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tjgray693599

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I’m building an NCR Big Brute and I am using epoxy for the first time. I measured (what I think were equal amounts) of the 15 min epoxy last night and I am seeing it cracking and falling apart on the motor tube/centering rings.
Can someone lend their experience and tell me what I did wrong? I’m afraid to continue using epoxy at this point.
 

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heada

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Looks to me like too much hardener or it cured too fast. Did you measure by volume or by weight? (and is that what the directions said to measure by?) Is the epoxy old? What's the temp where it was set to cure? I've seen older 5 min epoxy cure yellow and brittle like that. Slowing down the cure rate by cooling it down might help. Also adding milled fiber might help. Epoxy should not be brittle and crack like that.
 

Lugnut56

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Invest in a digital scale to weigh out the different parts of epoxy. It may not eliminate what you have going on, but it will narrow down the possible causes.
(I think I paid around $10 for this one from Amazon a couple years ago).

scale.jpg
 

Spitfire222

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Don't be afraid of using epoxy! It's a useful adhesive/tool to have in your building arsenal. Now, you may want to practice a bit more using it on scraps of balsa or cardboard to get the hang of it before using it on rocket parts again. Not all epoxies are the same, so read the directions carefully and more importantly, follow them, especially regarding mix ratios and temperature. If you post photos of the epoxy you're using along with the directions, it could help the troubleshooting process.
 

prfesser

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To get equal amounts by volume, turn the bottles upside down and squeeze either one as needed so that the resin/hardener are both right at the tip. Then squeeze both equally onto your mixing surface as you pull the bottles toward you. Objective is to get two beads of liquid that are the same length/width, as near as possible.

Assuming your mixing surface is flat (piece of scrap plastic), use both a stirring motion and a scraping motion. Scrape the stuff to the center with the side of your mixing stick, folding it over top of what's already there. Rotate the mixing surface a quarter turn and repeat (several times). A full minute or two of mixing is necessary for best results.
 

Woody's Workshop

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I had the thing happen to me, not on rockets but in my wood working.
I traced it down to a "Bad Batch" from the manufacturer. Whom it that might have been.
I had originally bought the stuff from an outlet store.
Probably was put out as defective by the manufacturer to recover cost.
It ended up on shelves to the public. They were simply 2 tubes marked A & B.
No instructions on the back, no packaging, just taped together.
Since you haven't replied which brand you were using, this may not be the case.
You might try again with some epoxy from rocket a supplier.
It might cost a little more than big box store's stuff, but it's top a quality product.
One bad experience shouldn't deter you from using epoxy because your result was out of the norm.
If you followed the instructions you did nothing wrong, and it's most likely the fault of the product.
 

tjgray693599

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Well, now my first mistake may have been squeezing it into a small cup and mixing it in there. The bottles are labeled TAYTOOLS, but the back of the bottles says it is distributed by Bob Smith Industries, which I have seen on the forums before

This was 15 minute epoxy, so I am going to try the BSI 30 minute bottles next time and weigh the qty and mix and scrape on a flat surface.

Thanks for the advise.
 

neil_w

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Well, now my first mistake may have been squeezing it into a small cup and mixing it in there. The bottles are labeled TAYTOOLS, but the back of the bottles says it is distributed by Bob Smith Industries, which I have seen on the forums before

This was 15 minute epoxy, so I am going to try the BSI 30 minute bottles next time and weigh the qty and mix and scrape on a flat surface.
For what it's worth: I use BSI 15 minute and 30 minute epoxies, mix them in small plastic cups, measure them out by eyeball, and have never once the problem you're seeing.
 

Edmiller

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I use Bob Smith epoxy quite a bit. Never had any problems.

I mix mine on a pad of large Post-it-Notes. Squeeze out identical lines of A and B. Then mix with a round tooth pick. or small dowel rod. When finished, I peel off the top paper and throw it away. Then I have a clean sheet to mix some more.

One thing to note- if you squeeze out the part A and then the part B, the part A will spread out and look larger than it really is before the part B is squeezed out. Then you will have a tendancy to squeeze out too much part B.
 

David Schwantz

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What was the little cup that you used? If it was a small bathroom cup, like dixie, it must be the ones that have no wax surface. Looks to me like that may have been what you used. I used the Dixie bathroom cups for all my mixing, glue, paint and just about anything. But they MUST be non- waxed.
 

BF Rockets

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The black centering rings look like they are plastic. Epoxy doesn't bound well to plastic.
 

rcktnut

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I use these, measure, mix, throw away. https://www.amazon.com/Epoxy-Resin-...TQCH49011XB&psc=1&refRID=K5WNM9SGZTQCH49011XB

I built mine about 17 years ago still going strong, read the directions and follow them and your rocket will turn out just fine. Rough sand the inside and outside of the rings and use CA instead of epoxy in the places where the instructions say. Gobs of epoxy in the wrong places will screw up the fit of the fins.
 
