I had another noob question for the casting gurus, Is is necessary to Vacuum degas your silicone, if you cure the rubber mold under pressure. I'm not asking about personal preference so much as actual fact. I don't see any need for a vacuum chamber if pressure curing will serve the same purpose. The pressure pot can force air bubbles into suspension. Enough even for clear resin casting. It should do the same with rubber. Let me know if I'm totally daft, Cheers,

Joe

Thanks Jase, I was actually asking if pressure casting the mold would work like degassing. I've heard people have done this, but didn't know how well it works. I'll have a vacuum pump for a vac former, but was just curious if the pressure thing would work. It's not so much about money as it is about using the pressure pot for double duty, I hate unitaskers, Like Alton Brown the food network guy, LOL. Thanks for the reply,


PS. Hey I went to the Smooth On site and found this,

http://www.smooth-on.com/faq_display.php?faq_id=81

Joe

(05-04-2010, 03:30 PM)bwayne64 Wrote: Thanks Jase, I was actually asking if pressure casting the mold would work like degassing. I've heard people have done this, but didn't know how well it works. I'll have a vacuum pump for a vac former, but was just curious if the pressure thing would work. It's not so much about money as it is about using the pressure pot for double duty, I hate unitaskers, Like Alton Brown the food network guy, LOL. Thanks for the reply,


PS. Hey I went to the Smooth On site and found this,

http://www.smooth-on.com/faq_display.php?faq_id=81

Joe

(05-09-2010, 08:40 AM)427 Cobra Wrote: Ron, and Jase are both correct. When vaccum degassing is done, this makes any tiny airbubbles rise to the surface. This keeps them from distorting the silicone mould, so that it can produce an exact reproduction. The problem with pressurising is that it only pushes the air into any available space, as Ron said. This only ruins the mould surface, and therefore making it impossible to make a good cast. This only leaves the surface uneven with bumps all over it. Not only that, but it might not even cure completetly with pressure casting, and this too will leave soft spots in the rubber.

Your best bet is that if you don't want to degas is what Ron suggested - use the OO-MOO brushable silicone, or a platinum based silicone to keep the airbubbles out, and the uniformity, or integrity of your mould intact. This way, you won't need a separate degassing chamber. This is only for small projects. If you have something big, you'll have to use a different method of making a reinforcing jacket for your mould, and use epoxy instead of urethane resin.

You can still use the regular silicone without degassing, but you'll never be able to pressure cast with this if it's not degassed. Meaning, that you can only use epoxy, or polyester type resins, or fibreglas with this mould. I hope this helps in some small way.

~ Cobra Chris

Everybody is missing my point Probably because of the way I'm phrasing it. What I'm suggesting is pressure casting the mold as your making it. You pour the rubber around your part and pressurize the rubber as it cures. This forces the bubbles that are there to microscopic size and into suspension. Then when you pressure cast the parts in resin, the bubbles in the rubber are to small to effect the casting. I hope that is clearer, This is why typing out a question on the internet sucks. I'm a terrible writer, and can't get the idea across, LOL. This is the same thing they discuss in the Smooth on article I linked to. I'm hoping there's a bunch of head slapping goin on, with people saying, " Oh I get what you're saying", I definitely know the that vacuum degassing removes most of the air from the rubber, This was just a question about the merits of pressurizing the rubber itself. i hope I explained it better. If we were on the phone, it would have been clearer. It's this damned typing thing. If we don't get it this time, well I give up, LOL. Thanks for all the replies, I'll just try it and see what happens, cheers,

Joe

(05-09-2010, 10:48 AM)427 Cobra Wrote: OK, Joe. This is what we're saying:

You can't pressurise the silicone as it's curing. This causes distortions in the rubber once the mould master is removed. The air isn't sucked out in the pressurising process, instead it's pushing the air into the rubber deeper to where it touches anything solid inside the liquid. Does this make more sense? This means that you will have a lot of air holes along the entire surface of your mould = RUINED!!!

