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  #1  
Old 05-03-2021, 08:14 PM
StressedOut StressedOut is offline
 
Join Date: Jun 2017
Location: Fullerton, CA
Posts: 161
Default Sealant mold project

While I wait for some parts to be delivered, I'm tinkering with a mold for some sealant to see if I can replicate cap seals like they use in building larger airplanes. This is also a good opportunity to sharpen up my Solidworks skills that have eroded since I took a class in 2017.

If you're not familiar with pre-manufactured cap seals, you fill the inside with some sealant and push them over the shop head. The sealant squeezes out the bottom to form a nice, clean seal around the shop head. It's less prone to leaks and much, much cleaner to use than smearing around a gob of sealant with a Popsicle stick.

I'm new to mold creation, so I thought I'd ping the community for your thoughts. I'm planning on using fuel tank sealant, like 3M AC-350. I can't seem to find AC-236 anymore except from Vans, but they don't have Class A. Class A is less viscous which makes it a better candidate for injecting into a mold, so I'll start with that and see how it goes.

I've already designed a prototype mold and my friend is going to fabricate it for me. (BTW, it really helps to have a friend who just happens to be a master machinist with his own milling machine.) It's a pretty basic design that will fit over the shop head of a #3 rivet.

Since sealant was originally designed as an adhesive, I want to make sure to use some mold release so I can pry apart the mold after cure without damaging the cured part. There are several types of mold release products out there, but I'm concerned about the release agent having some adverse effects on the sealant during cure.

What might be a good choice for mold release? Wax, silicone, crisco, or something else? It has to be something that won't react to a poly sulfide sealant.
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RV-14A Kit#140433
Completed: Vertical Stab/Horizontal Stab
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Pet peeve: "Lose" (rhymes with "booze") is the opposite of "find". "Loose" (rhymes with "juice") means "not tight".
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  #2  
Old 05-04-2021, 09:13 AM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,993
Default

1. You're looking for the wrong specification. Try AMS-S-8802.

2. Skygeek.com is a decent source for type A sealant as well as access hatch sealant, which permits easier removal. Van's has the best price on type B sealant.

3. The caps will prove to be awkward, I expect, especially compared to simply using a syringe and the type A sealant, which applies quickly and neatly around edges and rivets, as shown below.

4. There are several steps to sealing a riveted joint:



And



In the pictures, a "mil" is .001 inch. So 60 mils is .060.

5. Whatever mold release approach you choose, remember that it might contact the metal and impede subsequent sealing. Bit of a problem there. You need something that doesn't do that.

Dave
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  #3  
Old 05-04-2021, 09:24 AM
lr172 lr172 is offline
 
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I thought most single part sealants need either oxygen or moisture to cure. How would that happen in an air tight environment under a cap? I vaguely remember polysulphides requiring moisture to cure. Two part sealants typically cure through reactions with the included components and don’t eed O2 or moisture.
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Last edited by lr172 : 05-04-2021 at 09:29 AM.
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  #4  
Old 05-04-2021, 11:29 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by StressedOut View Post
Since sealant was originally designed as an adhesive, I want to make sure to use some mold release so I can pry apart the mold after cure without damaging the cured part.
Why not just make the mold from nylon, or a similar no-stick material?
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  #5  
Old 05-04-2021, 01:43 PM
StressedOut StressedOut is offline
 
Join Date: Jun 2017
Location: Fullerton, CA
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Quote:
Originally Posted by David Paule View Post
1. You're looking for the wrong specification. Try AMS-S-8802.

2. Skygeek.com is a decent source for type A sealant as well as access hatch sealant, which permits easier removal. Van's has the best price on type B sealant.
Thanks for the spec. Skygeek has AC-236 listed, but with a note saying it's not orderable. AC-350 was also listed as a fuel tank sealant. My background is not M&P, so I'm not fully up to speed on the various types of sealants and their appropriate uses so I'm trying to stay within the category of fuel tank sealants as opposed to fay surface sealants. I'll do a broader search using the spec number and see what I find.

We appear to have very similar backgrounds Dave, although I'm still working for that big Pacific NW airplane manufacturer for a few more years. I became aware of seal caps when I saw a presentation on a fuel tank leak investigation and the steps they took to develop a fix.

Quote:
Originally Posted by David Paule View Post
3. The caps will prove to be awkward, I expect, especially compared to simply using a syringe and the type A sealant, which applies quickly and neatly around edges and rivets, as shown below.
It's possible that the small size of the cap may turn out to be a problem when installing. I'm going to test it out before I commit to any larger scale mold designs. I'm not going to start on the wings until early next year, but I'm really looking forward to doing the fuel tanks. Yeah, I know - I'm weird.

