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Making Silicone parts out of 3D printed molds

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#1 jrazz



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Posted 18 May 2024 - 10:11 PM

I went on a rabbit hole trying to make soft silicone eyecups for my binoculars. The commercially available ones didn't fit and didn't feel right. These ones I made fit perfectly, are soft and safe to use (skin safe) and are very very comfortable.


I wanted to share my experience since I couldn't find a tutorial for this stuff. The process is tricky and the silicone is rather finicky when trying to use with SLA molds. You want to use platinum cured silicone because it is stronger and also food/skin safe but the SLA parts tend to inhibit the curing of this silicone. This can be overcome with careful planning, material selection and diligent curing of the molds.


Here are the molds I made:



And the resulting product:




Materials you will need

Silicone: Siraya Tech Defiant 25 silicone - https://siraya.tech/...cone-defiant-25

This is made to overcome cure inhibition against SLA printed parts. You can try other silicones but this one works. I also wanted to avoid tin cure silicone as that doesn't have the mechanical properties and is not always considered skin safe. This material is food grade so no worries here.


Optional colorant: Mica powder - https://www.amazon.com/dp/B09CTHFSBW
You need only a tiny amount per batch but it makes a huge difference IMHO. There are other dyes but beware of alcohol dyes as they can inhibit cure.


Obviously you need a 3D printer. You can use a "regular" 3D FDM printer. PLA seems to work well with the silicone but finishing the mold is time consuming and the accuracy can be hit-or-miss. I prefer using a MSLA printer as these can produce much more accurate parts. My tutorial is based on these but if you prefer using a FDM printer all this still applies with less worry of cure inhibition.


3D printer resin: Elegy ABS like 3.0. They Siraya Tech ABS like is also an option - https://www.amazon.com/dp/B0CRDP8C2
These are tough, relatively odorless resins that don't have many volatiles that inhibit cure. I've had issues with other materials.


Sealing: I did seal the molds with a Krylon Matte spray - https://www.krylon.c...h-spray-coating
Though not necessarily a must I found that this improves the finish considerably and helps prevent cure inhibition. (see a trend here with this cure inhibition - yeah, it's a problem)


Making the molds

  • Print your mold parts as per the default config. Err on the side of more exposure, You want the resin to be as cured as possible.
  • Clean the parts in two baths of IPA. A "dirty" bath to take away the bulk of the uncured resin and then in a clean bath for final cleaning.
  • Cure the parts for 30 minutes under water. Do not skip this part! I did not know about curing underwater but it seems that oxygen can interfere with resin curing. Submerging the parts underwater while curing prevents this and the result is definitely superior.
  • Dry the parts. You can use a paper towel and dry them in the sun. I placed them on my FDM printer's bed while being heated to 45°C for an hour to fully dry them.
  • Do any of the finishing steps here. Sanding and flattening if needed. Make sure the molds close completely. The resin is very thin and will run out of the mold easily. Ask me how I know.
  • Clean the molds completely again. You can use IPA but make sure you wash it with water and dry the molds again.
  • Spray the molds with 2 coats of the lacquer paint.
  • It's a lot of steps but you are really fighting cure inhibition here. The good news is that this tends to work really well.


Making the parts

  • Clamp the mold halves together. Again, make sure they fit really well.
  • I design the mold halves to fit a 60ml syringe which makes filling much easier.
  • Use a scale to weigh the 2 parts of the silicone. Accuracy maters here and missing on this will ruin your parts. I usually start with part A, add the colorant and then add part B. You only need the tiniest amount of colorant, like a smidgen on the tip of your mixing stick.
  • Mix well for 5 minutes. The Siraya-Tech silicone has a relatively long pot life and you don't have to rush.
  • Let stand for a minute or two to degass. Again, the resin is thin and will degass without a vacuum chamber.
  • Pull the rising into the syringe. I pull a vacuum on the syringe by capping the tip and pulling the plunger. This really helps degass the silicone more.
  • Once satisfied the silicone is sufficiently mixed and degassed, inject it into the mold.
  • Wait 24 hours. I know it is supposed to be done in 6. Wait 24. You don't want to pull out goopy parts. If your shop is cold, you didn't do a great job of prep or whatever, give it time to fully cure.
  • Pull your parts and repeat to make more!



I hope this can help anyone trying to make soft silicone parts. The result is wonderful! Just another tool in your ATM arsenal :flowerred:

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Posted 18 May 2024 - 11:16 PM

Nice! I used to make aspheric lens prototypes out of clear General Electric Silicone, which was a 2-part mixture that would would cure in a mild oven. We used a liquid ~release agent~ to treat our molds. Then (for production) We would make a silicone negative mold from a positive machined master and then use the negative molds to make the positive production parts in large quantities. So that one single master mold resulted in thousands of final parts. The most astonishing thing was that the final parts retained the specular finish and sub-microscopic details of the original --- confirmed by electron microscopy.     Tom

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#3 jrazz



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Posted 19 May 2024 - 11:04 AM

Here is the completed result:



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#4 Tom Stock

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Posted 19 May 2024 - 09:00 PM

Those looks great! Nice work.

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#5 Jim in PA

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Posted 19 May 2024 - 09:19 PM

That's really nice, doubly so that you didn't need a vacuum chamber.  Thanks for posting your process!waytogo.gif

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#6 jrazz



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Posted 21 May 2024 - 10:20 AM

More eyecups - Canon 15x50 IS



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#7 jrazz



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Posted 21 May 2024 - 12:16 PM

The really nice advantage of this method is that you can have some pretty severe undercuts and the Silicone just takes it.

These pulled out with zero complaints!





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#8 jrazz



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Posted 21 May 2024 - 01:21 PM

For those who want to try, here are the parts for the Canon in Thingieverse: https://www.thingive...m/thing:6631473


Also the CAD is here: https://cad.onshape....a82007277442a4e

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#9 Navy Chief

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Posted 21 May 2024 - 02:23 PM

Thanks for documenting this and sharing the files. I'm going to have to add making new eyecups for my IS 15x50 to my to-do list.
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#10 mmx_4


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Posted 26 May 2024 - 09:25 PM

Nice work !

#11 PEterW



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Posted 27 May 2024 - 01:17 PM

Tom, just like Repliset, used for producing copies of surfaces for surface measurements.. good to 0.1micron accuracy. I need to do some silicone molding from some FDM parts, so will give your notes some study. Interesting the underwater cure, had a colleague trying to use all sorts of non-IPA solvents to remove uncured resin, without swelling, so it wouldn’t poison silicone subsequently.


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