Printable one-way air valves: Need advice for removing very thin PETG from smooth steel sheet
 
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pipe2null
(@pipe2null)
Active Member
Printable one-way air valves: Need advice for removing very thin PETG from smooth steel sheet

I've been messing with my 1st layer Z setting trying to optimize the printing of very thin PETG on a smooth steel sheet heatbed.

I'm working on 3DP printable one-way air valves, so the flap part has to be very flexible (thus using PETG), but also must have a very smooth finish on the air-seal-side of the flap.  For flexibility, the flap has to be very thin appx 0.1mm - 0.2mm thick, the thinner the better as long as it has a minimum of 2 layers to fill in 99.9% of the microscopic gaps and give the flap a bit more structural integrity.  The flap directly connects to a more rigid frame so the overall part is rigid with known dimension to be used in other designs.  For example, replacing the "rubber glove" valves of the HALO PAPR.

The ideal for the flap design is 2 layers at 0.05mm each, for total 0.10mm thick flexible flap.  3x 0.05mm layers could also work, possibly the sweet spot between flexibility and structural integrity (don't know yet).  2x 0.1mm layers does work, but the flap suppresses air flow more than I'd like for small diameter valves due to the stronger "spring action" of the thicker plastic.

Here are example initial prints of my ultra basic proof-of-concept design:

Upper left removed cleanly from sheet but you can see the traces, which make for a poor air-seal.  Upper right is a bit more smooth, but was deformed while removing from sheet.  Lower left is the smooth-sided frame for the intake side of the valve.  Lower right is (a not great print) how the 2 parts stack together to form a one-way air valve.

My problem:  Too much AND Too little bed adhesion:  With vertically higher 1st layer Z, PETG print quality and the air-seal side (1st layer printed on smooth sheet) of the part suffers a little with less squish of individual traces and more print fails from not enough adhesion, but I can get the delicate part off the sheet without damage.  But lower 1st layer Z gets a great smooth and really good air-tight seal but 99% of the delicate parts are damaged or deformed attempting to remove them from the steel sheet.  A yield of 1 usable part out of 5 or maybe even out of 10 prints might be good enough, but 1 out of 100 is NOT...

Advice?  The only solutions I can think of, none of which I know of, is either use a post-process that maintains part dimensions to get a smoother finish after printing less-ideally-smooth parts, or to use some cleaver way to safely remove ideally smooth parts that are REALLY stuck on the sheet without damaging them.  I want to make these designs and any outside-the-norm post-processing requirements accessible and do-able for anyone with a Prusa (or any 3DP).  I'm using PETG since more people can print with that than TPU.

Posted : 21/04/2020 9:50 pm
kennd
(@kennd)
Estimable Member
RE: Printable one-way air valves: Need advice for removing very thin PETG from smooth steel sheet

Be careful of replacing the flutter valves on the Halo PAPR! They have been tested in a medical environment using monitoring equipment to verify airflow, SpO2, and CO2 removal from within the PAPR. All the components of the Halo PAPR are designed and chosen to work as one. Unauthorized changes could endanger people. Also. the licensing does not allow derivatives for that reason. Best you speak directly to the doctor that designed it.

Posted : 22/04/2020 12:02 am
Bunny Science
(@bunny-science)
Noble Member
RE: Printable one-way air valves: Need advice for removing very thin PETG from smooth steel sheet

I could never get a thin sheet to reliably come of the plate with good flatness to go with a 3D printed one-way valve. If my printer can't do it easily, I could not expect others to produce parts with that method. That's why I went with the time tested Heimlich flutter valve that is easily reproduced and closes with very little negative pressure. 

@Kenn is correct in pointing out why I did not allow derivatives in the license. The flow resistance of the output has been balanced against fan and filter in the Bunny Science Halo PAPR. The PAPR airflow needs to be balanced because the filter flow rate is limited compared to a loose fit PAPR.

If you do manage to create a reliably printed one way valve with fast, low pressure closure, it would be great. I could never get such thin layers to come off the plate reliably flat enough to ensure closure even with larger pressure differentials.

Posted : 22/04/2020 12:24 am
pipe2null
(@pipe2null)
Active Member
Topic starter answered:
RE: Printable one-way air valves: Need advice for removing very thin PETG from smooth steel sheet

Understood, and thanks for the Halo PAPR!  It is awesome goodness!  I'm not trying to replace anything on the PAPR, I just used that as an example use for a printed one-way valve.

The project I'm working on is a printable air regulator of sorts.  The intention of the project is to reuse the head straps and face pillows (with minimum to zero modification) of standard CPAP/BiPAP full face masks, but using a printed air regulator instead of the normal hose->"face pillow" connection.  The goal is to capture contaminated exhaled air and pipe it away for (possibly UVGI) sterilization or at least pipe it somewhere else that is safer for medical personal or family members if used at home, whether or not the user is actually known to be sick.

The air regulator is pretty tricky.  It needs a one-way valve for inhaled air to provide extra volume from ambient environment beyond the volume rate provided by CPAP, another one-way valve to prevent backfeed to the CPAP machine, and maybe/possibly another one-way valve to prevent reentry of exhaled contaminated air.  The extra tricky bit is using the positive pressure provided by the CPAP machine and the variable pressure within the facemask to create a balanced piston or similar type valve that controls the release of exhaled contaminated air down the out-flow pipe but still maintains the level of positive mask pressure set by the CPAP machine settings.  Since it's a "balanced" piston valve, it should work for any pressure level the CPAP provides.  The CPAP provides constant positive pressure, but the actual pressure inside the facemask varies slightly on inhale and exhale, which provides the imbalance needed to open the valve (slightly higher mask pressure caused by exhale) and allow air flow out of the mask and into the "drain pipe" so-to-speak.  Wherever the exhaled air is piped, it has to be at the same ambient pressure as the CPAP machine and user.  I'm not sure how BiPAPs deal with the lower secondary pressure level, so this type of setup with pressure regulation at the mask might not work with BiPAP, but CPAP should work ok.

