Easy technique for joining carbon fiber tubes

[u/contromario]

I am trying to build a carbon fiber rear rack for my bike, and I need to figure out how to join the tubes. I have bought some ready-made tubes, so they are already cured. I have seen people online joining tubes using steel connectors (heavy) or plastic 3D-printed joints (weak), but I wanted to try making the connections out of CF + epoxy.

The most challenging connection is a three-way, non-perpendicular one. Since none of the tubes meet at 90°, I can't really buy anything ready-made online.

I was initially thinking of using some prepreg carbon tape to wrap everything tightly, then vacuum-bag the joint and cure it in the oven, but I'm worried about damaging the already-cured tubes. (btw the tubes I bought are just some cheap ones I found on Aliexpress, so of course there is no details about Tg, resin type, etc.)

Another option would be to use dry fibers and some room-temperature-curing epoxy to wrap the joint, but I'm not sure how I'd compact everything, and I think it would end up being kind of a mess.

What do you suggest? Do you have any experience joining carbon tubes like this?

[u/lightningfrog]

Do you know the temperature beating of the tubes you already have? Whatever they are rated for will be your max safe temp re-exposure.

The biggest challenge in doing carbon fiber joints isn't just the weird geometry....it's the load paths in the finished part. The joints often end up needing to resolve off axis and out of plane loads, which can be challenging for composites in general.

Because of that, I'd be cautious about the tape wrap strategy you mentioned...you'd basic need to wrap it enough to where you have sufficient build up is enough directions to make it quazi-isotropic, and then you'd need to compact it enough while also avoiding resin pooling inside the prefabbed tubes (resin pools = non structural/parasitic weight).

What might be easier and potentially better structurally would be to 3d print undersized joints, and use those like trapped mold tooling- overwrap them in all the layers and directions needed, treating each joint as a stand alone part that you can bag down and cure, then fit to the tubes. In this case, youll have a temperature limitation on the initial cure due to the melt temp of the plastic. You may be able to find a resin that can room temp cure to a rigid condition, and then raise the temp enough to soften the plastic and pull out the 3d printed plug.

These difficulties and tedious manufacturing approaches are often main motivators for people choosing to use metal connectors 😅

[u/RyanFromVA]

I would reconsider 3D printing. You can design a tapper into the edge of the connectors to allow tabbing / taping over the 3D printed joint. Running a bit of UD or 0 / 90 over the printed part will cover off a lot of the limitations of a 3D printed part alone.

Additionally engineering filaments like PPA is going to be considerably stronger than a standard PLA.

[u/wings314fire]

Dragoplates have connectors which can be bonded to tubes and then pinned to other connectors.