I have a 5×7 piece (6mm) that I slumped into a fairly deep rectangular mold – it came out just “curved” versus down into the sloped shape. I slumped it again at a higher temperature, and it literally shrunk by 1/2” on all sides, and thickened up in the middle (it still didn’t take the form of the mold). Any ideas what happened?
Deep square and rectangular molds will almost always end up with curved edges that dip in towards the center. Slower slumping will minimize this. You can probably get to straight edges if the flat piece has sides that are bowed out – but it would require testing.