Pulling carbon tubes from their mandrels

[Essayee]

Hey guys,

These forums have helped our team out significantly in our attempt to put carbon control arms (and countless other projects) on the car. We've followed the advise of great posts such as this one http://fsae.com/eve/forums/a/t...48/m/15110476641/p/1 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/15110476641/p/1) that suggested we needed to guarantee consistent reliability out of our bonds before we worried about anything else on the subject.

Well, we've performed plenty of Instron testing and can reliably get our adhesives to perform as indicated on the manufacturer's data sheets. With that accomplished, we decided to move on to predicting laminate properties and laying up our own carbon tubes as an exercise in laminate analysis. That brings me to the reason for posting. After breaking two comealongs in three days (thank God Home Depot's return policy is so lenient), I've decided that the method displayed below can't possibly be the right way to go about doing this.

http://i5.photobucket.com/albums/y159/Bibliotequa/SAE/b5048c80.jpg

I realize most teams just purchase tubes (and that option is looking more and more appealing every day,) but I also would imagine some teams have been manufacturing custom tubes. How have you guys managed to pull tubes from their mandrels before? What material is your mandrel and how are you prepping it?

We're currently laying up a 3/4" tube with lengths between 9 and 15 inches. Our mandrel is steel bar stock. We're more or less following this guide http://www.solarcomposites.com/MakingShafts.html as far as the mandrel prep goes, and it is not treating us well. At the moment we fear we're putting so much force through that comealong that it's about blow and have a go at someone's leg.

I've heard pulling the tube mid-cure, or using tapered mandrels might be the way to go. We also have ideas of laying up on a weaker mandrel that we might be able to crush like a thin-walled plastic, or laying up on a cardboard tube and leaving the cardboard in there. Anyone have any ideas with which they have been successful?

[Essayee]

Hey guys,

These forums have helped our team out significantly in our attempt to put carbon control arms (and countless other projects) on the car. We've followed the advise of great posts such as this one http://fsae.com/eve/forums/a/t...48/m/15110476641/p/1 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/15110476641/p/1) that suggested we needed to guarantee consistent reliability out of our bonds before we worried about anything else on the subject.

Well, we've performed plenty of Instron testing and can reliably get our adhesives to perform as indicated on the manufacturer's data sheets. With that accomplished, we decided to move on to predicting laminate properties and laying up our own carbon tubes as an exercise in laminate analysis. That brings me to the reason for posting. After breaking two comealongs in three days (thank God Home Depot's return policy is so lenient), I've decided that the method displayed below can't possibly be the right way to go about doing this.

http://i5.photobucket.com/albums/y159/Bibliotequa/SAE/b5048c80.jpg

I realize most teams just purchase tubes (and that option is looking more and more appealing every day,) but I also would imagine some teams have been manufacturing custom tubes. How have you guys managed to pull tubes from their mandrels before? What material is your mandrel and how are you prepping it?

We're currently laying up a 3/4" tube with lengths between 9 and 15 inches. Our mandrel is steel bar stock. We're more or less following this guide http://www.solarcomposites.com/MakingShafts.html as far as the mandrel prep goes, and it is not treating us well. At the moment we fear we're putting so much force through that comealong that it's about blow and have a go at someone's leg.

I've heard pulling the tube mid-cure, or using tapered mandrels might be the way to go. We also have ideas of laying up on a weaker mandrel that we might be able to crush like a thin-walled plastic, or laying up on a cardboard tube and leaving the cardboard in there. Anyone have any ideas with which they have been successful?

[Boffin]

Have you thought about laying the tubes up over a wax/plastic that can be melted out after curing?

[MalcolmG]

I've made a few zero-draft parts with aluminium moulds, and then have frozen them to shrink the aluminium. I'm not sure how successful it would be on a tube, but I reckon it would be worth a try.

[Moke]

First off I hope you used release.

I make a lot of carbon tubes at work and as Malcolm suggested a night in the freezer will help. We push our tubes off with a hydraulic press, as you can get greater force and less chance of injury.

[Travis Garrison]

Use mold release, for wet layup I would recommend PVA as that actually leave a thin water soluble barrier film.

Use a metal mandrel, and a heated cure or post cure whenever possible, then as recommended by others stick them in the freezer...take full advantage of CTE.

Lastly, layup handles of some sort (I've done kevlar tow) or plan on bonding something to your carbon tube (steel ring over the excess?). Don't just press it off, you'll damage that laminate you just put so much effort into.

[BuckeyeEngines]

Two part female mold with a rubber tube through the middle. Incorporate the tube into the vaccum bag and it will expand pushing the carbon into the mold in the autoclave.

