Reducing Bubbles in Plaster Slip Casting Molds

In school we would tap the bucket holding the plaster after mixing and before pouring to agitate the bubbles to the surface. Then you can swirl the liquid to group the bubbles that have come to the surface and skim them out before pouring. We also pour to the side of the mold interior directly onto the palm of a hand touching the side of the mold so the plaster kind of just slides in so as to avoid any additional bubbles collecting along the form of the body you're casting. As a last step we would pound on the table to float the remaining bubbles up or you can stick a hand in the liquid plaster right at the surface and kind of just bounce it to agitate the liquid lightly and bring the bubbles up. Granted the way you're casting the mold is pretty different from any way I've ever done. If you're making simple drop molds you can actually just throw a solid form on the wheel upside down (so think the lip of the form would be what is attached to your wheel head, and you would be defining the foot at the top) and then throw your cottle walls out of clay around the exterior and cast directly into that. Ceramic Jim has a video that's exactly that https://www.youtube.com/watch?v=7n7zU0BZsLw although it occurs to me just now that your might not have a wheel. Either way you're going to want to make sure you get all your bubbles out before inserting the top silicone piece as that is most likely a recipe for catching those surface air bubbles.

For another purpose, I ordered a 5 gallon vacuum pot which is supposed to arrive tomorrow. I'm going to see how that works (intent is similar to epoxies and silicon).

Maybe if you put a long straw in the middle of the inner plug the air underneath can escape?

Or is there any way to make vibrate table on my own ?

From where to get the vibrate table ?

Pour the plaster over the item and then shake it so the bubbles rise away from the item you’re moulding instead of into it

Try methylated spirits

You are introducing too much air due to the way you are mixing the plaster. The entire mixer paddle needs to be deeply submerged at all times during mixing in order to not let any air into the plaster. You need either smaller mixing paddle or taller container and to keep the paddle as close to the bottom as possible

I made 2 molds the other day and I'm very unscientific about the whole process. What I did find is that the mould with the thicker plaster had much fewer bubbles and the wetter plaster mold had quiet a lot.

i saw some people use alcohol to fight the air bubbles - but i cant think of any scientific explanation

I think all of your problems would be solved by not trying to create a mold right-side-up. The important surface is the inside of the mold, not the surface that touches the outer container you hold everything in. I'd put the smaller silicone master and the form it sits in upside down, glued to a surface, then put a round tube around it that's tall enough, and glue that down. Then just pour the plaster in over the pot and vibrate. Bubbles will rise AWAY FROM the form, instead of upward against it.

Keep it simple. Yes you are adding air when power mixing - so don't power mix - mix by hand gently.