Printing molds/forms for PDMS

Projects & Prints
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We have had some trouble using the Form2 to print molds for PDMS casting as something in the plastic (using the Clear V2) prevents the PDMS from curing, but finally we have come to a solution. Most likely the problem lies with some of the photoinhibitors in the resin (see safety data sheet). Though as the material is proprietary this is difficult to be sure of.

The protocol can most certainly still be improved on, but this is a start for anyone out there trying the same thing.

Protocol:

  1. Let the mold soak in deionized water for 2 h
  2. Cure in oven at 85-90°C for 18 h
  3. The surface will be covered with a thin oily film, wash this away with dish washing detergent and deionized water.
  4. Dry the mold and it is ready to use for PDMS casting.

As always, use protective gear - gloves, eye glasses, respirator etc. - when appropriate.

Let us know if you try this protocol and how it works out for you or if you manage to expand upon it…

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As an old-school propmaker, your problem fairly screams “Mold release!”. If you don’t have two hours to spare for soaking, just coat your mold in any number of release materials:

  • Spray-based mold release (normally thinned Vaseline and propellant)
  • Solution of Vaseline and mineral oil, brushed on
  • Dish soap, brushed on

If there’s one thing resin printing has disproved, it’s the old adage that “nothing sticks to silicone” :-\

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Thank you for the great suggestion. The problem in our case is that we are making multiple molds, then bonding PDMS to PDMS or glass. To bond these we use plasma activation of the surfaces so we want to minimize any contaminants to the surfaces that might prevent the oxygen plasma from working (or worse - reacting with it). Also the real problem isn’t that the silicone casts stick to the mold but that the molds actually prevent curing. But spray coating the mold surfaces could be a way to go as we might be able to isolate the mold surface from the PDMS. Thanks for keeping the discussion going, the more knowledge aggregated the better!

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You could also try a thin coat of spray lacquer or polyurethane on the parts.

I use spray lacquer on my molds with only a slight degradation in details.

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Hi Telliria!

I have the same problem and I will try your solution.
Previously, I also tryed to silanize the mold (a solution used for silicon mold release). It has worked 4 times over 5.
I read the SDS of clear resin, but they don’t talk about photoinhibitors but photoinitiators. Is it possible that photoinitiators provide PDMS polymerisation?

Julien

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I hope this will work for you too. Since my last post we have noticed that models made from the clear resin we used will deform slightly at 80 degrees when curing the PDMS. Probably the glass transition temperature is around this point. We are therefore moving to the high temp resin, though I haven’t had time to try that one yet. Still the previous mold from clear resin has still been able to provide multiple casts successfully after the cleaning protocol. I will report back when I know more.

You’re right about the photoinitiators, just too hasty typing on my side. =) I would guess the problems with polymerisation to the photoinitiators. I have made many PDMS mold from acrylic by CNC milling and never encountered this problem before. Of course there is a chance there is uncured polymer, but as far as I can remember I also tried curing the molds for 10+ hours just to make sure everything was set and that didn’t help either. So I believe there is some sort of residue like softener or this photoinitiator that is the culprit.

Let us know how the PDMS works out for you.

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I try your protocol @ 60°C in the oven and it seems to work. I think glass transition of resins is next to 60°C. At this temperature I have no problem but if it’s warmer, my designs become to deform.

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PDMS curing inhibition is done by amines usend into formulations as crosslinking synergist.

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That went so far over my head, @BlueCat!

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@BlueCat I am strongly considering using BlueCast for modell that I will send to a casting house. They warned me about the issue of silicone curing inhibition. Is this an issue with the BlueCast ? I recieved a feature datasheet from Mr. Reale today and there is no mention of this. If it is an issue, what can be used to minimise the potential effect?

I will also send other prints with Black to try for a silicone mold, but the casting house prefers a lost wax model.

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@Telliria how did the switch to high temp resin go? I’ve been using high temp resin and have the same issues you’ve been having with the clear resin. The PDMS fails to cure, especially at the interface with the mold. I tried your 18 hour post-bake protocol, but didn’t see the oily film. The PDMS still failed to cure after the bake and soap wash.

@UNISENSOR what did you use to silanize? We were considering trying a spray-on silicone release agent like this.

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@DLab I regularly cast PDMS in clear resin molds. I use a mold release agent that I spray on them. Overnight at 60°C I can take the PDMS part out and post-cure them roughly with a heat gun for 2min to fully cure the PDMS at hight temp.

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Hi, I also was unable to cure PDMS using a tough resin mold. Design was very a simple template using Sylgard 184. Curing for two days @ room temp failed, and for 2 days @60C . BOth failed.

Sylgard notes indicate that inhibitors include: Organotin and other organmetallic compounds, Silicone rubbers containing organotin catalyst. Sulfur, polysulfides, polysulfones and other sulfur containing materials. Also Amines, urethanes, amine containing material and other nitrogen-containing materials. And finally unsaturated hydrocarbon plasticisers.

