Print Cooling, Fusion, and Bridges

Here’s a quickie to give your brain some exercise - but consider that I might be wrong.

What I think is happening is this:

On most printers the part cooling fan blows a stream of “cool” air (ambient) at the nozzle or thereabouts depending on how well designed it is. In the case of the Ender 3 for example, there’s also a blast of warmish air coming down from the cold-end of the hot end (?), the temperature of which depends on how long the printer has been active.

So here’s the thing:

If we want to bridge, it’s not just how fast the nozzle can whizz in the direction the bridge is meant to go, it’s more a case of how fast the extruded plastic gets cooled. Because if it’s cooed too slowly, the plastic will sag; cooled too quickly and it won’t fuse.

Now given that different prints require different amounts of bridging, it struck me (and please, don’t feel bad to correct me, I’m still quite new to all of this) that when we’re bridging we need to run the part cooling fan at max speed and hope that it cools the extruded plastic quick enough to solidify it before gravity does what it’s done to my entire face… BUT when we’re building walls and such, we need the fan to cool less because that increases the fusion between layers. In fact, infill might even benefit from having almost no cooling at all for this very reason.

Stefan at CNCKitchen has done some tests to prove this but it’s a bit of a no-brainer to me. Fusing plastic is no different to welding and if you don’t melt the two parts sufficiently the weld doesn’t hold.

AND here’s the $65M question: can we do this in any slicers or is this the sort of thing that needs g-code or (gasp) separate fan designs so that the machine can switch when it’s bridging - I guess that would require new g-code but it’s a serious question from me as a noob. Seems to me that with good cooling and a great hotend, it should be possible to bridge gaps of many mms and possibly even 10s of mms but at the cost of good fusion.

What say you all? I’m just putting this out there for you experts to have a gnaw at, but please don’t all bit me at once. I’m still learning!

There has been a discussion about allowing different fan speeds to be used in PrusaSlicer for printing different features. You might want cooling on for external perimeters for finish, but less cooling for internal perimeters for better inter-layer adhesion.

Another factor for bridging is speed. You want to keep just the right amount of tension on the extrusion as it’s stretched across the gap. It’s a balancing act between heat to melt but not snap, cooling to firm it up, speed sufficient to pull without snapping.

As you say, a fine balance. I think Prusa (and Voron) have this “licked” better than Creality’s (and clones) stock hot ends which are laughably bad. Hence I was wondering on the practicality of having two, independent blowers for part cooling. One for doing fusable layers (aimed as a downward rush) and another which is spun up only during bridging and is aimed directly at the nozzle - if that makes sense. But as I’ve said, I’m obviously a noob at this and while I’m good at seeing things others don’t, it’s entirely possible others have better solutions.