Printing Flexible Filament Through a Bowden Cable

Introduction: Printing Flexible Filament Through a Bowden Cable

About: Developed a new type of filament drive system

Printing flexible filament through a Bowden cable on your desk top printer need not be an impossible task.

Regardless of your preferred filament choice whether it be 1.75 or 3 mm

Not just semi flex material (i.e. polymer with a shore A hardness above 90) but fully flexible material - right down to 60A in 1.75 mm if you desire.

Not at ridiculously slow speeds but the full speed that you would print with a direct drive extruder -and probably even better than what you are achieving already.

Plus with full retraction capabilities.

To achieve this there are two areas that have to be addressed -

Firstly - Eliminate all cavities along the extrusion path - A lot has been discussed about this topic but I will add a few tips in steps 1 to 3 that are likely to help a lot.

Secondly - Get the right extrusion drive system that will be able to control your filament and produce the high degree of force required for this process. In this case I will show case my innovation the Flex Wheel Extruder.

Step 1: Choice of Couplings for PTFE and Tube.

Firstly chose the type of coupling that will allow the PTFE tube to be pushed all the way through. Being able to push the tube through the coupling is a very simple way to eliminate the small cavities that exist in the crucial areas at either end of the tube - i.e. where it leaves your drive system and where it terminates within your hot end.

Ideally a good quality coupling with a collet that when pressed easily releases the grip on the tube. This will allow you to easily adjust the length of the tube making sure you eliminate all gaps.

Tube dimensions have been discussed numerous times else where and is pretty well standard and easily sourced

Length - The shorter the better, however I have tested lengths up to 800 mm.

Step 2: Attaching PTFE Tube to the Hotend.

I prefer the hotends which allow the PTFE tube to go down the barrel so that they terminate closer to the hot end tip. The maximum temperature design limit of this type of hotend should not be a problem when printing flex as most flexible filament is printed between 200 and 230 degrees c. If you have an all metal hotend these still work well, I have just found the PTFE lined model seem to preform better on a larger range of flexible polymers.

The E3D lite is a great reliable low cost high production grade hotend,

Attaching the tube to your hot end -

If your hot end is not Bowden combatable a quick design and print of a coupling holder to suit your set up is quick to achieve, a stl. for a simple coupling holder for the Flex Wheel extruder is included for inspiration or a quick search on your favourite file sharing site will probably also prove fruitful.

If your hotend is Bowden compatible you may find, as I have, that the collet to hold the tube is a bit sloppy and even using a collet clip does not always eliminate the movement or that access to fit the collett clip is impossible, in this case I simply double up by installing my own second coupling holder before the hot end as well. Here is a link to a quick vid demonstrating what I mean.

Step 3: Attaching PTFE Tube to the Extruder Drive System

Aim: To make a seamless transition for the filament from your drive system into the PTFE tube with zero cavities and no resistance.

Fit the PTFE tube 'snuggly' over the stainless filament outlet tube of the Flex Wheel. As the ID of the tube will be different from the OD of the stainless steel tube preform the following steps.

  • Very carefully using a 3 mm bit (1.75mm) or a 4 mm bit (3mm) drill approximately 5mm into one end of the tube.
  • By hand use a drill bit corresponding to the ID of your tube and twist it inside drilled portion to remove any burrs created.
  • For really great results remove the filament guide from the flex wheel slide the PTFE tube over the outlet and using a length of rigid filament run through system and feel for any resistance, if some is present repeat the above step a couple of times to create a seamless tranisiton.
  • Finally clean and blow the tube to remove any debris.

Attach the coupling holder and the slide through coupling to the body of the Flex Wheel. The coupling holder for the flex wheel can be downloaded and printed from here thingiverse coupling holder.

Step 4: The Extrusion Drive System

I found that being able to effectively constrain your filament along the feed path to be only part of the problem when printing flex through a Bowden Cable. The other part and what I believe to be more crucial is the ability to generate a generous amount of force upon your printing filament - for this I use The Flex Wheel - which is a revolutionary new way to grip, control and print with rigid or even the trickiest of flexible filaments with your existing 3D printer.

