Yo Straw Dog: Been thinking about this myself. Have decided to get into the silicone mold experimentation and have done some snooping around the web. Seems for best results (concerning air bubbles) in silicone, the experts advise degassing the product prior to pouring the mold. They show how to pull a vacuum on the liquid silicone to pull out all the air prior to pouring. Logical to me and I wondered if the same process would work on a thin "slurry" of 'crete as well ? Haven't built my vacuum chamber yet but certainly plan to in the near future. Don't have any idea how thin it will have to be for the vacuum to pull the air out but intend to find out. I suppose if the vacuum was increased enough it would probably pull the air out of cured 'crete !! No, just kidding but I'm going to try anyway. The only thing I'm not sure about is just how many of the "bug holes" are actually the result of air in the mix and how many are due to something elas ??
Later
Packrat

Hey Packrat,

I'll be very interested to hear about any experimentation that you get up to with the vacuum. For the moment I'm avoiding anything like that, but if it seems enough of a priority, or worth the trouble, I'll be checking it out. Do you have any good diy links for vacuum processing filled molds?

s_d

I have made several latex molds and haven't had big issues with air bubbles.I have flatter ones and rounder ones, some of them I have made back up molds using that insulation in a can product, others I use packing peanuts surrounding them to hold their shape. I wonder if you are doing to thick a mix? I do my pour with my mix slightly on the thinner side, if it is a flatter mold then I only pour enough to cover the bottom by say half an inch, and then vibrate. Then I will top it up after I have vibrated it. On the rounder shaped ones I pour the mix in very slowly with the mix running down the side towards the bottom. I use my little electric sander and vibrate the work board or table that the molds are on.I have read that too much vibration will cause buubbles to. You can also have a small amount of your dry mix set aside to use as a patch kit if the bubbles are too much. Just mix it up and I would apply it with my gloved hands . Then wipe off the excess. The couple of local places here that do concrete statuary do not use anything more than a vibrating table to remove air bubbles. Cindy

Thanks for that suggestion Cindy. I've not really tried that beyond a few quick wiggles. I'll be a bit more aggressive and see how that goes,

Sean

Sean,
I have had a problem with air bubbles also, using plastic molds. You might want to search the archives in this forum as see some of the suggestions. It may be too much release agent.
Something that has worked for me somewhat is after your mix bang your container on the floor, wait a couple of minutes and watch the air bubbles come to the surface, then pour.
Rhonda

Straw Dog: No diy website for the vacuum process but saw the commercial version for $600.00 + and decided I would probably be able to do something close with a 5gal. bucket and a small shop vacuum. May be pipe dreaming and will waste my time but am going to give it a try anyway. Will post results when finished. My only concern is that I'm going to go to all this trouble and find out the holes on my finished product was due to something besides air in the crete !! Try this site for some basic info: smooth-on.com/plugin3.htm

Packrat

Packrat - go for it! Think of the thrill you (and the rest of us) will get if your 5 gal bucket and shop vac can come even close to what a $600 version could do.
Laura

I don't think a shop vac will pull a hard enough vacuum.

Here's a link to get started. At one time I was the proud owner of a Werther Silent-Aire (?) compressor. Tapping the input allowed it to serve as a vacuum. I never measured the vacuum, but it was substantial. My ex threw it out, along with some expensive squirrel cage blowers, thinking that it was garbage ... sigh

Here is a link that might be useful: refrigerator compressor as vacuum source

Hi,

happy ending to this part of the story.

I prepared two moulds yesterday, brought every trick in the book to bear. Used a nice thin cement, banged down the jug of mix, painted it around diligently after pouring it on (which I hadn't been doing consistently, in spite of the thread title), then gave it some vibration. I peeled away the mould this morning and wow, very nice. What had been acceptable is now .. stellar! Nice way to start the day,

cheers and thanks for the input,

s_d