Hi, ASA1000. Reading about your processing steps, it seems a bit strange to me that you flatten your prints for four minutes at 165 deg F in a press, and allow them to cool between two sheets of glass. I have never heard of allowing prints to dry under glass, which is completely impermeable to moisture, and seems likely to lead to different amounts of retained moisture in the paper in the edges compared to the center--whether it is from the middle trying to get out, or from the environment trying to get in.
You do not tell us what is between the heated platen of the press and your prints. That could be important.
I will tell you what I do, and I have no problem with wavy edges or getting my prints flat when unmounted. It is this:
Washed prints, after being hung by the corner by a clip for a few minutes (basically until I have the last print from the washer hung up), are then put image-side down on a clean piece of plex and wiped with a clean cellulose sponge. The mopped-off print is then placed face-up on the drying screen and mopped on the emulsion side with a clean cellulose sponge, then flipped over image side down and allowed to dry overnight, or longer as the RH demands and the print seems uniformly dry and "crisp" to the touch.
When it is time to flatten them, they go face-down on a clean piece of archival cotton rag matte board in the press, with four or five sheets of archival "interleaving paper" on top of it. The press is heated to my usual 190 deg F setting (checked with a thermometer). The press is closed for 30 seconds, and then the print removed. Note: The matte board and cotton rag interleaving papers have been thoroughly cooked beforehand and allowed to dry with the press open for a minute or so before the first print is done.
The prints treated this way come out perfectly flat, with a slight curl to them that relaxes a bit in time. You could mount them with corners and they would be as flat as they would be dry-mounted.
BTW I have gotten the same results if I just let the prints dry hanging from the corner and flatten them right from there, but I don't usually do that because they tend to dry with a random curl in them that can be a little challenging to deal with in the press.
When I (years back) would see curly edges, it was because the print had too much moisture in it that I tried to cook out of it too quickly. The different rates of shrinkage got "frozen in" in the form of edge ripples. It is a sign that the drying process has occurred unevenly. Check the temperature inside your press with a digital cooking thermometer, provide pre-dried absorptive materials in the press, and lose the glass plates, you don't need them.
I should say that I do not know of any process that leads to a "perfectly", geometrically flat print. The procedure that I use yields prints with a gentle uniform curve to them (BTW I use Ilford MG Fiber papers), which I find unobtrusive to detailed examination or subsequent dry-mounting.
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