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May 22, 2019, 07:36 AM
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Vented bladder in vacuum bag


I found a post on RC groups a few days ago that showed a fuselage being molded. Instead of using a bladder and positive pressure, the poster used a bladded inside the mold and vac bagged the lot, with a vent going to the bladder (for the life of me I cannot find the post/pic again and it's frustrating the heck out of me). can someone explain how this works and if it's a good approach.
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May 22, 2019, 08:08 AM
Themadartist
It basically works the same way any air leak happens. You pull vacuum and anywhere there's a hole, air will be drawn in. The only difference here is that the air is being drawn into a vessel that will contain it, rather than it being able to reach the 'frog' and be drawn back out again. Hope that helps.
May 22, 2019, 08:51 AM
Barney Fife, Vigilante
tom43004's Avatar
I have done this in the past. It's a good use-case especially if your molds aren't super strong and you're worrying about the safety factor of putting XX psi in them.

You vent the bladder to the atmosphere, then put the whole mold in a vacuum bag (keeping bladder vented to outside) and pull vacuum. It works very well, but your max pressure on the bladder is around 14psi obviously.... which is more than enough for most of our applications.
May 22, 2019, 05:44 PM
Registered User
Thread OP
Thanks for both replies, they explain it well. Since my 15yo son is using dental plaster for a mold and my vacuum setup is working well, it sounds like the ideal solution for a two part fus mould. It means I don't have to devise a pressure system for the bladder.
May 23, 2019, 07:49 PM
It's time for me to fly
JimZinVT's Avatar
I've done a couple small DLG fuses like this. I like it, it works well. There are some posts and pictures scattered through this thread. Starting at post 102, pictures in #173. I think it was discussed in the Vindicator thread in the slope forum too.
If you're using plaster molds this is the way to go. I broke an Ultracal plaster mold with less than 5psi with a pressurized bladder. But with the vac. bag & vented bladder the pressure is equal inside and out so no worries.
May 23, 2019, 10:45 PM
Registered User
Thread OP
Thanks Jim, I had not seen that thread. It seems to answer all of my questions. I was planning to use hydrostone for the molds, but that is not available ATM and dental plaster is the only other option. Since we only want to pull 2/3 examples of the design, it seems a useful material for the cost, but as you not it is not the most rugged choice
May 23, 2019, 10:58 PM
It's time for me to fly
JimZinVT's Avatar
If there is a ceramics supply place nearby you can probably get Ultracal 30. If you do a search in the Composite Fabrication forum you'll find a recipe for a homebrewed version of Ultracal. If I recall, it's a combination of plaster of Paris and Portland cement.
May 24, 2019, 07:22 AM
Barney Fife, Vigilante
tom43004's Avatar
I made it once. Wasn't hard at all.
May 24, 2019, 06:30 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by tom43004
........................................... around 14psi obviously.... which is more than enough for most of our applications.
Yes.
I am using less and less pressure when inflating a bladder for fuselage moulding. The last one I did successfully using a hand operated balloon pump. But I should add, I don't build DLG or thermal composite fuses, just (heavy) slope soarers.

Quote:
Originally Posted by JimZinVT
..........................it's a combination of plaster of Paris and Portland cement.
I've mixed some and used it. However, I have never solved the problem of edges breaking away at the zero draft angle part of the fuse/mould.
As a result, I've given up on so called "Mud Moulding".
Perhaps if more than one parting line was included?...............but then, that gets complicated.

Jim.
May 25, 2019, 11:03 AM
supreme being of leisure
ZAGNUT's Avatar
here's an example of resin infusing a kayak in one piece....same basic idea: vacuum "bladder bag" in a closed mold.

Resin Infusion Demo - Carbon Fiber Kayak (7 min 46 sec)


looks like 50kg of consumables for a 10kg kayak...
May 25, 2019, 03:24 PM
It's time for me to fly
JimZinVT's Avatar
Quote:
Originally Posted by tom43004
I made it once. Wasn't hard at all.
Tom do you mean "it wasn't difficult" or "the finished mold was too soft"?
Easy to interpret that 2 ways

RaceMag - there is another thread in this forum by Adam (WyomingWingWorks, or is it ...WindWorks?) about making bladders. Pretty easy to make a custom bladder using PE film and a soldering iron. I don't know if a balloon will work at this pressure or not? I've never tried one.

EDIT: it's "WyoWindWorks"
Last edited by JimZinVT; May 25, 2019 at 05:02 PM.
May 25, 2019, 05:31 PM
It's time for me to fly
JimZinVT's Avatar
This thread has a great video by Adam. He's doing pressurized bladder molding, but the bladder fabrication and many of the layup tips are useful for the vacuum bag/vented bladder method as well.

For the bladder fabrication, I don't use the rubber pad underneath or the parchment paper on top. I didn't have either on hand when I decided to try it, so just used 6 or 8 sheets of newspaper underneath and 2 sheets on top. It works fine for me.
Here is the first thread where bladder construction was discussed and refined.
May 25, 2019, 05:42 PM
Entropy is happening!
Jim.Thompson's Avatar

Glad brand "Go Between" film.


Quote:
Originally Posted by JimZinVT
............................
For the bladder fabrication, I don't use the rubber pad underneath or the parchment paper on top. I didn't have either on hand when I decided to try it, so just used 6 or 8 sheets of newspaper underneath and 2 sheets on top. It works fine for me. ....................
I have used newspaper underneath, but never tried it on top. Good idea Jim, I am glad you posted that tip for us. I will be using it next time I make a bladder.

Quote:
Here is the first thread where bladder construction was discussed and refined.
Excellent thread.

GO BETWEEN film: This is my favourite bladder making film. No folding creases to deal with, readily available and cheap:

https://www.glad.com.au/glad-product...ad-go-between/

Jim.
May 25, 2019, 06:01 PM
It's time for me to fly
JimZinVT's Avatar
Jim I saw that "Go-between" film mentioned in another thread, but when I searched for a source, it seems to be an Australia-only product. Maybe it is sold under a different trade name in other parts of the world.
What is it that you like about it? Does it have some stretch to it? I have been using a stretch film we use at work for wrapping pallets. It's very thin, <1 mil (sorry Jim! <0.025mm ) but very strong and stretchy. The only drawback is that it sticks to itself very well, so it can be maddening to handle.
May 25, 2019, 06:23 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by JimZinVT
Jim I saw that "Go-between" film ................
What is it that you like about it? Does it have some stretch to it?........................
It does not cling to itself and has no creases is very easy to handle. The drop sheet plastic that I used formerly came supplied folded. The fold creases were difficult to remove and would often result in a leak after welding/joining. This stuff has non of that - it welds easily and at relatively low temperature.
It has some stretch, so far, I have found it to be enough.
I use it for vac bagging too.

Quote:
.............., <1 mil (sorry Jim! <0.025mm ) but very strong and stretchy. ..........................
No need to apologise I have now caught up with the term "mil". It only took me 10 years!....................
My education continues.
I also have some of that wrapping film. It has it uses, but as for making a bladder out of it, I would never have considered.

I have just sent off an email message advising GLAD of our current discovery of another non culinary use for their product. I also ask about international availability.
I'll advise further when I receive a reply.

Jim.


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