Molding a Nosecone Using a Pressure Bladder

[watheyak]

Staggered...



In the mold, balloon inflated. Excess epoxy is already running out the sides, which is a good sign.



I should be able to crack it open late tonight or tomorrow.

 

[tfish]

fingers crossed!

Tony

 

[watheyak]

Mostly a success! It turned out way better than I thought it was going too. Especially for the first try. I still have some (actual) kinks to work out, but it's a great start.

First sighting-



First evidence of some issues. You can see the areas that are more opaque. This is where the wrinkle in the balloon was. Also some pinholes.





I was able to patiently pull the balloon out in one piece. Satisfying. Like a big booger. And speaking of, in the next pic you can see the area about 3/4 of the way down whee resin collected in the space left by the kink in the balloon. I think it's above where the av-sled and associated o-ring is going to go, but not ideal.





Some things went right. The staggered seam looks pretty good. The inner surface is smooth save to the balloon wrinkle issue.

Some issues to fix- The wall thickness is way more than I had intended. It's .085, do I can probably eliminate a couple layers. Some Pinholes. The balloon wrinkle. And it's seems very resin rich.

Next time I an going to see if I can double the air pressure to about 30psi. I'll also be faster at the process this time around so I won't be running up against curing epoxy.

More later, still digesting.

 

[watheyak]

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?

 

[kbRocket]

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.

I am wondering if it would be advantageous to surround the mold with a vacuum bag, seal it around the balloon inflation pipe, and then inflate the balloon by sucking the air out of the vacuum bag. This would result in pulling in the balloon rather than pushing it in. It might be easier to not trap a volume of air and resin in the tip of the nose cone. Materials other than balloons may also work and one small pinhole in the balloon may not cause collapse.

 

[Reinhard]

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

 

[watheyak]

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?

 

[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?

I'm pretty sure your epoxy/sand compound is really stiff in compression, but I'm wondering how stiff it is in tension. Doesn't mean it's not stiff enough, it's just a thought that came to mind.

With conventional vacuum bagging, I'm referring to the regular vacuum bag material, instead of latex.

Basically, the vacuum bag is built up coaxially, consisting of an inner bag and an outer bag. On the end(s) of the mold, the outside of the inner bag gets sealed against the inside of the outer bag. When the bag gets evacuated, the inner part of the bag expands towards the nose cone (like your balloon) and the outer part bag compresses against the mold. As a side effect, a stiff mold is not required.




(You can't see it well on the pictures, but the inner part is just a piece of sheet, rolled up and sealed lengthwise, then sealed circumferential on the outside against the outer bag)


It is always a bit tricky to get the bag material into the nose cone, even more so when the cone is small. The smallest I've worked on, had 75mm outer diameter, so yours is a bit harder. Also notably different, the molds I've worked with where completely open (full diameter) on the coupler side, allowing access to the inside. Most of the time the tip end was open too (later closed with a metal tip).

Reinhard

 

[watheyak]

Veeeerrry interesting. I love it, and thanks for sharing.

I'm going to have to ponder this a bit. If I can't make this system work, your method is the next try. I'm sufficiently motivated (obsessed). I could pretty quickly make a mold open on both ends.

I was going to ask what vac bag connector that was. Then seeing you're in Austria I checked the R&G website and found it. They have all the cool stuff!

 

[stantonjtroy]

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.

 

[watheyak]

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.

Thanks for the lead, Troy. That place looks great.

It's the big-boy version of what I am trying to do here. This is all based on guys making fuselages for competition R/C sailplanes, and there's a few methods out there. If the balloon fails to work, the next step was to make a shaped bladder using Airtech Stretchlon. Folks have been using a soldering iron to fuse the halves together. But it's finicky and not reusable.

The silicon sheet might be a good option. I'm gonna spend the evening learning about it.

 

[stantonjtroy]

Never considered the Stretchlon. I wonder if, for simpler geometries, it could be rolled and lap bonded? Might have to play with that. Could be good for one off/development prototypes. HMMMMMM.........

 

[_kestrel_]

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.

 

[watheyak]

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.