Don't foget if you try PVA wax the mold before applying the PVA. PVA on its own is not a release.

[Hector]

As a recent FSAE grad, nothing like logging on and seeing a picture like that... then realizing it's from your shop.

I laughed.

[Pico]

A polished mirror finish would help with the demoulding.

[Lorenzo Pessa]

if you need a long carbon tube, you can also design your mould not as a cilynder but as a cone (with a very low angle with the axis).

[Adambomb]

To take further advantage of CTE, try Al molds and dry ice. However, if you later decide to make some dry ice bombs, don't let baja find out, or they will call the police, and you'll end up on the cover of the university paper.

[Superfast Matt McCoy]

h t t p : / / w w w .acp-composites.com/home.php?cat=4663

I normally like the DIY approach but these are too cheap to not just buy. we used the unidirectional stuff on our rear wing supports in '07. way lighter, but they are very fragile and i wouldn't recommend them for that reason. after our spectacular explosion, the track workers tried to push the car by the rear wing and they buckled.

also, i put these on the MTS machine and they buckled when expected based on calcs, and they failed at the bond between the lugs and the carbon in tension (also about where expected based on adhesive shear strength), so they are good quality.

[Auerbach]

After using PVA on a few nose cones and having to destroy a plug during my FSAE days, I tried Fibre Glast 1153 FibRelease on my roadster seat plug/mold and it popped off really easy. Plus you just wipe the FibRelease on and start laying up, no need to spray or wait for the right dry time. Pretty cool stuff. I also sanded to 1000 grit and used 3 coats of Turtle Wax.

[kapps]

How bout aluminum mandrels. Vacuum bag and heat to 350F to cure. Obviously, this means the mandrels will need to be undersized so the CTE will bring the aluminum to the nominal size at cure temperature. When everything cools after cure, the aluminum will shrink and you should be able to just pull the composite off the mandrel.

[cjanota]

I would also try using a slide hammer instead of a come-along. The impulse helps to get things loose. It worked for some parts of ours.

[Simon Dingle]

To echo Auerbach, we made some CFRP tubes only a couple of days ago. CFRP wrapped around a polished half-inch steel tube with release agent (12 coats) on the steel tube before CFRP was layed onto it. CFRP tube came off the steel tube by hand. Easy.

[Drew Price]

Echoing cjanota, some impact will go a long way.

The other thing you could try since you've got your (scary) setup built already, is put some load on it with the tethers, and then *whap* along the outside of the part with a rubber or plastic mallet, it may pop loose that way.

We used a slide hammer made from a piece of all-thread, some fender washers, and a failed attempt at a machined aluminum diff carrier (or something) to pull the halves of our restrictor, but that had a lot more taper than you guys did.

Plus we had some freshies do like 8-10 coats of wax on all the AL moulds before the layup, that probably helped a lot too.

Best,
Drew

[Demon Of Speed]

w w w . f o r m u l a s t u d e n t .de/public-relations/fsg-news/news-details/article/-6a9dfde268/

[Essayee]

Thank you for all the replies. After reading some of your feedback and speaking to some sponsors, we made a fixture to press the tubes off the mandrel on a hydraulic press. The presses went flawlessly. No injuries, no abrupt explosions as the press lightly pushed the tubes off the mandrels. Except for one tube in which the visqueen we were using as a release agent bunched up inside the tube and ruptured about 4 inches of perfectly good tube, the pressing process only damaged about 1 inch of each tube which we were going to trim anyway.

After speaking to a couple sponsors, it seems we made a few critical mistakes which I figured I'd post here so future teams could benefit from this knowledge.

Our first big error was using the visqueen as a release agent. Apparently as it heats up during the cure it sort of melts and stops being as slick as it normally is. This contributed to the insane amount of friction present in the pull.

Additionally, we performed post-cure while the tubes were still on the mandrels which made the carbon contract further onto that mandrel and increase the force with which it was clamping onto the mandrels.

While speaking to the folks at carbonfibertubeshop dot com who seem to be extremely experienced on the subject, they also suggested two big things that they found helped. The first is to make sure you have tight straightness and diameter tolerances on your mandrels. They said they typically grind their mandrels in house, but that McMaster's precision ground drill rod is an excellent substitute if you don't have the means to do your own grinding or get it sponsored. Lastly, the folks at tubeshop also recommended using the Chemlease 41 release agent system. They said in the past they've also had some success with products from Frekote.

Once again, thank you for all the help. And hopefully someone can benefit from our hard-learned lessons in future seasons. As Pat Clarke clearly pointed out, going about this the wrong way is a good way to earn yourself a ticket to the local ER.

[kapps]

I'll echo Frekote as a good release agent.