Introducing Lix, the world’s smallest 3D printing pen that allows you to draw plastic structures in 3D. It’s only been on Kickstarter for a few days now, and already it has garnered close to a million dollars in pledges. An astonishing achievement, especially considering we can prove – with math and physics – that it doesn’t work as advertised. However, we’re wondering if it could work at all, so we’re asking the Hackaday community.
The device is powered through a USB 3 port. In the video, the Lix team is using a MacBook Pro. This has a USB port capable of delivering 900 mA at 5 Volts, or 4.5 Watts. Another 3D printing pen, the 3Doodler, uses a 2A, 12V power adapter, equal to 24 Watts. Considering the 3Doodler works, and they both do the same basic thing, there’s something extremely odd going on here.
Just as a comparison, here’s a wirewound resistor commonly found in the heating element or ‘hot end’ of a 3D printer. It’s a 6.8 Ohm resistor powered at 12 Volts. That’s 21 Watts. Here’s a heater cartridge, also found in quite a few hot ends. It sucks down 40 Watts. Once again, the Lix Kickstarter clearly shows the pen extruding filament using only 4.5 Watts of power. Something is really, really fishy here.
Intuition doesn’t hold a candle to math, so let’s figure out exactly why it won’t work.
We’re thinking the easiest way to figure out if 4.5 Watts is enough for a 3D printing pen is from a purely thermodynamic analysis: a specific amount of filament goes in, is heated up to its melting point, and is squeezed out of the nozzle. The equation to calculate how much energy is required for a specific temperature change in a system is Q = cmΔT, where Q is the amount of energy in Joules, c is the specific heat of ABS (1.3 J/g°K, source), m is the mass in grams, and ΔT is the change in temperature. All we need to do now is figure out how fast this pen is extruding, and the mass of that extruded filament.
In this video, starting at 10 seconds in, you see the Lix extruding about 13 centimeters of filament through the 0.6mm nozzle included with the Lix in five seconds. A little bit of math happens (volume of a cylinder with a height of 13cm and a diameter of 0.06 cm), and we can figure out the Lix is extruding 0.038 grams of filament per second (ABS density of 1.04 g/cc, source). This calculation was done by counting pixels and frames, which can be inaccurate, but not by much.
An estimated 0.038 grams of filament extruded per second, a change in temperature of 210°C (20°C room temperature, 230°C extrusion temperature), and a specific heat of ABS of 1.3 J/g°C (source) means 10 Joules are required to extrude one second’s worth of filament from the Lix pen. Since 1 Watt = 1 Joule for 1 second, about 10 Watts are sucked down whenever the Lix is extruding filament. Once again, the Lix can only draw 4.5 Watts from a USB 3 port. The math simply doesn’t work, and no USB 3 powered device can extrude ABS filament that fast. The math is also generous, as it doesn’t consider the phase change of the filament which would require even more energy. I didn’t include this because I can’t find a reference for the heat of fusion for ABS. The math also doesn’t consider losses to the heater block, the air, and a host of other inefficiencies in any real-world device.
But ABS requires a fairly high temperature to extrude. Even though the Lix team claims the pen works with ABS, let’s say they’re using PLA plastic, extruded at 180°C. Doing the math for a ΔT of 160°C means 7.9 Watts are sucked down from a USB 3 port that can only provide 4.5 Watts. Something is terribly wrong here. That’s why we’re turning to you and asking the rest of the Hackaday community.
The only way we can figure the Lix actually works is if the extrusion rate is really, really slow. Halving the extrusion rate of PLA to 1.3mm/s gets us into the ballpark of what the Lix power supply can do; that only requires about 4 Watts, leaving enough left over to run the motor and electronics inside the pen. This is exceptionally slow for any plastic extruder – RepRaps can extrude plastic about 50 to 100 times faster. There’s a good bit of evidence the video of the Lix has been sped up dramatically, given the disclaimer, “some of video scenes have accelerated speed” appeared on the Kickstarter sometime between the first and second day of the campaign.
While we know the video is an outright misrepresentation of what any USB 3 powered device can do, We can’t figure out if the Lix is a viable product. We’re turning to you. Can you figure out if the Lix pen actually works? All we know is the Lix pen has a 4.5 Watt power supply from a USB 3 port. It’s possible for a USB 3 powered 3D printing pen to work, albeit slowly, but the engineering is difficult and we don’t know if the Lix team has the chops.
As an aside, this post has inspired us to consider a column on Kickstarters that seem to defy the laws of physics. We’re thinking about calling it Kickherder, as the vast majority of Kickstarter backers for these types of projects are mindless sheep. If you have a better name, leave it in the comments.