Casting help

Basherthrasher

Is there a resin,and silicone mould combination that can be used without having to worry about de-gassing? I've contacted smooth on,and there are some that are compatible,but I don't which ones I can use without making a mess being as I have no casting experience. Thanks for any help you guys can offer.

Simple

I've used products from Polytek for years now with great results.

I can recommend the PlatSil 71-20 for molding rubber. Its 1:1 mix ratio makes it easy to use, and you know its mixed right when you have a uniform purple colour.

On the casting side, I use EasyFlo 60. Again 1:1 mix ratio, and pours almost like water.

The trick to keeping bubbles out is don't mix either like you're beating an egg, go nice and slow. Also, pour the resin slowly to avoid bubbles.
Nothing beats a vacuum pot to remove bubbles, but if you're careful you won't end up with to many.

Good luck!

JPolacchi

Yes,Tap Plastics maskes a platinum silicone(brand new on the market) that does not require degassing in a vacuum chamber.It is very low viscosity and not "thick" like other silicone RTVs.It also has a much lower Shore A harness.I believe it is around Shore A hardness of 10.It mixes easily with a 50/50 ratio and is also easy to make 2-part molds as this silicone will not adhere to itself and its chemical resistance exceeds tin based silicones alowing it to make far more copies.

"So easy,a caveman can do it"

Firefoxm31

A shore hardness or durometer of 10??? way, way too soft in my opinion. Also for the enexperienced at RTV mold making, I would never recommend a platinum based RTV, way too finicky with primers, coatings and such. I'd say stick with a simple tin based RTV...
Will

JPolacchi

You'd have to be illiterate to not be able to know how to use this new Platinum Silicone.I'm certainly no pro when it comes to making RTV molds,but 90% of the trouble shooting has been taken out of this.

It's pretty strait forward and mixes via 50/50 ratio instead of by weight like some tin based silicones.Also,no mold release is required for making 2-part molds(if needed),it will not adhere to itself.There are no adverse curing problems with primers with this silicone (to my knowledge)unless something really strange is being molded that may inhibit the cure.

There are some things that will afect the silicone for setting up and curing(pretty much anything sulfur based) and a couple other oddities that most of us in the RPF won't typically be utilizing.It doesn't need to be degassed,it basically de-airs on its own inside of 2hrs when it sets.

The shore A hardness of 10 doesn't make it gooey like like jelly,it does make the mold far more flexible,which I think will actually make casting some parts easier.Some of those hard to cast parts w/undercuts can be cast without needing to make a 2-part mold.Well,I guess "pick your poison",whichever one is most comfortable with,but this new stuff...I'm telling you is pretty damn slick.

Firefoxm31

First, thank you for the quick witted comment about being illiterate and making tools with platinum based silicones. Seriously JPolacchi, I think you could have been less abrasive with your point about newer platinum based silicones. Second, making a 10 shore hardness tool probably won't have the ability to hold it's shape. Most likely, distortion will occur even in a open cavity block mold, so would most definitly require a jacket/ support combination. Real quick, as far as things that will inhibit curing, there are many, especially for a platinum based curing compound, at least the older ones. A big for instance, how about a typical glazing putty used by many prop makers and scratchbuilders, not the plastic modeling kind either. Priming your master patterns is a highly recommended for this reason. I guess the question should be asked , what is being molded and cast to determine the materials are best suited for the project.

Will

replicaprops

If you can fit the mold in a resin casting pressure pot, then no degassing is required since the pressure pot will eliminate air bubbles in both rubber and resin.

hodge1001

put a small hole in the bottom of a mixing cup, use gloves and cover the hole with a finger mix your plastic and pour into the mould thru the bottom hole. this gets rid of about 50%of the bubbles as they are all on the surface but nothin beats a degasser or vac casting machine

JPolacchi

That works too(the old hole in the cup trick) if you want to use the thicker tin based silicone.It will get rid of most of the bubbles.Just be sure to brush a thin coat on the part you are casting before letting it pour(from a good 12" above is best)You want to get a thin stream running.Someone probably already mentioned this.You can also(with some brands) mix in a thinner that will lower the viscosity of the silicone,but again...allot of this is all done "by weight" and you need a scale to calculate all of this.If you get your mixing rations off either by weight or plain measuring,your silicone will not set and cure.

BrundelFly

Dude. Are you KIDDING??
Try it. You will get a part full of little WARTS.
You cannot pressure cast..UNLESS you have evac'd your rubber.



Everyone remember Capt Cardboard? Remember all those XWINGS he sold?

NOT ONE was pressure cast. I dont think he vac'd his rtv either.

Here are a couple of tricks I learned:

1) Baby powder your molds. Use 100% corn starch..NO ALOE.

But the baby powder in an old TUBE sock (maybe 2 of em) ..tie it off..tap your mold with the BALL. Delivers perfect amount of Powder (ty Scott Brodeen)

2) When you pour your silicone...GET a ladder. In a DRAFT free room..pour the silicone from as HIGH up as you can...as THIN as you can...it eleminates ALOT of air. (Thanks Greg Jein)

3) A few drops of MEK in your resin..will thin it...but it induces a TINY bit of SHRINKAGE so be careful. Also..its cancerous..so..BE CAREFUL
ALWAYS where gloves.

