joining sheets of xps

General Discussion
1

First off, hello all at swaylocks and I hope this hasn’t already been asked. I’ve used the search function but couldn’t find anything specific, so here goes. Making my first board and have access to some free xps sheets. Unfortunately, they are only 600mm long by 100mm thick, so in order to make a board I have to join two sheets together. To do this, I’ve routed out a 30cm section of each sheet and then glued them together with gorilla glue. I’ve also put in a stringer and a couple of dowles for strength but I’m worried, what with xps’s reputation for delaminating, that it’s still gonna be a big weak patch in the middle of the board and it might snap. Anyways, hopefully the pictures will make sense. My question is, is there anything more I can do to strengthen it, and do I need to or will a heavy glassing schedule be enough? Was thinking 3 4s on top and 2 on the bottom, with a tail patch for the fins…

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Thanks all.

2

Gorilla Glue is tough stuff.  Just for kicks, glue a couple of scraps together, let cure, and bend at the joint.  My own experience with EPS and XPS is that the glue is the last thing to give up. As a security blanket, glass a strip of carbon uni reinforcement fabric down the center on both sides or better yet, on the bottom and in ‘love handles’ on the deck side.  

Here is one type of tape available through our friends down under…  https://www.sanded.com.au/products/carbon-innegra-unidirectional-tape-per-50m-roll  

3

Thanks, that sounds like a good idea. Hadn’t thought of using carbon tape.

4

Dont worry if you use PU Glue like Gorilla it is going to be supertough… I have a couple of fused ones, with and without stringer… No problem, all of them handled portuguese beach break well.

5

What is the XPS foam density/minimum compressive strength?

6

unfortunately the foam I’ve got hold of was just some leftover sheets from the building site I work on. The wrapping etc is long gone. I do know, however, that it was intended to be used for floor insulation, as opposed to wall, so should be pretty dense.

7

Weigh a small, rectangular/square piece of your foam with known dimensions (TxWxL).

Do you know the foam brand/manufacturer?

8

found it, actually took a picture of a wrapper ages ago. It’s kingspan, styrozone, n500r apparently. All I can find online about compressive strength is that it exceeds 500kPa at 10% if that means anything? It’s used for car parks.

9

Looks like you have some fairly high density/compressive strength XPS (2.2 pcf/72 psi; page 4 in link below).  So it should be less susceptible to pressure dings.

If you want to increase strength, adding a wood veneer to the deck (compsand/timberflex build tech) combined with a stringer should make your board pretty tough – more of a T-beam structure, helping to resist compressive stresses.  With that you shouldn’t need a super heavy glassing schedule.

https://www.just-insulation.com/downloads/kingspan/Datasheets/Kingspan-Styrozone-Heavy-Duty-Floor-datasheet.pdf

BTW joining the sheets together something like this should improve joint strength.

 

 

10

Compsand builld tech videos:

https://youtu.be/w9Iy3Y5h7Ww

https://youtu.be/MBMZ6DnATRg

https://youtu.be/xsgBUMUpCi4

11

Thanks mate. I think, seeing as this is my first board I’m gonna keep it simple but will look into that for future as I’m sure I’m gonna want to build more.

12

Exactly.

Don´t worry that much about carbon or complex dovetailing. A good PU glue with a tight glue-gap is usually stronger, than the EPS/XPS next to it.

Glue some scraps togehter, break it. It will break next to the glue joint, not in the glue joint.

 

13

I could be wrong.  But it seems like a straight glue seam joining the ends of 2 foam sheets will create a hinge, focusing stresses on either side of the glue line.

Maybe Lemat can comment/contribute.

14

I believe I have seen somewhere on Sways where plastic shrink wrap has been used to laminate the thin wood veneer to the deck (without vacuum bagging).

I am experimenting with 3M 78 spray adhesive for bonding skins to XPS.

15

So many ways to build a board using scraps of foam. I try my best to keep things as simple as possible to get a decent shape, but I also try to make the blank strong. I prefer a diagonal glue line when I join full width pieces to get the proper length. I also don’t like cutting through a scarf line, so I try to avoid gluing up 2 flat sheets to get the proper thickness if it will end up creating a scarf joint.

Here’s a board made from 4’ x 4’ x 13" blocks of EPS. I cut rockered sections 13" wide and 4’ long, then glued them up to get a nose section and a tail section. Then I did a V cut to join the nose and tail pieces. I did 3 boards using this method, but I’ll probably glue up halves joining a nose and tail for each side then glue that up next time.  

As far as the foam strength and needing a stringer for strength, no need for that. I ride stringerless EPS and XPS boards, and so far they’ve been pretty durable. XPS is stronger than EPS. I just make sure to have all layers of glass wrapping around, so there’s a lot of glass along the rails. I don’t worry about flex patterns and that stuff people talk about. If you worry about that, you shouldn’t be using crap foam to start with.

I rode this board during a big swell this past summer in Hawaii. There were a times I thought I was going to end up with a snapped board, but it held up.

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16

If you have more of the same foam and same dimensions, try gluing up pieces to get the proper length and then cut rocker slices. With 100mm thickness, you will have a decent width and stagger the glue joints when you cut the rockered slices. The glue lines running nose to tail will give you added strength. I add a perimeter piece of foam to get a clean rail, but I’ve done with out the perimeter rail too.

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17

Thanks everyone. Lot of interesting ideas. Will post some pictures when I get it finished. (If I don’t completely mess it up when I try glassing) It being the weekend, gonna go get working on it now.

18

I can see how a diagonal/wedge glue line/joint would be less prone to hinging than straight or rectangular.

A curved glue line/joint should be even less susceptible to hinging. 

19

Yes!  I have been stringering 9’4 Poly blanks with three piece ply that is only 8’ long.  I join two pieces of Ply with a male/female V cut joint.  When the blank is planed you can’t see the joint.  The V joint is stronger than a Butt joint.

20

I feel like I learned here that the blank is more of a frame for the skin from which the strength comes. I’ve been making xps blanks w/out stringer - I think it’s like 1.8 lbs, home insulation foam - no big problems, but I use a heavy glass schedule for duribility (long before going to xps), 60 grit and “post holes” pre glass, and vent holes…