What specific type of foam are you cutting. I’m assuming some sort of high density rubberized foam but specifics might be relevant.
I’ve had a recent epiphany about cutting foam. Materials like wood cut with a wider kerf at the entry surface than at the exiting surface. The understanding is that the wood fibers get vaporized by the laser. This makes sense for why the kerf would be wider at top than the bottom.
But as you have found foam behaves differently with a wider kerf at the exit surface. I think what’s happening is that the foam does vaporize near the top surface. However, the material toward the bottom is actually melting and contracting away from the center of the laser beam instead of simply vaporizing. This leaves a wider kerf at the bottom.
As for your specific situation it’s interesting because you’re saying it appears that the direction of the kerf can shift. That sounds like it could be due to irregularities in the foam itself. Either foam density or from air pockets. Or if the material is an aggregate of some sort.
Strategies for dealing with this I haven’t fully refined.:
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One school of thought is that you could try to increase power to see if you can truly vaporize the material before any melting occurs. In contrast, I’ve found that reducing power to the minimum required leaves a more uniform look.
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Additionally, if you’re okay with a stairstep look you could run multiple passes at very reduced power to dramatically reduce the size of the bottom kerf.
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You could try cutting half-way through the material, flipping it over, and completing the cut from the other side
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Try focusing at different depths in the foam; this hasn’t really worked well for me and also seems to affect top surface quality which is paramount for what I do.
I haven’t been truly happy with any of these strategies but have found the most acceptable results for my use to be to use the lower power required while cutting all the way through. The bottom surface quality isn’t nearly as important as the top surface for what I do.