Casting help

[replicaprops]

Frank, you have been in this business a very long time, and of course I'm not arguing with you. Way back when you were teaching me about this stuff when I was doing full size arks. I know you are extremely familiar with every aspect of molding and casting, and there is no dispute there at all.

The only thing im saying is as people build their experience they sometimes come across methods that work better than others. I used to vac my rtv just like every book tells you to. Then one day I got some clear RTV from smooth-on and after vac I poured my mold and because the rtv was transparent I could see all the air bubbles that simply were not removed from the vac.

So now I pressure cure all my rtv. If you dont think its a good idea then thats fine, but I know lots of people who use this method and have never had a problem with it.

Honestly though I think the analogy of the micro bubbles expanding after pressure is removed is not correct. The action the pressure does to the bubbles is not so much compressing a large bubble into a small one, but more like breaking a large bubble into millions of tiny ones. The reason I can say this is the case is because some of my molds are quite small and thin and I would see the distortion in the surface of the rtv if the bubble was actually expanding.

The only thing I can ask is that you mix up a batch and try it for your self.

Nothing personal...and text looks more hostile..so please.

This is a friendly discussion, if it appears otherwise, thats NOT the intent.

.......

I dont agree with you. physics is is physics.
However, I have no mission to change your mind.
If you think it works for you....great.

However, for everyone else...Your parts my LOOK ok...but I guarantee your mold is distorted. Even if ever so slight.

Obviously, the less air,...the less of the distortion.

THINK about it..you MOLD under pressure, any AIR BUBBLES will get smaller.

The Mold cures, now you REMOVE from pressurization, that air will want to get bigger.

Remember the BALLOON expirement in 6th Grade?

Its SO much easier...just to REMOVE the air from the silicone.

Now, to play devils advocate, IF the mold is completely cured, would the air dissipate through the silicone? How AIRTIGHT is SILICONE? last I checked...its pretty air tght. MAYBE... Microscopicly, over how much time? Why take that chance? So until it DID dissipate...IF it does....the mold is DISTORTED as your MOLD has expanded. You might not see it with your naked eye, but it has.

Physics is Physics.


Now...evac the RTV.....then PRESSURE CAST? I Might buy that.

However, pressure molding alone....no way.
Anyone here wanna try it....go for it.

Let me know how it works out for you.

 

[BrundelFly]

"Honestly though I think the analogy of the micro bubbles expanding after pressure is removed is not correct. The action the pressure does to the bubbles is not so much compressing a large bubble into a small one, but more like breaking a large bubble into millions of tiny ones."


More than one way to skin a cat I guess...
I just dont agree with your physics about bubble distribution.
Pressure applies uniformly, breaking bubbles has to be done directionally, from a single source.
If what you say is true....then it would be impossible for a nuclear weapon to detonate.

I'll guess I'lljust have to take your word for it.

 

[replicaprops]

"Honestly though I think the analogy of the micro bubbles expanding after pressure is removed is not correct. The action the pressure does to the bubbles is not so much compressing a large bubble into a small one, but more like breaking a large bubble into millions of tiny ones."


More than one way to skin a cat I guess...
I just dont agree with your physics about bubble distribution.
Pressure applies uniformly, breaking bubbles has to be done directionally, from a single source.
If what you say is true....then it would be impossible for a nuclear weapon to detonate.

I'll guess I'lljust have to take your word for it.

lol, funny you should say that, I was a nuclear weapons technician in the navy

 

[BrundelFly]

lol, funny you should say that, I was a nuclear weapons technician in the navy

Then you should know how uniform compression works on a gas.



Cheers to the Navy!

I was an E2C Flight Officer.
VAW-112 off of USS NIMITZ

;-)

 

[Utinni]

Try Silpak. I only use their stuff, never have needed to de-gas the RTV or resin. Just measure, pour and cure! Several well known prop builders on this forum use their product to make small to very large molds and never have had a problem. They're very helpful and easy to work with.

- Jeff

 

[replicaprops]

Ive been on the Nimitz for a party. It was a much nicer ship than the Kitty Hawk that I was stationed on.

Then you should know how uniform compression works on a gas.



Cheers to the Navy!

I was an E2C Flight Officer.
VAW-112 off of USS NIMITZ

;-)

 

[Basherthrasher]

BASHER....
Ok..You cant make a pressure pot out of Plywood.
I about shot my soda through my nose....


Research what you need. Otherwise, you could really hurt yourself, or..worse.

Frank

OK,so this means that I can't make plywood work. What else can I use to make parts with? I thought that making this with RTV in between would make an airtight seal. Is there an alternative that I can use safely?

