PDMS vat floor refresh method for DLP projector based 3d printer. Maybe, possibly, could be…

Note: see the updates at the end of this post.

I had a couple of ideas about methods of refreshing my 3d printers pdms vat floor knocking around in my head for a couple of weeks and I finally decided to try one. The root of this particular idea was a thread about the Solidator vat release patent over at buildyourownsla.com. Instead of going over what was discussed I will refer you to the thread: http://www.buildyourownsla.com/forum/viewtopic.php?f=16&t=44#p728 this thread was started by Anderson Ta who I met this weekend at the Midwest RepRap Festival along with Jordan Miller and other members of Jordan’s team at the AMRI institute at Rice University (I think my conversation with Jordan and Anderson spurred me to actually try the vat refresh method I had been thinking about). I also met Josh Ellis from Makerjuice at the RepRap Festival.

A PDMS (sylgard 184 etc) vat floor coating prevents an exposed layer from sticking to it by “inhibiting” the cure of any resin touching it (any resin touching the pdms stays liquid in a thin layer, resin above the layer solidifies during a layer exposure).  Oxygen diffused into the top layer of the pdms is what causes the inhibition. As the oxygen layer on top of the pdms is depleted the pdms vat floor becomes less and less effective as a release layer (this is why the B9 Creator uses a wiper to clear resin from the surface of the pdms between exposures, so the pdms can breathe and refresh the oxygen layer).

So what does all this have to do with refreshing my vat floor?  If maintaining an oxygen layer at the surface of the pdms is what is needed to maintain a good release when printing then why not “force” oxygen into the surface of the pdms before printing (pdms is oxygen permeable) and let it diffuse back out to the surface during a print run.

My printers vat floor was starting to get “sticky” (I can tell by how certain layers release during printing) so I was considering removing the pdms and recoating the vat but instead of doing that I decided to try the vat refresh idea I had been thinking about.

I took my printers vat and cleaned the pdms vat floor with a microfiber cloth and 92% alcohol and set it aside to dry for about a half an hour. After drying the vat I placed it in a vacuum chamber (see update 2 below) and put it under vacuum for about 5 minutes (my rationale for this was to boil off any remaining alcohol or moisture from the surface of the pdms).  After removing the vat from the vacuum chamber I then placed it in a pressure pot and pressurized the pot to 40 pounds and left it for 30 minutes. During the time the vat was in the pressure pot I setup a print run in creation workshop for a part that was a slight modification of the last part I had run. After 30 minutes I removed the vat from the pressure pot set it up on the 3d printer, filled it with resin and began the print run. Each layer was releasing so smoothly that for a time I thought the print had failed and had come off of the build plate. Even the first two long exposure base layers barely stuck at all. The printout was 442 layers using makerjuice subg+ yellow, the part printed out perfectly.

So, this is what I think is happening:

The non stick property of pdms, as it applies to 3d printing, is a surface effect caused by a thin layer of oxygen on the surface of the pdms that inhibits curing of the resin, ensuring that a layer of liquid resin is between the build plate and the vat floor at all times, sticking occurs when that layer of liquid resin is not maintained (the oxygen on the surface is depleted). PDMS is an oxygen permiable material, I believe by pressurizing the pdms oxygen in the air is diffused into the pdms to a depth determined by the amount of pressure it is exposed to and the length of time. When the pressure is released the oxygen diffused into the surface of the pdms “sees” normal atmospheric pressure as a slight vacuum and begins to slowly diffuse back out thus refreshing the oxygen layer at the surface of the pdms until the pressure is equalized.

Whether this holds up as a vat refresh method only time will tell.  I am going to use this method or some variation of it before making prints unless I find over time that it is not effective (if so I will let you know) but so far it seems to have worked well for me. Anyone out there that tries this let me know what your results are.


Update 1:

I found that vacuuming the vat then putting it under 20 pounds of pressure for 5 minutes rather than 40 pounds for 30 minutes seems to work just fine. I have used the vat several times since I originally wrote this post and it still releases very easily during printing and does not seem to be “wearing” very much. I did have a bubble appear between the pdms and the vat floor glass one time when I used 40 pounds of pressure for 30 minutes (which I was able to get rid of by placing the vat under vacuum for 30 minutes). I don’t know whether it was because I skipped the vacuum step while prepping the vat for the print run or too much air pressure but I will find out and let you know.

Update 2:

Skip the vacuum step it may cause bubbles to form under the pdms especially after the first pressurization. What I’m doing now is after a print run I clean the resin out of the vat with 91% alcohol then a water rinse then a spray of alcohol to displace the water from the surface of the pdms (to eliminate water spots) then I wipe down the pdms with a microfiber cloth while it’s still wet with alcohol  Dawn dishwashing liquid water and a microfiber “washcloth”.  I let the vat dry then pressurize it to 20  pounds for 5 minutes right before the next use.

I have found that the trick is to difffuse oxygen into the pdms but not all the way through it. If air makes it all the way through the pdms you will get bubbles forming between the pdms and your vat floor window glass/plastic. It appears that the pdms will outgas from it’s top and bottom surfaces if over pressurized so if you try this method start with conservative pressures and time and work up from there. I suggest 20 pounds of pressure for 5 minutes to start with for a 2-3 mm thick layer of pdms. It does not seem to take much time under pressure to “charge” the pdms.

So far so good.



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