Epoxy Question and a Technique to Share

Discussion in 'Techniques' started by deangelo54, Jan 14, 2020.

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  1. Jan 14, 2020 #1

    deangelo54

    deangelo54

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    In the morning I mixed up some 30 minute zpoxy and installed the motor mount in my Loc Lil Ditter.
    An hour later, the epoxy that was left in the mixing cup was hard and the motor mount was securely installed.

    Later in the day I mixed up more epoxy for a fillet for the top centering ring inside the body tube. Twelve hours later, the epoxy never hardened. The next day I mixed up a test batch and it hardened after an hour and a half.

    I was able to remove the uncured epoxy using dowels, sticks, rags, paper towels and alcohol.

    What could I have done wrong when I mixed the epoxy for the CR fillet? I have been using epoxy for mid and HP rockets for 15 years.

    Technique:
    When using a dowel or stick to apply epoxy down a body tube, I roll up some paper from my printer to make a tube that I insert into the body tube. This prevents me from dripping epoxy inside the tube. Plus the white paper reflects light which makes it easier to see what you are doing down the body tube.
     
  2. Jan 14, 2020 #2

    neil_w

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    I can’t believe I never thought of that. Good tip!
     
  3. Jan 14, 2020 #3

    deangelo54

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    To be honest with you I never thought of it until yesterday. I hope you and others find that technique helpful.
     
  4. Jan 14, 2020 #4

    bmonagle

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    The only thing I can come up with is wrong proportions or incomplete mixing. Making sure it is thoroughly mixed is easy.
    IIRC, Z-Poxy is a 50-50 mix, and there are some differences in viscosity. If you squeeze out the same diameter puddle of each on a flat surface, the more viscous puddle will be taller, throwing off the ratio.
    I always measure out using a digital gram scale with a tare function. For $10 - 15 they are hard to beat and really help you make consistent mixes.
     
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  5. Jan 14, 2020 #5

    deangelo54

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    Sounds like you determine the portion of each part according to weight instead of volume. I have a small scale that I use to weigh Pinewood Derby cars and rockets.
     
  6. Jan 14, 2020 #6

    deangelo54

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    I found this video on YouTube.
    Looks like he wants to go by volume but compensates for the difference in the weight of the two parts.

    Given that when you buy epoxy each bottle has the same amount of liquid, I would think that one should mix according to volume instead of weight.
     
  7. Jan 14, 2020 #7

    blackjack2564

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    I always count to 60 when mixing. Most trouble is due to incomplete mix.
     
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  8. Jan 14, 2020 #8

    bmonagle

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    Yes, the densities are similar enough that weighing the portions gives a lot of consistency. I just weighed new bottles of a similar product and they were within 2 % of each other. Good enough for me. Better than trying to eyeball volume.
     
  9. Jan 14, 2020 #9

    deangelo54

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    That makes sense. Eye balling can be difficult. I think that is what the person in the above video is doing. He also adjusts for the difference in weight of the two parts. I'll run a few tests and see how it works out and report the results.

    To reply about mixing, from the videos I saw online it looks like 2 to 3 minutes is recommended. Thanks to all that responded.
     
  10. Jan 14, 2020 #10

    Steve Shannon

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    Is it possible you took both portions from the same container? Resin + resin, or hardener + hardener?

    Given the successes you had every other time that’s the only thing that makes sense to me.
    I like it when my hardener and resin are obviously different colors. It makes it easy to see that it’s mixed well and that I have approximately the right ratio.
     
  11. Jan 15, 2020 #11

    deangelo54

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    Although that is a good theory, I distinctly remember the epoxy changing color as I mixed it. The proper color change during mixing is a very good indicator that all is well.

    After reading Bmonagle's post I did a test with 30 minute epoxy. In a 1 oz cup I measured 2.5 ML of resin that weighed .003 kg and 2.5 ML of hardener that weighed .004 kg. I mixed it well for 3 minutes. After 35 minutes it was as hard as a rock.
     
  12. Jan 15, 2020 #12

    deangelo54

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    Your posts led me in a good direction concerning weight and volume. When I did the test I mentioned in my post on 1-14-20 at 9:17 PM both weight and volume came into play. I just ordered 100 one ounce measuring cups similar to the one I used for the test. The cups only cost $1.29 but shipping was $7.99. It's worth 93 cents a cup to get it right. Thanks for your inputs.
     
  13. Jan 16, 2020 #13

    jqavins

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    Well, there's your problem! That printer paper shield seems like a good idea, but you got printer cooties in the epoxy, which poisoned the hardening reaction.

    Seriously though, measuring by mass is always better than by volume whenever practical. From the kitchen to the rocket bench to the foundry, mass rules.
     
  14. Jan 17, 2020 #14

    deangelo54

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    From reading other posts and some YouTube videos there are those that agree to go by mass not volume. Then there are those that say go by volume. When I did my test (see my Tuesday 9:17 pm post.) I went by volume at the epoxy was hard as a rock in 35 minutes.

    What is the rationale for using mass as a guideline instead of volume?
     
  15. Jan 17, 2020 #15

    jqavins

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    I didn't mean to say that volume is not OK. In many cases it's very nearly as good, so nearly as not to matter. But mass is always good if you can do it, where volume is only usually good.
    • With some items (not so much with epoxy) density can vary. Mass by definition is the fundamental measure of "how much stuff" and does not depend on density.
    • Some things, (like very thick, viscous liquids, irregular solids, etc.) are difficult to measure accurately by volume, but their mass is their mass no matter their shape.
    Of course, when the directions are given using volume, and you're able to measure the volume with sufficient accuracy and precision, then use volume it'll be OK. When the directions give mass, then use mass and be glad.
     
  16. Jan 17, 2020 #16

    prfesser

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    Going by mass is the accurate way...but you need the actual densities of the resin and hardener, or the mass ratio of the two. The SDS usually gives the density.

    Some hardener I got from FiberGlast gave both the volume ratio and the mass ratio. Different densities for the two components, so the two ratios were significantly different.
     
  17. Jan 17, 2020 #17

    deangelo54

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    The good new is that I gathered some decent data from my test. I have volume and weight documented. If I repeat the conditions I should be good to go. The measuring cups I ordered came in today so the build will go back into production mode this weekend.

    For you scientist out there, here is a great video on Mass, Density and Volume. As you will see, all three are related.

    As a matter of fact, we are rocket scientists. LOL. Thanks for the inputs. Wish me luck with the build.
     
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  18. Jan 27, 2020 at 3:08 PM #18

    deangelo54

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    I think I am back on track with using epoxy. I just got some Bob Smith Industries 30 epoxy.
    I used a measuring cup so that I make sure I use equal amounts of resin and hardener.
    I found that the longer I mix it the shorter the working time. I started doing the filets on the Lil Diter. I mixed the epoxy in the measuring cup for 45 seconds. That seems to work fine.

    I know we talked about volume versus weight. The directions on the BSI bottle specifically state to go by volume. As bmonagle stated, the densities are close enough that you can also go by weight.

    Thanks everyone for your inputs. They were all very helpful.
     
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