0.2 mm nozzles, PVA and you

Hi all,

TLDR; 0.2mm nozzles with PVA is hard, I'm increasing the modulus of elasticity by reducing the filament temperature (dry ice, freezer) as well as experimenting with extruder designs

Long version:

Problem: I've been printing with PVA with 0.4 mm nozzles with some success around 185 C. I'm looking to switch to a smaller nozzle since I'm creating single strand PVA bridges (for vascular systems) and I'd like to reduce the diameter of the extrusion. Once I switched to a 0.2 mm nozzle, the filament requires more force to be extruded and since PVA is very flexible, it bends after the feed motor and does not extrude.

Solutions:
- Lower modulus of elasticity
Currently, I'm experimenting with freezing the PVA filament in order to increase its modulus of elasticity. The filament is noticeably stiffer upon freezing (-18 C) but I still have the bending problems. I'm currently looking into dry ice to reduce the temperature even further. I'm not sure how much I can increase the modulus on thermoplastics, but anecdotal evidence seems to be positive. Here's a good paper on thermoplastic stress-strain [ PDF ]

- Filament guide on extruder
I've purchased a new tension design with a filament guide for my stepstruder, similar to this one Thingiverse MK8. It should help solve the problem, but until it gets in, I'm unsure if the filament will just buckle against the walls and the friction will stop the extrusion.

If you have any experience with 0.2 mm nozzles / PVA feel free to chime in. Otherwise, I hope you enjoy my adventures as I update this with my progress.

Are you using 1.75mm or 3mm? I've used 3mm PVA with a 0.35mm nozzle, nothing smaller though. Is there a gap after the drive wheel or hobbed bolt that could be filled with something to help prevent buckling?

I'm using 1.75mm filament and yes there is a gap after the drive gear which I've ordered some parts to fill. When the parts get in I'll update. Using 3mm filament might be a good idea, but I'm not sure how to retrofit my Makerbot Replicator Dual for that.

Because PVA is nearly incompressible, you are dealing with a buckling issue (as you stated).

PVA is close to incompressible. The Poisson's ratio is between 0.42 and 0.48.
[en.wikipedia.org]

For guidance/additional support, I would be inclined to use a brass tube ground to nest/rub against the hob.

Dry the filament, water acts as a plasticiser.

polyvinyl alcohol (PVA)
...with higher humidity more water is absorbed.
The water, which acts as a plasticiser, will then reduce its tensile strength,
but increase its elongation and tear strength.
[en.wikipedia.org]

What type of hot end are you using, post a picture.
You would want to use a hot end that is short as possible to reduce the dwell time in the barrel, and with a fan.
I would use some thing like this [www.thingiverse.com].

You could try to device a method to prestress the filament, i.e. stretch it to increase the stiffness.
If you're printing a small object, you might be able to do this manually or with simple tools.
Note: the center of the filament between the clamps will have the highest stress, so keeping the segments shorter will help even it out.

Tks for the Thermoplastics – Properties PDF, I added it to my list.

I'm using the standard hot end with the Makerbot Replicator Dual which looks like this. I believe they call it the Stepstruder MK8. The hot end in this extruder is looks pretty short, though I don't have any specs to compare it to.


Here's the list of techniques I'll try next week

- dry filament
- dry ice
- filament guide
- prestress? (looking for information on how this works and how to do it)

I'm also looking into pulling the PVA across a gap during extrusion in order to reduce the diameter. I haven't seen any one do this besides Jordan Miller with sugar glass.

Edit: I found this blog post from nophead/HydraRaptor 2007 about exactly what I described above! It looks like Makerware provides a pretty simple way of altering the feed rate to travel speed ratio!

Edited 1 time(s). Last edit at 03/14/2014 04:33PM by ironchief.