This system effectively addresses the draw backs and problems associated with the hob gear type of extrusion system.

You can attach the Flex Wheel directly to your printers frame or print a stand so that it can be placed wherever convenient.

Technical note: Why I found that using a hob gear and length of close fitting tube to be an ineffective solution.

  • In order to produce sufficient force on true flexible filament which is required for Bowden set ups you have increase the pressure exerted on the filament by your hob gear. This increase in pressure 'squishes' the cylindrical filament profile and makes it elliptical.
  • You then can not force an elliptical shape into the round PTFE tube.
  • If you back of the force so you don't squish the filament profile then the system struggles to over come the resistance of the tubing system.
  • If you leave a little gap between the hob gear and the tube to allow the filament profile to regain shape you then have cavity. This then causes a problem as the filament will buckle under the high pressure, some thing we are familiar with when trying to print flex.
  • The solution I arrived at was the flex wheel which removed the Hob gear problem altogether and provides a seamless path for the filament the entire way through the extrusion process.

Step 5: Reduce Micro Steps on Your Extruder Driver

The Flex Wheel is powered by an awesome little geared nema 17 stepper with a 30:1 High Precision Planetary Gearbox. This motor coupled with the larger diameter gear box means that the E steps need to recalibrated. The new steps for your extruder will be a high number.

To reduce this number of steps to a realistic level I have cut back the micro stepping function to 1/8 th step, this will give your motor more torque - which is great for pushing flex through a Bowden tube.

For specs on this motor or if you wish to source one we have an affiliate link HERE

Step 6: Setting Retraction

Flexible filament experiences a great deal of hysteresis, much more so with Bowden cable, however from my experience the amount required to 'unwind' and 're tension' during retraction and deretraction movements is definable and constant.

The Flex Wheel makes these movements possible due to different mechanism used to generate force upon your printing filament .

How much to retract - this will be material and length of tube dependent, a good starting point to determine the minimum amount is by the following experiment -

  • Heat your hotend to about 15 degrees below what you would print with.
  • Slowly extrude filament by small increments until the filament just starts to form at the tip of the hotend.
  • Unscrew the Bowden coupling at the flex wheel side - it should spring up as the tension is released.
  • Measure this amount and that will give a figure on how much needs to be retracted just to overcome the hysterias - then add 2 mm to this length, check printing quality then fine adjust if required.

How quick to make retraction movements -

  • The standard 4988 chips are sufficient however the power of the Flex Wheel requires everything they have and acceleration and speed are limited by them.
  • If you want to get the most out of retraction I would suggest upgrading to the higher power driver which is compatible with desk top electronics we all use.

Flexible filament by its nature is very stringy and while the above settings will reduce the majority of 'blubs' and reduce stringing to an acceptable level, even better results can be achieved with the following.

  • No wipe - this just seems to spread the molten material over the hotend tip even more and adds to the stringing problem.
  • Lift the Z axis a little during retract -this helps pull and snap that little bit of polymer left at the tip - hopefully having it stay on the printed par.
  • Quicker travel - so oozing is reduced.
  • Reduce print temperature by a couple of degrees- The flex wheel has power to burn so can easily over come this extra amount of resistance.

For refence I have used the following parameters in my slicing firmware.
4988 chips - Retraction - 100 mm /sec Derataction - 80 mm / sec.

High Powered drivers - Retraction 180 mm /sec - Deratraction 100 mm / sec

Step 7: Final Point: Changing Filament / Bowden Cable.

A common problem when you change flexible filament is when you withdraw the filament from the hot end a thin tail can be created, this may sometimes have blobs.

This tail and blob can easily break off when travelling through any extrusion mecahanism.

I find a quick simple way to avoid this problem is to unscrew the coupling at the point were the tube meets the Flex Wheel withdraw the filament from the tube snip off the deformed tail, good practice is to make the cut at a 45 degree angle so ready to feed next time.

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    Great first Instructable. Thanks for sharing with the community.