DC812 - Inner Tube Bladder 101 - RC Groups
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Jan 20, 2011, 01:07 AM
Dan.Cummins's Avatar

DC812 - Inner Tube Bladder 101

SUMMARY: Basic story is that some local San Diego slopers were looking for a quick and dirty way to obtain better wet seams with out being a proctologist, rocket scientist, a millionaire, or some sort of mechanical genius.

Goal is to find a bladder material that is readily available and can be inflated without elongating in the mold. If the bladder elongates, all that material that you so carefully laid into your mold just got moved around. So after a bunch of trial and error, Throne and AJ found that you could use a bicycle inner tube IF you prestretch the inner tube such that when inflated, it doesn't elongate.

Technique works best on closed molds, but can be used on existing molds with canopy openings.....but at much lower pressure...which works fine for most applications. Inner tube will conform 100% to inside of mold at relatively low pressures (as little 5 to 7 psi is plenty for most applications.....more pressure is better up to a point). Minimum tail boom diameter is limited to the width of your deflated (into a ribbon) inner tube. For a 5/8" bung hole, I use a 1.125 to 1.250 ten speed inner tube. I have given up trying to use used inner tubes, new ones are cheap and pop less frequently. Limitation of standard butyl rubber inner tubes is that they are limited to about 3X diameter inflation. Ward Hagaman is planning to do some testing using some Latex inner tubes that are 3X the cost but have a potential of 7X diameter burst inflation. Last goal is to have a bladder that holds pressure indefinitely so that you aren't tied to a compressor.


- Readily available
- Cheap
- Holds pressure indefinitely
- Easy to assemble
- Easy to execute/use
- Doesn't elongate when installed and inflated
- Works with readily available release agents
- Non-permanent (removable after part is cured)
- Effective on relatively small fuse cross section less than 1"

So I thought I would outline the steps with words, and fill in with some pictures later on.

Step 1: Find a new bicycle inner tube that when deflated into a ribbon can fit into your tail boom cross section by only bending into a crescent moon or less.

Step 2: Cut the tube at the valve....don't worry about the valve, you aren't going to be using it.

Step 3: clean about 1.5" of the ID of the inner tube end you just cut with denatured alcohol.....set aside your inner tube.

Step 4: Go to your local tire store, and get some "Truck" or long schader valve stems....the long ones are about 2.5" long, and are bulbous on one end. Using a razor knife or power disk sander, cut/sand bulbous end so that it is even with the stem of the valve....ie, the whole length of the valve is now one diameter.....about 0.375" in most cases. Dull up the rubber on the valve stem with sand paper, and wipe it down with denatured alcohol.

Step 5: Apply a thin coat of Goop to the id of the inner tube end that you just cleaned.....smoosh it together to make sure you have a even coating on the id, and then pull it fully open. Take your prepped valve stem, and apply a similarly thin layer of Goop. Let the Goop set up for about 10 minutes.

Step 6: Have a pair of alligator clamps on hand. Insert the valve into the id of the tube all the way up to the brass threads AND all the way to one side of the tube ID. Pinch the tube flat like a ribbon so that it stretches around the valve stem. Use your clamps to pinch the tube and stretch it over the valve stem.....let the Goop set up for another 10 minutes with the clamps on. Now the Goop is well set, but probably not completely dry...that's ok. Apply a thin layer of goop to one side of the covered valve stem and one side of the flattened inner tube. Let the Goop set up for 10 minutes, and then fold the flatten part of the tube over onto the valve stem and hold in place for a minute or so. Tube should be stuck to itself pretty well now. Use electrical tape to stretch wrap over the inner tube valve area.....one or two layers and/or as much as it takes to build up to the id of the end of your tail boom. Take a small zip tie, and secure the inner tube directly past the brass threads.

Step 6: Guesstimate about 2/3 of the length of your mold. Use one of your alligator clamps to seal the other end of the tube by folding it over on itself and clamping that folded end to the table. Stretch the tube to the length of your mold, and inflate it to about 2.5 X original diameter. If the tube is the right length, it should not get longer, and it should be easily stretchable to the length of the mold.

