Introduction: Ultrasonic Misting 3D Print Vapor Polisher
Vapor polishing is nothing new. Experienced 3D printing enthusiasts are well aware that you can smooth the surface of 3D printed objects by exposing them to the right solvents. There are a few existing methods for doing this, but each of them comes with unique drawbacks.
What's needed is a better solvent application method, and that's what this instructable is about. (Be sure to check out the video in the last step!)
The current vapor polishing methods and their drawbacks include:
1. Hot treat via boiling solvent in a crock pot or similar chamber (Potentially dangerous and very hands-on process)
2. Cold treat via slow release of solvent from paper towels in an enclosed container (Very time consuming, can't observe parts during this process.)
3. Spray canned solvent aerosol onto part. (Inconsistent surface finish, must be done outside)
4. Dipping parts directly into liquid solvents (Unpredictable/inconsistent finish, Likely over exposure of part)
All I wanted is a machine that lets me quickly drop parts into a transparent container and be able to press 'go' and have the machine produce a predictable finish on its own. I do not want to have to put together a really involved setup that may be a fire hazard, fume hazard, or something that produces unpredictable surface finishes. Essentially I want something as convenient as a microwave. I'm also forgetful so I don't want my parts to be destroyed if I forget that I left them in the machine.
The Ultrasonic Misting 3D Vapor Polisher is the solution to all of these problems.
This key component of this machine comes from ultrasonic humidifier, which uses a piezoelectric transducer (like a speaker) to create a high frequency mechanical oscillation in a liquid. This vibration forms an extremely fine mist of droplets in a fog/mist. The density of the fog is controlled by varying the intensity of the vibrations via a potentiometer.
This fog mist is very dense and wont move far on its own, so I used an aquarium air pump to blow it from the misting chamber into the glass finishing chamber where it can condense on the 3D printed part. This airflow keeps the air moving inside the finishing chamber, which helps produce a consistent finish on the part. The airflow system is open by necessity but we do not want the exhaust air to become a fume hazard so there is a water bubbler on the exhaust to absorb excess solvent. (Note that this only protects you when using water miscible solvents such as acetone.)
If a spot treatment polish is desired the tube going into the finishing chamber can be disconnected and manually directed onto the part where ever you want. This a great way to apply chemicals for solvent welding (aka gluing parts plastic together).
The other cool thing about this design is that the fog creating components are separate from the fog applying components. This means that you could use a micro controller to do some very precision applications of the fog. As it is, using a simple mechanical timer as shown in the drawing will still give you a ton of control.
Step 1: Polishing Chemicals & Solvent Science
Obviously you want to be really careful around solvents. Read the MSDS sheets for any chemicals you use and always wear proper PPE when handling them.
While this machine was designed to allow for use indoors the only solvent I recommend doing this with is acetone. All other solvents should be treated like poison.
Why is acetone special? Actually acetone is some pretty amazing stuff.
Acetone is miscible with water (they mix well), so the bubbler does a decent job of removing it from the air. The absorption of acetone by water is about 1 to 1 by volume, so keep your water fresh to minimize the smell. In any case acetone is perfectly safe for humans under normal circumstances and normal amounts. Believe it or not, the human body actually produces acetone in small amounts!
Acetone is the only commonly available solvent other than water that isn't classified as a VOC or HAP. This means that there are no regulatory restrictions on how much of this solvent we use. No other commonly available solvent is so free of restrictions!
Acetone is the fastest evaporating and one of the strongest of all commonly available solvents and it is very dry (non-oily). So it makes an excellent cleaner and degreaser and this is how it is used in most industries.
Though it doesn’t smell good, you would have to consume a lot of this stuff to be harmed. It is just an irritant in low concentrations, all the bad stuff happens at high concentration levels. Acetone is not regarded as a carcinogen, a mutagenic chemical, or a concern for chronic neurotoxicity effects.
Cool, so what solvents work best for 3D printed plastics?
Acrylic: Most Solvents
PLA: MEK or 'MEK Substitute'
PVC: Most Solvents
Polycarbinate: Pretty solvent resistant
Nylon: Pretty solvent resistant
Polypropylene: Pretty solvent resistant
Polyethylene: Pretty solvent resistant
*Always clean your chambers before switching to a new solvent. This is no joke, mixing chemicals as common as ketones & alcohols can cause an explosion. When in doubt consult a chemical compatibility chart: http://www.safety.vanderbilt.edu/chem/chem-compati...
*Solvents do not "melt" plastic, but rather they dissolve them. Melting means a physical change from solid to liquid caused by an increase in thermal energy. The reaction here is more similar to what happens when you pour water onto a sugar cube.
Step 2: Bill of Materials
(1) Air Pump & Check Valve: Aqua Culture 1/2 Gallon Betta View with Base (Same one available at Walmart)
(1) 8"x6"x3" Project Box: www.radioshack.com/project-enclosure-8x6x3-...
