I’m already doing the “cast a tube between two tubes” process and you’re right it’s a pain. Apart from ensuring the tubes stay coaxially centred there is as you say not much clearance.

Loads are variable and dynamic, meaning they may no always act down the axis of the tube...bending and twisting may also occur.

Can’t use two CF tubes as inner one needs to be transparent - or at least translucent.video show the effect I’m trying to achieve, really just looking for a way to restore some (I accept restoring all is impossible) of the strength lost by cutting in the design.

https://www.youtube.com/watch?v=dYT2Kv628D8

I think either a thin walled grp tube/layer or a pre-made clear plastic tube bonded in is the best simple solution.  How much load are you trying to take?


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

There is another problem with your idea. Glass and carbon got greatly different young's modulus. Wich means that your resin layer will take huge loads under stress, because the outer layer will deform less than the inner one. And every matrix failure will make it less translucent. Worse: the cut offs on the outer tube will cause stress concentration on the already weakest parts of the tube. Additionaly you will get high shrinkage stress due to the unfilled casting between two solid tubes.
If you are really planning a  structural part, you should take a grp tube, that can take more than the calculated loads, as the carbon tube will be more of an additional load, than an reinforcement.

All excellent points. It all comes down to the actual load this thing needs to handle.

What about a polycarbonate inner tube then? Significantly stronger than acrylic, and would be incomparably stronger than a FRP consisting of one layer of 80g fibre and a resin fill. 

Loads are hard to estimate, but maximum I guess could be 60kg or so...but as I said it could act in a number of directions.

Nominally, think of the tube as a prop, so in perfect use it’s loaded down it’s central axis, compression only. However in practice the tube may not always be vertical so there could be elements of flexural and torsional forces, but no tensile load.

The outside of the tube needs to be black, including the design. At the moment I’m putting a 20mm frosted acrylic tube inside a 25mm Carbon fibre tube and filling the void between the tubes with crystal clear polyester (no, this probably isn’t the best material) with about 20% ATH powder added (the powder gives some opacity which helps diffuse the LEDs).

Once the resin has cured, I stick an led assembly down the middle and cut the design with a dremel until I can see the light shine through (ensures I don’t go through the tube completely). At this stage the design is very visible even when unlit which is aesthetically displeasing, as well as being rough to the touch; hence I reinstate the design with clear polyester, this time with no ATH but instead some translucent black pigment. No, reinstating a design on a curved surface is NOT easy with a very fluid resin, but I’m working my way towards fixing that.

The idea for the 80gm weave was two-fold. I thought it might add some strength, plus I thought it would provide the diffusion that the ATH powder currently provides, without the settling out that is a problem with the ATH (one of my solutions to introducing the resin is to stand the rod vertically and fill it from the bottom using a syringe, so the design is effectively 1m deep, meaning there’s a lot of gravity at work!)

I’m toying with the idea of filling the void between tubes and the design at the same time, which would give more volume to fill and cast the etched design monolithically with the void fill material, effectively locking the two tubes together.

I’ve also thought of using an inflatable bladder as the central tube, meaning it could be deflated and removed leaving only the resin behind (and also meaning I could add the resin with it deflated, then inflate it to “press” the resin more effectively into the void and design).

Right now I’m doing a lot of thinking, not much doing! Sorry that I’m still being vague about the application, but I think there’s a gap in the market for this, but with low entry requirements...someone who already has more knowledge/experience than me could launch a product quite quickly, so I want to keep the exact application to myself for now.

Hopefully though I’ve outlined what I want to achieve above, but to recap;

Must be strong.
Must be uniform black on the outside until the LEDs activate.
Design “windows” must diffuse the light (think of it more as a backlit screen - as long as it glows light transmission isn’t that important)
Void down the middle must be maintained and I’ve discovered for the purposes of diffusion, the LEDs can’t be pushed right up against the “screen” material or they just look like point sources; need at least 10-15mm of clear air between them.

All suggestions as to materials and methods gratefully received!

600N is quite a load for a 25mm rod including bending and torsion. If the light should be opaque anyway, a solid grp tube might be enough. After you cut out the design, give it a laquer finish, and the cutted grp should become translucent again.

I have considered that...but I’ve been distracted by the pretty woven finish on Carbon fibre rods. Might have to source some GRP tube...suppose can always paint it black lol.

I got confused by “solid tube” cos to me that sounds like a rod...I still need a void down the middle...or does solid mean opaque?

I’ve just received my square metre of 80gsm weave and more good news...light transmission and diffusion is much better than expected...Eight layers still lights up very well, and that’s without resin which I assume will only improve transmission (as doing it “dry” I’m probably losing a lot of light to internal reflections).

If I’ve got eight layers I’ve got more strength and it’s also not quite as critical that I don’t “nick” the reinforcement when cutting the design into the outer, black tube.

Is there a commercially available 20-ish mm diameter GRP tube made with about eight layers (how many mm would the wall thickness be?) of 80gsm weave? That would leave a 1.5-ish mm gap between it and the CF tube that I could fill with resin with the black translucent to lock the two together after the design is cut...which also improves the reservoir of resin available to ensure flow as opposed to trying to squeeze it through a gap that’s only the thickness of the CF tube wall.

Is there a better resin for the bonding operation if so? The tendency of the polyester to remain tacky for a long time makes it very hard to finish...

I guess at a push I could try making my own tube...but it sounds like hard work!