silicone (PDMS) slipperyness? - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 15, 2019, 11:54:49 pm
content: Hi! Not strictly speaking electronics related (yet), but I need a rather involved liquid valve - 6 port rotary valve for sample introduction in a scientific instrument i'm designing.I'm thinking of making it myself, having a flat sheet of soft polydimethylsiloxane gel, the standard 2 component silicone, with in/out ports punched in it, and then have a round top disc of acryllic with the interconnecting channels milled in it. It's going to operate at very low positive pressure and low flows.Is it realistic to get an arrangement like this to seal, and would you think the slipperyness is sufficient that the valve can be turned without any lubricant?Here's an image of the valve I'm trying to recreate. The commercial ones are MAD expensive.https://www.vici.com/support/images/app11.gifThanks!--Chris

author: coppercone2
author_rank: Super Contributor
timestamp: May 16, 2019, 01:52:30 am
content: I think this might be useful, not for design but application specific from the best.https://www.swagelok.com/downloads/webcatalogs/en/MS-02-479.pdfI think others may be illuminated by this, you probably know most of those details.I think you want to polish that acrylic after it was milled to make sure there is no burrs that will dig into the gasket. But for your main question I don't know.

author: coppercone2
author_rank: Super Contributor
timestamp: May 16, 2019, 02:21:17 am
content: make a sample without the channels and leave it compressed at your expected operating tension for a while.All valves should be periodically cycled because they stick as part of a valve maintence procedure.The friction should not change much because of a small sampling system that is built into it.See if it operates and what the recommended maintence cycling time should be, then make a test groove and put the valve on tester to test 10k actuation to see when failure occurs and inspect visually.\should be a non expensive testTeflon vs acrylic too. or hhigh density poly ethylene

author: jeremy
author_rank: Super Contributor
timestamp: May 16, 2019, 02:40:58 am
content: In my experience, PDMS is the exact opposite of slippery; heck, I’ve used itforit’s stickyness. Situation would be worse under pressure too.I think the key polymers used in this area are PTFE and POM

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 16, 2019, 11:35:58 am
content: Okay, so PDMS is out. The idea with the testing is good, and the acryllic being translucent, any leak will be very apparant.I'll try putting a layer of soft slippery polymer (PTFE, PE) between the port layer and the flow layer. See attached.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: May 16, 2019, 12:16:30 pm
content: I've seen a similar rotary valve in a scientific dosing application that is very similar to what you've just drawn. The porting groove was milled into a piece of teflon and the plate connected the tubing. The teflon piece could be turned to align the porting groove with different tube ports on the plate. The teflon piece was pressed against the plate with a spring to provide sealing between the two. In that case, the plate was something like a finely polished piece of ceramic but I'm sure acrylic would work much more easily.The spring was pretty stiff: almost a die spring or about what you'd find for a valve spring in a lawn mower. However the valve did not leak even with a few atm on it.

author: Rerouter
author_rank: Super Contributor
timestamp: May 16, 2019, 12:24:52 pm
content: PDMS is also a right pain to clean up, It loves to climb up the walls of pipes, and I suspect it would not stay put.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 16, 2019, 01:14:42 pm
content: Paul, that sounds very similar to what I'm trying to do, and as you say, I can skip the top layer of acrylic, just mill the channels into the PTFE block.The stack of 2 layers could then just have a common M6 machine screw with a heavy-ish spring. I'll attempt that!thanks for the advice!

author: Paul Moir
author_rank: Frequent Contributor
timestamp: May 16, 2019, 05:16:23 pm
content: One thing I'd say about the valve I was describing: after a couple years it began to leak internally. The spring pushed on the periphery of the teflon and I suspect its previously machined flat face had become concave since teflon is kind of gushy. A non-warping backing would have helped if this was the case.EDIT: its/it's scallywag.

author: frogg
author_rank: Regular Contributor
timestamp: May 16, 2019, 05:31:31 pm
content: Does this have to be a rotary valve?The schematic looks like it's possible to accomplish via some combination of check valves, 3-port 2-way valves, and some method of synchronization (mechanical linkage or electronic)

author: coppercone2
author_rank: Super Contributor
timestamp: May 17, 2019, 01:13:31 am
content: add a dye to the water to test for leaks, even if it leaks for a bit, stops (i.e. a 'fart leak') and evaporates you will get a stain residue.Your method is not great to detect a transient leak during a long run time.

author: coppercone2
author_rank: Super Contributor
timestamp: May 17, 2019, 01:16:13 am
content: Quote from: frogg on May 16, 2019, 05:31:31 pmthat thing is like an ASIC. Why its good for hydraulics? Because you get less 'dead space' in the circuit. Basically all the excess space in a sampling system that is continuous will accumulate previous sample and dilute it so you get a lag in your response, so you need to flush it more frequently. And if its an organic material or food it can rot in there etc. You can think about it almost like RF design. You want to make it easy to flush too.They have 'sampling valves' that are manual, you basically turn it and it scoops some stuff up and releases it into the other section like a manual pump with a single turn. Check valves with springs and stuff are difficult to flush.

