I have issues with it pushing and bunching up the layup where the balloon first contacts the layup. I tried inflating the balloon tip into a small ball shape....to encourage that to inflate first..which helped some.
One thing that's going to drive me bonkers is the outer diameter.
The motor case is 2.13" in diameter.
The plug is 2.13" in diameter.
The finished piece is 2.17"
What the foxtrot?
If your next attempt at 30psi results in and even bigger nose cone, your mold is probably not stiff enough. If this turns out to be the case, you can counteract it by lowering the pressure after you've squeezed out most of the excess resin but before the resin starts to set.
The vacuum bagging approach that @kbRocket proposed should also help, because it will result in a countering force on the outside of the mold. A similar approach, but with conventional vacuum bagging instead of the balloon, has worked well for me in the past.
As far as the extra epoxy in the wrinkles is concerned, @tfish's approach with peel ply can help there. If the epoxy pools between the balloon and the peel ply, it will get removed with the peel ply. Removing peel ply from a nose cone is not always funny, but it can be done.
Reinhard
That's a great point about the peel ply. It's likely to be incorporated into the next attempt.
I wish mold stiffness was the issue. That'd be easily fixable. The mold is already a bunker buster at 40lbs of sand and steel.
When you say conventional vacuum bagging, do you mean in two halves that are later joined? Or do you have a trick to do it all in one piece?
There is a company out west, Torr Tech, who specialize in reusable Vac bags, Vac tables & inflatables (and everything you'd need to make your own). To have them make one is pricey but they do have the sheet material available by the sqft in cured and uncured. The advantage is you can make a bladder that is a little more predictable as to how much and where it inflates and it's silicone so de-molding is a snap. I've used both cured and uncured to make custom shaped vac bags and bladders for fabricating various aircraft radomes when I worked for a company that specialized in such. I've also looked at sections of plugged and/or crimped fire hose as a bladder material (this better for mold controlled tube surfaces). FWIW
https://www.torrtech.com/Pages/Silicone-Sheet.htm
P.S. Just noticed the uncured sheet has a minimum order of 30 sq ft so probably out of most peoples price range. Cured is available by the sq ft, 1-99, and they do sell the adhesive & primer.
Have you given any thought to using a silicone plug rather than the bladder? That was another technique used on glider fuselages. I believe the plugs were made using Oomoo 30 silicone from Smooth On. It has been a few years since I saw this technique on RCGroups and I can't find the thread. I'll keep looking. I recall that the method was used to solve some of the issues you are experiencing.
esit - Nevermind, the silicone plugs were used when molding the fuselage in 2 pieces. It would be nearly impossible to remove the plug from a completed nose cone.
I may have missed some details in this thread as I skimmed through pretty fast. So I apologize for missing any key points or any suggestions already made.
Caveat - I'm one of the people who has done bladder molding before, for DLG fuselages (nicknamed the "Bunker Buster" fuselage by a few people due to its durability).
Kevlar by itself isn't a good material for much of anything structural where there may be any compressive loads. The normal solution is to alternate layers of Kevlar and S-glass. The resulting structure is much better under compression, and avoids Kevlar's tendency to fail by delaminating from itself.
If you make the outer layer an S-glass layer, this also makes cleanup of the parting line easier. If the first glass layer is oversized and then trimmed to one side of each mold half, then there is no exposed Kevlar at all. Cleanup is then with a sharp knife to shave off the parting flashing. Note - if you are not getting parting flashing, you have closed the mold too tightly and excess epoxy cannot escape where it is supposed to - outside the part!
In that vein, do not fully close the mold initially. Close it down on some playing cards to space it open. Pressurize moderately, say 15 - 20 psi, and let it sit for a while to push out excess epoxy. Then close it down the rest of the way, and pressurize up to your chosen pressure. I used 42psi routinely, but my ratio of mold thickness to cavity diameter was much greater than yours. I wasn't worried about it going boom.
