when to use a core? (and when not to?)

General Composites Discussions Engineering Talk
#1

Basic question from a beginner:

Let’s say I’m making a large flat CF panel. I could:

A) just lay 8 sheets of CF cloth, infuse them, and call it a day.

B) or lay 4 sheets, add a foam core, then lay another 4 sheets.

Obviously B is stiffer than A in flexure. But I’m sure there are some drawbacks to that solution too. So, in what situations I should go “I don’t think it’s a good idea to put a core in the middle of the CF sandwich, better make it plain”? Conversely, what are some typical situations when using a core is indicated?

#2

See my post on reddit :stuck_out_tongue:

About drawbacks though, if you are confined to a specific thickness, a core might not work for the stiffness you require. All my tables that I make for sale are CF and core (normally foam) because it just makes it so much stiffer than straight up CF. But for something like extra long ceiling fan blades, a core will make it too thick, so I use mostly unidirectional fibers to keep it stiff when supported on one end only.

#3

How about fasteners?

Let’s say I need to join various elements together with steel nuts and bolts. To me, the foam core composite seems easy to crush. What are the do’s and don’ts in this situation?

How about drilling holes in a CF composite for the bolts? Wouldn’t that weaken the piece? If I plan ahead for everything, could I lay the CF fibers around the hole?

#4

In normal circumstances drilling is not that much of a problem. Tech-wise, glueing is better. Loads are spread much more.

Also take a look at www.bighead.co.uk, they have nice fasteners, also for sandwich panels.

As for bolting through a sandwich panel: Depends on the load. For very light loads you might be OK, for bigger loads use a high density foam locally, and for very high loads or very dynamic loads, revert back to single skin.

#5

Clickbond and Swiftbond makes various glueable large flanged bolts and nut housings for gluing on the outside for light loads. i think they make things for internal fastening as well (again, you will insert during manufacture, OR cut out hole and core, and then pot in hardpoint.

For those, you can do several things, all depending on your end part, and loads. Just glue it on the sandwich skin, bulk up several more layers, larger than the flange you are gluing, or adding a solid laminate/hardpoint inside the sandwich. I have used thin foam core, but where I bolted to, added the same thickness pre-made fiberglass board (FR4) to where I need to drill and glue fasteners too.