author: vindoline
author_rank: Unknown
timestamp: May 17, 2019, 01:13:59 pm
content: Quote from: ChristofferB on May 15, 2019, 11:54:49 pmSo is precision electronics test gear . . . if you pay retaileBay is your friend! I can't imagine why you would want to DIY one of these, there are so many solutions in the life-sciences. Valco valves are the big player. Look for derelict or broken HPLC equipment. I've bought them for under $100.If you're thinking about amateur HPLC, I'll warn you that the cost of EVERY SINGLE PIECE will make you weep. You won't believe what a single nut, connector or piece of plastic tubing will cost. Then there's the cost (and sourcing) of high purity HPLC grade water and (typically) acetonitrile. You also can't pour the waste down the drain! Hazardous waste disposal fees!I'm definitely NOT trying to be a wet blanket, just making sure you go in eyes wide open.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 17, 2019, 01:55:47 pm
content: Quote from: vindoline on May 17, 2019, 01:13:59 pmHaha I'm well aware that it's not on the cheap end! I'm also not really trying to do HPLC, more like LPLC//flash/FPLC? so HPLC-ish in setup, but at much lower pressures. But you're right, the sample valve might be the one place where finding one used is unbeatable.There are a lot of truly great scientific papers on analytical standard "DIY" hplc detectors; UV absorbance detectors and fluorescence detectors made from single PEEK cross fittings with column in/out, and optical fibers in the two other ports going to a simple UV LED/photodiode system that I want to try to mess around with.I'm also hoping to source parts from Ebay, but everything I can avoid buying is a win. For low pressures, I've designed a very cheap fluid connection system consisting of 1/16" PTFE HPLC tubing mashed over lenghts of hypodermic tubing. This gives me both ports to epoxy into things, syringe ports, fittings etc. See attached.I'm also planning on using non-toxic solvent systems, you can get a long way with water/ethanol or water/acetone mixtures. At this stage I'm not super concerned about solvent purity either.Thanks for the interest!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 18, 2019, 01:00:19 am
content: Okay so I made a quick prototype to see how the valve would behave, both elements of acrylic, and to noone's surprise, it leaks badly. It does hold at VERY slight positive pressures though, it seems, although this might just be because the leak becomes too small to notice.I'll attempt different rotor pieces on this base, maybe LDPE or HDPE and definitely PTFE.If nothing else the prototype looks kinda cool.Enjoy!

author: LukeW
author_rank: Frequent Contributor
timestamp: May 19, 2019, 08:43:56 am
content: I would just consider finding a commercial HPLC valve (Rheodyne or similar) on eBay.https://www.ebay.com/itm/Thermo-Scientific-Rheodyne-6-port-injection-valve-MHP9900-601-1/283169374615?hash=item41ee35c197:g:TnYAAOSw0c9booSWOr maybe you can use real spare parts / rebuild kits - e.g. the packing, the seal etc, and partially build your own?I wouldn't use acrylic. What happens if you want to run DCM as a solvent? I don't think the epoxy will be very solvent resistant either.Maybe PEEK would be a good choice?PTFE is probably the best choice - for slipperiness as well as chemical resistance.PDMS seems like a bad choice, unless you're trying to make microfluidics.

author: kony
author_rank: Regular Contributor
timestamp: May 19, 2019, 10:43:57 am
content: OP, the PMMA prototype design you showed is absolutely unsuitable for purposes of liquid chromatoraphy or anything that operates with larger than negligible overpressure to ambient as you already discovered. Had you ever repaired HPLC injector valve? The lapped ceramic surfaces with massive support stainless plates and thrust bearing with a very substantial preload are there for a very good reason. Sourcing scavenged parts ex Ebay is really your best (and cheapest) option.Not even PEEK will be good choice if you try to replicate it with correct setup due to long term creep under the preload (teflon and PE being absolutely out of question due to this). Ceramics and glass-ceramic composites are used for such purposes, lapping of the sealing surfaces not being optional but rather a must (at least one of the surfaces if you do get bit clever in the design).Has this project expected commercial output?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 19, 2019, 12:37:06 pm
content: Quote from: LukeW on May 19, 2019, 08:43:56 amI'll probably end up doing that. Just aquiring the high performance polymers might reach a price where getting a second hand rheodyne valve is better.Quote from: kony on May 19, 2019, 10:43:57 amI completely agree, it was more or less made to see if i could fabricate the channels and ports with the tools i had on hand, intending to do the entire "real" valve in another polymer. I never intended the valve to function at higher pressures, the desired working pressure was something like "very slight push on syringe".and no, in no way at all does this project has any commercial output, it's purely for personal amusement.Thanks for the interest! I think I'll just look for a 2nd hand 6-port valve. It was an interesting excercise nonetheless.

author: coppercone2
author_rank: Super Contributor
timestamp: May 19, 2019, 03:50:37 pm
content: I found your post interesting because I was not sure what would happen. Its too messy of an experiment to deal with for me.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 19, 2019, 08:55:25 pm
content: Thanks! Doing "wet" projects like this is always an issue in a home shop setting, but keeping it in the realm of microfluidics, having tiny volumes of fairly benign liquid, and having a closed waste liquid receptacle helps a lot. My ealier gas chromatography project was more pleasant that way, only waste was helium and a tiny amount of analyte.I was wondering however if it would be possible to implement a sample valve such as described on a microfluidic chip, using air or liquid pressure ports that compress a PDMS membrane and block flow.. Would be neat.--Chris