Charlie:
Thanks for the datasheet. It looks a bit complicated though I might just have to join them to the biggest chunk of metal I can find. A fan might help too.

It does make sense that power devices will get hot and I wasn't expecting them to be completely cold but I just didn't expect this extreme heat from such a small percentage of it's capability. I've also used some of these same rectifiers in an audio circuit powering 30W speakers without them getting the slightest bit warm. Speakers don't draw constant though.


amgen:
The LEDs do use PWM which helps but they have the potential to be turned on full for long periods of time so that's my test condition.


Archangel:
I've never had an electric blanket but I do own a few smoke machines that are designed to heat up and are rated at 700-800W each and they don't seem that much hotter.


AvionicsMaster1
That's correct. 12VAC 50Hz into the rectifier and 100Hz out of it. I have 14 rectifiers running and they are all identical. I'd be really unlucky if all 14 were faulty.


Thanks everyone for your help. It's clear now that this temperature is normal and the largest heatsink I can fit in (and maybe fan) are needed.

I've attached a picture (from before the ABS melted) in case anyone is curious about the project
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