Lay up and wet out the fabric layers in the mold. NOT outside the mold. Brush on a coat of your epoxy of choice. Yes, it may bead up. Note, DO NOT TRUST any wax by itself, not even partall, to release the part. I have had that fail and destroyed a mold. Use PVA on top of the wax. Yes, there is an art to using it. Sorry... Or, use some other mold release on top the wax.
You lay up in the mold because you want to be putting dry fabric on top of damp layup. Pat and brush into place everywhere and it starts to soak up excess from the previous layer. Then add more epoxy as needed to complete the wet-out.
Doing it this way greatly minimizes the trapped air between layers of fabric. You don't want trapped air. You want only epoxy. Compression via balloon will squish out excess epoxy, but it won't squish out the air just squish it to smaller bubbles. Bubbles are not a good thing regardless of size. They weaken your layup.
Note - use more layers of fabric than you think you need to. The stiffness of a part is greatly related to wall thickness. Under good compression the wall thickness is minimized as unneeded epoxy is squished out (or should be, see following paragraphs)... So add more fabric!
For the tip of the nosecone, I recommend cutting up some S-glass to make a load of strands in a cup. Add epoxy and mix well. Place the gelatenous mass in the front of the nosecone before closing the mold. This helps form a more solid tip. The balloon will force it into a nice shape under pressure, and the excess epoxy will get oozed out the front of the mold.
Your mold is lacking channels for epoxy flow. This is causing you some issues. You can add them after the fact though that is harder. I'll explain how, but first I'm going to explain how to get them in during making the mold.
When making the mold, get the first half molded etc like what you did. Then when prepping to make the second half, use layers of quarter inch wide tape to make a channel perhaps a quarter inch away from the nosecone. Then every inch or so make right angle channels that extend out to the sides of the mold to provide outlet channels. I recommend about 1/16" thickness for these channels at a minimum! Now you have a flow path for excess epoxy to get squeezed out so the layup can be compacted by the balloon.
For after-the-fact, do you have the router attachment for a Dremel? Use a small milling bit and mill the channels into your parting tooling coat. Make sure to block the dremel so it cannot inadvertently traverse into your tooling cavity or you will have a very bad day!
Ok back to process.
Once you have the layup in the mold, the mold loosely closed, and mild pressure applied, heat the mold. I used a bunch of heating pads on high. Much better is to build a heat box and put the mold inside. A warm mold allows the excess epoxy to flow out better, and gives a better cure. How warm and what heat cycle to use depends on the epoxy, and of course the tolerance of your mold for heating. You did heat-cure your mold, didn't you? If not, your options for heating are rather limited. And actually, you need to make the mold with an epoxy that can take at least as much heat as the epoxy you are going to use for the parts produced from the mold.
Heating the mold is a prime reason why sometimes wax is not an ideal choice for mold release.
Leave it in the mold and under pressure longer than you think you might need. The cure is better then and the part can't really warp any while in the mold unless the mold itself warps.
When I mold at 42psi (let's say for argument 30-60psi as I've done that range) and one has evacuation channels for epoxy, and warms the mold, the balloon can get quite solidly attached to the inside of the part. To get it to release, let the air down and let it sit for perhaps 15 minutes. Then hook up a vacuum pump and pull a vacuum on the balloon. If it still doesn't release easily, twist the stem out and that helps release the balloon.
BTW, normally when one is making parts one may do the painting in the mold itself. Then the part comes out already painted, polished nice and shiny smooth! The paint should be cured to the touch (but never ever touch it as finger oils are BAD) but should not be hard cured before starting the layup.
Speaking of which, you should be wearing gloves when handling the fabric even, and doing any of the layup work. It keeps finger oils off the fabric and mold, and keeps epoxy off your hands. Exposure to epoxy in various uncured forms can lead to sensitization and a really nasty allergy. If you get that allergy, you won't be able to be anywhere near any uncured epoxy - ever. That's a high price.
I hope something in this post is of use to you! Have fun!
Gerald
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