Unusual Resin Issue

Hey all, long time lurker but first time poster. I've recently gotten into silicone mold making and resin casting (thanks in part to a lot of good advice I've found on these boards). However, I've run into a resin casting issue I've been unable to find any guidance for on my own.

I'm aware of some resin issues resulting from improper mixing ratios, and even experienced a bit of that as a result of early trial and error. The issue I'm getting seems different from what I've read about. Basically what seems to be happening is, on only a few casts, I find that the resin on the outer surface of the mold seems to have fully cured, but the material on the inside at certain spots on the cast is still soft, to the point the outer surface is flexible and can crack with enough pressure. If I leave these casts along for a few days, the inner material does seem to harden eventually. The issue is, when I try to paint these casts, air bubbles will form under the paint after a day or so.

I'm using a mold made out of Mold Star 15, and my resin is Smooth Cast 300. I've been tinting the resin to a light-medium gray using Uvo Black pigment. I've been doing my best to be accurate with the mixing ratio for the resin (using the same mixing container which I've marked to help me pour out the correct levels, cleaning it between each cast) and following all relevant instructions. I have noticed that the leftover material in the mixing cup will sometime exhibit a similar behavior, with the surface exposed to the air hardening and everything below remain hard.

Right now the only thing that my gut is leaning towards is the pigment. I've made a few casts where the resin is not tinted and never seen this issue, but it's only a handful of attempts and there are quite a few times where casts with the pigment have turned out okay.

Thank you for any advice you can give.

Hey, I use Smooth Cast 300 almost every day and I once had the problem you're experiencing a few months back for me it was due to me forgetting to shake the containers before I made my mix. It seems such a trivial thing but now I make sure I shake up my resin first thing in the morning and I haven't had the problem since.

If that isnt your problem then it could be a bad batch, try contacting your supplier and explain the situation. usually they will have had reports from other people and they tend to replace it.

Hope that helps

DaveChaos said:

Hey, I use Smooth Cast 300 almost every day and I once had the problem you're experiencing a few months back for me it was due to me forgetting to shake the containers before I made my mix. It seems such a trivial thing but now I make sure I shake up my resin first thing in the morning and I haven't had the problem since.

If that isnt your problem then it could be a bad batch, try contacting your supplier and explain the situation. usually they will have had reports from other people and they tend to replace it.

Hope that helps

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exactly what i was going to say. Its very important to mix both parts before combining them. And like davechaos said, it could be a bad batch. Its not old by chance is it? Was it stored properly? Not too hot or too cold etc?

Thanks for the replies. I usually shake each container a few times before each pour, maybe I wasn't shaking it well enough.

The resin was used within a week or two of it being delivered, and stored in a room kept at 70F, so I think things are okay there.

Trying to think about whether all the problem casts came from the same kit of Smooth Cast (unfortunately this was from a couple of months ago and I've just gotten around to looking more into since I'm starting to think about new projects I want to work on). Maybe I'll try reaching out to Smooth-On and see if they have some additional info.

One thing that makes me question the batch as being the cause was that the problem was only occurring on one of the three molds I was working with. The other two did have a pretty sizable undercut at the sprue, which was causing me to fill the mold part way, slosh it around to coat the walls, refill and then squeeze it a bit to help with air bubbles. Going back to the first option of it being a shaking issue, I wonder if that procedure was helping to make up for the lack of pre-shaking on the two "good" molds? Just throwing out ideas.

Thanks again for the feedback.

Could high humidity have caused it to go off? I remember reading something years ago about a problem with resin coming from too high humidity when casting.

It’s hard to say for certain but it sounds like a mixing issue as stated above.

Another thought I had.. you could not be mixing the UVO in quite well enough. You need to put it in the B-side very sparingly and mix it very very well. It’s very goopy and it takes a while to break down. Pour your B into your A cup.. if you have a lot of the UVO sticking to the bottom of the cup, you’re adding too much or not mixing well enough. Take your time mixing both components as well.. the potlife is quick, but I’ve never not had the time to spend 30 seconds to mix it properly.

Thank you all for the additional replies. I'll definitely take all the helpful advice to heart on my next project.

One other thing I was thinking about: Can the depth or general geometry of a mold impact the curability of the resin? I ask because I was pouring three separate molds simultaneously, and only one of the molds had this issue. Granted, I mixed a separate batches for each pour, but the batches were from the same resin kit and employed the same process each time. The mold that was having the intermittent issues is about 6 inches tall, the the highest points taper at the ends. It was always in these narrowed tapered sections where I found the issue I described in my first point. I know resin cures fastest in the largest cross sections, but I generally waited over twice the documented cure time before pulling.

Just another thought I wanted to bring up. I do so appreciate all the help I've received so far.

From what you’re saying, it definitely doesn’t sound at all like a bad batch of resin...

It’s been said but I’ll put one more vote on it:

Either you don’t mix the tint properly. Really it does need a lot to break down and not having goopy streaks in your B part. Take the time to stir it well!

OR you don’t shake your container of A and B parts well enough. I literally shake them in the air for like 30 seconds. Not one or two shakes. More likely something along the lines of 50 back and forth! It introduces a ton of air bubbles in the mix but I have a vacuum chamber and a pressure pot to fix that. If you don’t, shake it a good 30 min before using it and you’ll be golden!

Good luck!

I’m casting extra thin parts or parts that don’t have large volume. My cure time is pretty much 4x what it should be. As you said, the thicker the resin volume is, the faster the cure time and the more exothermic reaction will happen in the mold!

For me, ONYX slow should be 90 minutes but a volvo panel will take about 8h to cure. Making the whole process of casting a set soooooo slow!!!

If you wanna try to fix the already soft parts. You can hit them with a heat gun or hair dryer.
Just be careful because it will re soften them, but should start the heat reaction again and fully cure.
I've had issues with casts before coming out soft and then I hit them with a heat gun and they fully cured.

Jerichai said:

I'm using a mold made out of Mold Star 15, and my resin is Smooth Cast 300. I've been tinting the resin to a light-medium gray using Uvo Black pigment. I've been doing my best to be accurate with the mixing ratio for the resin (using the same mixing container which I've marked to help me pour out the correct levels, cleaning it between each cast) and following all relevant instructions. I have noticed that the leftover material in the mixing cup will sometime exhibit a similar behavior, with the surface exposed to the air hardening and everything below remain hard.

Click to expand...

Ive never worked with or handled any of the materials you are using so I cant comment on them.
But,
you do state you are reusing the same mixing cups after cleaning them. Some materials dont respond well to that and its considered contamination to those materials. Again, ive never used nor handled any material youre working with. I did have similar issues years ago using Por-A-Kast but never once while using MPK high impact plastic.
http://hobbysilicone.com/resins/

Ive also seen the same issues with material when its been exposed to moisture or higher temps/humidity. Bad batches happen. Anyone thats worked with Por-A-Kast knows this. Thats why there is always extra of one side left over (A or B I forget its been so long since using it) as some products are to be measured by volume not weight. Volume vs weight comes into play with larger castings. In the end when you find a material that works and you know how to use it, stick with it.

Try using new mixing cups for every mix. What are you cleaning the containers with?
Take in account moisture and PU resin are not friends.

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Try using new mixing cups for every mix. What are you cleaning the containers with?
Take in account moisture and PU resin are not friends.
I assume the tint is specific for urethane?.