Getting air bubbles out of undercuts in molds/casting.

[plissken1997]

Hello. Im new to this form and medium. I recently started mold and casting resin. Im working with a pretty basic setup, and do not have any thing like a degassing pot.

Im trying to cast small human figures, and the problem im having is i keep getting air bubbles in the resin casting right underneath the chin of the figure.
The resins is being poured through the feet of the figure of the mold.

The molds im making are simple 2 part block molds.
im using Alumires (RC-3) resin, and OOMOO 30 silicone

Any any suggestions or tips or tricks would be very much appreciated!

Thanks.

[mangozac]

Hi Plissken, welcome to RA!

Have you tried squeezing the mould a few times as you pour/after pouring the resin? That often helps in dislodging bubbles and air pockets like that.

[plissken1997]

thank you for your response . I've tried squeezing the mold tapping it on the table the the air seems to be trapped underneath the chins.

[Fleafa]

Sounds like a flaw in the mould design. Try pouring to fill just the head and then give it a few really good squeezes. When you tap the mould angle it so that the air can escape easier. You could also try rotating the mould around at this point. Then pour the rest.

[Subtle Discord]

Yup, everyone else has the right idea. You have two major options; add a small vent to the chin so the air can escape, or take care to tilt and pinch the mould to get the bubble to role out off the stubborn spot.

An added vent will likely need to lead into the chest of the model, and it will need the removed and cleaned up after the cast.

If you are tilting and pinching try to remember/visualize the orientation of the model and be sure to tilt it the right way and to a significant angle to really help the bubble roll free. Partially filling the mould, tilting, and then finishing the pour will likely be needed. Also note that the shape of the chin might be such that the bubble gets stuck because it conforms so well to the shape of the chin. Nothing you can do but really squeeze, tilt, and bump/tap the mould in an effort to get the bubble free.

Im not sure of the cure time on that resin, but if it's very fast you might also have better luck with a slower kicking resin that will stay liquid longer, and bubbles will more freely roll out of stubborn spots.

Note that even with pressure and/or vacuum a few small bubbles in tricky spots is quite common. A bit of filling and cleaning is almost always needed.

[johncast]

Hi Plissken

Always a common problem with figures with just one spot that won't fill .

All the above is good advise and worth trying. Another method I could suggest is to very lightly coat both halves of the mould cavities with resin before putting the mould together.
Put the mould together as normal and then very slowly fill the mould don't rush , and if this can be done drop by drop , so much the better.. I would expect the mould cavity can be filled by pouring slowly down one leg and hopefully getting the resin to push any air out up the other leg cavity. Some tilting of the mould may be needed. With the mould surface already coated with resin this can assist in getting a good casting.

Johncast

[plissken1997]

Wow. Thank you all for your advice.I started using smooth castt 300, i have notice its a very brittle resin. is this a genereral consistency?

[mangozac]

I don't remember Smooth Cast being especially brittle. Generally the higher the Shore Hardness of the resin the more brittle it will be.

[Fleafa]

Smooth-Cast 300 series resins actually have a decent flexural strength/modulus. They degrade significantly when they start to age (due to the two parts being exposed to air). Post-curing can help.