That is INCREDIBLY helpful! Man, you just never know what someone else's valuable experience is. Case closed on the relevance of random musings. Just wish I'd heard this $30 ago. In thinking about it I was beginning to doubt it was a heat sink effect, and thought it was just due to the thickness of the 1/4 stock and the fact I couldn't heat beyond the fulcrum.. so that is great to know.
Luckily the 3/16" stuff softens broadly enough to let me mold (mould?) it to the panel beneath, since I don't think I could ever measure and have a hope to get it right.
That slip technique is clever. If I'd have double checked my work after the first bend I could have saved that sheet. Live and learn.
After re-reading my post this morning, I realized that I left out some info...
When I am using wood for a heat sink, I'm not using a flame. (obviously, the wood would catch fire, unless wrapped in foil, which somewhat defeats the sink
) I use a heat gun for thin stuff, and a "heat tape" made for bending plastics for the thicker stuff. (I normally do not need or use wood sinks when using the tape.) The thin stuff is actually harder to bend, as the bends tend to "follow" the shape of the heat line. If the heat line is not straight, the bend will have a dip or swell, and it's off to the trash with your piece
Usually I make "bucks" to bend around and clamp wood to the top side at the bend to form a narrow channel. This forces the heat into a straight line, and concentrates it.
Your fulcrum comment actually is probably more of the reason that the bend fell in the wrong place. The center of the heat spread will define where the center of the bend occurs. This is why the "heat tape" works so well. The bend always falls in the center of the width of the tape. If you were heating from the bottom, the center of the heat was probably not centered over the other bend.
Linkage to US Plastic page with mentioned "heat tape" AKA plastrip heater
http://tinyurl.com/cksaqThis tool is the "stuff" for no worry bending.
Dave.
Edit: I keep thinking of stuff to add. I'm done now.