Resin expanding whilst curing?

[NiceGuyAdi]

I’m casting up a storm shield and made a push mould for an icon to go on the front with blue stuff/oyumaru.

 

My first attempt with a green stuff/millionth mix was ok, but I wanted to see if pouring resin would be better. What a horror show. It was full of bubbles, which was my fault, but what was weird was that I could see the resin bulging out of the top of the mould as it cured.

 

Is this normal? Is that why you need channels running out of a mould when you make two-part moulds? Just trying to understand the material a bit better as I get back into casting.

 

I used Polycraft SG2000 (polyurethane). The kit hasn’t been used for a few years and I think I contaminated one of the bottles, as it had developed some slop in the bottom of the bottle.

[Subtle Discord]

No, polyurethane resin should not expand when your casting, in fact, as the molecules of the plastic mix and start forming chains a tiny amount of volume should be lost resulting in a very small amount of shrinkage. The vents in moulds are only to provide paths for trapped air bubbles to escape and to create a structure to hold small/delicate components, and that's about it.

 

After several years of casting and using product of differing age I can say it sounds like the resin has expired/spoiled; polyurethane resin has about a 1 year shelf-life unless you use a product to extend it and even then it only buys some more time. The 'A' component (which I think is the catalyst that 'kicks' the solidifying process of the plastic) is quite volatile and it starts forming chains in the polymers on its own as it absorbs any moisture in the air that seeps into the container. I've had this happen to me on a few occasions and what your describing sounds familiar; as the resin cures it will froth and bubble for no good reason (it's even worse if you try to vacuum de-gas the resin) and that will cause it to expand rather than create an accurate copy, and naturally this will also add all sorts of extra bubbles. If the resin gets really old the cured plastic will also weep and ooze a strange liquid even long after it should be fully cured. It's very strange to see, to say the least.

 

The best solution I've found to avoid this is to store the resin in glass Mason jars with proper canning lids for making preserves and use a product like Xtend-It to add a dry gas layer to the resin. If the bottles are full and the seal is good this can slow the spoiling considerably, but nothing can stop it totally. If the 'A' compound forms sludge, sediment, and/or crystals at the bottom of the bottle it's a sign it's starting to absorb moisture; if it's just a little bit you can mix it back up and it should be fine but if it becomes considerable than odds are good that you'll get the stubborn frothing, bubbling, and deformed casts.