REMOVING A MANDREL IN FIBERGLASS LAYUP

[AP aroma]

Hello Forum,

Yesterday I did a fiberglass layup using a 3" phenolic coupler tube as a mandrel. The mandrel was covered with 2 wraps of 2 mil Mylar. I had the hardest time removing the mandrel when the fiberglass had cured. On a wood lathe I turned a wood plug the same diameter as the the mandrel and tried driving it out with this. It didn't work well at all. The phenolic coupler splintered and pieces got lodged between the plug and the fiberglass. This made it all the more difficult to drive the phenolic tube out. I ended up drilling holes in the wood plug to make it less ridged, and with continued difficulty I was eventually able to get it out.

Does anyone have a good technique to remove a mandrel like this?

Thanks In Advance,

Jim

 

[Scrapmaster87]

When I made my 4.5"OD mandrel from a PVC pipe, I carefully measured the high and low spots every 2-3" down the pipe. I sanded down the high spots and re-polished. Then I re-measured and repeated the sanding. I ended up getting the pipe consistent to a few thou down it's length, even better would be a slight taper.

 

[G_T]

If you do not have a draft angle it is going to be very difficult no matter what you do, short of using a single-use mandrel that you can dissolve out.

You could try something clever like using a little bit smaller mandrel, greased, wrapped with the tooling layer sheet, and then make your layup on that. Pull the central mandrel when done, then pull out the tooling sheet. You'll have to be very careful about where the grease is allowed to go. It will be tricky. And you'll lose fine control of a nice round inner surface which may or may not be important to you.

You can use variations of this with for instance several layers of parchment paper or similar, waxed on the outside. That would be easier.

Gerald

 

[kalsow]

I'm not a pro, but I've got enough scraped knuckles, bad tubes, and destroyed mandrels to have an opinion...

First, make sure the mandrel is as straight and "flat" as possible. If it's go a visible bends, kinks, or dents, don't use it. Second, make sure the Mylar layer(s) slide easily along the tube. If they're wrapped too tightly, you've already lost the battle. Finally, slide the tube along the mandrel when the epoxy reaches its leather stage. You don't need or want to remove it completely from the mandrel, just move it an inch or two. Most epoxies shrink, just a little, when they cure. In my experience, most of that shrinking happens in the last stages of curing. If I slide the tube before it's rock solid, I can usually remove it when it's cured. Not always!

Did you try to compress the setup? Heat shrink tubing or tape are very tricky. I don't know that I've ever used them successfully on a tube wrapped from plain cloth. They work OK if your fibers are from a "biaxially woven tube" or you cut your cloth on the bias. If you have fibers that wrap straight around the mandrel (90 degrees to the long axis) they have no where to go when you add the compression. So they bunch up, creating a tiny wrinkle or bump that will bind with your mandrel. You lose. If the fibers run at 45 degrees along the axis, they have some hope of adjusting to the compression without binding on the mandrel.

Good luck! and keep trying!

 

[SDramstad]

I have done exactly what you tried except I used a full length Blue Tube coupler. Have been able to remove all of them intact although some took more wrestling then others. It can help to remove them before they are completely set up although they should be at the tough non sticky leather stage before you remove them.

 

[cls]

You didn't say, did you lube up the mandrel with PVA before the layup?

I've done it both ways, lube the mandrel, lube the mylar. May be easier to pull the mandrel, then unwrap the mylar from inside.

 

[G_T]

PVA isn't a lube. It is a mold release. I've used it extensively. It won't decrease your sliding friction, though it mostly will prevent adhesion. If you have an expensive mold and rely on the PVA eventually it will let you down. You want an initial coating of a mold release wax or a semi-permanent mold release, and follow that with PVA, if you are using PVA. That is better and safer. In any event, don't expect low sliding friction. You won't get it. But even if you come up with a coating which does, you'll still have a large surface area and pressure. You need to exceed the stiction. Have a hydraulic press handy?

Low temperature wax has also been used. Warm part up in hot water bath to above melting point and slide part off mandrel. I've done this as well... it is nowhere near as easy as you'd expect. Wax viscosity is not very low when melted, you have a lot of surface area, and a VERY thin gap. It still takes lots of force. Then you need to do the solvent cleanup of the wax on the inside of your layup.

I was making a small carbon composite tube with slight taper. More recently I made a mold, made the layup in two sides with single side staggered overlap on each, assembled around a pressure bladder, and made perfect high compaction high strength low weight tubes that way - complete with polished outer finish upon demolding. Less than half an ounce and a couple feet long would cantilever over 7 pounds without breaking. and this was a small diameter thin walled tube.

Pulling mandrel and unwrap mylar (or whatever) works. The release isn't a sliding release. It is removed normal to the surface. But you still might need mechanical assist to get your mandrel out. If that is a problem, mold a flange on one end of your tube. Cut it off when done.

