It doesn’t have to be a large cavity in order to be useful. Imagine being able to reliably fill a hole that’s 5mm deep. Not amazing, but that could mean 25 layers. That’s 24 layers more than what we can fuse together now.
I added a SAT solver to refine the tube endpoints so that nearby tubes don't start or end on the same layers. Even the default greedy solver (CP-SAT is SLOW) does a pretty good job at "staggering" the tube ends.
The part is "knitted" together in 3D at different Z layers.
Try the binaries. They've only been tested in Linux but they build fine for all platforms.
Slice something with MAGMA infill turned on and flip off the visibility of everything but injection lines. You'll see how the U shaped tubes knit the part together
You can’t inject filament into a hole without it cooling and solidifying at the top. That’s the problem with this whole idea.
You can set tube height to something tiny like 5mm. It takes a fraction of a second to fill that.
As someone who's printed a lot, on fast printers you have to set a minimum layer time because it takes up to 20 seconds for the layer below to solidify, cool to below the glass transition. Plastic is a bad conductor so heat loss through the walls probably isn't as bad as you'd think