 

[propologist]

Sooooo did we ever figure out which resin is best or that we all have gas.

 

[Basherthrasher]

I'm still up in the air about silicone mould material. This will be decided when we can figure out what the options are for it.

 

[Utinni]

Again, try Silpak. I love their products. They're very helpful too! I've never needed to de-gas my RTV, just mix up and pour and I've NEVER had a problem with any of my molds.

- Jeff

 

[BrundelFly]

You need an AUTOCLAVE for pressure castings.

They work GREAT.

Mine SORTA looks like this...
I found mine on EBAY. CHeck local medical industrial liquidators in your area.

 

[Basherthrasher]

An autolclave is what we used to sterilize medical instruments. How do you use this for pressure casting? Why wouldn't plywood sealed with RTV sealant work? This stuff is rated at 500 LBS. psi.

 

[JPolacchi]

Because plywood isn't air tight no matter what you do to it.It also wouldn't be able to handle the 29 HGs of pressure.I would guess that it would either colapse(it might even implode or something?) or you would loose your seals and loose pressure. Some good steel pipe(big stuff...12" diameter or bigger,like what's used for sewer pipe) and solid welds can make a pressure pot,then you just need your fittings(pressure gage) and vacume pump.There are commercial ones,but they are very pricey.Tap Plastics has one for around $600-$650 I think for small molds.

 

[replicaprops]

You dont need anything as large or expensive as an autoclave unless your casting huge parts.

Most everyone buys a 2.5 to 5 gallon pressure pot used by painters (painters pot) . Add a ball valve and it's now a casting pot. You can usually buy them brand new for under $100 and on ebay for even less.

The larger the pot the stronger the steel and clamps need to be. You need to cast under a pressure of 55 to 65 PSI depending on the viscosity of your resin or silicon.

Ply wood will crack after 10 psi and you will create a bomb. Don't do it.

 

[BrundelFly]

Ever hear of...the DARWIN awards?

You dont want one.



Yes...Paint pot will work well.

Just to note: I paid 150 bucks for my autoclave on EBAY.

 

[Basherthrasher]

Because plywood isn't air tight no matter what you do to it.It also wouldn't be able to handle the 29 HGs of pressure.I would guess that it would either colapse(it might even implode or something?) or you would loose your seals and loose pressure. Some good steel pipe(big stuff...12" diameter or bigger,like what's used for sewer pipe) and solid welds can make a pressure pot,then you just need your fittings(pressure gage) and vacume pump.There are commercial ones,but they are very pricey.Tap Plastics has one for around $600-$650 I think for small molds.

I was going to reinforce this,but I understand what you're saying. Since this can't withstand that much pressure,it would cave in,and split the wood.

You dont need anything as large or expensive as an autoclave unless your casting huge parts.

Most everyone buys a 2.5 to 5 gallon pressure pot used by painters (painters pot) . Add a ball valve and it's now a casting pot. You can usually buy them brand new for under $100 and on ebay for even less.

The larger the pot the stronger the steel and clamps need to be. You need to cast under a pressure of 55 to 65 PSI depending on the viscosity of your resin or silicon.

Ply wood will crack after 10 psi and you will create a bomb. Don't do it.

Unfortunately,this will have huge parts that need to be either fibreglas,or cast from resin in a big vaccum chamber. Yeah,I agree. I don't want to ruin my parts,or waste money on a faulty process trying to create models.

Ever hear of...the DARWIN awards?

You dont want one.



Yes...Paint pot will work well.

Just to note: I paid 150 bucks for my autoclave on EBAY.

The paint pot is too small! Even a 10 gallon pot would be too small lengthwise. I'll see what feeblebay has to offer in autoclaves.

 

[SUPERprops]

here's one I got a while back, and it works great:
http://cgi.ebay.com/PRESSURE-FEED-PAINT-TANK-POT-SPRAY-GUN-SPRAYER-SYSTEM_W0QQitemZ190360983085QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item2c5266162d

You just have to saw off a tube inside to give you more room and make shure your compressor is bigger than the tank.

 

[replicaprops]

Just remember the larger the volume the chamber is the more reinforced it needs to be.

 

[Pzak]

I have pressure molded for years. It works fine for smaller items that will fit in the pressure pot.

BTW, Harbor Freight has great deals on 2 1/2 gallon pots and pancake compressors.

 

[Mostly Fantasy]

I've found I have to either de-gas, or pressurize the silicone. Doing both is overkill, but doing one or the other is a necessity. I prefer de-gassing, especially if I don't know what the master is made of when I'm making a mold for someone else. You don't have to worry about voids with degassing.