My main reason for wanting to go down the seal cap path is that smearing around sealant with a Popsicle stick tends to get it everywhere. Bits of sealant will elongate and drop away when you draw the Popsicle stick back leaving a mess.

A seal cap has all of the required sealant inside of it and when installed, the sealant will ooze out from under the cap and there is nothing to smear around further. Done properly it uses no excess sealant, is more reliable than smearing around a gob and produces significantly cleaner results.

Quote:
Originally Posted by David Paule View Post
4. There are several steps to sealing a riveted joint:



And



In the pictures, a "mil" is .001 inch. So 60 mils is .060.
I've seen these illustrations before in the sealing specs I have access to. I've purchased a Semco gun and accessory kit (used) for a song ($80 IIRC) that has nozzles, and I've gotten some specialized nozzles that will do edge seam fillets like the ones in the illustrations to the correct dimensions with a single pass. These nozzles are very inexpensive, around $4, and they're reusable with proper cleaning. I found them on SkyGeek.

Quote:
Originally Posted by David Paule View Post
5. Whatever mold release approach you choose, remember that it might contact the metal and impede subsequent sealing. Bit of a problem there. You need something that doesn't do that.
Great point. I should thoroughly clean the caps after they're cured to insure this doesn't happen. My prototype mold is being fabricated by my friend as we speak.

The other option is to fabricate the mold from something that doesn't stick to sealant. I'll do some checking at work with my M&P contacts to see what's a good choice or do some experimenting on my own.

Thanks for the feedback Dave. I appreciate your suggestions.
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RV-14A Kit#140433
Completed: Vertical Stab/Horizontal Stab
Scrapped: Rudder
Working on: Empennage (Elevator, Rudder #2, Aft Fuselage)
Ordered: Wing Kit (Arrives March 2021)
Construction log - mykitlog.com/ajackson
Dues paid on March 2021
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Pet peeve: "Lose" (rhymes with "booze") is the opposite of "find". "Loose" (rhymes with "juice") means "not tight".
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  #6  
Old 05-04-2021, 01:44 PM
StressedOut StressedOut is offline
 
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Location: Fullerton, CA
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Quote:
Originally Posted by lr172 View Post
I thought most single part sealants need either oxygen or moisture to cure. How would that happen in an air tight environment under a cap? I vaguely remember polysulphides requiring moisture to cure. Two part sealants typically cure through reactions with the included components and donít eed O2 or moisture.
The sealants I'm considering are all of the two part variety.
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Art Jackson
RV-14A Kit#140433
Completed: Vertical Stab/Horizontal Stab
Scrapped: Rudder
Working on: Empennage (Elevator, Rudder #2, Aft Fuselage)
Ordered: Wing Kit (Arrives March 2021)
Construction log - mykitlog.com/ajackson
Dues paid on March 2021
Member of EAA Chapter 92 (KCNO)
Pet peeve: "Lose" (rhymes with "booze") is the opposite of "find". "Loose" (rhymes with "juice") means "not tight".
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  #7  
Old 05-04-2021, 01:45 PM
StressedOut StressedOut is offline
 
Join Date: Jun 2017
Location: Fullerton, CA
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Quote:
Originally Posted by DanH View Post
Why not just make the mold from nylon, or a similar no-stick material?
Excellent suggestion. I need to find a material that is machinable that won't stick to cured sealant.
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Art Jackson
RV-14A Kit#140433
Completed: Vertical Stab/Horizontal Stab
Scrapped: Rudder
Working on: Empennage (Elevator, Rudder #2, Aft Fuselage)
Ordered: Wing Kit (Arrives March 2021)
Construction log - mykitlog.com/ajackson
Dues paid on March 2021
Member of EAA Chapter 92 (KCNO)
Pet peeve: "Lose" (rhymes with "booze") is the opposite of "find". "Loose" (rhymes with "juice") means "not tight".
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  #8  
Old 05-04-2021, 11:02 PM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by StressedOut View Post

My main reason for wanting to go down the seal cap path is that smearing around sealant with a Popsicle stick tends to get it everywhere. Bits of sealant will elongate and drop away when you draw the Popsicle stick back leaving a mess.

A seal cap has all of the required sealant inside of it and when installed, the sealant will ooze out from under the cap and there is nothing to smear around further. Done properly it uses no excess sealant, is more reliable than smearing around a gob and produces significantly cleaner results.


.
Our recently published fuel tank build video has a simple tip for encapsulating shop heads of rivets that alleviates most of the typical mess and other problems that youíre talking about.
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