It's been over a year since I published a popular Instructable about ultrasonic misting ABS 3D printed parts with acetone vapor. A couple months ago I was inspired to revisit this project by the professional 3D printing "Polysher" machine on Kickstarter.
In this instructable I've redeveloped my original ultrasonic polisher proof of concept into a more polished and professional tool with key functional improvements.
Additionally, after perfecting the machine I performed experiments involving mechanical testing of acetone polished ABS specimens in various orientations; and I observed some very interesting results! (See final step)
In case you didn't see the original project, my goal has always been to build "...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."
The original project involved repurposing the electronics from an ultrasonic water humidifier so that they could be used for acetone instead. The acetone mist/fog was then blown through an open system and filtered through a water bowl before venting into the room.
The result of that project was a functioning polisher that I could operate inside the house, but not without a bit more effort than I had wanted. In practice the water in the filter would readily absorb the acetone, but the water needed to be changed after every use and the process was wasteful with the acetone. Additionally, the form factor of the entire machine was not visually pleasing or fun to lug around.
So after a couple design iterations I've improved the old design into something even more awesome!
The user interface is now simply an ON/OFF switch and a single momentary button for 'go'. The new design now uses a closed pumping system and has been optimized for maximum fog production; So it is completely sealed allowing for indoor use, it takes less time, consumes less acetone to do the job, and no water ever needs changing!
And finally the entire machine is built into one single transparent assembly, so it looks good and is easy to transport to Makerfaires! (See you at PDX-mini this fall!)
Step 1: Polishing Chemicals, Safety First!
The first thing that must be said here is that the solvents used by this machine are flammable and irritating to your eyes, skin, and respiratory system. Always read the MSDS sheets for chemicals before using them, always wear appropriate personal protective equipment when handling chemicals, and always keep solvents away from open flames, sparks, and other possible ignition sources!
Personally, when pouring chemicals I just wear safety glasses and make sure to have adequate ventilation. If you have super sensitive skin then use latex or viton gloves are resisn(not nitrile or PVC).
Despite what is commonly repeated, acetone (and isoproptyl alcohol for that matter) is actually a fairly safe chemical as far as human exposure is concerned. Though it doesn’t smell good, you would have to be exposed to a lot to actually 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, nor a concern for chronic neurotoxicity effects.
The device in this instructable uses materials which enable the safe indoor use of acetone for polishing ABS or isopropyl alcohol for polishing 'PolySmooth' material. Specifically, the tubes are EPDM rubber and latex rubber. All clear containers used are polypropylene. Please do not mix chemicals or use different chemicals than I've recommended without doing appropriate research first!
Step 2: Gather Tools & Bill of Materials
Tools: Electronics soldering equipment & wire strippers, drill & drill bit set, light duty hand tool set.
None of these parts are optional nor are many easy to substitute for. Fortunately they are all readily available online at the links below. If you are upgrading from having built the original device then you'll need all new parts except for the humidifier itself.
(1) Crane Adorable Ultrasonic Cool Mist Humidifier - Frog/cow/pig/owl/duck/etc :)
(1) Solvent-Proof 12V Pump Assembly (This critical part can only be found through this link. I know because I searched forever for a solvent proof pump, until I ended up sourcing all the parts and building it myself! This little baby turned out great though! It can run continuously pumping liquid, gas, and anything in between.
(1) RTV Silicone Glue (Silicone is very resistant to chemicals)
(1) 12V Digital Timer Delay Relay Module (The 3D printed case files are provided in step 5)
(1) Computer Case Fan (I used 40mm^2 x 20mm deep) (A fan is NOT optional, the piezo-electric shaker from the humidifier creates heat that must be removed from the system.)
(4) Adhesive Mounting Strips or a roll of Double Sided Foam Tape (For attaching bins together. Most glue wont work for this because polypropylene is so chemical resistant!)
Recommended Tube Fittings (Likely available at your local hardware store. Mcmaster Carr Linked.)
(2) Nylon or Acetal Barbed Tube Fitting, Straight for 3/32" Tube ID x 1/4"-28 Male Pipe
(1) Nylon or Acetal Barbed Tube FittingStraight 1/2" Tube ID x 3/8 Male Pipe Size
(1) Nylon or Acetal Barbed Tube FittingElbow 1/2" Tube ID x 3/8 Male Pipe Size
Step 3: Take Apart & Prepare the Humidifier
Dissect the frog (humidifier)! The humidifier comes with a power cord, power switch, potentiometer, float switch, and on/off lled. 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!
The potentiometer controls the fog intensity. In this version i decided to max it out and just leave that control inside the box. Better to make more fog and adjust the timer for less time!
When it is all apart all you have to do is cheat the float sensor into thinking it is ON all the time by zip-tieing the float in what was the up position. Note: Do not to operate the humidifier dry! Im sure there's a reason they installed this float switch.
On that note, the humidifier circuit has been pretty durable in practice. I've only ever bought one and it got heavily handled while I iterated designs to get to this point.
Step 4: Prepare the Liquid Solvent Reservoir
In this step you drill the holes in the bottom of the plastic 1.2 cup tupperware so you can mount the vibrator. Measure and drill 4 mounting holes and one big center 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.