Step 7: With the required length of the bladder determined, make the net length, cut to length plus one inch or so. Clean the ID of the cut end with denatured alcohol. Apply a thin layer of Goop to the last inch of the ID, and smoosh together to get an even distribution. Wipe excess that squeezes out. Open up ID , and let it set up for 10 minutes. After 10 minutes, pinch end together into a ribbon with your fingers. Apply a thin layer of Goop to one side of the ribbon, let setup for 10 minutes. After 10 minutes, fold the pinched end into thirds...left over middle, right over middle. Use a small zip tie off the end at the marked length. Cut off excess tube leaving about an 1/8" past the zip tie.

Step 8: Trim zip ties at both ends of your bladder. Use a sanding block or file to smooth cut off zip ties on both ends.

Step 9: Tie about 12" of kevlar two to the non valve end. Tie off the tow behind the zip tie on the non valve end.

Step 10: Just before you start laying up in the mold, inflate the bladder until it elongates to length of mold. Use petroleum jelly to lube OD of entire bladder....be generous, apply with feeling. Hang the bladder until you have completed your layup in both mold haves. Deflate the bladder so that it forms a tight ribbon shape.

Step 11: This bladder technique works best with the high/low layup strategy

Step 12: After you have laid up both halves, attached 1/2 of the kevlar tow on the non-valve end to the screw you installed on the nose end of the mold. Proper tie off length will place the non valve end of the bladder exactly at the nose of the mold when the bladder is stretched towards the tail.

Step 13: Place a clamp on the edge of your work bench as a stop for your mold. Put you mold half that has the bladder tied to it against your clamp/stop. Stretch the bladder by grabbing the valve or string tied to the valve past the end of your mold. Tie off the valve end by clamping the string to your work bench. The bladder is now laying in your mold half. Adjust the bladder so that the ribbon is perpendicular to the parting line.

Step 14: Close your mold as you would normally do. Once close, release valve that you tied off, and carefully allow the valve assembly insert itself into the bung hole of your mold. This is where is is important that you size the OD of your valve assembly such that it cannot turtle back into the mold....if in doubt, make sure your valve string is long and tied off.

Step 15: Now the easy part......inflate your bladder to taste. Any positive pressure will press your wet seam. Shoot for 5 to 7 psi on your first go. If your mold can handle 15 to 20 psi, go for it. If you go for 25 psi or above, you have to make accommodations to retain the valve so that the bladder doesn't grow outside the mold and potentially burst. If you have managed your resin during the layup, 5 to 15 psi will provide you a wet seam that is as good as your mold. You can play with leaving your mold open VERY slightly to bleed off a little resin, but I would wait to try that until your third or fourth bladder layup.....you just don't lose that much resin at these pressures.

Step 16: After your layup has cured, deflate the bladder, pull your part, and then grab hold of the valve, and pull with a steady firm pressure, and watch your bladder peel off the ID. When you get to the nose end, give the bladder a little extra tug, and the kevlar will release or break.

Step 17: After your fuse has FULLY cured, cut open your canopy area, and clean, rinse, and/or wipe out inside of fuse with denatured alcohol to remove any release agent from the inside of the fuse.

Step 18: Reach through your canopy, and use your finger to feel that smoooooooth homogeneous seam that is consistent all the way to the end of the tail boom.
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Apr 30, 2011, 10:22 PM
Gliders, Helis, and 3D
Eric Zanutto's Avatar
Pretty cool, I was going to mention those ultra light high performance road bike inner tubes but sounds like you already figured that one out. They also usually have less of a seam and should go up to 120psi so long as you seal up any cuts you make. By goop do you mean shoe goo? Just wondering if its the same stuff. That stuff, jb weld, pop rivets, and duct tape are the only tools you need in this world!
May 01, 2011, 11:35 AM
Dan.Cummins's Avatar
Goop and shoe Goo are the same thing Eric. It ends up that Goop is marketed under a bunch of different names, and the only real difference in chemistry is the Marine Goop that has a UV protective additive.

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