Peculiar resin problem

[Dr Mathias]

Hello,
I came upon an issue with some resin casts that I've not encountered.
A few days ago I made two molds to cast some small resin parts. Mold A had styrene masters in it, Mold B had some ABS/Plaster parts. I used Smooth-On MoldMax 25. The rubber cured very quick, and was developing small 'chunks' by the end of the pour. It's relatively hot and humid here.

When I de-molded the masters the next morning everything seemed fine and the molds appeared to be cured as expected, so I went ahead with casting.

I filled the molds twice, starting with SmoothCast 305. 'Mold A' produced tacky casts with glossy surfaces- two days later they're still sticky and somewhat soft. Parts from 'Mold B' don't appear to have that problem. At the same time I did these I cast a few other older molds- with similar parts in terms of thickness- and they're not tacky either, so it doesn't appear to be the resin.

I then tried a quicker cure resin, SmoothCast 300, and a slight dusting with talc. 'Mold A' didn't produce a tacky, glossy cast that time. I haven't been using talc lately since I switched to vacuum then pressure. So that is a variable.

Any ideas what might be happening here? It seems odd.

Thanks!

[RageofAchilles]

I have used both those resins and find them to be quite unforgiving if the mix isn't quite right or they come into contact with moisture. It could just be that one of your moulds was sweating. It's usually recommended to wait 72 hours for a mould to cure before use, but honestly most of us are too excited to get the first cast out.

I did have a period last year when I had sweaty casts and pieces that didn't cure, I have not had the problem at all this year and have even used old resin from that same batch! All I have done is give the resin a really good shake before I mix it.

sounds like your plaster leached the moisture from your mould so when you poured into it you got no problems, while the mould that had styrene in it must have been sweating. You had no problem with old moulds because they have dried out more and leached out silicone over multiple casts.

[Dr Mathias]

That sounds like a good explanation- I didn't seal the plaster before making the molds.

I usually don't wait to cast after I make a mold- too anxious

Thanks!

[mangozac]