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Old Dec 09, 2008, 05:41 PM
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Easiest way to make a fiberglass tube?

I need to make a tube that is 130mm O.D. and 118mm I.D. and about 30 inches long. I've done some epoxy work before (vacuum bagged surfboards and boat repairs) but nothing of this nature before. I have a limited number of tools in my shop, so no lathe. 118mm is just barely bigger than the O.D. of a 4" Schedule 40 PVC pipe, so I was thinking about using a section of that as a mandrel.

Another thing I might also try is to add a layer of carbon fiber to the layup (between 2 layers of glass). I've never worked with CF before, any particular weaves I should try?

Lastly, I'm on the coast here in Central North Carolina, so no local suppliers of lighter weight fabrics. Any on-line suppliers with better than average prices?

Any other ideas or tips?

Thanks!
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Old Dec 09, 2008, 05:52 PM
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Go to the Slope forum and search for Target's thread called Ballast Tubes 101 for info on how to make a tube using a pipe for a mandrel.
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Old Dec 09, 2008, 06:36 PM
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Thanks Cory, I saw that. Do you think the same technique would work for a tube 6x the diameter? I'm wondering if I'll have problems trying to pull the pipe out?
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Old Dec 09, 2008, 06:38 PM
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Soller Composites sells cloth, plus fiberglass and carbon sleeves that would make the layup seamless and easy. You can wrap the layup with vhs tape or inside-out electrician's tape to compress it while curing.

http://sollercomposites.com/
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Old Dec 09, 2008, 07:06 PM
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I've only used Target's method once so I'm no expert. I would think that the thickness of the waxed paper and packing tape wrap should allow you to tet the pipe out OK.
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Old Dec 09, 2008, 07:12 PM
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If you can find a foam cylinder of the right diameter, just wrap the cylinder with the fabric then use acetone to melt the foam out when it is set. Might not be the best way but it has to be the easiest.
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Old Dec 09, 2008, 07:14 PM
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You could stack foam board together, bonding with 3M77, make a circular template for each end, then hotwire it to get any size you need for a lost foam tube.
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Old Dec 09, 2008, 07:41 PM
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Wow, lots of great tips, thanks! Fortunately, I have a vacuum pump so compressing the layup shouldn't be an issue. By the way, if you ever need to shop for a vacuum pump, try the air conditioning repair shops, they use them to vacate coolant from AC systems. That's where I got mine for $20 and it's served me well for about 8 years now.

Ward, thanks for the link to Soller Composites. The carbon/fiberglass sleeves sound interesting, but I'm not sure how I'd use that in this situation. I'm assuming I'd slide the mandrel through the sleeve, but wouldn't it have to be an exact fit, otherwise there would be a lot of puckers in the fabric?

EDIT: After studying the Soller site a bit more I see now that they post a range of diameters the sleeves can be used for. So I'm guessing that once the mandrel is in place you can smooth out the sleeve and make it a tight layup. If so that will definitely make this easier!
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Old Dec 09, 2008, 08:59 PM
I DS slower than I build!
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The sleeves, also known as socks, work just like a Chinese finger cuff. Pull them tight and they get thinner.
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Old Dec 10, 2008, 03:55 AM
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Great tip, thanks. I'm definitely going to give socks/sleeves a try. It'll make laying up a lot easier plus almost no waste, awesome.
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Old Dec 11, 2008, 06:22 AM
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Harbor Freight has the AC pumps for about 60 to 90 dollars new.
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Old Dec 13, 2008, 02:16 PM
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The difference between your ID and OD is 12 mm. That would be too thick to lay up completely using FG only, wouldn't it?
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Old Dec 13, 2008, 03:52 PM
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Yeah, good point. The layup would be 6mm thick (about 1/4"). Although I'm sure that's way too thick for an RC airplane, it's not too out of the ordinary for what I'm designing (a submersible). if I go with CF I can probably taper the layup towards the middle to save weight.

Incidentally, while I was visiting the Soller Composites website I read that they sell and recommend using heat shrink tubing as a cheap alternative to a vacuum bag. They even sell heat shrink tubing as big as 7" in diameter! Although I'm pretty comfortable using a vacuum bag, I gotta say that this sounds a lot easier and is probably a lot cheaper in the long run.
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Old Dec 13, 2008, 05:40 PM
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Hi,

Heat shrink tube doesn't create the compaction pressure that bagging does so you will have a much more resin rich (heavier and not as strong) layup with that method.

