Issues with Smooth-On's Oomoo 25
Over the last 6-7 years I've had some great experiences with Oomoo and it's my normal go to choice.
However I've recently run into an issue with Smooth-On's Oomoo 25 formula where the casts were all coming out sticky. I ran into this problem about 6 months ago as well, I tried talking to both the Reynolds Advanced people and the Tech support at Smooth-On and pretty much got nowhere as they claimed to have never heard of this specific problem occurring before and kept hammering the point that it was probably my mixing technique despite the fact I've done well over 15k successful pours over the last 6-7 years.
I've isolated every aspect of my process and tested each stage to see where the issue might be coming from, I made sure the resin and mold release were performing properly and rerun several new batches of molds and all of them are producing sticky casts so there is something different with the material batch. The product is well within date code and when I bought a second gallon kit with a different batch number it was still producing similar sticky results.
I suspect that the alcohol content within the mix is too high which collected on the surface of the mold cavity while it was curing. Areas of the mold that I cut away to make a reservoir and wider channels seem to be unaffected and that specific area of the casts are not sticky. I ran a 4 hr post cure bake on the molds which didn't seem to help.
What seems to have solved the issue was soaking them in cold water with dish soap for about 2 hrs, followed by a scrubbing with a toothbrush. I did a second quick cleaning with rubbing alcohol and toothbrush, for the second cleaning I did not soak but applied small amounts of the rubbing alcohol with the toothbrush, scrubbed it, then immediately rinsed them and air dried them in front of a fan which seems to have removed whatever residue was causing the stickiness and the casts are now coming out like they always have in the past.
It is very evident that the issue was being caused by some sort of variation in the silicone mix, which Smooth-On simply refuses to admit to the possibility of a flawed batch. Their customer service really sucks in this regard, all I wanted was to swap product for stuff that worked normally. I've literally used hundreds of gallons worth of their materials and when I spoke with then they constantly acted like I was a fresh faced user making beginner mistakes and their only suggestion was to buy more material or a different line and would do nothing to offset my loss on buying a bad batch, very frustrating.
If you find that you get a batch of the Oomoo that is producing sticky casts there is a work around by cleaning the molds. It adds some extra steps but at least it won't be a total waste of the material.
You can also clean off some of the sticky casts with soap water, but it does require soaking it for several hours and often a repeat cycle or two before it takes.
Also be aware of a possible interaction on master parts that have been molded in their Moldstar material. While it does not damage the master it leaves behind a trace amount of residue that needs to be cleaned off before you can use it in other lines of silicone. If left on the master it can cause it to bind with tin cure silicone (like Oomoo) which can potentially ruin the master. The residue can be cleaned off with dish soap and scrubbing with a toothbrush.
I ruined a set of 3d printed masters this way, fortunately I was able to replace them. Upon discovering this issue I cleaned my other masters and there were no further negative interactions. So if you switch between different lines of silicone material make sure to give the masters a cleaning in between.