I have been presented the opportunity to build oval tubes but im not so sure about the logistics… The I.D. is critical and the O.D. need to be show quality. I also need to have only one seam line. oh, and its on a budget… The tubes will be used for motorcycle exhaust so machined parts will be fitted into each end to capture the components. Thanks for looking!
Braided carbon tubing and heatshrink? just a thought, but it should work.
http://www.solarcomposites.com/
What are you planning on for a mandrel?
wow good site. I think that handles the composite half of the job, now to answer your question. What the heck do I use as a mandrel???
Get a plastic tube extruded. Making an oval die is as cheap as cutting a plate. It won’t work for high temp curing but if you’re taking Capt Ken’s suggestion it should work fine.
a silicon bladder and two piece mold would work well.
That is the way to go, if it’s a large quantity. For small runs I suppose you… Wait a minute do you have a piece to make a mold from?
This was my question too. An oval with two specific id’s would be difficult to come by if not impossible.
Wouldn’t this not necessarily be a good idea since the ID is critical? it might work out, but if his tolerances are tighter than plus or minus one ply he could possibly have trouble here, right? I was thinking the opposite might be better (assuming a straight tube). Having a metal mandrel which you can heat and cool to get the part off with thermal expansion and silicon piece to cover the outside and throw in a bag (with all other necessary steps, of course).
True, I didnt catch that the first time reading. it really depends on his tolerance then. If it is very tight your correct that you would need a solid mandrel or a wash away type. if he has a little room to play I think this would be one of the better ways to get a nice outside finish.
All good stuff. here is one idea i’ve had:
-use a rolled piece of aluminum to make the oval with a split down the middle. tape the splt and cap each end to hold the mold in place and make the part. when cured, close the split on the mold making it smaller than the part and separate the two.
I like the idea of an expanding mandrel but remember that I need to keep my cost low. I want to make a straight tube at 7.5 feet long and machining that piece would be a nightmare!
extruded plastic sounds good too but how do I get the part off when im done?
Silicone bladders are cool but my tolerances are a little too tight for it, unless i had a sleave around it to keep it at the right size.
I have a part in hand and more importantly, I have a CAD file 
Is it a straight narrow piece? If so you pick a material that has a higher coefficient of thermal expansion and once you cure the composite, freeze the mandrel and the part and pull out the mandrel.
Or maybe a two piece mandrel with a diagonal split down the middle. When the part is done hammer out one of the pieces and it should pop out. The attached diagram is a quick side view with the cut. Add some hooks or latches to one end to keep the two pieces snug during molding, then when done release the latches and remove one side. The other will easily release after that.
====Edit====
Ignore narrow. The wider the part is the more it will shrink. Should be straight or have a constant curvature though.
Look up Smart Molds…they are solid molds that are shaped to size, so your ID will be a match. Lay up on the outside of the mandrel. Lay in 2 part mold (or shrink tape, whatever), cure with the mandrel under pressure, heat the mandrel to collapse it, and then you reshape the mandrel.
So, you would need 2 molds. One for the mandrel, and one for the part.
I forget the exact name or company. But for high runs, I think it would be good.
Also, a split aluminum male mold. You would expand it to size, wrap it in film/tape. Lay up your material (pre-preg only I’m sure). Cure in female mold, or again shrink tape. When cured, you unexpand the split mold, and pull out. My old company did that for square tubes. Think of 4 corner rails, and turnbuckels to make the mold frame. It was wrapped in mylar tape to make the inside surface. They prepregged it, and shrink taped the outside.
the company is CRG, they do some really cool stuff
Don’t do it. I had a very similar offer from an exhaust company… i basically turned him down for making his tubes. He wanted perfect quality and at less than Chinese prices. If it’s for some guys working with Billy Boat… don’t do it.lol. i quoted the guy $140 for two parts and he almost swallowed his tongue… and he would not guarantee me at least 20 units up front. for $140 i would have needed him to buy at least 50 units.
We were going to either bladder mold with two part female outer mold. The bladder would have been latex or similar bladder material, then i decided a clam shell part bonded together after trimming would be better… customer said no to two seems though and that’s when i basically let him know we wouldn’t be able to meet his requirements.
those CRG Smart Mandrels… great invention they have there.
Canyon, that looks like a good idea for a mandrel.
Ha thanks for the advice. This is for a proformance motorcycle exhaust company. The tubes are oval, 14 inches in circumferance and 18" long. they want me to build 50 units at $42 a piece… I can do it with material cost and time but I would only make back around 13% (which would pay off my mold after 2 years) and im not sure if thats enough to cover the “what if”
Then it’s probably not worth it for you… unless you can find other customers looking for the same part?
Easy, with the right tooling. 
I’m sure the I.D. dimensions are not critical, as there will be some tolerance with the slip fit of the metal ends. The inside texture with a roughness of +/-.005" should be just fine, and is achievable with a bladder.
No reason to have more than 1 seam, if any at all. This is where experience and knowledge goes a long way. I see too many shops start up and expect big/complex jobs right away. Won’t happen.
So what are you trying to say? 50 tubes a month is too big for an “inexperienced new shop?”
You have no reason to think that we are either inexperienced or new. Nor should you take the oportunity to make an example of yourself. This thread is simply for suggestions. Do you have a suggestion??
I’ll let TET defend himself if he wants, but I think you took his statement the wrong way