The rate of exothermal reaction is partially governed by the rate of the heat loss. If you mix in a bucket the surface area is minimal so the heat buildup at the "core" is high. Pour in a shallow container after mixing. If your potting volume is high maybe you have to keep it in the fridge while curing.
The other (snipped) suggestions make sense, especially adding a filler.
I would not deviate from the specified mixing ratio until your epoxy is proven to behave well if you do this. Deviating from the specified mixing ratio can result in an incomplete cure.
If the epoxy cures slowly at first and then suddenly with a nasty temperature rise at room temperature, it may take ages to cure if chilled.
Another idea: If the layer is barely thick enough to get a nasty temperature rise, you may be able to head off the temperature rise by blowing a fan at it. But this will not work for potting something in a bucket.
You need a slower curing hardener. Varing mixing rations is risky - the ingredients are quite reactive and you do not want too many of the reactive bits to remain.
Slower curing hardeners exist especially because of this problem.
Also, above some thickness it will be very difficult to achieve sufficient cooling.
"epoxy retarder" shows some google hits in the direction I'd search.
Note that for polyester resin this is more easily available in the form of 'clear cast' resins.
1) Obtain one or more (depending on the size of your casting base) CPU heatsinks and fans .
2) Obtain some heatsink grease
3) Affix fans to some hard surface (wooden grid maybe) so all the heatsink plates are aligned .
4) Coat heatsink plates with heatsink grease
5) Sit the mould on the heatsink plates . Be sure that the two are pressed together properly .
6) Attach fans to 12V (or 5V) power supply.
Put the object to be casted in the mould, and then pour in a maximum of 1.5cm of epoxy . (I haven't tested it with anything larger than this yet) . Put the completed assembly outside in fairly bright sunlight , so the epoxy gets a chance to set .
The advantages of this method are that it ensures a reasonably clear cast as all the bubbles rise to the top, with no "bubbling" caused by overheated epoxy (plus you can re-use the mould) .
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