Adrian Adamson was going to do something like this recently. I'd like to find out more but also couldn't find more info.

Do you remember if the plug is slightly smaller to account for wall thickness? Or does it compress?

 

[_kestrel_]

The discus launch glider fuselages were mostly done without shrinking the silicone plug. They were much thinner layups though. I’ve searched on and off today but haven’t found much for doing single piece layups with a silicone plug. I’ll keep looking as it may turn out to be a less equipment heavy way to make a custom nose cone.

 

[watheyak]

It occurred to me that since the nosecone shape is, even though VK, essentially wedge shaped. If I had an open ended mold it might work.

I do have an open ended mold I made from this plug about four years ago. It's nowhere near as nice, but with a little work it might be.

So now I've got the balloon, the Stretchlon shaped bladder, and now a silicone plug. I've got 8 days on the road to weigh my options and do some research.

Thanks for all the input everybody, this has been a good discussion. Keep the suggestions and knowledge coming!

 

[watheyak]

Good progress has been made with the balloon setup. One more is in the mold that hopefully has a few minor issues fixed. Next up is trimming and adding the phenolic tip. Trimming Kevlar and the resultant fuzzies is no fun.

 

[watheyak]

I practiced a bit of trimming. The phenolic tip, resin printed av-bay and carbon fiber inner shoulder are in their approximate positions.

 

[David_Stack]

Impressive work! Curious, did this nose cone also come out ‘plus sized’ as compared to the plug? If not, did you ever determine why that first pull from the mold was a few hundredth larger that planned? (Or why these parts are coming out the size they are)

 

[watheyak]

Yes, they're still coming out slightly fat. The short answer as to why is I screwed up.

It really is a tiny amount, but I am thinking about adding a few coats of urethane clear to the motor case and it might make it fat to match. Loki blue might look pretty nice clear coated. And the gloss would allegedly make it more aerodynamic

 

[G_T]

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

 

[watheyak]

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

Funny, at the start of this endeavor I had toyed with the idea of contacting you directly as you came up in both sides of the research for this project. Coincidentally this also re-ignited my interest in DLGs and competing in F3K events. My old Maple Leaf Encore has been refurbished and is back flying. I'm currently in the market for something a tad newer...

Thanks so much for taking the time to freely give so much information! I'm about 50/50 on things you've suggested. Some I can incorporated on the next go, others will have to wait until the next project and mold. The most recent part I pulled from the mold is serviceable, I think. But it's not without it's problems. It could have benefitted from another layer and there are pinholes galore. The latter issue is complicated by the fact that i don't want to paint this, I'd like to have the glossy clear. I was able to fix the pinholes with a wipe of epoxy after the fact, but this is not ideal.

Further complicating things is the fact that this will not get painted, so I am looking for that glossy and perfect finish out of the mold. I know it can be done, but this, again, complicates things. On top of that, I am using high temperature epoxy. Cotronics 4461, that is slightly lower viscosity than normal laminating epoxy. It cures at room temperature, and then is post-cured out of the mold at 250 degrees F to get it's heat resistance. Any paint will just burn off this rocket.

I am having a hard time understanding why the pinholes are being formed. I am brushing a coat of epoxy in the mold before any fabric goes in. But I think perhaps part of the problem is that I'm wetting out the fabric beforehand. This is the only way that I can accurately align the edges for a staggered wet seam. I must be trapping air in the weave of the layers when I do this. There is also a very this fiberglass sanding veil on the outermost surface. It's actually attached tp the 2x2 twill with a very this mist of 3M77, Phil Barnes style. That's been helping keep the twill from skewing. Any tips on getting a staggered seam when adding dry fabric pieces to a mold? This has kept me up at night...

I think on my next go, I am going to alternate layers of Kevlar and S-glass, keeping the fiberglass tissue as an outer layer. I'll also heat the mold and keep it heated as it cures. I was also only using 30psi. I'll bump it up to 42psi. While I don't have channels, I have been leaving a gap when I close the mold. Plenty of epoxy leaks out, but you playing card suggestion is great. I may yet make a channel in this mold if I'm going to make many more parts with it.

