Vacuum to make moulds

[mangozac]

Now I'm asking for some advice for a change

So I've always cured my moulds under pressure to make them bubble free. It's easy because the Pinkysil silicone I use cures in 30 minutes. While I still ended up with the odd bubble in an undercut or deep cavity, it was never bad enough to make the mould unusable.

Skyping with Alan (Deadmeat) a few weeks back he insisted that I try using vacuum to make the mould. I shrugged it off at the time with the excuse "if it ain't broke then don't fix it", but after a bubble issue on a recent mould and email discussion with RA member Timdp I decided to try moulding under vacuum.

Now we all know that silicone expands when placed in a vacuum. Quite a lot. General rule of thumb is to have a container of at least 3 times the silicone volume to ensure that the degassing silicone does not all spill out over the top. With this in mind I build my mould box just under 3 times higher than the amount of silicone I planned to pour.

This morning I mixed the silicone, placed it in the chamber and applied vacuum. I let it get to -27 inHg and then decided to return to room pressure and check that everything was OK. The result:

Overflow

Of course I had anticipated the possibility that I would get some overflow. But what surprised me is just how much silicone I lost. There's barely any left in the bottom of the mould box! It must have expanded to over 5 times its volume to lose that much silicone. I thought maybe it had somehow all leached out through the spaces between the Lego bricks (not that I've ever had an issue with that in the past) but closer inspection revealed that not to be the case - the silicone down the sides had only come from overflowing from the top.

So what vacuum do you guys take your silicone to? Since I don't have a clear chamber lid for vacuuming I can't watch the silicone degas. Do you guys with clear lids simply stop the vacuum once the silicone level reaches the top of the mould box?

[Pinselknecht]

Why dont you build a clear lid for your chamber? Take acrylic with 20 mm thicknes.
And yes, every silicone is different. I have some with 3 times others with 6 times rising. Use vacuum first and then pressure and you will get the finest molds

PK

[Blindhorizon]

I was actually curious if it was possible to do all castings with just vacuum and get good results. On topic but off at the same time. Can you cure resin under vacuum? Will it be bubble free, similar to pressure casting?

Blind-

[Fleafa]

Did you degass before pouring?

[Pinselknecht]

it is possible to cast resin with vacuum. but only if the shape of the part you want to cast is not to complicated! And you must release the vacuum before the resin cures. Otherwise you will deform the mold or get very big bubbles insted of smaler ones!

Try vacuum and then pressure and you will have perfect results. I do vacuum and then 10 bar pressure

PK

[mangozac]

[TomG]

I use vacuum for creating moulds. It has two stages. I mix needed amount of silicone, then put into Vacuum chamber in container for 8 minutes on maximum vacuum (I use 3cfm oil pump 70 litres per minute) to have mix almost without bubbles. Only few remain on surface. Then slowly return here air.
Don´t forget to have big enough container. Silicone can expand 3x-4x of original volume. When you have this done, you can slowly pour from height it into mould via thin flow and then put into vacuum chamber once more. But only for 5 minutes. You need some reserve on moulding frame, because silicone can expand a bit, but not more than 1/4 of original volume. Because you evacuated almost bubbles in the first stage.
Then again slowly return air here and you can put it somewhere for hardening. There will be few bubbles on surface that will pop in time.
These times depend on used silicone.

[Fleafa]

I tend to pull the vacuum until collapse and then pour into the mould. Still get the odd bubble, but only on very detailed surfaces. Then I tend to pull a second vacuum after the pour. All curing is at 1 atmosphere. My condensation cure takes around 4 hours and my addition takes 12. Not a fan of this addition though

[factor40]

I've just started doing this as well, I've noticed a few nasty bubbles creeping into my molds as of late. I do have a clear acrylic lid so I can watch it but I also just let it rise until it collapses for the first time and that usually does the trick. Then I just pour the mold and apply pressure.

I will add that I don't usually need to even hit -27.

[mangozac]

[factor40]

[mangozac]

[factor40]

Zac, my vacuum setup has an inch thick acrylic top and it fits right on my casting pot. I mix and degas in a large container and then pour directly into the mold box. I then switch the tops and apply the pressure.

[paulson games]

I only use pressure when creating the molds and don't have issues with bubbles. When I cast I use a combination of both vacuum and pressure and I don't get any surface distortions or BBs.

When pouring the molds I just use a slow elevated pour that helps eliminate the bubbles and it helps the silicone flow a bit slower so air won't trap as easily. Positioning of the parts also helps avoid air pockets. If you angle a piece it can help avoids pockets. I haven't had any bad molds in years due to bubbles although I've ruined a couple by not mixing enough

[johncast]

I wouldn't have a vacuum chamber I couldn't see into . It is vital you have a Perspex cover or as I have a vacuum chamber that has a large vacuum dome .I can view all sides and top of all the moulds at all stages of vacuuming. Careful observation of the material being vacuumed is a must. Depending on the volume mixed and viscosity of the rubber all play a part in the explosion or expansion of the rubber.
In some cases I have had to let the vacuum off around 5 to 6 times before I can get the rubber to expand and then drop down to settle for a 20 to 40 second boil. Bigger container need , maybe . But when I mix rubber I really mix well, and that I know produces a lot of air to the mix but a good vacuum and then just pour into the frame of the mould slowly and have bubble free moulds.

I seldom vacuum the freshly poured mould haves. only those with solid brass patterns..


WHY WELL , TAKE NOTE.
For those of us with masters in styrene, 90% of us, and there are parts in those masters with laminated pieces , if some of those laminated gluded sections of the masters are quite large they can contain air. Small amounts of air which you cannot see. If there is air trapped in between these glued sections, the air will want to escape when the mould is vacuumed. Trapped air just blows up inside the styrene part distorting the part. This can lead to a hours of hard work being blown apart by the trapped air when these type of patterns are vacuumed. I admit I got caught with this many years ago , I vacummed a mould containing a styrene pattern made with 2 layes of styrene and the air trapped between the 2 halfs just blow the pattern apart. I couldn't see this as all this happened underneath the vacuuming rubber. When the mould was turned over , disaster. So I never vacuum rubber that has been poured around a styrene pattern. Good lesson learned.

Just mix the rubber ,then vacuum the mixed rubber and slowly pour over the pattern, I have rearly a mould with any air entrapment .


Johncast