So, that's one of my current overly ambitious projects.  It is completely possible to create a printable regulator like this, but whether or not I manage to figure out a viable design is a totally different story.  So, NO, I WILL NOT RELEASE PRINTABLE FILES unless the design has been thoroughly reviewed by someone with a helluva lot more medical experience and knowledge than I have.

 

So, comments, thoughts, and ideas are welcome, especially if they help make membrane-thick plastic parts easier to successfully print.  😊 

Posted : 22/04/2020 2:29 am
Bunny Science
(@bunny-science)
Noble Member
RE: Printable one-way air valves: Need advice for removing very thin PETG from smooth steel sheet

Ah. OK. Was just concerned you were going to do that with the PAPR. It's not just a valve there, but the Heimlich valve's finger volume is also a small reverse flow volume to allow cut-off time before bad air can be entrained.

Keeping environment around a COVID+ CPAP user clean is a great project idea.

The only thing that lets me get a thin layer of PETG reliably off the plate is to heat shock the hot print plate with alcohol sprayed upon the printed part. That will get the cleanest release, but it still pretty far from flat enough to be a reliable sealing membrane. Sometimes, 3d printed isn't the best solution. You are probably better off die cutting material that is already a flat sheet rather than printing that part.

Posted : 22/04/2020 4:30 am
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RedDawg
(@reddawg)
Reputable Member
RE: Printable one-way air valves: Need advice for removing very thin PETG from smooth steel sheet

It may be a bit counterintuitive, but I find that "extra-hold" hair spray has some curious, useful properties. For instance, it can act as an "adherence enhancer" for filaments such as PLA but can serve as a release agent for PETG, Flex and other filaments that tend to have release problems. It also is readily dissolved and removed with 91% IPA. Can't hurt to try it for your situation.

1. "Creativity is the sudden cessation of stupidity." and "Any problem can be solved with the materials in the room." -Edwin Land, inventor of the Polaroid Land Camera. 2. "If I can't fix it, it isn't broken." -Me....
Posted : 22/04/2020 5:34 am
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pipe2null
(@pipe2null)
Active Member
Topic starter answered:
RE: Printable one-way air valves: Need advice for removing very thin PETG from smooth steel sheet

Thanks for the input, and I'll have to look up the Heimlich valve tomorrow and see the technical details of how it works, might be able to use some of the concepts.  If there is a book or resource that has a bunch of different valve type designs and how they work, might be very useful.  I'm struggling to remember my college physics on pneumatics and such (there's something about Pascal in there somewhere...), but that was decades ago...

The other half of my project is "Simply" a UV-C light in a can for in-line air sterilization.  I thought it would be simple, 'cause at the end of the day, it is just a light in a can.  But...  More complicated: The geometry of the "can" aka bioreactor dictates both air flow and UV power efficiency due to specular geometries and the reflectivity of materials used as enclosure as well as any interior coatings, etc.  In order to inactivate viruses in real-time, the can geometry, UV-C source irradiance intensity, and air flow through the can must deliver a sufficient UV dose of germicidal wavelengths at a rate equal or greater than the peak air flow rate of typical breathing.  It should be possible to build one with parts from the local hardware store plus a non-ozone UVC light from eBay, and theoretically at least you should be able to place it on the floor next to a patient's bed, with hose up to previously mentioned CPAP mask with printed air regulator.  That's the general overall thought, no idea yet if it will work out.

I've been using Wladyslaw & Kowalksi's "UVGI Handbook" to figure out a design, but the single biggest issue is not knowing what UV dose is needed to efficiently kill COVID-19.  There are many other nasty bugs and microbes that require MUCH higher UV doses than viruses, but I'm only interested in the  minimum safe D90/D99/+ dose for current problems.  There are multiple UVGI, gamma radiation, elevated temperature, plus various other virus killing studies on SARS-CoV, and the D90 dose of required UV to kill other coronaviruses range from 3 to 3046 J/m^2 depending on the specific virus and medium (air/water/agar/plasma/etc).  But for the life of me, I have not found any specific "how to efficiently kill COVID-19" kind of study or preliminary findings that I would expect to have this many months into a pandemic.  I know it is a new virus and all, but knowing how to effectively and efficiently kill it seems like good information to make public.  But I probably have missed something since I'm not in a medical field.

 

For my current printing problem with really thin PETG, I tried using an upside down can of dust-off (with a lot of ventilation) to try out the temperature shock.  It did help a little bit, but I suspect my main problem is the way I designed the valve, it is just too fragile the way I made it.  Guess I have another design challenge for tomorrow.  😀 

Posted : 22/04/2020 6:22 am
Bunny Science
(@bunny-science)
Noble Member
RE: Printable one-way air valves: Need advice for removing very thin PETG from smooth steel sheet

Use alcohol and completely soak the piece while the plate is hot. Then peel it off using a small kitchen knife at about 35 degrees. That's how I get my thin pieces off. The combination of knife and soaking wet with alcohol accomplishes the task.

Actually, these days I use the knife a lot more often, especially taking off the skirt and intro line from well worn PEI plates.

Posted : 22/04/2020 11:35 am
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