I understand what you’re saying about the thickness of the CF layer, but counterintuitively it needs that thickness so that the effect of the translucent black pigment is maximised when the design is infilled. With only 0.2mm of depth, with the lights turned off you’d be able to see the white tube through the cutaway. It needs to look like “magic”; not like bits of white plastic with LEDs behind them...it needs to be a coloured design that appears out of nowhere...

...and now I’m just wondering if I just typed total rubbish...just because the CF layer is only 0.2mm, doesn’t mean the design has to be that shallow...hold the phone, Batman, this may change things...thicker walled (4mm?) GRP tube, wrapped in CF for finish, I etch the design through the CF and into, but not through the GRP, meaning I’ve still got a continuous “tube” of 2mm thickness “underneath” but still a good 2mm depth of design to take a translucent black infill resin. Strength AND aesthetics, problem solved!

OMG that would work I’m sure...as long as 2mm of GRP will give me the backlit screen effect I need. Easy Composites people...do you sell a 25mm OD, 4mm wall unpigmented GRP tube? How easy is it to wrap such a tube in a couple of layers of CF and achieve a finish as good as the commercially available high gloss, woven finish CF tubes?

Oekmont, my apologies for being dumb...just hope I can source a material as described now!

I would still recommend using a grp tube and get a layer of braided carbon on it. This will be a few times quicker, much stronger, cheaper, better to replicate the results, and likely just as appealing as the cast version. It might be almost impossible to get an air free casting in a mm wide ring gap with reinforcement in it. Only way I could think of to make it work would be with "vari". Additionally you waste to much of your precious wall thickness for a carbon tube, wich you cut away any way. A braided layer around the tube would be only 0,2mm. Or you could use a steel mandrel, and build up the laminate with ud glass, braided glass and finally braided carbon. This would be even stronger, and likely very clear, as you could use clear epoxy.
Polyurethane usually got the best overall bonding propertys, but to be honest, I don't know much about casting polyurethane.

Acrylic is fairly easy to bond to as far as plastics go, but considering the loads you are speaking of you may actually struggle a bit here too. Epoxy will bond to it reasonably well, but you will want to ensure you scuff up the surface very well. 40 or 60 grit sandpaper, and I would go so far as to use a razor blade to make a series of deep scores up the length of the tube. This should all get filled in by your casting resin, so cosmetically it won't make much difference, but it would certainly make all the difference adhesion wise.

In all honesty though, I'd be a bit wary of the bond under those loads. 

Oekmont...and anyone else, you’ve all been very helpful...does using the internal acrylic tube affect whether you’d recommend pu or epoxy for the bonding? Adhesion to the inner acrylic tube is probably more critical. If I can cast the bonding layer and infill layer as a single operation, there will be considerably more physical “lock” into the outer CF tube

Thanks oekmont, you certainly know your stuff.

For now I’ll continue with my frosted acrylic inner tube/carbon fibre outer tube and a bonding layer between them that contains some (as yet unspecified) layers of 80 gsm weave.

Can someone at easy composites let me know the best clear epoxy/pu from their range to do this operation please? Thanks - by the way I’m assuming it will be January for delivery now?

Polyester is the worst choice in this application, as polyester has really bad bonding characteristics to epoxy. Epoxy or polyurethane will give much better bonds.

8 layers of 80g glass fibre results in about 0,4mm thickness. This will not hold 600N of dynamic force on the end of the tube.

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!

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.