I tried flattening all the plastics on my Stern Meteor this morning. It worked PERFECT! This was the first time I've tried flattening plastics in the oven. (I've tried outside in the sun, but that was just fair, results wise.)
Here's my step by step process:
1. Preheat oven to 200 degrees.
2. Place plastic facing upward (you can look down on the artwork and see it just like you would if it were on the machine) on a baking sheet with a piece of parchment paper between the sheet and the plastic.
3. Put it in the oven for 10 minutes.
4. Remove from oven and place a piece of parchment paper on top of the plastic.
5. Place a couple of heavy books (I used a dictionary and a medical dictionary) over the plastic.
6. Wait a good 20 minutes or until the baking sheet and plastic has completely cooled.
7. Look at your beautiful flat plastics!
All the dangers apply. If you don't watch this and forget you could have a melted blob. Also, oven temperatures can vary, so your mileage may vary as well. I was kinda scared on the first attempt. But it went so well that I did all the other plastics and every single one of them came out perfectly. Now, don't forget to get those bad boys nice and cleaned up before you do this. You don't want to get imbedded dirt and gunk into the plastic. Cheers!