Pressurizing a Mandrel

[Chris TNR]

In my endeavor to produce a mandrel for a 4" diameter BT, I have begun work on a piece of 6061 aluminum tubing, 4" dia.x 36 long. My idea here is to cap off (airtight welds) both ends, with one end having a drilled and tapped
(1/2" pipe thread ) center hole to accept a fitting for my airhose. As you can guess, I will then use air pressure to relieve the BT from the mandrel after the CF is applied. My only question is, "How many holes get drilled into the tubing to produce even air distribution throughout the mandrel?" Also, "What is the diameter of the holes?"
I was thinking, 6 (1/4") holes on all 4 sides of the mandrel would be sufficient, but then maybe thats too many, or maybe not enough. Although I'm definitely stepping into new territories with this project, I felt there must be someone here in TRF whom of which has had some experience in this field.

I have done some searches on this topic (both TRF and EMMR) but unfortunately haven't come up with anything that would help me.
Has anyone ever done something of this nature?
__________________________________
-Chris

 

[ADiehl]

I can't speak from experience, but I would make a guess that a lot of small holes would be much better. If the holes are any larger than 1/16" I would be afraid whatever you lay up on the mandrel would at least slightly sag into the holes, and if you are vacuum bagging or shrink taping your tube, I think any size of hole will make your tube impossible to remove. Perhaps a different method would be better, like running hot water through the tube while laying up and curing, and very cold water to remove? A temperature differential of 90deg F should give a .002 shrink in an 6061 tube. Still, your pressure removal idea sounds interesting and definitely warrants a try.

 

[Chris TNR]

I can't speak from experience, but I would make a guess that a lot of small holes would be much better. If the holes are any larger than 1/16" I would be afraid whatever you lay up on the mandrel would at least slightly sag into the holes, and if you are vacuum bagging or shrink taping your tube, I think any size of hole will make your tube impossible to remove.....Snip.

It's funny that you mention that!!
Literally minutes after I posted this, another team member said the same thing about the material sagging into the holes (he also recommended using 1/16" holes) during the curing process. I agreed......right up to the point in which I said,"What if the tube was slightly pressurized before my layup was cured?" .....and..... I used wax paper (somewhat snug) as my release material?
I guess the reason why I'm doing this is because I have had so much trouble removing the mandrel after it cured, even with freezing it. Just thinkin'.....there's got to be a better way.
-Chris

 

[WAC_Hal]

You might try posting your question on the Home Shop Machinist forum. There's a wealth of information with the people who post there.

https://bbs.homeshopmachinist.net/

 

[Stones]

What about sleeving the mandrel with a rubber type wrap that is sealed to the tube? Something that could be coated (Freecote) for ease of removal. You'd pressurize the mandrel to get a slightly oversize mold for the tube. After curing, you release the pressure and the tube should come off the wrap. Just a thought.

 

[Sailorbill]

Chris,
Try Mylar in 0.002 or 0.005" thickness. Heavily waxed or sprayed with a release agent, Also wax the mandrel, You could warm the mandrel when applying resin and cloth. This will expand the tubing and promote faster cure then when you want to remove the tube just cool it there should be more shrink that way.
Also in case you haven't already done so look at joining the Yahoo Composite Rockets Site https://groups.yahoo.com/group/CompositeRockets/ .