Acrylic usage in SLA printers

AD001AD001 Member, Backers
It's bothered me for some time that acrylics are so prevalent in SLA machines of all flavors (ours included). I see it in vats, mounts, shields, covers, etc depending on the machine. Unfortunately, this particular material seems susceptible to spalling if not properly cleaned immediately after resin contact- a bad choice for printers, I'd think! Over time vats become tacky, appear to slightly dissolve on the edges, etc.

Why is it so prevalent? I'm aware that it is inexpensive and easy to find in everyone's favorite orange, but it just doesn't feel like the right material. UV transmissability isn't a huge concern I'd think, since most of us are actually referring to the low blue spectrum when we say 'UV' curing (~400nm+), which other plastics can handle (PC, for example). Even then, with flex vats becoming more popular, the only transmission-critical part of the printer is FEP. Color and chemical resistance could be applied via a coating or film treatment, but this isn't done, to my understanding. One of my easier Draken mods was to layer my big acrylic door with a thin vinyl film, for example. No more spalling!

The mix of chemical resistance and mechanical properties in an inexpensive, available plastic is the best reason I can come up with- and I don't consider it a bad one in many instances. What confuses me is why it is used in places that don't require it, and where other materials might perform better.



tl;dr many parts of the printer could be made out of a more chemical resistant plastic but aren't, and also aren't even coated with a protective layer (spray, film, etc) in many cases. Is acrylic just 'good enough?'

Comments

  • rkundlarkundla Member, Moderator, Backers, Ultimate Backer
    Acrylic is easy to get in all sorts of sizes, thicknesses and colors. You can cut it with many types of tools and it is easy to assemble it with cement. That all equals low cost and higher margins! 

    In a way, that ease of use does have disadvantages. Resistance to certain chemicals, in particular the acrylates in the resin, varies based on the chemical composition. I used that garbage resin from Bucktown Polymers that ate one of my acrylic vats - nasty chemical. Now my 3Dfacture green castable resin if left on the powdercoating too long will eat that off to bare aluminum, as well as the finish on the wood table my printer is sitting on. :o It hasn't hurt my vat though, so I got that going for me. ;-)

    I like the idea of a coating and wish there was one that was transparent that could be sprayed onto the acrylic. I wondered if wax could help, but never tried it. Attaching something like FEP or other PTFE-family film would work, but OMG, putting that sticky backed film on top of a new PDMS layer in the vat was a job I wouldn't wish on my worst enemy. The film always looked like crap when I applied it and was happy to get a section that didn't have slight wrinkle, a piece of small unidentifiable debris or a bubble that would not move and you were not allowed to pop since resin will find its way under the film.

    My two covers have various damage from the resin, and I even think that fumes are enough to cause cracking at the cut sections, but, the lid is just there to keep some of the dust out of the print. Most ambient light can't cure the resin sitting in the vat from my experience so far and having two metal doors that open like a cabinet is probably a good enough way to do it. Maybe it is more about the form than the function? ;-)



    Ron
  • AD001AD001 Member, Backers
    Ron, 

    Thanks for the notes! I'll agree re: form vs function. I think a lot of folks like to be able to watch prints? I've seen resins eat through the powdercoating as well. 

    Off topic, but what are you using for your adhesive-backed FEP? How did your PTFE experiment go? I've been using bare FEP sourced from FlexVat.com and it has served me far better than the stuff MuVe sells. I find the bare stuff easy to apply but, as you might imagine, it can get resin under it, significantly reducing its usable lifetime.


  • rkundlarkundla Member, Moderator, Backers, Ultimate Backer
    I had that MuVe film and it kinda worked alright. I bought film at McMaster-Carr in 12"x12" adhesive sheets as well and that seemed okay when I could put it on right.

    I couldn't find a coating that seemed to make a difference using the acrylic vat. I was hoping to get something to fill the microscopic gaps in the surface to help with the peeling but there was no significant difference. I do now after every couple of prints put a small amount of isopropyl alcohol on the FEP film in the vat and wipe it around with my finger tip. I do this to try to clean the surface of any fine resin that might be trapped on the surface or in small anomalies that could increase the peel resistance.

