Could you please give me some advice with making composite tubing as Im at my wits-end for getting a repeatable process going. I only make a small number for R&D / 1 off projects but things are proving to be waaaay to non-repeatable for my liking.Tubes are generally in the 25mm to 60mm ID range. I would like the tube to be a pre-preg tube so can cure between 80-1200C with our oven. I manually wrap shrink tape on the tube.So a couple of questions:Ply Orientations - Im assuming weaves that incorporate fibres to resist hoop stress (eg a plain weave 0/90) make the tube harder to remove from the mandrel than tubes that have just axial fibres (e.g. UD at 0) Would this be correct? Mandrel surface finish – must the tube be perfectly smooth and if so, how smooth is perfectly smooth?!?My biggest nightmare is getting the tube off. Im using Aluminium mandrels usually. I started with tubes then moved to solid bar in the hope that the additional material would help it expand a bit more.However, then, I couldn’t cool the aluminium quickly enough so went back to tubes with water passing down the middle to cool it. (I don’t use the freezer as Im worried about thermal shock – is this justified?) But, still wouldn’t slide off. I tried whacking it out with hammers but nope, that didn’t work either. Occassionally it does, when I obviously match the aluminium contraction rates perfectly, but that window is so tight, its just not repeatable. So, to cut a long story short, what are you aware of for- What temperature to remove the pipe from the oven at- What method of cooling the pipe is used- What method of pulling/ pushing out the mandrel is used- What method of holding the tube in place while the mandrel is being pulled/pushed out is used?Greatly greatly appreciate your thoughts. Generally, this is a low cost / whats lying round the workshop solution Im looking for!
I wrap my mandrels with a layer of bagging material. After curing, I freeze it which makes pulling it out easy. In Stubborn cases I use a block and tackle to get more pull.
1: 80-1200 c is a large range. You won’t find any resins that can bake at 1200 
or CF that would last that temp!
2: surface: smooth as possible NO tooling marks! Sand perfectly from 200-1200grit. Clean VERY well, and make sure you sand ALL OF THE MANDREL! I have seen some failed ones where they sand the middle, where the prepreg is going. You now have a concave surface, and the ends are larger than the middle. I use semi-perm (Frekot) and never have a problem. Longer and smaller diameters will always cause more problems.
3: pulling should’t be an issue if your release is good. Aluminum will shrink, carbon has a near 0 CTE. The BEST way is to pull the part from the oven warm, and freeze the mandrel if you have problems. I have taken my tubes from my mandrels (normally thickwall tubes) while warm enough to hold. Normally I just hold the carbon tube, bang the mandrel on the table, and the resin flash breaks off and the tube comes out. I would not use release, incase you get a wrinkle…that wrinkle will bind when pulling off the tube, and starts creating a mass that puts pressure on the part, and seizes it.
4: manual wrapping of the fiber and tape sucks, but there are ways. Like a lathe, or high torque motor that is slow, and simulates how a lathe works. That way tape winding is easy and fast!!
5: The ply ori. all depends on your part. Many tubes are Uni with an outside hoop wrap. I have done tons of 0/90, and 0/90/45/90/0 fabrics, both are easy to get off. The worst I had was 0 uni running the length, with a 90 hoop on the outside. I changed it by doing a 90 on the inside, and then 0’s, and last 90 to hold it together.
Release well, and have a smooth PERFECT surface (tapers help), and you should be able to pull the tube at room temp.
Thanks folks
Firstly, 1200C is a typo, I copy and pasted that from word and seems to have placed the last zero as full size instead of superscript. It meant to say one hundred and twenty degrees c! also the reason why there seem to be no paragraphs in that text!
Just a few clarifications if thats alright:
Removing it warm, ie at say eighty degrees?
Freezing the mandrel - how do you do this? Hollow?
It must be surface finish that’s my ‘sticking’ point and poor lathed finishes
Must also look into a lathe modification for tape wrapping!
Ask for "High slip"release agent from your supplier
Have reasonably high-slip Frekote 700NC so dont think release itself is the problem.
One other question, say I removed the whole arrangement at 100 degrees C and ‘quenched’ it in icy water, is the carbon likely to thermally shock? Its high elongation carbon usually in an epoxy pre-preg
It will be hard to shock carbon and it’s resin, especially at such a low temp. again, carbon should have near 0 CTE, so the aluminum will shrink.
I just pulled a tube from a fine lathed VERY thick walled tube, about 3-4" dia. Popped right off even though the mandrel was about 80c.
I was worried about the surface (you can hear that ribbed tone when you rub your fingernail down it), but the lath marks are only maybe 20-30um deep. A good sanding could help.
So…check your tool, sand as needed evenly.
Thanks guys, appreciate it so will give you an update during the week 