It's getting late, I've spent all evening working on the second iteration of the fin can. I'm sure I'll think of more overnight.

Thanks again for all the info, Gerald. Much appreciated!

 

[G_T]

You do not want to wet out the fabric beforehand! That is going to guarantee pinholes and bubbles. Aligning is easier than you think really. What you want is a little pair of scissors with a sideways curved edge. You lay up the first layer of fabric (glass, not Kevlar) in the mold so that it extends above the edge of the mold on both sides. Trim one side flush with the mold once wet out. It's that simple.

Wetting out and drying the fabric down beforehand is a production technique used to speed the process. It doesn't make a superior product to hand layup in this situation.

Oh, forgot to mention - when you wet out the mold initially before the first fabric, you may want to have your epoxy mixed with some colloidial or fumed silica. It keeps it in place better and acts as a bit of a filler. You are just thickening up the epoxy a little so it stays in place with a first coat. The rest of the time you use epoxy without filler. Since your mold does not have features, this may not be as important.

BTW, if you want to avoid Kevlar altogether, S-glass should be fine. Basalt fabric is another one to consider, if you can get it in the form you want at a price lower than S-glass. It will still be RF transparent. It was originally developed by the Soviets for ICBM nosecones, IIRC. I haven't looked in years to see what might be available now.

The only reason to use Kevlar in the layup is for its rip-stop properties. It is as good as S-glass in tension but not anywhere near as good as even E-glass in compression. So your part is weaker but crack propagation is a little slower with Kevlar, for the same weight. However for a nosecone if it forms a crack in flight I don't think Kevlar will make much difference in the outcome...

On the layup, you'll want the outer layers on a bias so they conform well with the mold. But structurally you would be better off with most of the layers at 0-90 orientation. That of course won't conform to half a nosecone unless the weave is so open it is worthless for structural purposes. So instead cut into convenient width strips and lay down strips lengthwise. The outer surface will still be smooth. The nose will be thicker than the base. And the slight ridges on the inside won't matter.

Conforming any fabric to a 2D curve is going to require distortion of the weave. Do what you have to do, while avoiding wrinkles. If you can't avoid wrinkles, use smaller pieces!

Back to mold making - it is useful to also run carbon tow around the perimeter of the opening to help prevent breakout.

Also, before I forget, I'd recommend you not go up to 42psi as it is increasingly likely that your mold will CATO sending pieces to embed in your walls. I'm not kidding. Perhaps 35psi. It's too bad we're not using hydrostatic inflation rather than air, but that would be more work. That at least would only break the mold not make it blow up!

Gerald

PS - All my DLG flying fields are gone. That's why I haven't been very active with DLGs these last couple years. Plus there hasn't been a real need for me to develop any new airfoil families as what I've published is still basically the state of the art and the requirements have not changed. I may consider making something for F5K or whatever electric 1.5m DLG-ish is called or going to be called. That has different requirements and the current airfoil families are not ideal. But it takes me about a thousand hours to fully develop a new airfoil family so I'm not exactly eager to do another one for free for everyone. I've got man years into the existing ones that nearly everyone is using either as-designed or slightly modified.

 

[G_T]

PPS - I use about twice the clamp points on my mold that you have on yours. If you make another one I suggest increasing the number to spread the load better. Molds made this way are fairly stiff but brittle.

Note, when closing up the gap in the mold once it has been under pressure, close progressively rather than each bolt at once. Otherwise you may blow up your mold with you right next to it. That would not be good.

Gerald

 

[watheyak]

Any ideas why the part would come out of the mold "less than round"? The sides with the seam are somewhat flat.

The plug was round, the mold was round. I'm confused.

 

[G_T]

Is the mold still round when under pressure? Is it still round when warmed under pressure? Do you leave the part in the mold until it is fully cured? FWIW, full room temp cure is roughly a week! Epoxy shrinkage occurs mostly in the last 10% of cure. At 90% cure it will seem fairly solid and that can fool you into thinking it is ready. That's why post-cure should be done in the mold if possible. You don't have to maintain pressure, usually. But it is good to leave it in for a lot longer than you think you might have to. How long are you leaving it in? The mold helps handle warpage tendencies. Symmerty of fabric can also make a difference. Very few fabrics are really symmetrical even though they might appear to be. So shrinkage may be more on one axis than another. Also the greater the epoxy content compared to fiber content, the greater the warpage potential.

