Heat treating your 3D printed parts can be a fickle job. The filament has a narrow temperature range that produces optimal results. Too hot and your part quickly distorts, too cold and the strength increase does not occur thru the whole part.
After some experimentation I found that the typical electrical kitchen oven was a very poor choice. The on-off cycling of the thermostat results in large temperature swings and exposure to the direct radiant heat of the coils can overheat your parts too easily.
A new method was needed! Enter the sous vide. Sous vide is a cooking method involving placing your food in sealed bags and then cooking it in precisely controlled hot water. The sous vide device is a simple water heater and pump, with an extremely precise temperature control, easily holding the water temperature to within +/- 0.2 deg F.
This slow consistent heat is perfect for precision heat treating your 3D printed parts! And it's reasonably cheap. If you don't have one, a sous vide device can be found for about $50 on Amazon. Link at the end for the one I purchased.
Here's my method:
1) Using any suitably large container (pot, bucket, or a cheap styrofoam cooler) filled with water, start the sous vide device up to your desired setpoint temperature (160 to 180 F for PLA).
2) To better control part distortion I placed my parts into a container and then filled with fine white sand. The sand will support your part and allow the part to soften temporarily without sagging and coming out distorted.
3) If you have an oven thermometer I'd recommend monitoring the sand temperature. You want you part to be held at your target temperature for around 30 minutes or more. Depending on the size of your container the sand could take some time to reach temperature. Pre-heating the sand before you put your part in can help speed things up.
That's it! And then end of time remove your container and let it cool before removing from the sand.
If you're doing this often or on very large parts I'd recommend using a water container that is insulated and has a lid to control evaporation. A cheap styrofoam cooler with a hole cut in the lid for the sous vide would probably work excellently.
This method rocks! It produces a very consistent result that you can count on without much fuss. And when you're done you can still cook with it; toss in a steak for dinner!