Silicone rubber reacting badly, unsure why?

[Anglia105E]

I have been producing molds with no issues, using silicone and 3% catalyst in plastic containers, but today something has gone badly wrong.

This time I used the same silicone mix but used a latte glass as the container, and I have a Shapeways figure that was slightly damaged so I thought I could make a copy of the original. Now I don't know if it is the glass that is the problem or the material that the 3D printed figure is made from. What I got was a spongy gooey mess that was extremely difficult to extract from the glass container. The silicone mix had set properly on a plastic spoon that I used up some left over mix, but the mold in the glass was a complete disaster. I can't see how the glass would be a problem, so I assume that there must be something in the 3D print material that has reacted with the silicone? Thought about repeating the process using Blu-Stuff but I am worried the same thing will happen, and so more time and money wasted. Any thoughts from forum members would be most welcome as always.

Thanks, David

[1930fordpickup]

Could it be a the release that you used? 

[OutaFocus]

If the master contains sulfur it will react with certain RTVs. Platinum cure RTV is very sensitive to sulfur and some other substances. I try to only use a tin cure RTV as it is more tolerant of most substances. I use Kleen Klay if I need clay to make a mold because it is sulfur free. I have no idea of the chemical makeup of the 3D part, but I would assume it's something in it that is reacting with your RTV.

[Greg Wann]

Scrub your original part with a toothbrush and alcohol and try again.  The person that handled it before you did might have been having fried chicken for lunch.

[Anglia105E]

Thanks guys for helpful suggestions. I didn't brush the master part with release agent because it is a one part mould, so no silicone to silicone. You may well be right about the sulphur Scott as it would be quite reasonable for Shapeways to put some chemical in the 3D printing material to discourage modellers from copying originals. I have now given the master figure a thoroughly good clean, during the process of removing all the failed silicone gunge using warm water and washing up liquid. Need to try again for a mold using Blu-Stuff and a plastic container. I only chose the glass because it allowed me to stand the figure vertically rather than lying it down horizontally.

[peteski]

I doubt that Shapeways puts any RTV inhibitors on their printouts. They are simply a printing company.  They don't know or care about how the printed items will be used. Plus, what if the designers themselves wanted to use their printout as a master for castings?  Out of curiosity, with what type of material was the figure printed?

More likely the bad chemical reaction was due to either the material used for the printed object or maybe the wax or rinsing agent  residue remaining on the object.

[Anglia105E]

Thanks for your comments Peter, and the figure was printed as described below...

• 3D printed in White Strong & Flexible: White nylon plastic with a matte finish and slight grainy feel.

I think I might just bite the bullet and have another go, as surely the latte glass can't be a negative factor?

[Greg Wann]

I would not think the glass was the cause.  The silicone next to your original part did not cure properly.  Perhaps spraying some dull coat sealer on it will help.  You could mix a small amount of your silicone and just pour a layer over it and see if it cures.  If it does not then you will know the silicone does not like this material.  In this case you will need to seal it and try that test again to see if your outcome changes.

[bill w]

I have been making molds for a long time, and from what you described it seems to be a bad mix. If the container was the problem then the silicone touching the container would be

soft and gooey, and that would only be 1/16 to 1/8 inches soft all around the container... the rest of the silicone would be cured. If the silicone around the part is soft then the part is the problem.

I may be wrong but I never heard of using 3% catalyst for silicone unless it's a new product. I use both tin base and platinum base silicone and the mix ratio is 10 to 1 (10% catalyst)

which leads me too believe it was a bad mix. Again I don't know what brand of silicone you are using.

If you primer your part that will do the trick.

Edited March 21, 2017 by bill w
correction

[Anglia105E]

Good advice from Greg Wann and bill w

Excellent idea Greg, to pour only a thin layer of silicone mix over the part and see how it reacts, much better than my idea to pour 130 ml into the latte glass! - Also, I was considering painting the figure with grey primer before producing the mold, so I agree that coating the part with sealer should help. As regards the 3% catalyst bill w, this is definitely the correct ratio according to the suppliers instructions. I have previously produced a large two-part mold using 3% catalyst and it turned out really well, nice and firm all the way through the silicone. The Blu-Stuff is different because it is a 1:1 mix of 100 ml blue to 100 ml white, so equal amounts of that product. Only one Blu-Stuff mold failed on me, and that was because it was my first encounter with the product and I didn't mix it for long enough or well enough. Again, I agree with bill w that a coat of primer should stop the material of the part reacting with a good mix of silicone.

[peteski]

White Strong & Flexible material is IIRC Nylon. It is also fairly porous. Maybe it is absorbing the catalyst from the RTV mix?  As others said, I would test how the rubber hardens by just immersing a small part of the figure in it (head or hand)..

As far as painting/sealing it, I expect it to absorb paint like a sponge.

[OutaFocus]

Thanks guys for helpful suggestions. I didn't brush the master part with release agent because it is a one part mould, so no silicone to silicone. You may well be right about the sulphur Scott as it would be quite reasonable for Shapeways to put some chemical in the 3D printing material to discourage modellers from copying originals. I have now given the master figure a thoroughly good clean, during the process of removing all the failed silicone gunge using warm water and washing up liquid. Need to try again for a mold using Blu-Stuff and a plastic container. I only chose the glass because it allowed me to stand the figure vertically rather than lying it down horizontally.

Yeah, as was said in a post above, I doubt Shapeways adds anything to the printing material, but who knows what it's made from. Perhaps something in the chemical makeup is reacting with the RTV. Sealing the master is a good start if it can be done without destroying the detail. As was already suggested,testing it on some RTV before making another mold is a good idea.