    Other than that, I've pretty much stuck with the flex vat prototype. Due to the film sticking to the acrylic window in the original work, I had a new acrylic window that was like 0.5 - 1.0 mm thinner. This put an air gap between the stretched out FEP film and the acrylic window and has given me some better performance during the peel process. I don't have a gasket so if I leave resin in there too long, it will start to wick outside the vat.
    Ron
  • AD001AD001 Member, Backers
    Gotcha, thanks! I've got a small container of cell phone screen adhesive- I think I'm going to try using a nearly-nonexistant layer of that to adhere the FEP to my silicone. Will report on results!
  • rkundlarkundla Member, Moderator, Backers, Ultimate Backer
    Oooh that sounds like an interesting idea! I wonder how difficult it would be to remove the film if it was damaged?
    Ron
  • AD001AD001 Member, Backers
    LOCA used in phone screens becomes easier to remove upon heating, so I'd assume that would help. I'm also assuming that the resin heat conditions won't be sufficient to critically affect its performance (based off of my experience with finger-measuring resin temp). As a minor update, I straight up couldn't get the LOCA (liquid optically clear adhesive) to cure between an aluminum foil/FEP test sample with daylight, flashlight, or postcuring chamber. I'm going to check it on a few different materials; my LOCA might be very specific in its curing wavelength but I want to see if it might be the FEP/aluminum that is preventing it.
  • rkundlarkundla Member, Moderator, Backers, Ultimate Backer
    I guess something could inhibit it, maybe the aluminum...

    What brand of LOCA are you using?
    Ron
  • AD001AD001 Member, Backers
    X-factor sticky-icky 1000 ultra low viscosity. I got it a bit back. I haven't actually verified that the tube is still good- I'm planning on doing so tonight/tomorrow!
  • AD001AD001 Member, Backers
    A minor update: I've tested my LOCA on plastic wrap in a thin-spread film and blob under post-curing lighting for maybe ~30 minutes for thoroughness. The thin film barely cured, if at all, except for a few spots where it was thicker. The blob cured besides for a film on top of it. This leads me to believe that LOCA (or mine at least) is heavily affected by oxygen inhibition. 

    I'm going to try it again with FEP covering the thin film and a thicker 'blob' section and see if that is sufficient to cure. I may also deposit some LOCA into a small dish of water and see if that is sufficient.
  • AD001AD001 Member, Backers
    edited January 23
    Update: I've tested with FEP covering a thin film and a thicker layer backed by plastic in air, LOCA in water, and finally FEP covering a thicker layer backed by plastic in water. 
    The FEP with a thin film of LOCA provided no noticeable curing. Nonviable.
    The FEP with a thicker layer provided good curing with a minor uncured layer at the FEP/LOCA interface sufficient to prevent adhesion. Nonviable.
    The LOCA in water cured but dispersed.
    The FEP covering a thicker layer immersed in water fully cured but, unfortunately only provided a minor benefit with regards to keeping the FEP stuck down. I would conclude that if a thin layer of LOCA were spread and properly squeezed out between the FEP and silicone, the tank filled with water, and postcured for 1/2 hour to an hour (maybe less, I simply was testing work/not work vs time to work), we may see noticeable, but not significant, increase in retention of the FEP to the tank. 

    I conclude that this is not worth rushing out to try yourself considering the marginal-at-best benefit and that this has not been tested at scale. When it comes down to it, there's a reason we use FEP- stuff doesn't like to stick to it! I'm somewhat curious about the adhesive application process used in the adhesive FEP and teflon films.
  • rkundlarkundla Member, Moderator, Backers, Ultimate Backer
    @AD001 looking at the following 3M datasheet for one of their LOCA products:


    It is acrylate based chemistry... very similar to the UV resin we print with on the Draken. They mention the following:

    "Cure of surfaces exposed to oxygen may be inhibited resulting in a more tacky, under
    cured surface. Curing with high intensity short-wave UV light or covering the surface to exclude oxygen is suggested for
    a more completely cured surface."

    So you either need a high intensity short-wave UV light (like a unshielded mercury lamp) or cure without oxygen. Easiest way I can think of for that is to put the vat in a vacuum chamber with the UV light. However, what I have read indicates it is very difficult to remove the oxygen without significant equipment and even flooding the container with inert gas like argon. :-O

    Maybe the mercury lamp would be better. :p

    Page 5 of this document has some good information:

    Ron
  • AD001AD001 Member, Backers
    An argon flood isn't out of the question; I have access to a welding lab and argon is used as a shield gas. A bit out of the way, though!
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