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prfesser

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tjgray693599

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I had success with the 30 minute epoxy. I still need to get a scale, but I’m happy with the results. The small cup is what I used before.
 

catman001

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Like others have stated, Bob Smith epoxy is used by many with good results. Without knowing what happened, it almost looks like the parts might have moved while curing. The BSI (Bob Smith Industries) website states when using the 15 minute epoxy, wait 45 minutes to handle and a full cure in 2-3 hours. Before tossing the epoxy, I would try mixing some up being careful to get equal amounts and mixing well and try on some scrap material.
 

H_Rocket

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To get equal amounts by volume, turn the bottles upside down and squeeze either one as needed so that the resin/hardener are both right at the tip. Then squeeze both equally onto your mixing surface as you pull the bottles toward you. Objective is to get two beads of liquid that are the same length/width, as near as possible.

Assuming your mixing surface is flat (piece of scrap plastic), use both a stirring motion and a scraping motion. Scrape the stuff to the center with the side of your mixing stick, folding it over top of what's already there. Rotate the mixing surface a quarter turn and repeat (several times). A full minute or two of mixing is necessary for best results.
What Terry said.

As there is no instruction for hobby-poxy, I'm not sure if you can measure it by weight (they don't specify on the bottles I have). I suppose if you were weigh equal volumes, you could determine the ratio and go from there.
 

beeblebrox

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Well, now my first mistake may have been squeezing it into a small cup and mixing it in there. The bottles are labeled TAYTOOLS, but the back of the bottles says it is distributed by Bob Smith Industries, which I have seen on the forums before

This was 15 minute epoxy, so I am going to try the BSI 30 minute bottles next time and weigh the qty and mix and scrape on a flat surface.

Thanks for the advise.
I have been using either the Bob Smith 30 minute or previously the Devcon 30 minute for decades. As for mixing remember this: The resin is slightly more viscous than the hardener. Use this to your advantage. I use a piece of paper to mix the stuff on. I squeeze out a puddle of the resin about the diameter of a nickel or quarter. Then with the hardener, I squeeze it out drawing a circle the same diameter as the other puddle, then fill it in. (The resin flows out slower, so the two resulting puddles will be almost exactly the same diameter and height, therefore the same volume.) Using this method yields consistent results, you can tell when you get to the bottom of the bottle and you have the same amount of each left. The 30 minute seems to be less susceptible to minor errors in mix ratio. The 15 minute is crap - period. The 5 minute is not good either and you have to be fast to mix it and apply it. Not worth the hassle as it is not that strong. Remember, the longer the cure the more time for bonding and for porous materials, soaking in as well. Use a 3/16" or 1/8" dowel for mixing. when thoroughly mixed, I fold the paper in half and squeeze the epoxy out of the paper slowly onto where I need it. Just remember to make sure you don't use too much, and that it will keep flowing for at least 20 minutes. Tack fins for example with CA, then fillet them with epoxy in horizontal position.
 
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PromptSupercritical

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Most epoxy are mixed based on volume not weight. Small batches can cure differently than large ones. What I used to do with the small squeeze bottle type was make a mark on the bottle at the level of the fluid at the meniscus (low curved point of the fluid) and then pour out the same amount of resin and hardener. Older bottles had graduations you could use.

Another way to get small batches of equal volume is to get a set of measuring spoons (don't steal your mom's, wife's, or significant other's). This way you can pour anywhere from 5 to 30mL (or more) of resin and hardener at a time into your small cup - clean up with 70-90% isopropyl alcohol.

A rule to live by is: If you have mixed the epoxy-hardener for 3-5 minutes, mix it for 3-5 more!

Stay away from 5 and 15 minute epoxy until you get the process down - they really don't give you much time to work (pot life).
I also went away from hobby store brands and use West, System 3, and/or Areomarine. You spend more but they perform consistently.

By the way, were you trying to bond plastic and paper? I think someone said epoxy joining not the best choice.

Always remember large batches mixed in small spaces = high heat.

Good luck!
 

rklapp

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I use these, measure, mix, throw away. https://www.amazon.com/Epoxy-Resin-...TQCH49011XB&psc=1&refRID=K5WNM9SGZTQCH49011XB

I built mine about 17 years ago still going strong, read the directions and follow them and your rocket will turn out just fine. Rough sand the inside and outside of the rings and use CA instead of epoxy in the places where the instructions say. Gobs of epoxy in the wrong places will screw up the fit of the fins.
I mix small amounts on a post it note but I like the cups for larger amounts.
 

tomsteve

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scuff up that plastic with some 60 grit to give the epoxy something to bight onto
 
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