Battling Bowden Tube Physics
Those of you who've seen our new Gold V6 will have seen the swanky new Capricorn PTFE tubing and collet clips that it comes with. This blog post goes into more detail on the need for these upgrades, and how they work.
We have a new high-performance bowden tube, and ways of making our tube couplings grip the tube more securely using a pre-tensioning clip. These will make your bowden system more responsive and reliable. Read on to find out how.
What is a bowden system?
In the original incarnations of 3D printer extrusion systems the motor, gearing, filament drive and HotEnd were all built together in a single assembly. The extruder (motor, gearing, and filament drive) thus fed filament directly into the HotEnd.
But back in 2009 a guy called Erik (who went onto start a company called Ultimaker) came up with the fancy idea of removing the heavy, bulky motor and drive mechanism from the print-head. The filament drive mechanism would instead push the filament into a slick tube, which would guide it all the way into the HotEnd which was located at the print-head. It’s a bit like a bicycle brake cable, but backwards.
This results in some useful benefits:
- The print-head that moves is lighter, so it can move faster and with higher accelerations. This means quicker prints, less ringing and a cheaper motion system.
- The amount of “stuff” on the print-head is much lower, and takes up less space. This can mean a smaller print head, a larger usable print volume, more extruders on one print head, or any combination of these factors.
However, there are also some notable downsides:
- More resistance to extrusion due to increased ‘drag’
- More lag/lash/hystersis/slop/whatever you want to call it. The extrusion system is simply not going to be as responsive as a direct extrusion system
- You can’t print flexible or soft materials as easily
Not to mention a fair few bowden myths:
- "You just need to increase retraction and your bowden system will be as precise as a direct extruder"
- "Bowden systems improve print quality because the resistance of the spool to unwinding no longer tugs on the print-head as it moves around"
By diving a little deeper into the physics of where the downsides to a bowden system really come from, we can begin to understand how to improve these systems by minimising the negative effects.
What needs fixing?
Your extruder system has a finite amount of pushing force. You want as much as possible of that pushing force to be used to generate pressure in your HotEnd, as it’s what causes the molten filament to actually extrude from the print-head (pretty crucial to 3D printing!).
The friction of filament sliding its way from the filament drive to the HotEnd saps away some of this force, leaving less force to create useful pressure. When we use a bowden system the filament has to travel on a longer and more convoluted path, and so we get higher losses to friction. We tackle this by using a tube made of PTFE (a very slippery material indeed), which reduces but does not eliminate friction. This problem is fairly easily mitigated by ensuring you have an extruder mechanism which can exert enough force on the filament.
This leaves us with the rather more complex matter of response and control. A printer needs good control of the amount of flow from its nozzle in order to accurately build parts. It also needs to be able to carefully control when extrusion starts and stops. We maintain this control by using a stepper motor, which moves the extruder filament drive wheels very precisely. However, there are still some errors that occur.
The first is what we might call “pure backlash.” This might be something like backlash in your geared extruder, or slack in the couplings of your bowden system. To counter this you have to increase your retraction distances to take up the lost motion upon direction change, and when doing so there is some delay.
Another error source in extrusion systems is the compression/buckling of the filament. As we apply pressure to the filament in order to extrude it will bend a little (but a little is all it takes) between the extruder drive wheel and the HotEnd melt zone. Because of this spring-like compression we can advance the filament by 1mm at the drive wheels, but 1mm’s worth of filament will not immediately exit the nozzle in perfect synchronicity. In a direct extruder with a well-constrained filament path there is very little flex and so our control system is very “stiff”, with changes in extruder motor speed being quickly translated to changes in nozzle flow. However with a bowden system this filament-spring is much “softer” due to its much increased length. The subsequent deformation under force results in extruder motor speed changes taking longer to be reflected at the nozzle, and the errors being greater in magnitude.
Retractions try to counter this by “pre-pumping” at the start of extrusion and relieving pressure at the end of extrusion. The more nuanced pressure advance algorithms do a conceptually similar thing, but are continuously compensating during acceleration and deceleration. Whatever control method you use, it is not possible to gain control authority over a system that is inherently less responsive. You can compensate for the errors to an extent, but you can’t eliminate them. Your extruder has a finite amount of speed and acceleration which it can use for compensating.
So this takes us back to improving the physical bowden system. We want to increase the stiffness of our filament-spring, and remove any lash in the system.
Making it better
Lash is easy to deal with. If you have a geared extruder you want to ensure your gears are well meshed and with no slop. With a modern extruder like Titan this is pretty easy. For the bowden couplings (which can have some wiggle room in them) we are introducing collet clips, which slide under the toothed collet part of the coupling that physically holds the tube and locks it into place with a little pre-tension to boot. This reduces coupling lash to near zero as the collet and tube it is holding are locked into place.
To increase the stiffness of the filament spring we are introducing Capricorn PTFE Tubing; a new, higher tolerance tube, with a tighter 1.9mm internal diameter and added internal lubricant. This has a twofold benefit - it reduces sliding friction of the filament in the tube, but most importantly it makes the filament 'spring' stiffer, by quite a lot. The tighter internal diameter permits the filament to have far less flexion than in our standard 2mm ID tubing, meaning that it can't bunch or coil up. This really firms up the response of the whole bowden system. Because the filament has no space in which to bend within the tube, movement produced by the extruder is much more efficiently translated down the tube, into the HotEnd, and out the other side.
You just have to make sure that you don’t use poor-quality filament with an uneven diameter, which these days is far less of a concern than when we originally introduced bowden versions of E3D HotEnds.
Other things you can do to improve performance include having the shortest bowden tube possible, while keeping bend radius no less than around 150mm.
Don’t forget that bowden tubes can and do wear over time. Abrasive filament will gradually eat away at the inside of the tubing, and just the passage of filament with drive-wheel toothmarks on it will have an impact. Additionally, the ends where the couplings grip can get chewed up over time, but this is much reduced if you use our new collet clips.
The spool weight myth is countered because you get the same no-drag effect with a direct extruder by running a guide tube from the spool holder to the extruder in the normal way, thus keeping a constant filament length between extruder and spool independent of carriage position.
When all is said and done, what this really means is that you are going to get better prints with our new bowden tubing and collet clips. Particularly a much reduced extrusion start/stop ‘dimple’, along with reduced oozing during travel, all achieved with lower retraction distances. This is going to be extra-noticeable with somewhat more flexible materials like nylons and semi-flexible materials where the buckling has a more significant effect in the extrusion system, but even with stiff PLA we’re still impressed at the very noticeable difference that switching to this stuff makes.