Air bubbles and ‘mercury’ effect after pressure pot

[Sevv]

I am trying to cast a simple cube of Smooth Cast 325 surrounded on 5 sides by 1/2 mm of Crystal Clear 202.

No matter what I do I still seem to still get the odd air bubble and 90% of the time a shimmery glass/mercury effect that forms between the cured crystal clear and smooth cast a few hours after cure time and almost spreads over the next day to cover a large portion of the join between the two materials.

I live in the UK and the temperature in my house is normal 18 ish degrees

I am really running out of ideas about how to avoid this.

Back to basics. I have mould A. Made from Mold Star 30. This is used to cast the cube from Smooth Cast 325. Simple, nothing to really go wrong here.

Next I have mould B.
Which is made from a negative shape and a positive ‘plug’. The Smooth Cast cube is sat snuggly into this plug. I fill the negative mould with crystal clear 202, turn the plug with the cube attached upside down and place it into the negative mould filled with crystal clear 202. I have escape holes built into the mould so the excess crystal clear pours out.

Placed in my pressure pot at 60 / 65psi. I leave it to cure over night.

Demould in the morning, I do still seem to get the odd air bubble (why?) and normally as it emerges… all is well. The cast looks great. Then as I said above, over the next few hours I see a mercury mirror effect appear between the two cured layers of resin. It’s almost like the crystal clear is pulling away from the smooth cast. But it can’t surely?

Side note. If I try this cast with only crystal clear in the mould. No smooth cast 325 cube. So essentially you have a 2 part mold star 30 mould. I never get air bubbles. I only seem to get air bubbles when I’m casting with my smooth cast cube and the air bubbles only seem to form on the upside surface of the cube.

It’s like the cube being upside down is forming a shelf and the air bubbles can’t escape up past it like in a vac chamber, yet this is a pressure pot and bubbles are supposed to be crushed! Plus the naked 2 part mold star mould has a similar shelf but that doesn’t get air bubbles forming on it. Grrr The cube is sanded to a very smooth finish so it cant be imperfections in the resin holding onto air bubbles.

Any help would be greatly appreciated.

 

[JPH]

Smooth cast 325 makes bubbles. I degass each half and then mix ssslllloooowwwwllllyyyy.

After cast, rough up the sides with some middle grit sandpaper, and does the outside HAVE to be Crystal Clear? Paint on XTC instead, or laquer even a rattle can clear coat

I wonder if more cure time may help, maybe shrinkage is happening.

I like EpoxaCast for clear resin.

 

[Sevv]

Thanks for replying. I do pressure pot my smooth cast cube as that's curing to ensure as minimal air bubbles as possible in my cube before I coat it in crystal clear.

You might be right and a longer cure time might be needed, shrinkage might be happening. Is that why you suggested roughing up the cubes surface so the crystal clear bites into the cube more to reduce shrinkage? Laquer is also a good idea ill try that.

I am not set on crystal clear no. I am defiantly up for trying EpoxaCast.

 

[JPH]

Thanks for replying. I do pressure pot my smooth cast cube as that's curing to ensure as minimal air bubbles as possible in my cube before I coat it in crystal clear.

You might be right and a longer cure time might be needed, shrinkage might be happening. Is that why you suggested roughing up the cubes surface so the crystal clear bites into the cube more to reduce shrinkage? Laquer is also a good idea ill try that.

I am not set on crystal clear no. I am defiantly up for trying EpoxaCast.

I recommend roughing up the sides in case retained release agent or too smooth a surface is causing the separation.

If it is gonna shrink, that is another reason to extend cure time. Let it shrink completely before covering it. A slick surface and shrinking inner piece is recipe for separation.

I know these products can get pricey, I hope we can find you a great solution without hurting your wallet too much.

 

[Chaank]

This is an interesting issue. I'm wondering if maybe you are getting a little condensation on the sides of the internal cube as the resin heats up. What size are the cubes you're casting? Sounds like an interesting project

 

[Sevv]

Thanks all. I really hope we can figure out a solution as well. I'm currently taking your thoughts about making sure the smooth cast cube has had time to shrink before I coat it in crystal clear. I've cast 2, one to try with lacquer and one not... both roughed up with 220 sand paper. I'm giving them a good few days to sort themselves out before I move onto the next step. Ill defiantly update when I see if this helps or not.

The cubes are only 25mm across, really not very big. I don't think they have any moisture on them but its a good thought.

 

[Sevv]

So far, no signs of the mercury effect. I left the smooth on inner shape for 3 days to allow it to properly shrink before I applied the crystal clear 202. Going to leave it a few more days but I would have thought I’d have seen something by now… exciting so, right now thank you very much for the tip!

 

[Sevv]

Had a look this morning. Still perfect. Calling that a win! Thank you for helping me solve this issue, it’s been pissing me off for ages

 

[jbro44]

I know this is an oldish thread, but now that the temperature in my basement is on the cool side, probably not far from your 18 degrees C, I started to find my cure time dramatically increase. I noticed this with jet black urethane resin. Normally two hours is more than enough time to leave it in the pressure pot, but suddenly 2 hours wasn't enough and it emerged bulbous and filled with air pockets. So I bought a heating pad, called a "Brewing & Fermentation Heat Pad." It's round and is the same diameter as the pot. It only heat's up to about 90 degrees Fahrenheit (32 C) so it seems to be quite safe in terms of any risk of increasing the air pressure. I taped it to the bottom of the tank. It fixed the problem remarkably well. It wasn't very expensive and, whether or not it solves your problem, it's likely to help prevent slowed curing issues in general.