It overheated
Updated
On this page8
Depth is the usual variable
Standard resins carry a maximum pour depth and exceeding it is the normal route to this. Deep pour formulations exist precisely because the general purpose ones cannot shed the heat from a thick casting. The limit is on the data sheet rather than a matter of judgment.
Leaving it in the cup is the other
A full cup is the worst possible shape: maximum mass, minimum surface, and an insulating wall. Mixed resin left standing will often go off in the cup long before the same material would in a mold. Pour promptly, and if you cannot, spread it out.
A warm room shifts everything
Higher ambient temperature shortens pot life and raises the peak, so a volume that was fine in winter can run away in summer. This is why the room temperature belongs in the log next to the batch.
What it does to the piece
Yellowing, internal cracks, bubbles driven out of solution, distortion, and sometimes a mold damaged beyond reuse. None of it is recoverable, so the cost is the resin plus whatever it was poured into.
If it is going, let it
Outside, on something non-combustible, away from anything that burns, and leave it alone. Do not pour it down a sink and do not carry it through the building. It is making its own heat and will finish on its own.
Prevent it by splitting the pour
Multiple thinner layers, each allowed to cool, is the standard answer for a thick casting in a standard resin. It takes longer and it is how deep work is done without a deep pour product.
Mix only what you will pour
Mixing a generous batch to be safe puts the surplus in a cup where the heat concentrates, which is the exact condition this page is about. Measuring what the mold actually takes, with a small margin, is the habit that prevents it.