Possible pdms vat refresh method

Various topics related to Vat design, construction and coatings.
HTL
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Possible pdms vat refresh method

Postby HTL » Fri Mar 21, 2014 11:56 pm

I have been refreshing my vats sylgard 184 anti stick properties by putting my printers vat under pressure for a half an hour before using it for a print run. It seems to work. I'd like to hear any comments.

Here is the write up: http://projectsinterestsandetcetera.com ... -could-be/

Finhead
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Re: Possible pdms vat refresh method

Postby Finhead » Sat Mar 22, 2014 12:20 am

Nice write up I will have to try that once I am up and running since I have both vac and pressure chambers.

Clinton

EdgE
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Re: Possible pdms vat refresh method

Postby EdgE » Wed Mar 26, 2014 8:13 am

Hi HTL,

Thanks for sharing your thoughts & experiments about this subject.
I also read your yahoo post: https://groups.yahoo.com/neo/groups/diy_3d_printing_and_fabrication/conversations/topics/9297.
This is also the reason, why I started a post for creating a list of vat-solutions, which might work, in the hope, some other solutions will popup.

Since I don't have either vacuum-unit or pressuriser-unit, it's very hard for me to repeat your tests.
I was thinking in the opposite direction. Since I didn't print for weeks (because of this issue), my Sylgard vat hasn’t seen air for weeks now. So I'll defiantly try to print with this oxygen-less vat and (hope to) damage it even further... ;)
I also like to ask you, how necessary it will be, to pressurise the vat, or just 'vent' the vat in normal atmosphere. Is there a difference? Or how big is this difference in 'number of reusing' the vat again?


I was also discussing this topic with a friend, while we both counter the same issues. (In the end, we all do!)
The bottom-up process is very nice, but not robust at all...

So we also thought about some solutions (wiper), which could be helpful, for getting air in.
(Something like: every 10 exposures, wipe the floor & wait 10 seconds for getting air in.)

Does anyone know, if this oxygen-issue also counts for FEP/Teflon sheets??

hp_
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Re: Possible pdms vat refresh method

Postby hp_ » Wed Mar 26, 2014 8:41 am

Hi Edge,

chechout the solidator patent, it mentions this part a little..

hp

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sgraber
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Re: Possible pdms vat refresh method

Postby sgraber » Wed Mar 26, 2014 8:51 am

HTL: Could you re-run your experiment and not do the pressure pot step? It would be very interesting to see if cleaning the surface with IPA followed by the vacuum-only step would yield the same results. It's possible that through use the PDMS is absorbing a bit of resin/monomer and that the vacuum step simply "pulls" this material out along with the residual IPA that has migrated into the PDMS surface.

I formulated UV resins for 14 years and I don't want to downplay what you've seen with the pressurization step, but IME it takes a fair amount of oxygen to inhibit surface cure. In my mind, pulling the residual resin/monomer out of the surface would have the same effect and it could be the pressure pot is not needed. Could you try this at some point?

Shane


HTL wrote:I have been refreshing my vats sylgard 184 anti stick properties by putting my printers vat under pressure for a half an hour before using it for a print run. It seems to work. I'd like to hear any comments.

Here is the write up: http://projectsinterestsandetcetera.com ... -could-be/
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HTL
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Re: Possible pdms vat refresh method

Postby HTL » Wed Mar 26, 2014 10:02 am

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 during layer exposures, sticking occurs when that layer of liquid resin is not maintained (the oxygen on the surface is depleted) and the resin solidifies on the surface of the pdms during exposures. It in fact does not take a lot of oxygen on the surface to cause this effect, if it did neither the B9 (expose the pdms to air for a couple of seconds every exposure cycle) or the solidator method (diffusing air through the release layer) would work. 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.

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sgraber
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Re: Possible pdms vat refresh method

Postby sgraber » Wed Mar 26, 2014 11:10 am

HTL wrote: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 during layer exposures, sticking occurs when that layer of liquid resin is not maintained (the oxygen on the surface is depleted) and the resin solidifies on the surface of the pdms during exposures.


Actually, the non-stick properties of PDMS come from the pendant methyl groups contained in the polydimethylsiloxane polymer chains itself that cause the non-stick properties and not adsorbed surface oxygen. I worked with a number of different silicone-containing additives over the years. The pendant methyl groups yield a very low surface energy and as such keep things from sticking to it.

Incidentally, I have not seen actual empirical data showing there is a thin boundary layer of oxygen at the resin/PDMS interface. If you have it, please show me a link as I'm most interested.

