[slabity]

So I'm pretty skeptical about this as well (see my other comment on this), but the particular failure mode you're discussing is not what's happening in reality.

> Anyone who has ever cleaned blobs of filament off of a nozzle after a print failure can tell you what happens when you try to pump hot filament into empty space. Filament cools below the melt temperature quickly, especially when it comes into contact with your print.

That's completely irrelevant because this isn't printing into empty space at all. This is injecting molten plastic into confined channels, with no active cooling, made from material that doesn't conduct heat well. You're saying that the plastic will cool too quickly, but I believe the opposite will be true.

The problem that the author is describing is that the plastic is actually far too hot when injected and causes wall collapse. This is because the author isn't taking into account that FDM walls don't handle the required pressure near/above glass-transition points.

The failure mode you're describing is the complete opposite. If you were correct, it would result in cold plugs or extruder jams. It wouldn't result in wall collapse or layer delamination.

[Aurornis]

> That's completely irrelevant because this isn't printing into empty space at all. This is injecting molten plastic into confined channels, with no active cooling, made from material that doesn't conduct heat well. You're saying that the plastic will cool too quickly, but I believe the opposite will be true.

The filament will cool on contact with the part. Think about it: How are you expecting this filament to stay as hot as the inside of your nozzle while it’s flowing through the part and touching all of those walls? How are you expecting the walls to not melt, but the melted filament to flow through them?

Even filament extruded into free space cools enough to become a solid string barely inches out of the extruder. It will cool even faster if it’s touching something.

The only way this works is by heating the entire part up to a temperature where the filament stays hot enough to flow. So you’d need to heat the chamber and the part to 210C to get PLA to flow through it.

[mgunlogson]

The injection probably happens faster than you think. With a high flow hot-end and 5mm deep channels its reasonable for the injection to complete in less than a second.

> How are you expecting this filament to stay as hot as the inside of your nozzle while it’s flowing through the part and touching all of those walls? How are you expecting the walls to not melt, but the melted filament to flow through them?

Specially formulated injection materials with viscosity near water at injection temps. Dual nozzle setups where PLA is injected near its breakdown temperature into a heat resistant shell like CF-Nylon or Ultem. I need people with more time and hardware to test this. This was a side project for me, out of a dozen I'm working on. And I already spent a bunch of time on it.

For a years there's been talk about doing some kind of 3D injection on the GCODE level, like Z-pinning https://github.com/OrcaSlicer/OrcaSlicer/discussions/4815 . But it was hard to experiment because slicers don't support it. Now there's a codebase with dozens of different knobs to play with.

I added support for dual nozzle printers and using a different material for the injection stage. I just don't have the hardware to test it

[mgunlogson]

> The problem that the author is describing is that the plastic is actually far too hot when injected and causes wall collapse. This is because the author isn't taking into account that FDM walls don't handle the required pressure near/above glass-transition points.

Yes exactly. And it may be a solved problem.

I need someone with a dual nozzle printer to try injecting something like PLA in a CF-Nylon part. I anticipated this and added support for dual nozzle and multi-material with a different material selectible for the injected channels. But be warned, it hasn't been tested it at all.