Suspect it’s the amines. But not really sure as who knows what’s in the resin. Apparently there’s more info on Sylgard bulletin 10-022 "but couldn’t find it!

I tried a simple experiment placing two droplets of PDMS on pre-printed objects made from high temperature resin, tough, clear and a glass control. In one of each kind of droplets I added an “inner droplet” of the relevant uncured liquid form2 resin (without mixing). I then put it in oven for 48 hours at 60C. The droplet on glass on its own cured fine. The droplets of PDMS on glass with high temp and clear resins monomers cured almost up to the edge of the resin inner droplet. The droplet of pdms on glass with a drop of tough resin cured around the outside of the pdms droplet but became very tacky toward the centre in quite a wide zone.

The pre-printed clear resin object I used as a substrate had been around for a while (1 year old) and the PDMS droplets on it cured fine although were a little tacky after 48 hours@60 C. (incidentally making it very transparent!)…

The droplets of PDMS on the tough resin object totally failed to cure - they spread out and merged into each other forming a thin layer of uncured PDMS that pooled at the lowest point.

The droplets of PDMS on the high temp resin also didn’t cure very well. In part this was due to the poor design of pre-printed object. I just used some old prints I had lying around. It had holes in it. Interstingly the PDMS dribbled through the holes to form hanging droplets which then cured, except for where they contacted the mold. Forming a solid blob that could move around on the mold, even when upside down. The uncured PDMS close to the mold formed a sort of adhesive.

Conclusion: clear molds OK if they have been sitting around for a year!. High temp and tough: No!

I’m currently printing my mold out of clear resin, and will follow the protocol given above for soaking in water for 2 hours and curing to see if it worked on freshly printed clear objects. I’ll report back in a few days. Also ordered some PTFE mold release agent, which might help.

HTH

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I second the lacquer motion, minus the spray part. I also had issues with cure inhibition using molds made with high temp resin. Spray lacquer didn’t seem to work, but a couple of coats of brush on lacquer fixed the issue. And it also made it easier to get the silicone out - no need for release.

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Grey pro resin worked for a pdms mold with excellent results.

Just rinsed it lightly in IPA. No curing. No spray. Pdms cured well. Got the imprints of the print lines on the pdms though.

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Very interesting discussion all. Thankyou.
Just my tuppence ha’penny worth. I have been experimenting with CLEAR V4 and MODEL V2 resin for moulds. I am a big fan of the Model resin because of the resolution I can achieve. The CLEAR has been less successful as the resolution when compared to the MODEL is relatively poor.

We have been moulding RTV silicones in the MODEL mould with a lot of success. We have had no issues with Silicones not curing in areas of contact with the washed and cured. resin. we on purposely use low temp cure platinum cure to avoid shrinkage and issues with the resin glass transition as we require "precision"moulded parts… We are validating the range of silicones we use in MODEL V2 moulds.

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@Telliria I am trying to make microfluidic device using mold made by clear resin on form 2. But roughness on mold still an issue for me which can hinder plasma bonding.
When I tried to print it as parallel as possible to the build plate, I am getting accurate dimensions but rough lines at an angle (same as FDM) on molding surface. but on the contrary, printing at 45 degrees giving good surface finish but inaccurate dimensions. Can you please share your protocol for printing a mold for microfluidic device with orientation and other settings so that we can get good dimensional accuracy as well as glossy finish ? Thanks in advance!

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I agree to that. Grey resin comes with its own challenges. We tried to print parallel to the build plate, but the mold surface produces rough lines at an angle (Same as FDM) on molding surface which gets replicated on pdms surface which hinders its plasma bonding.

By the way, did you get any success printing pdms molds using any other resins of formlabs which produces very good surface finish?

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The only resin I’ve been able to make a successful mold from is grey pro. Although that sounds like that’s because my rinse and cure process is not good enough. If surface finish is important to you there’s a whole section in the forum on making lenses from clear resin, with ultra clear finish. Indeed I tried to make fluidic devices directly from clear resin in the hope that we could monitor internal processes with optical techniques. One trick you can try is to varnish/polish your object after you’ve finished which fills in irregularities on the surface, but obviously reduces your dimensions a little (which could be a good thing!). Varnishing and polishing is in itself very difficult especially with complex geometries. However, with simple geometries I’ve been able to spin coat a thin layer of fresh resin onto objects which creates a smooth glossy and highly transparent finish - although sometimes with airbubbles trapped in the layer. But for a mold I guess internal airbubbles may not be as much of an issue as they are for optical applications of clear resin. HTH

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Thanks a lot for your response. Have you checked microscopically the casted pdms device? Were you able to do any plasma bonding with imprints of lines on pdms surface?
Actually I am doing cell culture in casted microfluidic device. I have tried standard clear and grey resin but microscopically the cell culture channel in replica molded pdms shows rough marks. It would be great if you can share images of your grey resin pro molds or casted PDMS, then I will be able to decide whether grey pro resin can satisfy my requirement or not? Also can you let me know with what orientation did you print the molds in printer?