What went wrong?

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Beautiful day, 72 degrees. 8 pumps of resin, hardener ran out after 4 or 5 pumps. Got out a new jug and finished the job, though now not exactly sure of the mix ratio. 10 minutes into the job it gets real hot in my hand and actually starts smoking. You can see the result. So was it a bad mix or did the plastic cup react with the epoxy?
 

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What resin are you using ? and what ratio.
I never mix more than a 9 oz batch.

When ever you mix resin, it will kick sooner, if you leave it in the cup. 10 minutes, is too long.

Best to pour it on the hull, or a paint tray, to spread it out. Both will slow the reaction.

In my experience, old epoxy generally has a shorter pot life.

It's not the cup.
 
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Once you add the hardener, the resin and hardener combine and produce heat as part of the epoxy formation process.

The larger the volume, the larger the amount of heat generated and the longer the path to radiate that heat. As the temp of the solution/cup goes up so does the reaction rate ... you see the results in that pic.

That is why you will generally find the instructions to spread the epoxy out quickly after it is mixed, this increases the surface area and allows the heat to escape, keeping the reaction rate manageable and the working time to what the manufacturer states.

If you had that mixed in that cup for 10 minutes, that is exactly what I would expect to happen with most epoxies ... so there really isn't an issue, just that your process of leaving it in the cup allowed the heat to build up too much. Even if you are dabbing it or can't pour it thinner on the project, get it into a shallow pan so that it is spread out, if you create a situation where the heat has to travel through a lot of epoxy to escape the mix, you will get this every time.

Brian
 
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Yes. As others have said, you had your epoxy in a cup with a fairly narrow bottom so all of the heat from the exothermic reaction radiated up from the epoxy at the bottom into the epoxy above. Letting the epoxy sit while you went to get more hardener allowed the epoxy you had mixed to "cook off".

I mix as small batches of epoxy as is practical, often only one mini-pump worth of resin and one of hardener and use a wider, shallow mixing cup.
 
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I dunno, can you count to four (or three) hardener pumps after taking a short break? I think you would have to make a very “hot” mix, with more than just an additional pump of hardener, especially when starting with 8 pumps of resin.

It may have been the exothermic reaction wait time, especially if the hardener to resin mix was rich.

In the old poly-resin days, with far less pot life, I’m sure folks enjoyed a steaming, Ow-too-hot-to-hold, soon-to-be hockey puck of poly resin, even when it didn’t eat through the mixing pot.

If it wasn’t way too much hardener/too long a wait I’m gonna lay melted pot blame on the cheap Dollar Store knock-off Solo cup. Do you and Doug shop at the same Dollar Store?

Although. . . . . you say “pumps”. Have you cleaned those pumps, and checked the (5/1) calibration anytime recently? At any time?

To clean the pumps West recommends that the resin pump be flushed with a solvent, they suggested alcohol. To clean the hardener pump flush with hot water (the hardener is water soluable), then flush with alcohol.

Good opportunity to check the pumps calibration while you are at it.

Unless mixing a really rare big batch of epoxy I only use two sizes, and two varieties, of mixing pots. For “larger” batches, Chinet Crystal 9oz cups. “Larger batches” usually meaning 4 – 5 pumps each, especially is I am mixing in flyaway thickeners.

https://www.walmart.com/ip/Chinet-C...uS7UT2Rbwxi-guNwi6RoCUcEQAvD_BwE&gclsrc=aw.ds

Yes, we set out the finest crystal in my shop. Those Chinets are transparent, which is nice for mixing different epoxies and additives, not too static-clingy with silica or graphite powder, reusable at least with the same mix over a few days. Don’t squeeze to hard, they become brittle.

And, for wee amounts of one of the G/flexes or etc, 30cc/1oz graduated medicine cups.

https://www.amazon.com/Disposable-M...pY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU&th=1

200 each for $9 quickly pays for itself in less wasted resin, especially pricey G/flexes. Trying to mix small amounts in larger pots I have a habit of making too much epoxy, especially 50/50 G/flex.

I don’t even use the graduations; it is easy to eyeball 50/50 G/flex 650 mixes nozzle dribbled into the cup. With thickened G/flex 655, in the toothpaste tubes with a toothpaste consistency, is even easier to swipe off a pea sized Part A dab on the edge of the cup, where it clings, as you squeeze out an equal pea size dab of Part B alongside and mix.
 
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That happens pretty much every time I use epoxy. It is the nature of the beast. Plastic cups are not the way to go, unless you use the batch very quickly.
 
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there is nothing inherently wrong using plastic cups ... any epoxy left sitting in a mixing cup will kick faster than if it is spread out. The bigger the batch, the faster it heats and kicks.

Fancy container or cheap container, the epoxy is going to kickoff .... the more epoxy you have, the more heat, the faster it will kick ..... unless you get it spread thinner so the heat can dissipate
 
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I don't use plastic cups, I use those little plastic condiment (or gravy) tubs for small batches and for larger batches, ones that takeout mashed potatoes come in. they're shallow enough to brush right out of the container and slow down the kick, are made to handle some heat, and if it does kick, I just let it harden because they're tapered enough to just pop out the puck, then give a quick wipedown with the appropriate solvent (the black ones even handle wiping down with acetone)
 
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I have never ever ordered takeout mashed potatoes. Am I missing something? Takeout anything is cold as the nearest takeout is 35 min away.
 
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I have never ever ordered takeout mashed potatoes. Am I missing something? Takeout anything is cold as the nearest takeout is 35 min away.
Neither do I, but I picked up 50 of them for $12 at the local "super" grocery store, and the condiment ones cost me $10 for 100 at a craft store
 
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