Fasteners shown are 2" long #8 pan head machine screws. But you can use anything you want.
Ive determined from research and lots of trial and error that silicone glue is the only way to go. Use it liberally on both sides of hole seals. 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 thread itself in and seal the hole mechanically as much as possible.
Maximizing Fog Production: During the new build experimentation process I noticed that sometimes the fog production would be inconsistent for seemly no reason. After some experimenting I determined that the amount of fog production was controlled by 3 critical variables in order:
1. Everything needs to be completely air tight. I even used silicone to glue the misting container shut to make 100% sure it was sealed.
2.Depth of the acetone in the misting chamber (shallow = more fog)
3. Diameter of the fog delivery tube (wide tube = more fog)
4. Length of the fog delivery tube. (short tube = more fog)
Step 5: Wire Up the Digital Timer & Solvent Pump
This design is a case for a digital programmable timer relay, which turned out to be a handy little tool. You can use it to turn electrical devices (AC or DC) on and off at programmable intervals, or when activated by a switch. For detailed wiring, see diagram.
You can program the time from 0.1 sec up to 999 minutes. It can also switch AC OR DC loads. Anyway I sat down and created some instructions as well as the 3D printable case model attached to this step.
*The Pump and cooling fan get 12VDC power directly. They are ON as soon as the main power switch is turned on.
*Fogger circuit is the only thing controlled by the timer relay. The momentary button is wired to trigger this timer relay to turn ON the fogger and begin a countdown. When the countdown is done the timer relay switchs OFF the fogger. (If you press the momentary button while a fogging is in progress then the timer starts over
Step 6: Prepare the Big Tupperware
Drill 4 tiny holes and 1 big hole to mount the fan so that it can suck in fresh air. Drilling accuracy is not important but you do want to make sure you locate the fan so that it blows onto the bottom of the solvent reservoir.
Drill a couple dozen small holes on the opposite side so that the ventilation air flows all the way across the box.
Drill two holes for the fittings and use the fittings themselves to thread their own holes. Make sure to position the big fitting (fog inlet) below the small fitting (air re-circulation), otherwise you will suck out fresh fog!
Use the double sided tape to attach the two bins together. I recommend this rather than glue because glue doesnt stick very well to polypropylene because it is so chemical resistant.
Step 7: Drop Everything in the Tupperware
At this point you should have a big ball of functioning electronics and all you need to do is jam them in the box. Use the double side tape to hold the electronics where you want them so they dont shake around.
The pump will function sideways and upside down, but not if the tubes are connected backwards. The shorter tube is the pump output. This gets connected to the solvent reservoir as shown in the diagram in the introduction.
Step 8: FILL 'ER UP, PLUG, AND PLAY!
I got a funnel from the dollar store to aid filling. Only fill it halfway, less fluid = more fog but too little fluid may hurt the misting hardware. Don't run it empty! On that note I have noticed that this design is much more conservative with the solvent because it is being recycled within the system. That giant can of acetone will last a lonnng time.
EDIT: Since I forgot to say this anywhere else, I should note that it doesn't necessarily take 24 hours to smooth a print like I showed in my video. The actual exposure time is limited to the programmed time (360 seconds here) that the ultrasonic is ON, and for a little longer for the residual fog to clear out. The longest step is waiting for it to dry.
Acetone leaves the plastic surface too squishy & sticky to touch for a while. You can open the lid to let it dry sooner, but I just let it sit for a day all sealed up. Being able to do that right there is one of the biggest benefits of this contraption, that it only applies a dosed amount of acetone. If you forget to take your part out you won't come back to find a puddle of goo.
Step 9: Mechanical Testing of Acetone Polished ABS Parts
Curious about the mechanical effects of exposing ABS plastic to acetone, I used a previous Instructables project to perform some experiments. Last year I built a small universal load testing machine (the Testrbot) that can measure force and displacement to high degree of precision.
So over the last week I 3D printed ABS specimens in two different orientations, treated half of them with acetone, and broke them on the Testrbot to see how the orientation and acetone affected the printed parts.
The results were very interesting! (complete report attached.)
My testing shows that acetone treatment has two measurable opposing effects on ABS FDM 3D printed parts:
1) A chemical weakening of the material structure
2) A mechanical strengthening of layer bonds via the reduction of surface stress concentrations. (This effect was anticipated but up until now it was never tested for.)
This testing has shown that effect #1 outweighs effect #2 to decrease the part strength by 9% in all stress conditions other than Z-axis loads, in which effect #2 outweighs effect #1 to increase the part strength by 31%.
Regarding effect #1, these new results agree with previous testing done (by me in 2015) in effect but not in magnitude. My hypothesis is that the additional drying time that I gave the new specimens helped remove all traces of acetone which may have contributed to additional softening of the specimens in the previous testing. (Well, that and because I put all the parts into a DIY plastic dehydrator for 3 hours to aid drying!).
The overall effect of Acetone vapor polishing on ABS effectively makes parts somewhat more isotropic. That is, they react more uniformly to applied loads from various directions. In this case, polishing sacrifices strength in their strong axis to increase strength in their weak axis.
Well that's all I've got today. That was a few months of work there so I hope you enjoyed it and that you take a second to vote for this project in the contests!