Comment by Animats | Hacker News Reader

Comment by Animats | Hacker News Reader

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Pushing filament with a gear was never a good design; it was just something that hobbyists could build easily. Injection molding machines use a screw drive to push molten plastic. This new machine uses a scaled-down version of the injector screw of an injection molding machine.

Design and manufacture of injection molding screws is complicated but well understood.[1] They're not easy to machine, but that's what CNC mills and four-slide machines are for. It's like making a drill bit, something that's done industrially millions of times a day but is very tough for hobbyists.

I'd wanted to use a laser for heating in a 3D printer, but not to heat the plastic being deposited. The fundamental problem with filament-type 3D printers is that you're welding a hot thing to a cold thing. That never works very well. That's what causes cold solder joints and weak welds in metal. In 3D printing, it's why the bonds between layers are much weaker than the plastic's inherent tensile strength. I wanted to have a small laser (about 10W) aimed ahead of where the filament is being deposited, to heat up the material already placed. That way, both sides are hot as the weld occurs, which should yield a much better weld. This is what the heated build plate and heated build chamber designs are trying to accomplish, but they're not putting the heat the one place it's really needed.

As lasers in the 10W range become more affordable, this looks like a better idea. A few years ago, it was an insanely expensive idea, but now, you can get 10W for about $300.

[1] https://www.plasticsportalasia.net/wa/plasticsAP~zh_CN/funct...

Heated build plates and enclosures are primarily to prevent the part from warping and pulling itself off the build plate.

The problem with trying to bond two hot layers together is that they distort. Typically the previous layer has not solidified enough and so most printer employ a cooling fan to lower the temperature of the previous layer, not raise it.

It's very hard to find the right balance of extrusion temperature, fan speed, feed rate, and a myriad of other factors that affect print quality.

Honestly your comment makes me think you've never actually used an FDM printer.

Actually the fan is to cool the current layer, immediately after depositing, to solidify it as quickly as possible. That it also cools the previous layer is just an accident.

I think Animats is right. It would be better to locally heat the previous layer right before depositing, and then cool the joint right after.

Unfortunately that requires highly precise heating and (even harder) cooling, so is probably still a few years away.

Localized cooling isn't hard. Laser cutters do it; they have a tiny nozzle spraying compressed air (in some machines, an inert gas) on the cutting site.

For temperature feedback, a non-contact IR sensor would work. Operate the IR sensor only during the off time of the laser (presumably pulse-width modulated) and you can read the temperature while heating.

The screw mechanism they describe is quite a bit different than an injection molding screw. The screw in an injection molder is like an auger, designed to push plastic pellets. A hobbed bolt is not an option because it can't pick up the pellets. Here they are grabbing the filament from the outside with threads, kind of like rifling.

Reading the paper, it seems like they managed to double the force compared to a traditional pinchwheel. However, there are now extruders that used geared pinchwheels on both sides of the filament, doubling the force without requiring threading the filament.

That's what the article says, but there must be some trick to make that work. Trying to drive a filament just by threading it and running it through a rotating nut will mostly twist the filament or tear off the threads. Maybe an inside conical thread, like the negative of a wood screw...

It's actually just a standard 4-40 thread cut into the filament. They use rollers to prevent rotation of the thread when cutting the threads into the filament, and then when they actually use the nut it's got enough thread engagement that it's better than a hobbed gear. If you think about it, it's almost like a hobbed gear that attacks from all sides instead of one or two.

Feel free to ask me questions on it, I work in the industry and have spoken with one of the authors of the paper, it's a shame it's behind a paywall.

I wouldn't compare it to the screw drive they use on injection molding. Those are for pushing pellets and melted plastic, not for pushing a rod (or strand) of continuous plastic.

This design uses threads on the outside of the filament [1] to grip it with a nut.

The main difference is that the design and manufacture of this system is extremely simple, contrary to what you've said. They use off the shelf 4-40 [2] taps and nuts to create a triangular thread. I would even go as far as to say this is _easier_ to manufacture than hobbed gears.

[1] http://www.sciencedirect.com/science/article/pii/S2214860416... Excerpt for those without access: "in the FastFFF system, this limit is overcome using a rotating nut to drive a threaded filament (Fig. 2c), analogous to a leadscrew linear actuator"

[2] http://www.sciencedirect.com/science/article/pii/S2214860416... Excerpt for those without access: "The ABS filament is cut with a standard 4-40 thread (Craftech Ind.), which has a triangular profile (Fig. S1, Table S1) [10]."

This is actually something you can address by annealing the printed part after producing it.

http://justinmklam.com/posts/2017/06/sous-vide-pla/

It seems that improving the annealing process of printed parts from current 3D printers could, at best, reach the maximum crystallization of pure stereoregular PLA polymers.

Due to the two enantiomers of lactic acid, there are four different PLA polymers with different stereochemical structures: PLLA, PDLA, racemic PLA (PDLLA), and meso-PLA. It is only the the PLLA and PDLA stereoregular polymers that show crystalline characteristics. However, research indicates that mixing the PLLA and PDLA polymers allows the formation of a stereocomplex that has a much greater degree of crystallization. Note that this is a physical mixture of the two stereoregular polymers rather than a polymerization of the two lactic acid enantiomers, so the PLLA/PDLA stereocomplex (greater crystallization than either of the PLLA or PDLA polymers) is very different than the PDLLA polymer (which is amorphous, meaning non-crystalline).

Perhaps what we need is a 3D printer that uses one PLLA filament and one PDLA filament. The two filaments would be melted and combined in the nozzle of the printer. As the mixture of the two polymers annealled, allowing the stereocomplex structure to form, the resulting material might have mechanical properties that are superior to either of the parent filaments.

I wanted to use the same design to deposit copper powder to printed circuit board. The laser would draw a temporal heat mask and nozzle would blow copper powder over the workpiece.

Laser sintering works something like that.

But really, if you want PC boards, use a board fab house. There are so many good services, they're cheap, and you get drilling and plated through holes.

I have 3 printers of various quality and design, and by and large the issue with printing I find most difficult to optimize is a constant wall quality. As you mention, extruding hot plastic is a solved problem, but doing so a layer at a time is complicated by travel time of the print head, previous layers, and ambient air temperature.

Without that it is very hard to get consistent strength and durability at any speed.

Have you seen this conversion of a Cetus3D printer to a laser PCB engraver? Maybe the laser could be added as a bolt-on peripheral to the head.

https://hackaday.com/2017/11/27/entry-level-3d-printer-becom...

The problem with combining a laser with a print head to perform some kind of heat-ahead function is you cannot predict the vector direction the head will be traveling with current hobbyist firmware, like Marlin, without modifying it sufficiently enough to buffer the GCODE so it moves the focal point of the laser to the next vector direction while still extruding in the current vector.

You'd be effectively operating two independent print heads at the same time, otherwise if you're just focusing the laser at the extrusion nozzle, you're just heating already heated plastic, or worse, burning the current layer of plastic behind the traveling printhead.

The problem with combining a laser with a print head to perform some kind of heat-ahead function is you cannot predict the vector direction the head will be traveling with current hobbyist firmware.

That's not a particularly difficult problem, although doing it on a machine with Arduino-sized RAM might be difficult. Large machine tools which interpret Gcode do considerable look-ahead, because they have considerable inertia and have to slow down well ahead of stops and turns.

A four-slide or mills for making screws? Not sure what you mean... a Swiss Lathe or milling attachment?

Most I've seen are one large piece machined on lathes with a tool attachment, then ground. And a some that are made of smaller 2"/51mm blocks that you slide together over a shaft for different configurations or to replace worn sections.

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