Unfortunately, many people claim they've done this without a problem, which is a lie!!! There's no way to pressurise silicone, as this would push a lot of the material out of the mould box, and onto the sides. This too, will compound your problems. There's no way to control the liquid once under pressure. These compounds are formulated to be used one way only, and any deviation from it will be disasterous!! Does this answer your question more thoroughly? Don't give up - if there's a way to get around something, we'll try to help you find it. That's what we're all here for.

~ Cobra Chris

Oh ok, I see, said the blind man, LOL. I guess you did know what I was saying, And I wasn't going to give up on casting just on this question, I wonder why Smooth On recommends this on their site. it says right in there Faq, that you can do it. That's weird. Oh well, i wasn't questioning you guys knowledge, just throwing around ideas. I always try to find the easiest way to do things, cause I'm lazy, LOL. I also understand that there's a right way to do stuff and anything else will fail. I will have a Vac pump, so I can de gass. I was just looking for a shortcut. I guess it doesn't take any more time to de gass, versus pressurizing anyway. Thanks again guys, for the responses. I bow to your wisdom. I'm a total beginner at resin casting, haven't cast anything in silicone yet. Done plaster, fiberglass, and the assorted other mediums, no silicone though. Don't think this is the last crazy idea you'll hear from me. I'm a mad scientist, Cheers,

Joe



PS. Here is the section on the Smooth On site I referred to.

Quote:Suggested by professional model makers; if you are going to use a rubber mold on a regular basis for pressure casting, make the rubber mold under pressure. This means mixing and pouring mold rubber over your original model, placing the structure in your pressure chamber and applying 60 psi until the mold cures.

Reason; If you pressure cast resin in a rubber mold that has not also been pressurized, any bubbles present in the rubber mold may implode under the pressure and the collapsed bubbles may be reflected in the finished casting as bubble positives or "dimples".

Limitation; when making a mold of a hollow model under pressure it may collapse under pressure

Warning; use commercial pressure chambers only. More than one resin caster has learned the hard way that "home-made" pressure chambers can explode violently and cause serious injury.

Important; ensure that the air running from your compressor (used to pressurize the pressure chamber) is dry. Moisture in the air line will react with urethane resins and rubbers causing bubbles or foaming.

Air line driers are available from industrial supply stores.

Yea that's the reason I was asking about the pressure casting. I don't know what's goin on there. I'm gonna try a small mold when I start casting, just to see if it 'splodes or not, LOL. I figured if it works on resin, it should work on rubber. But from what the guys are saying here, it won't work. I'll let everybody know if I have to clean out my pressure pot afterward, At least silicone won't stick to the pot, should just peel off. I know how it is, once you find a technique or product that works, you don't want to do anything else. It could be a waste of time, but hey what else is time and money for. I think vacuum de gassing is cool, I like watching it bubble up and then fall in on itself. And it definitely is the tried and true way. I think though that some air gets into the rubber when you pour it. Maybe the pressure is to get rid of that. It's an experiment, I will probably come back and say " I was wrong, it blew out of the mold, like She'ite through a goose", LOL. Thanks guys, I appreciate the feedback, Later,

Joe

(05-20-2010, 11:17 PM)modelnutz Wrote: B-man...the short answer to your question is.... yes, you can cure a mold under pressure and yes, you will get a decent mold...as a matter of fact, the mold will actually have better strength properties due to the simple fact that the molicules of silicone are packed a bit tighter together. ( higher density = higher strength )

However... ( doesn't it suck that there's always a "however" to these things ? ) if you do pressure your molds...your masters had better be very solid items....ANY air spaces that have the tinyest openings will be completly filled in the process. ANY porosity of the master will cause the silicone to lock in to the master ( ask me how I know

Also, if the silicone is not FULLY CURED, you will have a real fun time watching the air bubbles expand inside your newly poured mold.
( again, ask me how I know )

So, to sum it all up...yes, it's possible....but it's a much more certain outcome to vacume de-gas a mold.