Those are just some general thoughts on the subject.

Just a suggestion here... When laying up fabric in the mold, leave the inner layer fairly dry. Use a pretty dry acid brush to get the fabric firmly in place and to force epoxy to wick up through the fabric. Then put down a layer of paper towels and blot them into place as if it were another layer of fabric. Let it soak up epoxy. Throw them away afterwards. If the paper towels came out wet rather than damp, do it again. This helps get rid of the bulk of the excess epoxy early when it is thinner and easier to remove, rather than the pressure of the balloon having to make it migrate inches through a very thin region of fabric then through a thin gap to exit the mold cavity. Note that if excess epoxy cannot exit the mold, all the pressure in the world will do nothing to make the epoxy content uniform throughout the part. What makes it uniform is the incompressibility of the fabric itself. It will only squish so far.

If your epoxy is sticky, the paper towel thing can become a nightmare. If it lifts a fabric layer then you are going to have air problems. AKA the layup work has been ruined.

Along those lines, mix up your epoxy in smaller batches as you go along. That way it is easiest to apply the minimum amount needed as you are working because it hasn't started thickening any yet.

If it is thickening much before you get the mold closed up and under pressure, you need to take steps to slow that down. Work at a lower temperature - each 10 degees C temperature drop should roughly double your working time. Or better, use a slower cure hardener if such an option exists. I used MGS mostly, and adjusted the ratio of fast and slow hardeners to give a working time suitable for whatever it was I was doing.

Gerald

 

[watheyak]

Unfortunately it came out of the mold at about the 24 hour mark. The next version will spend a week in the mold.

All great info, Gerald. Thanks for taking the time.

Here's a couple pics of current version. Hoping V4.5 is the final.

 

[G_T]

Generally 24 hours is sufficient for most epoxy systems if temperature is elevated at least somewhat - say 110F. I found in practice that with temp a bit higher than that, at 12 hours there would be some small warpage. But I don't know for the system you are using. Longer is generally safer if you have the time! Given that you are using a high temperature system, I expect it may not even reach enough of a full cure at room temp. Note if you don't do the post-cure heat treating cycle, the high temp epoxy may no longer be high temp epoxy. The glass transition temperature will be low compared to specs. Potentially quite low compared to specs.

If you have a part a day old, and one a week old, and squeeze each, you can readily feel the difference between the cure states. Ditto if you have one part cured at room temp, and one at a somewhat elevated temp. Ditto for post-cure heat treat cycle.

Note - for anyone reading along, if using West Systems boat epoxy, the above does not apply. The Tg (glass transition temperature, when the polycrystalline solid epoxy undergoes a phase transition to amorphous solid epoxy with considerable loss of structural properties) is always low with that system and there is nothing you can do about it. It is not suitable for high performance airframes, or any structure that may have to take heat and stress at the same time.

Your nosecone wall thickess is not all that thick. If you can afford a small mass penalty up front which is a good place to have extra mass, perhaps add some more layers of fabric? That will make a stiffer stronger part that will be a little more resistant to warping.

I suppose in worst case you could try something crazy like pull the part at the end of a day, trim off flashing, and put it back in the mold turned 90 degrees and leave it there for a while.

Anyway, looks pretty good from here! Good luck with your project!

Gerald

 

[watheyak]

A quick update-

The most current nosecone flew successfully at our clubs November launch on a Loki L-1040 to just over 28k' and Mach 3. The nosecone survived the flight in great shape. No scorching, bubbling or charring. It did not survive when the rocket separated as per plan at apogee, but the parachute stayed stuck in the nosecone. Much was learned.

A new version is in the works, incorporating suggestions from Gerald as well as other modifications I've come up with.

More soon!

 

[G_T]

Good news and bad news

Good luck on the next flight!

Gerald