joining sheets of xps

MiWie wrote:

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.

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.

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.

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.

sharkcountry wrote:

I prefer a diagonal glue line when I join full width pieces to get the proper length.

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. 

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.