Large Bubbles in Castings with Smooth-Cast 327

[mrnabisco]

Hi!

I'm new to this board and just getting back into working with resin. I do have working experience with resins and fiberglass some years ago... Including homebuilt aircraft part fabrication. But there is always something new to learn and I'm excited to join in here.

In brief, I'm getting some good sized bubbles in castings made with Smooth-Cast 327 and green pigment.

The casting in question is Smooth-Cast 327 in a Mold Star 30 mold. It is dyed with So-Strong green.

My procedure: Apply Mann Ease Release 200 to the mold (let dry). Mix the resin, add the green then degass in a vacuum chamber for 2 minutes. I have a good pump that reaches 30lbs. I then pour a small amount of resin into the mold and slowly flow it around to line the mold with a thin coating. Then I slow pour to fill up the mold.

The casting is 2" x 5" in size and about 10oz in resin volume.

The final castings are consistently getting 1/4"-1/2" sized bubbles. Just a few. These are happening in the same general area each time, usually near a seam. They appear to be hollow bubbles. ??? I've done four castings now with the same result. I am not getting small bubbles, only these larger hollow cavity type bubbles.

FYI: I am also doing similar castings of two other molds and getting better results with only a few castings showing any large hollow bubbles. These castings are with So-Strong black and purple dye.

Could it be an issue with the green pigment?

Could it be an issue with the seam in the mold? Not closing tight enough?

Should I be degassing the resin in the mold instead of before pouring?

Or do I need to pressure cast? Which I would hope to avoid having to invest in...

Thanks for any and all suggestions.

Bert

[rattrap]

In my view degassing before pouring into the mould wasts time, especially if you use a resin with a short potlife. I always degass resin in the mould, that will get rid of the big bubbles. If you are then getting microbubbles, you need to add pressure or if you don't want to get set up for pressure try adding an anti foaming agent which will help.Everything else you are doing,I don't think, would cause this air trapment you are getting.

Try degassing in the mould and let us know the outcome, good luck...

[Fleafa]

I degass in the mould too. It will be messy if you haven't designed the mould for it! What you're getting sounds like trapped air rather than bubbles. Can you share a photo?

[mrnabisco]

Thanks for the responses!

I just poured a set of molds and degassed them in the molds. Actually, I degassed the resin before pouring and then again in the mold.

Lots of foam and messy... Since the molds were not designed for that and I needed to top off after degassing.

Waiting for them to set to see how it worked.

Will advise.

Bert

[mrnabisco]

Just pulled the casting from the mold... Air pockets in the exact same places and a few more.
???

Will try to post pics.

Bert

[mangozac]

Possibly because of the way that the resin overflowed. Either that or the mould shape just won't facilitate air pocket removal from that spot?

[Fleafa]

Try a part-pour and degass, then top up. This can minimise mess and the trapped air may be easier to dislodge if the surface of the mould has already been coated by resin which expanded during the degassing.

[mrnabisco]

Thanks for the advice.

I cast one of the smaller molds by degassing in the mold with success.

Will experiment with the problem mold by partially filling, degassing and then topping off and degassing again.

Otherwise, I'm assuming a pressure chamber is the next step. I do have a compressor, so I would just need to source out the correct pressure tank.

Bert

[mangozac]

[Fleafa]

Pressure won't fix trapped air.

[mrnabisco]

:/ If pressure won't fix the trapped air problem then I'm stuck...

I cast the problem mold again and still got a few air chambers. Smaller and in different places this time.
I poured in three steps, filling about 1/3 each time. I degassed after each pour.

So I'm at a loss now as to what to try next.

I am considering mixing in marble dust and trying. Perhaps that might displace the air pockets.

[mangozac]

[Fleafa]

Adding powder to the resin will increase viscosity and release trapped air even more reluctantly. Talcing the mould may help prevent the air 'clinging' to the mould.
Photos!

[mrnabisco]

Update: I did try one more pour with clear resin, i.e. no pigment. This gave the best results yet, but there is still one small air cavity.

I find this all quite confusing, since the other molds I am casting are coming out perfect with the degassing in the mold technique.

I will try to post pics of the problem air chambers...

Otherwise, I think it is time to abandon resin for this project and switch to hydrocal.