Gerald

 

[tfish]

Jim

I agree with @kalsow
"First, make sure the mandrel is as straight and "flat" as possible. If it's go a visible bends, kinks, or dents, don't use it. Second, make sure the Mylar layer(s) slide easily along the tube. If they're wrapped too tightly, you've already lost the battle. Finally, slide the tube along the mandrel when the epoxy reaches its leather stage. You don't need or want to remove it completely from the mandrel, just move it an inch or two. Most epoxies shrink, just a little, when they cure. In my experience, most of that shrinking happens in the last stages of curing. If I slide the tube before it's rock solid, I can usually remove it when it's cured. Not always!"

Also do not tape the mylar to the mandrel in a place you can cut the tape free after the layup..meaning only tape it down on the very ends, so you can untape it later..This way the mylar can slide off the mandrel.

A good mandrel to use are motor case with at least 2 wraps of 2 mill mylar.

I have some "how to" video on you tube..tfish38 as does John Coker

Tony

 

[Tractionengines]

Have a hydraulic press handy?

As a LAST RESORT...The mast of a forklift works well...

Choke the tube with (2) slings at 180° to each other. Straps catch under base of mast. Cross pin or bar sized to ID of Tube/OD of mandrel at bottom pass another strap thru ID of mandrel and over fork. SLOWLY raise forks. Will pull it out one way or another... (We had mylar tear and bunch up between mandrel and tube binding it in place... this pulled it apart. Took 45minutes to set-up the pull and 45seconds to actually make the pull.)

 

[rocket_troy]

Yeah, I've used a fork before, but the mistake I made was I held the mandrel with the jaws of our 6" tube cutter (like $12K worth) and pulled the 2 apart horizontally. The 2 came apart with enough force, but only after moving the jaw alignment out of square [oops].
PVC is a good mandrel material, PE or PP even better, if you can either tolerate the roundness of pipe or have the ability to turn the mandrel material to your requirements, because if things get really stuck, you can chill the mandrel out.

TP

 

[watheyak]

I've had great luck with an aluminum tube mandrel, Frekote 700 mold release and a freezer. After the the layup is cured, chilling it in freezer shrinks the mandrel enough to let it slide out easily.

 

[Zertyme]

Rotating the nut on the right will push the Extra tube section against the layup, pushing it off the mandrel. A bit of ice in the mandrel can help.

 

[Reinhard]

I've had great luck with an aluminum tube mandrel, Frekote 700 mold release and a freezer. After the the layup is cured, chilling it in freezer shrinks the mandrel enough to let it slide out easily.

That's also another good reason for curing at elevated temperatures.

Reinhard

 

[AP aroma]

There are many opinions and ideas here. I am getting that this is no an easy task! I'm wondering if an aluminum mandrel is not the best way to go because it can be subjected to cold and the diameter reduced that way. I think the downside of using a motor case is the length of the tube that can be layied-up on this mandrel. But a good and helpful discussion.

 

[rocket_troy]

If your primary method of mandrel removal is chilling, then Al-Alloy is really not optimal. Workable, yes, but its coefficient of thermal expansion isn't anything great for this. Where that might show up to bite is when the the fit is tight and perhaps some resin has found its way between the composite layup and the mandrel. That's where a more slipperier high expansion material like polyethylene will shine.
Of course, finding the right diameter for your mandrel will probably constrain you to more dimensionally stable materials... alas.

TP

 

[astronwolf]

If you are starting out with a coarse/rough surface like that phenolic coupler, I'd say that you need to prepare the surface of the mandrel better, and use something more than just PVA. https://www.trmoldrelease.com/procedures/

 

[techrat]

I saw a good technique on Youtube -- the idea was to tape down 1" or 2" strips of plastic that are LONGER than your tube is going to be, to the mandrel. Then wrap that in another layer of plastic, circularly, around the mandrel. After verifying that everything is smooth, with no wrinkles, lay down your fiberglass. After that has hardened up, untape the strips and start pulling them out, one by one. You should be able to pull them out, and then get the tube off the mandrel easily.

 

[watheyak]

The PVA may also be the problem. It's great for releasing from a negative mold, but unsticking then dragging a mandrel out of a tube is not it's intended purpose.

It's a gummy film that has to go somewhere. I can see it bunching up and impeding your progress. Especially if it was applied thick or unevenly.

 

[cavecentral]

The few tubes I made, I slide the mylar off the mandrel while the epoxy was green, but not fully cured. Then removed the mylar (5 mil) and slid it back onto the mandrel to finish curing. Not sure if what you are doing requires tolerances where this chance of warping matters, but I never had issues once I got the mylar out.

Waiting too long, and it was a struggle. I think I hung the mandrel vertical in my garage ceiling and hung on it to get it loose...