Thanks Man . I'm always trying to find the fastest and "Cheapest" way to do things. If I could use a pressure pot for double duty, that would be great for me. Of course cheaper, can mean you have to Vac degass and pressure pot your parts. Other wise you spend the money in bad parts, i.e. wasted material. I'll probably give it a try, and if it doesn't work, I'll go back to the tried and true. I am always willing to earn from others mistakes as well. I mean that's what these forums are about. You guys know your stuff. I'm just a beginner in some of these processes. I've done everything you can do with fiberglass and composites. Including building Minesweepers for the Navy, but haven't used Silicone yet. The only molding I've done, is with Frp, and plaster. Used latex once or twice. So I defer to the collective wisdom, I know one thing. I've learned more in a few years on forums like this, than any where else. So thanks again, Cheers,

Joe

(05-21-2010, 05:34 AM)427 Cobra Wrote: That's another reason why I wouldn't trust pressurising the mould material, just for safetys' sake! Most people make two part moulds that are hollow and don't have the durability that I incorporate into all my things. I want durability, which is why I'd just use an epoxy fibreglas mixture for the product, and forget about pressurising, or degassing the mould entirely.

What are you trying to make with this, if you don't mind me asking? I mean is this a two part mould, or is this a really big solid part that will need two sides to complete?

~ Cobra Chris

(05-09-2010, 10:48 AM)427 Cobra Wrote: OK, Joe. This is what we're saying:

You can't pressurise the silicone as it's curing. ...

Unfortunately, many people claim they've done this without a problem, which is a lie!!! There's no way to pressurise silicone, as this would push a lot of the material out of the mould box, and onto the sides. ...

(12-14-2010, 04:41 PM)THX1138 Wrote:

(05-09-2010, 10:48 AM)427 Cobra Wrote: OK, Joe. This is what we're saying:

You can't pressurise the silicone as it's curing. ...

Unfortunately, many people claim they've done this without a problem, which is a lie!!! There's no way to pressurise silicone, as this would push a lot of the material out of the mould box, and onto the sides. ...

Just for reference purposes, this is false information. You can place a silicone mold under pressure while it's curing. I've done it many times, with good success. The process does not push the silicone out of your box. Gravity still works in a pressure pot.

(12-14-2010, 10:01 PM)erospawn Wrote: One more thing to note on that as well.. You don't necessarily need to degas the rubber, although doing so will make your life much much easier... I did it this way a few times while getting the vac stuff sorted out. The trick is mix slowly... then get as high as you can and pour the thinnest stream of rubber in to your mold box, pour slowly. When you pour it, don't pour directly on top of the part. Let it rise up and slowly cover the part to be molded. It will take time and usually does well under pressure...so, if you're desperate to get started, you can try that.. But, degassing is much a better way to go.

Thanks Eros, Sorry don't know your given name, I've seen that technique on a few videos. The small stream to the low part of the mold. Someone described it as the bombs away technique, LOL. As much as these products cost I don't really mind an extra step of degassing. Better that than getting a failed mold. As soon as I get some rubber and resin, I'll give her a go and see what's what. I'll probably get a small kit and practice a little. I have enough of one of the Roco models, to make a gang mold. I'll need a bunch of those little suckers, so it's a good place to start. It's been cold here lately, for Georgia that is. Hopefully as long as the room is above 75 or so, I'm guessing everything will cure alright. I'm still an absolute noob at the casting process. Thats why I'm here picking your noggins, Thanks for the great input. It really is helpful, Cheers,

Joe

(12-15-2010, 10:07 AM)bwayne64 Wrote: ... I have enough of one of the Roco models, to make a gang mold. I'll need a bunch of those little suckers,...

Joe[/color][/b]

(12-15-2010, 04:21 PM)THX1138 Wrote:

(12-15-2010, 10:07 AM)bwayne64 Wrote: ... I have enough of one of the Roco models, to make a gang mold. I'll need a bunch of those little suckers,...

Joe[/color][/b]

Ganging Rocos? Wouldst thou be building a Galactica??

Thou art correct. I'm one of those crazy people, LOL. Started a mockup, back in 1986, and just now starting to get it together. I may get it done, before I die, Cheers,


Joe