How would you make a hollow box?

General Composites Discussions Composites Talk
#1

I’ve been trying to figure out what is the best way to make a completely hollow box. For explanation sake, lets leave out draft angles, or even a ball instead of box. I understand how to make the 2 part mold (is there a proper term for this? split mold? closed mold?) with the flanges and all but my question is how do you connect the two halves?

remember since it is fully closed, an inflatable bladder will not work.

I have two ideas how this may work…

  1. over hang the edges onto the flange then when you sandwich the two parts, the flange surfaces will mate creating a bond while having everything cure all at the same time. the downfall of this is you’re adding “thickness” to the part which in some cases might not be acceptable.

or

  1. layout both halves, let it gel then trim along the flanges to create a perfect seam, but now how would you connect them if the thickness of the part is very thin?

I like the idea of #1 better since i think it will create the strongest bond, but what if the tolerance is crucial and you cant “add” that extra thickness?

#2

Part A and B can have a milled or ply-dropped flange. Thickness will not increase. Don’t know the term for this joint, but common in metal and woodworking. Secondary bonding is done after cured parts.
The angles can be short, or very long. You can even mold in something, like an offset, on ONE of the halves. But then you will risk having a thicker seam.
No way to have a solid one piece hollow part. If you can make holes, go the route of dissolvable core for a one piece.

#3

interesting… but will that bevel be enough if the part is thin?? would some resin to the trick or would a 2 part structural epoxy be more appropriate?

#4

You would need a 1:12 bevel at least.

However, if the box is fully closed, there is hardly any reason not to have it filled with a core.

Other options are bladders or dissolvable cores, but that needs holes.

#5

maybe i was being a bit too broad lol, I am trying to make a spoiler, once i make the molds from top and bottom half, how should i join them?

#6

Couple of options, including foaming them together, use a lip-method,or even use large puddles of bog, which can be quite effective on the thin end.

#7

Thank you for the replies herman!

for the foam method, what are you referring to? off my head how I would do it is, lay up both halves, and trim the edges, once cured use a spray foam then trim that once its cured, then mate the two haves with some epoxy… Its probably not the best way but thats what i came up with lol…

Can you explain the puddles of bog? (whats bog?)

I will most likely used the lip method but interested in learning the other methods you recommended!

#8

bog is a paste like mix from epoxy and fillers,
for the epoxyfoam and bog method, look into this file, near the bottom: http://www.kgenterprise.fi/files/pintecokg/pdf/Sicomin%20foaming%20epoxy%20PB%20170%20250%20user%20manual.pdf

page 18.

front lip is done with pressure from the foam, back is too sharp and is done with bog.

#9

wow, thank you for that link susho, thats fascinating! good read!

#10

bog is probably a “marine composites” term. In Dutch it might be referred to as “skbabi” (pun intended, but still true).

#11

I would use a joggle gasket seam on the leading edge of the spoiler and “sploog” at the trailing edge.

#12

Just looked up joggle gasket seam. Great stuff thank you for helping me learn a new technique! At this point I’m looking to hybrid a bunch of these techniques with my own, thank you all for helping get more ideas flowing!

When you say sploog/bog exactly what are you guys referring to? In my head I’m thinking of resin mixed with cabosil… Am I right? I wish there was a glossary of terms haha in so bad with terminology:o

#13

For a spoiler wing, I think my idea should be ok if you are good at molds. use the filled epoxy system to make sure everything is joined well, and the outside is smooth. Simple post-sanding and you are golden!