[maestro]

Hi,

I have a project which requires me to 3D print vascular models derived from computed tomography data of patients. After printing, I embed them in transparent silicone. When the silicone solidifies , I need to dissolve the printed model, which will create a map of the vascular tree in the silicone. This mold is going to be used for angiography training. My problem is in the dissolving process. I use a PVA (Polyvinyl Alcohol) filament, which is supposed to dissolve in water. The dissolving process takes too long (1-2 weeks) and I have to manually extract the softened pieces, they don't flow themselves, they don't liquefy. As a result, there are places that I can't reach due to the tortuous structure of the vessel and these parts remain in place. My question is: How can I enhance the solubility of PVA? 

[Arkcon]

Maybe with some heat you could dissolve it better.  Maybe ultra sound would shake the partially dissolved filaments free?

[maestro]

Maybe with some heat you could dissolve it better.  Maybe ultra sound would shake the partially dissolved filaments free?

I have tried using an ultrasonic cleaner. The ultrasonic cleaner I have bought was imported form China and was a cheap one so I don't know how effective it was but it was taking really a lot of time to heat the water and apparently couldn't go over 55 degrees Celsius. Basically it didn't help. The major problem in my case is probably the surfaze area of the object in contact with water. As tho object is in a silicone block, only its upper and lower parts are in contact with water. This wouldn't be a problem if the dissolving part would liquefy by itself but it doesn't and requires me to pull it manually (which becomes impossible in the inner parts due to the tortuosity of the object). Would boiling the part help dissolving it? Some sources on the web tell that the dissolved PVA should change the color of the water to white and and act like glue but in my case it doesn't . 

[billnotgatez]

I am curious
What brand of PVA are you using
is it
AquaSolve Water Dissolving PVA

[pgk]

PVA is rather quickly dissolved in warm water 40-50oC. Supplementary, you can add a little amount of a co-solvent e.g. ethanol or acetone, if compatible with silicone and other materials.

[maestro]

I am curious
What brand of PVA are you using
is it
AquaSolve Water Dissolving PVA

I have tried 3 different brands: Actually I am using Esun's PVA which is very easy to print but very hard to dissolve. I have tried a sample of Aquasolve but I had a lot of difficulty in the printing process. While Esun's PVA is quite flexible after being printed, Aquasolve sort of crystalizes and becomes very very brittle. This is problematic because my model gets destroyed while trying to remove the supports (it might be due to my inexperience in printing with it tough). I've also tried PVA by Orbitech, which was behaving like Aquasolve, slightly more printable. I have asked in a few  3D printing  forums about the solubility of different brands of PVA but I didn't recieve an answer yet.

Today I have tried to put the silicone block in a pot filled with water and inserted it in an oven. Heated at 120 degrees for 2 hours. I still can't get them to liquefy. Still need to use a metal tool to remove them and they can only detach piece by piece.  I wonder if it is due to an additive in the PVA. I've observed that different PVA brands have different colors and consistencies which might be due to additives.

[pgk]

I am afraid that the PVA filament that you use is not polyvinyl alcohol but rather polyvinyl acetate or a polyvinyl alcohol-polyvinyl acetate copolymer.  Anyway, polyvinyl acetate swells in water but it is not water soluble, but soluble in ethanol and ethyl acetate.
Please take a look to the links, bellow that refer to water soluble filaments for 3D printing.
http://printelize.com/en/MaterialGuide/PVA
http://www.pla3dprinterfilament.com/sale-3083098-water-soluble-polyvinyl-acetate-pva-3d-printer-filament-1-75mm-nature-color.html
http://www.fazendomedia.com/sale-china-pva

[maestro]

Thanks for the links. I wasn't aware that PVA could also be used as an abbreviation for Polyvinyl Acetate. You might be right about Esun's natüre, I can't verify it online (as the website doesn't tell if it is acetate or alcohol). The links you have provided are for polyvinyl acetate filaments and they all tell that they are water soluble. Isn't that contradictory with what you have told in your post?

I can try to dissolve it with ethanol if it is soluble. I don't know where to find ethyl acetate (or even if it is safe) so I can't try that one.

The Aquasolve filament seems to be polivinyl alcohol by the way... 

[pgk]

The links were indicative and refer either to polyvinyl alcohol either to polyvinyl acetate for3D printing. Just, search in the Web.
Waterborne vinyl acetate can be made by adding a non-ionic emulsifier or an intermolecular (block) emulsifier such as:
-CH2CH(CO2CH2CH2OCH2CH2OH)-

[maestro]

I have found Aquasolve's composition on the web:

"AquaSolve™ - PVA filament is a compound of PolyVinyl Alcohol (>75%), Aliphatic Polyol (<20%) and Calcium Distearate (<5%)"

Does it give any tips for its solubility?

[pgk]

You have to check it by experiment, if you cannot find the technical data sheet or any other technical information.

[Intanjir]

Googling MSDS Esun PVA Filament confirms that it is polyvinyl alcohol. http://devel.lulzbot.com/filament/MSDS/esun/msds_esun_pva.pdf
It also mentions polystyrene, perhaps this is in the filament as well, but I think that more likely it is an error since the CAS number/formula given for it is just the one for the PVOH.

While PVOH is water soluble it is often not rapidly dissolved, particularly for longer length chains or highly hydrolyzed samples that are perhaps better mechanical materials for printing. To dissolve it quickly it comes in big granules that you try and add to water which you make very very hot(90 degrees C not 55 degrees) while keeping the granules separate from each other for as long as possible or else they will clump and form a thick gel with limited access to additional water. ie It is all about high temperature, high surface area, and plenty of stirring. What you are trying to do with it, having it in narrow long tubes is a worst case scenario for surface area and stirring.

With your only remaining weapon being temperature you might try putting it in a pressure cooker for a day. I notice a big improvement in the rate of dissolution near boiling vs. ~80 degrees. 120 degrees in a pressure cooker might actually do the trick.

Or you might just use a solvent with a higher boiling point in the first place. Glycerol will dissolve PVOH at high enough temperature, and won't boil even after the PVOH would have melted(though the air may smell sweet as the vapor pressure becomes non-neglible). You would probably want to stop short here though (maybe 180 degrees?) since your silicone will have issues at around these temperatures. This is unfortunate since the mixing would go much faster if the PVOH was liquified.