Yeah so something I didn’t make clear: the interface of laser and material is very complex and analog, and the dynamics of heat and material/chemical changes like combustion/ablation/sublimation all join together in a way that can’t be separated.
My explanation of the laser ignores hourglass shape and the messy interactions that @timjedwards pointed out in order to arrive at a simple “ideal best case” reason why the cut profile would be angled. The real truth of what’s happening involves a lot more, but since every other factor only increases the effect it’s a useful way for me to think of it.
Ultimately, exactly why isn’t so important: we’ve all seen it in action, whatever the reasons that cause it. For a uniform material, kerf’s always wider at the top surface.
Now…. For less uniform stuff all bets are off. See: foam core board cut profiles.
In addition to all of the combustive/chemical processes, coefficient of linear expansion (i.e. things tend to get larger as they get warmer) can also come into play, especially with acrylic. Check it out, it’s wild – it doesn’t affect kerf profile exactly, but it does show that the material+laser party is complicated:
This old discussion about acrylic kerf and thermal expansion has some applicable info in it too I think. (It’s one one of my favorite discussions on the forum becasue science is cool, even if I never got all the pieces to fit well without mechanical adjustments and/or glue. )
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