The laser penetrates the filament heating it internally instead of only heating the outer surface. It's near-infrared (808 nm) so it heats the small cylinder of filament essentially uniformly throughout its volume.

It's also nice because they can vary laser power much more effectively than you can vary the temperature of a conduction surface.

https://arxiv.org/pdf/1709.05918.pdf

And it is low friction since it does not need surface contact.

I'm guessing here, but it could be possible that the actual heating of the filament is done entirely with the laser, and the job of the heated nozzle is simply to keep the filament at temperature all the way until it is extruded. It may have not been possible or feasible to combine the laser heater and nozzle, hence the need for both parts.

When the Laser heats the filament, it's 1.75mm in diameter, so to get it down to the 0.4mm diameter for extrusion a nozzle is still required which also needs some heat so the plastic doesn't resolidify.

The temperature needs to be consistent first. There is a PID cycle the nozzle goes through to keep it consistent. If you look at the temperature on a graph during print, it's not a flat line but rather a sine wave as the PID cycles. My printer has an LED that flashes with the heating element, and you can see it work more during heavy extrusion to keep the temperature up. The frequency of the cycle is related to the mass of the nozzle/heater and speed of extrusion. I can only imagine at that speed they could not get a consistent PID loop with a traditional heater.

FWIW, a sin wave is usually a sign of a simple threshold based controller, not a PID loop (or if it is a PID loop, it's very poorly tuned). A well tuned pid loop will not exhibit any periodic tendencies, though I would expect that it would exhibit changes as the filament extrusion is stopped and started.

The mode of heat transfer is very important for applications like this. Higher mass flow means you need a hotter heater which can get tricky for long term usage or you'll have to use some sort of refractory or ceramic based heater. Also with conduction, the biggest problem is heat loss either due to the environment of the system . It is almost impossible to direct all your energy towards a single source with conduction. Radiation on the other hand works really well for this type of application. Especially for lasers since the energy sources is highly polarized and condensed. The other method might be to use induction heaters or microwave, but plastics do not play well with that.

It’s definitely possible that you couldn’t reach the speeds that they are with any kind of traditional heater cartridge. You need a lot of power directly on the filament to melt is at quickly as they are.

Titan 3D Robotics has a extruder that claims to be able to print 1 pound of filament an hour which for ABS would be able 400 cm^3 (I think?)

I believe it's water cooled but still looks to be using traditional heater cartridges.

Specs: http://www.titan3drobotics.com/atlas/

Video: http://www.youtube.com/watch?v=mKPKIGcrlFU

Right. That's what you get when you build a 3D printer like a CNC machine. Titan takes in pellets directly, rather than bothering with filament. They probably use a heating system and drive screw like an injection molding machine. They can print with more plastics than filament printers. They end up with a lot of moving mass on the print head, but that's what big motors and controllers are for. The Titan unit uses 14KW of power.

I'd once considered heating the surface to which you are bonding with a laser, just ahead of the extruder, so you weld hot surface to hot surface, not hot surface to cold surface. That's why filament type 3D printers make such weak joints between layers.

I think the 400cm^3 figure was for the non-pellet version while the pellet version can use 10mm nozzles and extrudes a ridiculous amount more. I wish there was more published information about both of their extruders, they look quite interesting.

I'm guessing that absurd power requirement is for the 85C heated enclosure variant, otherwise I'm no idea where the 60amps are going.

I remember reading about a slicer with an option to use the hotend just as a heat source that traced the previous layer to smooth/blend it. Using a laser may be a good method of improving the surface finish as well.

What is the $ per kilogram cost difference if you can buy bulk PLA plastic pellets vs. buying filament on a roll? For a benchmark comparison figure the generally well-reviewed Monoprice filament ranges from $18 to $20 per 1kg spool.

it's about half as expensive.

What's the best way to keep the laser pointed just in front of the nozzle?

Not the parent commenter, but I imagine mounting it so it rotates around the nozzle’s axis would work. Fiber optics would help if the laser unit is too big to have swinging around on the print head.

You might also need to be able to tilt the laser up/down to vary the distance from the nozzle.

It should be pretty easy from there to have the computer keep it pointed at wherever the nozzle is going next.

I was thinking 3 lasers 120 degrees apart, turned on as needed, rather than a mechanical solution. 5W laser diodes would probably be enough power.

That is almost steampunk, we need cyberpunk :-P