4) Consult the place where you buy your Silicone..they will probably be a great help.

Try that....
I have been molding and casting for 15 years. Trust me...I got a clue as to what Im talking about.

Basherthrasher

Thank you all for your advice. I'm looking to make this out of either polyurethane resin,or urethane resin and fibreglas resin parts. This is too big to go into a pressure chamber,at least the fuselage,and wings are. The smaller parts aren't a problem. I just need to make this the easiest way possible without making a serious mistake. If I can make a chamber that would be airtight - then I could cast the parts in the same chamber that the mould could be degassed in.

Has anyone here ever made one out of plywood with RTV in the seams? Would this be suitable enough to make parts,and be airtight while degassing? This would have to be screwed down with drywall screws,and sealed with RTV gaskets. Does this sound like it could be done this way?

stormtrooperguy

i've yet to degas or pressure pot anything, and have done ok. i've used smooth-on's oomoo and rebound silicones and both have been problem free.

for resins i've used a variety of the smoothcast 300 series and smoothcast roto (i tend to pour solids with roto too, since it cures fast and is good for shock resistance).

JPolacchi

I've never done it....can't afford a pressure chamber anyhow(nor do I have the space).Pouring works fine if you take the time to do it,but it is nice to work with something a little more trouble free which is why I'm switching to the platinum silicone.

BrundelFly

Sorry...Im also:
Platinum Silicone...is NOT easier to use.

More trouble than its worth. It DOES tend to shrink less.

JP what is it that you are casting that PLATINUM appears easier?

It used to be the standard...when making CLEAR castings.
SMOOTH ON CRYSTAL CLEAR uses TIN Based molds.
The castings come out, NON STICKY and clear!


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

replicaprops

Sorry brundelfly, but you are incorrect in this statement. When you mix up rubber and let it cure under pressure it will remove all air bubble from the mold.
What you are thinking about it curing mold rubber outside a pressure pot then trying to cast in it under pressure which will distort the mold.

Rob
[quote=BrundelFly;1060972]Dude. Are you KIDDING??
Try it. You will get a part full of little WARTS.
You cannot pressure cast..UNLESS you have evac'd your rubber.

JPolacchi

It's a new platinum RTV silicone made by Tap Plastics.It's an easy 1:1 ratio mix,very low viscosity(mixes easily) with no bubbles in the mold.It basically degasses on it's own with a Shore A hardness of about 10.I've seen/held some of the demo molds in the store,and they are not as flimbsy as you might think.They are firm enough that they will hold their shape and not distort or colapse.

The silicone will not adhere to itself, so it can easly be used to make 2-part molds.It has a higher chemical resistance so casting resins/epoxy won't attack the rubber and you can get many more copies than a tin based silicone will allow.I'm just going to be molding some kit parts and casting with urethane resin,maybe some epoxy&glass.Depends what part I'm doing.

Of course it has a list of things that will inhibit the silicone from setting and curing,so it does do well to know what it is you are molding.Will/can it be used to cast the clear urethane resins that Smooth On has?I don't see why it won't,but it would be good to ask them as well as some of your own research.There's more info on the Tap Plastics web page if you want to know more along with a video/demo.

Firefoxm31

I think Brundlefly is saying pressure verses vaccum, vacuum degasses the rtv, pressure does not...
Will

Thanks for the info on the new Tap plastics Platinum based RTV, always learning new things....
Will

BrundelFly

[quote=replicaprops;1062095]Sorry brundelfly, but you are incorrect in this statement. When you mix up rubber and let it cure under pressure it will remove all air bubble from the mold.
What you are thinking about it curing mold rubber outside a pressure pot then trying to cast in it under pressure which will distort the mold.

Rob

No..thats not what I said.

Go back and read your Post:

"If you can fit the mold in a resin casting pressure pot, then no degassing is required since the pressure pot will eliminate air bubbles in both rubber and resin."

I took that as...You do not have to EVAC your RTV....but you can still PRESSURE CAST it.

WHICH IS NOT TRUE. You will get "Hairy Parts"

Now..PRESSURE MOLDING..ie..what you described ...Making a Mold..then PRESSURIZING IT while it Cures....THAT does NOT ELIMINATE AIR BUBBLES.

Its makes em smaller...but when you TAKE your mold our..IT WILL BE DISTORTED,



You said:

"... When you mix up rubber and let it cure under pressure it will remove all air bubble from the mold."

WRONG. See above.


"What you are thinking about it curing mold rubber outside a pressure pot then trying to cast in it under pressure which will distort the mold."

No..it will distort the casting.


Hope that clarifies it for everyone else.

replicaprops

Every mold I have has been cured under pressure and I have never had any distortion or problems with air bubbles re expanding. The molds are also much stronger and resist tearing much better and last longer.

BrundelFly

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.

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.

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

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

BrundelFly

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