hollow panel with stringers

General Composites Discussions Composites Talk
#1

I am making a panel that should be lightweight and stiff. The panel will be 4" x 25" x 25" and will be exposed to water.

My plan is to cut 4" polystyrene sheet to the outline shape that is required. Next I will cut the outline shape again where I want my stringers. I glass (using epoxy) each individual section with one layer of weave before re-joining the pieces back together to form the outline shape. I will then glass another layer of weave over the whole thing. I would like a smooth surface on both sides and I am thinking if I clamp the foam and glass between 2 old waxed kitchen worktops it should give me a good finish . Once cured I will drill a small hole in each section and pore in acetone to dissolve the polystyrene leaving the hollow shape with stringers. I am dissolving the foam because some parts of foam surface would be exposed to water and absorb making it heavy.

Will the cut foam make the surface uneven? I am hoping that the clamping will give me a good surface on both sides. Any feedback?

Thanks in advance.

#2

Anybody? Is this a bad idea?

#3

Have you considered making the 2 face sheets and a “top-hat” or c-channel stiffener separately and then bond them all together with a structural adhesive?

#4

1: expanded PS should not absorb water. Ifyou are worried, cap the foam end.
2: using that much acetone is very dangerous, and disposing would require a HazMat disposal. Think og how much solvent goo you would have.
3: what infusible said. Make the stringers using a, say wood mold, wrapped in PE/PP film. Make face sheets, and then bond them together.
You can even make caps the same way, and glue them on the outside of the panel.
4: as long as you have two tool surfaces, you shouldn’t get much surface issues at all. Side clamps will clamp the stringer sections together, and top and bottom plates would clamp the faces on the C’s
5: if for some reason you go this route…25" isn’t that large, so just dig out the foam…don’t dissolve it.

#5

After a long break im getting back into this project and having a rethink along the lines of structural adhesive and component parts. This technique also let’s me use cheaper polyester resin.

Any adhesive recommendations and suggestions for size of gluing surface? I would like the joins to be at least as strong as the 2 layers of 200g mat I’m planning to use.

#6

I am guessing I would need about a 10mm wide gluing surface at the top of the c shape stringers recommended in the above posts.

With the 4 stringers I was planning on gluing c shaped ribs top and bottom so I would guess I’ll need about 200ml of structural adhesive.

Are my guesstimates way out? I hope so because pricing on website below would put cost at 20gbp just in glue! Why is this glue so much more expensive than regular epoxy eg laminating epoxy? Are there cheaper alternatives?

http://www.easycomposites.co.uk/Category/Resin--Gel--Silicone--Adhesive/Structural-Adhesives.aspx

#7

Any advice greatly appreciated

#8

Making glue is a tedious job, requires special machinery and additives.

Ever considered infusing the part with the hollow stringers in one go?

#9

OK if I decide to glue I’ll just buy that expensive stuff.

I thought it would be easier in one go hence my original idea and post. The only other way I can think to do it is make access holes and insert inflatable bladders (condoms? :slight_smile: ). Any other ideas?

In my head the job is so much easier and quicker if I do all the wet glass work in one go so I’m all for that

#10

Found cheaper polyurethane glue
http://www.easycomposites.co.uk/products/structural-adhesives/pu18-black-flexible-polyurethane-adhesive-sealant.aspx

Any comments on this stuff?

#11

You want to glue STIFFeners with FLEXible glue?

Better source some methacrylate glue, or epoxy glue.

#12

:slight_smile: the part will be subject to vibrations which this pu glue is good for. I had assumed that flexible term was relative and it would still offer a stiff final part given the size of the part vs stringer. I’m not going to loose sleep over flexing of 1mm in fact it may help absorb impacts that I expect will happen.

Does it still sound bonkers? :slight_smile:

#13

Standing on a sheet with wheels under it always seems bonkers to me, although I recently saw a picture of some longboarding girls, which made it all clear to me… (sorry, cannot post the picture here)

Yes, flex glue can help reducing vibration, but keep in mind flex pu is VERY flexible. I guess a stiffer PU is more suitable.