Carbon fiber would be quite pointless in the middle of your lay-up as it would add little to stiffness. CF is best utilized on the inner and outer layers of lay-up. You may consider one layer of light glass for protection but otherwise keep the glass in the middle if it is just for fill.

If you use a piece of tubing for a mandrel, you will have to consider that it will be very difficult if not impossible to remove it if it isn't tapered. I have had success with wrapping several layers of polyethylene sheet (like the stuff used in vapor barrier in house construction) as it it very slippery and epoxy or polyester resin doesn't stick to it. The longer the piece you are molding the more plastic you seem to need. At 30 inches long you may want to consider sectioning your mandrel lengthwise so you can remove it in sections.

Another technique I have used with success is to make the mandrel out of pourable polyurethane. You would need to make the mandrel by pouring it into tube with the same inside diameter as you need for the finished tube. It would also need a rigid tube support molded in to help it stay straight. The support should be heavily lubed with release agent before pouring the urethane so it is easily removed. Then when the glass/carbon fiber piece is set you pull the ridid tube that supports the urethane and then start to pull the urethane out. The urethane contracts in diameter as you pull the end out and makes it relatively easy to remove.

Finally, you may want to consider blow molding. To do that you need to get some tube (PVC or other plastic tubing works reasonably well, especially if it is thick walled) with an inside diameter the same as the finished outside diameter of composite tube you want. Cut it in half lenthwise to create the outer mold. You will have to cut two pieces if tubing that are the corect length so that you end up with true pipe halves and account for blade thickness if you desire absolute roundness of the finished product. Thus you will end up with 4 halves but only two of them will be actual halves, the other two will be short by the thickness of the blade. I usually make this cut on a table saw and hot melt glue wood forms to the tubing so it doesn't roll when cutting and ensure accurate cuts. The outer mold pieces should be about 5 or 6 inches longer than the tube you will need. Then you will need a tube 1/4 to 3/8 inch smaller than final inside diameter you want that will be used as a lay-up form. It should be a few inches longer than the finished composite tube length but two or more inches shorter than the outer form pieces. Take a mountain bike inner tube (new, no leaks) and cut it about 4 inches from the valve stem and pull it over the form tube. Trim the tube even with the form ends or stretch to fit if necessary. Use lots of rubber cement on the inside of the inner tube ends and the form tube to seal the inner tube to the form tube. This should only be an inch or so from the ends. Then use three or four wraps of fiberglass tape (the kind used for shipping that is usually gold colored, it is best because it doesn't stretch. Finally wrap any tape available over top of this until it is almost the same diameter as the inside of the tube you cut lengthwise for the outer mold. This will keep the inner tube in place during the final steps. Now drill a hole in one one end of the lengthwise cut outer tube pieces to create an outlet for the valve stem of the tube. You are now ready to lay up your tube. Wrap a single layer of Saran Wrap or something similar around the inner tube/form assembly; don't cover the valve stem. It will be best if the valve stem has the cap on it. Wet the glass/carbon out and wrap it around the inner tube/form, again do not cover the valve stem. Don't wrap it too tight and make each layer a little less tightly wrapped to reduce/eliminate wrinkles in the lay-up. When the glass/cf is on the form you can work out any trapped air (best done from the center working toward the ends) and wrap it in a layer of Saran Wrap. Place it in the lenghwise forms with the valve stem poking out through the hole previously drilled and use the metal hose clamps with the screw type fasteners to clamp the two outer forms together. Use lots of clamps . Then all you have to do is hook a bicycle pump up to the valve stem and pump it up. Best case vacuum pump is about 14 psi so if you go to 20 psi you will better that. The thicker walled the outer forms are (they can be reinforced with fiber glass before hand) and the more clamps you use the more pressure you can use. I have successfully used this method up to 50 psi which, in conjunction with peel ply and bleeder cloth yielded a very high fiber to resin ratio tube. The higher the pressure the greater the danger of ending up with a dry lay-up which is often much weaker than one with too much resin. I would probably stick with around 20-25 psi unless you are willing to experiment in which case you will also want to experiment with peel-ply as well. Also remember that the higher the pressure you use the more clamps and or reinforcement you will need. It is best to have a dry run and pump the apparatus up before laying up the reinforcement to make sure everything holds. This is essentially how many carbon fiber bicycle frames are made.

Good luck,

Bjorf
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Old Dec 13, 2008, 07:20 PM
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Wow, you've really done your homework Bjorf! I've got to admit that after reading this I'm a bit confused, but I'm saving this post and will take some time to study this thoroughly. From what I've been able to absorb so far this sounds very similar to a method I read on a few DIY rocket modelers' sites.

Thanks very much for your post!
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