huge bubbles in my pressure-cast moulds???

Antenociti · Post by **Antenociti** » Mon Nov 09, 2009 6:11 am

Hi all, bit of an oddity today...

i've pressure cast some of my production moulds "as usual" over the weekend.. however when popping them out of the pot this morning i have found 4 of them have huge (massively huge) pockets of air in them.

My build process is the same: mould blocks for the box (lego), with a tacky-glue seal around the bottom on to 2mm thick styrene. pour in the silicone, allow to degass on its own for 30-60 minutes, then whack in the pressure pot at 60psi and leave for 24hours +.

pressure this morning (48 hours later) was still over 40psi, so I know the pressure was on the whole time... but on the "top" surface of the mould has a massive 3cm wide and 15mm deep air pocket in it - with the surface bloated upwards.

this is the first time i've seen this in 4 years of doing this..and we are repalcing moulds on a weekly basis as they get used up.....

the only thing that i did differently this time was to cover the outside of the mould-blocks with sellotape. This was done because i was getting some wastage of silicone that escaped through the gaps between the blocks.

Does anybody have any ideas why and where these huge "bubbles" have come from...i'm at a complete loss as to what might have happened and have never seen anything like it.

(no camera at the workshop today so i cant take any iamges sorry)

Chacal · Post by **Chacal** » Mon Nov 09, 2009 5:26 pm

Are you supposed to apply pressure to the silicone as it cures? I was under the impression one would pressure cast the resin, and put the silicone RTV in a vacuum prior to laying it onto the mold box, then leaving it be until cured.

Antenociti · Post by **Antenociti** » Mon Nov 09, 2009 6:41 pm

been doing it this way for 4 years without any troubles at all.... cant explain why it went wrong this time.

Stu Pidasso · Post by **Stu Pidasso** » Mon Nov 09, 2009 6:44 pm

I'm thinking the huge bubble was just a collection of little bubbles that coagulated into the big one, popped as it was pressurized, but the RTV was cured enough to not "level out" into a flat mold top.

Got any pics?

Blappy · Post by **Blappy** » Sat Nov 14, 2009 10:43 am

Perhaps by putting tape on the outside to the blocks you trapped the air in between them because the rubber could not ooze out.

I tried pressuring the rubber after pouring years ago but stopped when I had a master I thought was solid collapse on me. It was a pretty ugly situation. I have however Vacuumed the rubber after pouring it when I knew for sure the master was solid. Thats how I got the detail on the neck here to turn out. http://www.starshipmodeler.com/newshop/ ... k_1917.jpg

Degassing your rubber before pouring it into the mold is a far superior method and it does not tie up a valuable pressure pot for 24 hours. On a good day a single pressure pot can make me over $200.00 casting.

vaderman · Post by **vaderman** » Mon Nov 30, 2009 4:40 pm

I agree with Blappy. Degas your molds rather than pressure cast. Use a vacuum chamber to degas the silicon first. Then pour into the mold box and the degas the molds in the pressure chamber. After that, you can pressure cast your parts.

Scott

Migmaker · Post by **Migmaker** » Tue Dec 01, 2009 11:33 am

It's true, I learned from one of the finest and not best mold makers out there many years ago....degassing (, de-airing,vacuuming) the rtv is a must, forget the pressurizing. Only need to pressurize the castings from the mold tool. I've been casting myself since 94, never did it the way you are.

Will