a problem. If you get a ding keep riding and fix it after your session. If you have eps and get a ding, get out of the water and tape it, then continue. Fix after your session.

Xps works ok for garage builders. Good advice here. Follow it and it will work.

[Trent hink]

Clear packing tape: the session savior!