Beginner's epoxy question...

[dr wogz]

the longer the cure time, the more working time you have and the longer the glue has time to work its way into the pores & voids to make it do what it does..

 

[BMcD]

I agree with Jim Hinton. I've had five minute leather up on me while I was futzing about trying to get everything together. OTOH, now that I have watched some of John Coker's videos, I am much better about getting all set up to actually USE epoxy before I MIX epoxy. Incidentally, John Coker, if you read this thread, thank you for making those videos. I learned a lot.

 

[Nytrunner]

Yeah, if you want to go the epoxy route, use a longer cure. Saturday I just had the forward centering ring on my L1 rocket pull out upon ejection. It was paper/ply built with 5min 4 years ago, and has had 7 flights on H-Js.

The booster spun in horizontally and bounced on landing, so repair is in its future!

 

[Devin Batten]

When building a Loc IV I went with the recommended 15min epoxy. It went up and landed on a concrete pad nearby. Popped joints on 2/3 fins. Rebuilt with Aeropoxy and reinforced all other joints. Was golden even under an H999 and multiple concrete landings. I exclusively use Aeropoxy for the critical components, motormount and fins.

 

[afadeev]

Another option would be "Fix It" epoxy clay. It's easy to use and not as messy. It allows you a 3 hours windows and you can do all the fillets at once. It doesn't stink, put off bad fumes or make a mess. You should check it out. It's as easy as playing with play doh.

The fixit clay (sold in sticks, you just cut off a chunk of what you need, and knead it to mix) as well as the epoxy putty/paste (like PC-7) are great for fillets or shaping pieces

I will add my contrasting experience with Epoxy clay here - using "fix it" clay is the absolutely WORST piece of advice I've received from Apogee, and later these forums.
Ever.
I had tried it once, and threw it away (actually, I offered it for free to anyone who wanted it, and there were no takers):

• It was a PITA to work with: massaging long fillet snakes is neither easy, nor fast, nor fun.
  • Perfect shaping is nearly impossible - surface gets dislodged if you pull a tool along the top, and without a tool, the surface is always uneven.
  • Sanding Clay is harder and takes way longer than sanding most epoxies.
• Clay did not bind well to the airframe (as should have been expected, since it doesn't seep into pores), and cracked upon first hard impact. I pried it all out, with surprising ease. It came out in solid long chunks with minor screw-driver prying. I replaced the filets with proper epoxy and never looked back.
• Clay is STUPID heavy, and all that extra weight goes into the worst (aft) part of the rocket, negatively impacting stability.

Madcow's Super DX3 instructions refer to "5 minute epoxy" in the "you'll need these items to complete this kit" section. It sounds like some people have gone this route, while others have gone the 30 minute route and still others have used simple wood glue to make this HPR kit.

All viable approaches, with a couple of practical pro's and con's to each one:

• Working with 5 minute BSI epoxy requires maximum speed and dexterity, since it starts gelling fast, and never exactly at 5 minute mark. Basically, 5 minutes working window is too short, unless you are attaching small pieces together, fast.
• 15-30 minute epoxies give you way more working time to weight in resin + hardner precisely, mix them properly (another problem with 5 min - you rush things, and mixes don't come out right), and apply them without rushing.
• Wood glue (TiteBond II being the best for strength and water-resistance) is great for structural attachments. It is also light, and flexible. However, it (TiteBond in particular) has two practical limitations:
  1. It "grabs" and freezes tightly fitting parts in place unpredictably, and permanently. Therefore, I avoid using it when gluing couplers, or sliding motor mount inside the airframe (epoxy those).
  2. It shrinks when it dries, thus making it a poor choice for cosmetic fin filets. Nothing wrong with a skinny filet, mind you, but if you want wider and thicker fillets, you are better off using epoxy.

Have fun with your build!

a

 

[ThreeJsDad]

I will add my contrasting experience with Epoxy clay here - using "fix it" clay is the absolutely WORST piece of advise I've received from Apogee, and later these forums.
Ever.
I had tried it once, and threw it away (actually, I offered it for free to anyone who wanted it, and there were no takers):

• It was a PITA to work with: massaging long fillet snakes is neither easy, nor fast, nor fun.
  • Perfect shaping is nearly impossible - surface gets dislodged if you pull a tool along the top, and without a tool, the surface is always uneven.
  • Sanding Clay is harder and takes way longer than sanding most epoxies.
• Clay did not bind well to the airframe (as should have been expected, since it doesn't seep into pores), and cracked upon first hard impact. I pried it all out, with surprising ease. It came out in solid long chunks with minor screw-driver prying. I replaced the filets with proper epoxy and never looked back.
• Clay is STUPID heavy, and all that extra weight goes into the worst (aft) part of the rocket, negatively impacting stability.

All viable approaches, with a couple of practical pro's and con's to each one:

• Working with 5 minute BSI epoxy requires maximum speed and dexterity, since it starts gelling fast, and never exactly at 5 minute mark. Basically, 5 minutes working window is too short, unless you are attaching small pieces together, fast.
• 15-30 minute epoxies give you way more working time to weight in resin + hardner precisely, mix them properly (another problem with 5 min - you rush things, and mixes don't come out right), and apply them without rushing.
• Wood glue (TiteBond II being the best for strength and water-resistance) is great for structural attachments. It is also light, and flexible. However, it (TiteBond in particular) has two practical limitations:
  1. It "grabs" and freezes tightly fitting parts in place unpredictably, and permanently. Therefore, I avoid using it when gluing couplers, or sliding motor mount inside the airframe (epoxy those).
  2. It shrinks when it dries, thus making it a poor chose for cosmetic fin filets. Nothing wrong with a skinny filet, mind you, but if you want wider and thicker fillets, you are better off using epoxy.

Have fun with your build!

a

Longer cure times also yield more strength due to more cross linking. Ever notice the heat ratings for JB Weld and JB Quick, there is a significant difference.

 

[Speaknoevil]

Longer cure times also yield more strength due to more cross linking. Ever notice the heat ratings for JB Weld and JB Quick, there is a significant difference.

I didn't think there was any correlation between strength and heat ratings. For instance, although JB weld has a HDT of 500 degrees, its tensile strength is ~5000psi. Compare that to System Three T-88 structural epoxy which has a relatively low HDT of 119 degrees, but a tensile strength of 7000psi.

 

[ThreeJsDad]

I didn't think there was any correlation between strength and heat ratings. For instance, although JB weld has a HDT of 500 degrees, its tensile strength is ~5000psi. Compare that to System Three T-88 structural epoxy which has a relatively low HDT of 119 degrees, but a tensile strength of 7000psi.

I agree 100%, I was pointing out that cure times are a factor in physical properties of epoxies. Longer cure times will normally improve the mechanical values of an epoxy. I was comparing the two JB products to each other and how the longer cure time improves it's properties.