I had an odd occurance today and wondered who else might be able to shed light on what happened here. Today I was making resin casts of RTV rubber molds. I had two different molds, and mixed up a batch of resin. I made it a light grey, and poured both molds at the same time. Curiously, once I waited about an hour or so, I started popping the casts out of the molds. One mold, all the casts came out crisp, dry and hardened. The other mold though, as soon as I touched the overpour (to allow for contraction and air), it was very sticky and still wet. Parts of the mold were hard, parts were not. I waited another hour, and popped one piece out of this mold, It was hardened enough to remove, but was very squishy on the inside. All the casts in THIS MOLD had the same problem. Anyone have any idea why this is going on and what I can do to get the casts to cure?

It sounds like your resin wasn't mixed well, like half of the cup or whatever you mixed it in didn't get mixed well. Or the resin is old.

Unfortunately, the casts most likely will not harden any further. The best bet is to mix up some more resin and see if it happens again. If it does, the resin may be old and not reacting right with the catalyst.

Good luck.

Thank you Sir for the advice... second batch is drying as we speak!! It appears to be hardening well. I THINK I might have as you said not mixed it well, I think the coloring agent I put in the resin was clumped at the bottom of the cup and might have not mixed well.

Yes,extra mix time for pigment additions is a good idea. Also,pigments tend to slow the catalysis action so more time should be allowed for de-molding.(as a general rule).
Cheers!

well, had the same problem again today with an unpigmented set up. Ruined 3 molds!!! No curing at all. I tossed the resin. Unfortunately I had to toss 3 molds along with it. Does anyone have an average "shelf life" for resin A and B?

The last 2# kit I bought had a 1 year shelf life. No, it didn't stay good much longer than a year. It was 1lbA,1lbB part. Micro Mark brand I think.
Bummer about your molds. Try Acetone to clean the muck out?

I am using the same stuff.. barely lasted me 3 months. I didnt try acetone as I figured the easiest thing to do its to make new molds. Oh well... live and learn.

I've only used Alumilite 2-part resin, but I found that heat and moisture are the real killers. Storing it cool & air-tight extends the lifespan, but ultimately it is always a race against time once the bottles are opened for the first time.

I found that I really needed to use it up within about six months to avoid throwing any contaminated stuff away, so I now save up my casting projects until I have enough planned to use up as much of a full set of bottles as possible.

After all, every time you open the bottles to pour another batch, more moisture gets in...

Tom

That could explain some of it... I live in Massachusetts where its routinely 60-80% humidity

Couple of pointers for you Brian,
Any polyurethane that has been opened will only have a shelf life of 6 weeks before it starts to go off.
Depending on the manufacturer A and B should be a different colour and viscosity.
If the darker part of the resin (either A or B) has a "honey candy" look to it or sediment in the bottom, toss it.
Pigments should be added to the lighter half of the mix first before the second half is added.

I have cast thousends of parts over the years and have had this problem sometimes as well. The only thing that I have to add to all of this good advive is to make sure that when you pour the 2 parts into your mixing cup that you make sure that all of the resin from both parts is mixed. Sometimes a little of one part(A or B)gets on the side of the mixing cup and does not get mixed. It will ruin everything!

Thank you all for all the advices... most are RIGHT ON THE MARK!!!! I have changed the way I mix, and have had better luck with both the color consistency and the curing. I have come to the conclusion that my A and B werent BAD, but that I had over added color mold. I mixed up a batch last night ,using brand new A and B, and In a shot glass sized mixing cup. I started with A, filled with an exact amount, added 6 drops of color and mixed it. I let it set for a minute to let the bubbles out, and then added B and mixed. It cured, but just sticky enough and soft enough to not be usable. BUT, it cured me from using more than 4 drops of color for that sized pour again!!