(1) Ultrasonic Humidifier:Crane Adorable Ultrasonic Cool Mist Humidifier - Frog
(1) Airtight Glass Jar: A 1 gallon pickle jar would be perfect.
(1) Brewing Airlock: Make your own like I did or get a fancy one here: Cylinder Airlock
(1) Empty Paint Can: 1 Quart or 1 Gal, available at any hardware store.
(3 ft) 1/4" OD polyethylene tubing: available at any hardware store.
(1) 90 deg threaded quick disconnect fitting for 1/4" OD tube: available at any hardware store.
(3) Straight threaded quick disconnect fitting for 1/4" OD tube: available at any hardware store.
(1) 5 Minute Spring Loaded Mechanical Wall Timer: Intermatic FD5MW 5-Minute Spring Loaded Wall Timer, White
**Note: Some parts used in the photographs are different than are named here. I used a 7x5x3 project box but I think it is too small. Also, I used a 1 quart paint can but I think 1 gal would be better because it could hold an entire jug of acetone. Finally, it is critical that your finishing chamber is airtight. For this reason I do not recommend using the one I got from Walmart (seen in the main photo).
Step 3: Take Apart & Prepare the Humidifier
My wife was sad that I took apart her frog humidifier, but I insisted on dissecting it for science!
The humidifier comes with a power cord, power switch, potentiometer, float switch, and on/off light. We will use everything so there is no need to cut out any electronics. All you need to do is take apart the humidifier with your screwdriver.
When it is all apart all you have to do is cheat the float sensor into thinking it is ON all the time by ziptieing the float in the up position. Note: Its probably best not to operate the vibrator when there is no liquid, im sure theres a reason they installed this float switch.
In photo 5 i cut the power cord to the aquarium air bubbler soldered it onto the humidifier power posts. This makes it so that the air bubbler and the humidifier are powered by the same power cord, and are only on when the main power switch is on.
If you choose to install the optional 5 Minute Spring Loaded Mechanical Wall Timer that I recommend then you simply install it in series with the main power, like a switch. The installation directions will come inside the timer box.
Step 4: Prepare the Paint Can (Misting Chamber)
In this step you drill the holes in the bottom of the paint can so you can mount the vibrator. Measure and drill 4 mounting holes and one big through hole for the vibrator itself. The big hole will be leak proof because of the rubber seal but the tiny holes are trickier. Solvents will dissolve most any glue you use here so drill your mounting holes as small as possible. I used 2 part epoxy to help seal it in and I haven't noticed any leaks.
When you drill the holes in the lid for the tube fittings make sure to size them as small as possible. You want the fitting to screw itself in and seal the hole mechanically as much as possible because it will be difficult to glue over a large hole without the solvent destroying the glue soon after.
EDIT: Silicone glue is a great choice for solvent resistance.
Step 5: Assemble the Project Box
Notice from the very first photo in this instructable that I ran the air bubbler tube up the outside of the paint can, rather than hiding is on the inside. I did this to avoid having to seal up another hole on the base of the can.
Its difficult to see in the pics but I added a slice of foam around the air bubbler to prevent it from vibrating against the box and making noise.
After you jame everything in there you are done! I'd sujject letting all the glue dry for 24 hours before butting any solvents into the chamber. It'd probably be better to test the equipment out on water before you just straight to solvents. Also if you bypassed the float switch like I did then be careful not to turn the machien on when it is empty or bad things will probably happen.
Step 6: Mechanical Testing of Acetone Polished ABS Parts
I was curious what effect acetone vapor treatment would have on the mechanical strength of abs specimens. Good thing I happen to have a TestrBot Universal Test Machine to figure it out.
The testing was pretty conclusive that treating abs specimens with acetone vapor caused them to lose strength despite having given the specimens a full 24 hours to dry out. Skeptics can check out the attached pdf file for detailed test results.
This result was surprising to say the least. I expected the polishing to strengthen the parts by reducing surface stress concentrations. My highly speculative guess for the resulting loss of strength is that the exposure to the solvent caused a permanent chemical change to the surface of the parts resulting in a softening affect.
Looking at the inside of the parts and it is clear that the treatment did not penetrate very deep. It is interesting to note that the mass of the specimens was measured before and after treatment and right before they were tested. Specimens increased in mass by about 0.2g right after the treatment but they went back to their normal weight after 24 hours.
To be fair, this was not an exhaustive study. I tested a handful of specimens in a single orientation. But then again, my standard deviations were low so my setup is clearly capable of producing repeatable results.
All things considered, having an ultrasonic vapor treatment chamber could still be beneficial to anyone wanting to improve the aesthetic value of their 3D printed parts.
Check out this video showing the whole process:
UPDATE 9-2-15: I've just released a new batch of test results that show how to optimize 3D printed parts for strength.