The test I suggested above would shed light on whether or not the pressure pot is actually needed. If it is not needed, then the "pressurization" of the PDMS is not needed.

It in fact does not take a lot of oxygen on the surface to cause this effect,


Actually it does - especially given the long exposure times seen in UV SLA. Air is made up of ~21% oxygen. Even dropping it down to a couple percent is more than enough in most cases to get good surface cure in coatings.

if it did neither the B9 (expose the pdms to air for a couple of seconds every exposure cycle) or the solidator method (diffusing air through the release layer) would work.


Could you link the patents/empirical data on this? I'm not saying its wrong, but again in my uv formulating experience and background as a uv formulation chemist, it doesn't jive.

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.


Again, the test I suggested previously would shed light on whether or not this is true.

Here's a bit of interesting reading on PDMS:

http://en.wikipedia.org/wiki/Polydimeth ... patibility

"Solid PDMS samples (whether surface oxidized or not) will not allow aqueous solvents to infiltrate and swell the material. Thus PDMS structures can be used in combination with water and alcohol solvents without material deformation. However most organic solvents will diffuse into the material and cause it to swell,[citation: McDonald, J. C.et al. (2000). "Fabrication of microfluidic systems in poly(dimethylsiloxane)". Electrophoresis 21 (1): 27–40. doi:10.1002/(SICI)1522-2683(20000101)21:1<27::AID-ELPS27>3.0.CO;2-C. PMID 10634468.] making them incompatible with PDMS devices. Despite this, some organic solvents lead to sufficiently small swelling that they can be used with PDMS, for instance within the channels of PDMS microfluidic devices. The swelling ratio is roughly inversely related to the solubility parameter of the solvent. Diisopropylamine swells PDMS to the greatest extent; solvents such as chloroform, ether, and THF swell the material to a large extent. Solvents such as acetone, 1-propanol, and pyridine swell the material to a small extent. Alcohols and polar solvents such as methanol, glycerol and water do not swell the material appreciably. [citation: Lee, J. N.; Park, C.; Whitesides, G. M. (2003). "Solvent Compatibility of Poly(dimethylsiloxane)-Based Microfluidic Devices". Anal. Chem. 75 (23): 6544–6554. doi:10.1021/ac0346712. PMID 14640726.]" http://pubs.acs.org/doi/abs/10.1021/ac0346712

Here's the full paper: http://gmwgroup.unix.fas.harvard.edu/pubs/pdf/899.pdf

UV resins and monomers are not considered highly polar like water; they will swell materials like polystyrene, ABS, PVC, EPDM, etc, (the reference above suggests it's possible to swell PDMS with UV resins/monomers given my experience with them) and it *could be* that the UV resins/monomers used in a given UV SLA migrate into the very top surface layer of PDMS slowly over time and start causing the stickage problems seen. Washing the surface well with IPA followed by a vacuum chamber could draw these materials out of the surface layer thereby 'refreshing' the surface. The pressure pot may not be needed. The only way to know for certain is to test it.

If I'm wrong, I'm wrong but if I'm right, it's one less step to have to perform. :P
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HTL
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Re: Possible pdms vat refresh method

Postby HTL » Wed Mar 26, 2014 12:00 pm

Occams razor.

I go with what works. The method I'm using is a variation on the b9 and solidator methods of refreshing the release layer which work via refresh of the pdms/fep surface oxygen layer. The method I'm using works for me. If you have the equipment I would be happy to know what results you have using variations of this method.

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sgraber
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Re: Possible pdms vat refresh method

Postby sgraber » Wed Mar 26, 2014 12:06 pm

HTL wrote:I go with what works. The method I'm using is a variation on the b9 and solidator methods of refreshing the release layer which work via refresh of the pdms/fep surface oxygen layer. The method I'm using works for me. If you have the equipment I would be happy to know what results you have using variations of this method.


I can test it once mine is done. ;) That said, yours is done and you have the equipment to do it. If you've got time to try it the next time you need to refresh your PDMS, try it without the pressure pot. If I'm right, it'll save you a step. If I'm wrong, then I'm proved wrong. :P
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HTL
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Re: Possible pdms vat refresh method

Postby HTL » Wed Mar 26, 2014 12:21 pm

Having over 40 years of successfull hardware and software projects I no longer feel the need to be proven right or wrong as you seem to. The projects and methods I discuss here and on my website are what I do for fun. I will leave it to others to determin if the suggestions I make are useful to them.


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