All the LEE 6C srue plates I have laid my hands on have been aluminum. I've never been fond of that idea, but the I've never had a problem with one either. I HAVE had problems with the (old style) 2C steel sprue plates.
I had a couple LEE moulds on which the sprue plates would warp occasionally, even though they had not been over-heated. No mallet - just a gloved hand to open/cut the sprue. Those, I had to lap, or bend AND lap. I eventually junked those two moulds because the alignment began to get touchy. I DID get a lot of bullets out of them over the years, but I also had to work on them from time to time to keep them viable.
