I’m interested in filling small plastic item with epoxy to secure a threaded rod in it. Epoxy seems like it would work well, as it would pour in easily and then cure to a rock-hard state. The catch is that it generates heat while curing, and I don’t want to damage my plastic item.
Thus the question: how hot can 5-minute epoxy get while curing, assuming one starts at room temperature? I can put my plastic item in a water/ice bath to keep it cool, but would rather avoid the hassle if it’s not necessary.
I’d estimate the void to be filled is maybe 0.75" in diameter and 1.5" tall. The plastic is maybe 3/16" thick, and may actually be skinned foam, which could be an issue for heat dissipation.
I think I found a pretty comprehensive reference here:
See graph #1 near the bottom of the page showing peak exotherm temps for 4-ounce cups of various epoxy blends. Looks like this stuff could get hot, as in several hundred degrees if no cooling measures are taken. I will add some steel bearings to the mix to reduce the total epoxy volume and add thermal capacity.
Just as a point of reference, I used to use a decent amount of Devcon 5-Minute (and 2-Ton) in my shop, and when I’d mix up a decent amount using a small plastic cup and didn’t use it all, the cup would deform as the epoxy cured. Not enough to melt through, but enough to warp the (thin) plastic severely.
Steel balls would help, as would any other chopped-up stuff that won’t float in the epoxy. The added weight may distort your small plastic object, though. Alternatively, you could add the epoxy in multiple thin layers, allowing each to cure before adding the next while keeping the heat from any one layer low. Or you could build a plug of some other material (wood? Epoxy putty?) around the threaded rod, so that the volume to be filled by epoxy would be small in the first place.
If you don’t mind using a 2 hour epoxy, or one that cures more slowly, that would help a lot. I think they’ll both release about the same amount of energy, but turning 5 minutes into 120 minutes will lower the rate and the temperature rise by 20 X, at least roughly.
Is your plastic item something which could be buried open end up in a small container of VERY wet sand? The sand would hold the item in an inverted position while you pour in the liquid epoxy and set the threaded rod while the water should keep the shell of the plastic cool enough to prevent warping?
See post #3: The item is small enough, and the plastic shell thick/rigid enough, so that I’m not worried about distortion due to the weight of the steel balls.
The epoxy I bought is this stuff. I bought two tubes of it, and each comes with two mixing nozzles. The problem is that each mixing nozzle is single-use, so at most I could only do four layers of epoxy. In addition, the threaded rod needs to be sunk about 7/8" deep into the epoxy, which means that’s how thick the final layer has to be. I might do two layers.
Yes. This seems like a great way to secure it while also providing some extra cooling capacity. Given the thickness of the plastic the cooling rate will be limited, but anything helps. Thanks.
I’m pretty confident about the steel-ball solution. Even with a (low) packing factor of 0.5, that still eliminates half of the heat energy that would otherwise be evolved. Plus, with a specific gravity of about 7 (compared to 1.25 for epoxy), the steel balls will add a whole lot of thermal capacitance.
I will go ahead with this sometime in the next week or two and report back on the results.
Consider adding cotton flock to your epoxy. This will both reduce the volume of epoxy needed and increase the cured strength.
Also note that you can increase the curing time and thus reduce the peak temperature by allowing the item to cure where it’s cold (e.g. in your refrigerator). A common rule of thumb is that the reaction time doubles for each temp reduction of 10 degrees C (18 F).
It’s been a good 25 years since I used epoxy, so my memory is hazy. Should I worry about offensive odors being released into the fridge by the curing epoxy? Would I be better off simply pre-cooling my item (in its little box of wet sand) in the fridge, and then conducting the cure out in my relatively cool garage?