# Casting question(s)



## Chrisisall (May 12, 2011)

Hi, I'm getting ready to mold & cast my scratch-built phaser, and I have a last minute question or two for the experts here: I'm going with Smooth On tin silicone for the mold, but is there any real difference between 30 & 40 shore hardness for my purposes?


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## J_Indy (Jan 28, 2013)

This would be a good Q for Steve Neill. I posted a link to his Google Chat in the "I Hate the JJ-Prise" thread. He switched it to Monday nights - so maybe you can get in if you log in early (it gets filled).

Not an expert - but I would think the softer silicone is better if the object has complex undercut details, and the harder is ok if it doesn't...


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## Chrisisall (May 12, 2011)

J_Indy said:


> Not an expert - but I would think the softer silicone is better if the object has complex undercut details, and the harder is ok if it doesn't...


My thinking as well; i was planning on 30A hardness, I just didn't know if 40 offered any plusses I wasn't aware of.
Steve's so busy, I didn't wanna take up his time with such a noobie question.
There are some undercuts, but not many- I'm molding each piece separately...


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## J_Indy (Jan 28, 2013)

Very nice!! :thumbsup:

From what I have seen, Neill is a cool guy - if you love models and making stuff, then he loves you.

Are you going to be casting under pressure? One of the things I heard him say is that, no matter how you pour, you get micro-bubbles at the surface of the cast. After a while, the ragged edge made from pulling the piece/breaking the bubbles tears the surface of the silicone and makes the molds unusable.

Pressure crushes the bubbles down and makes the mold last for more pulls.


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## Chrisisall (May 12, 2011)

J_Indy said:


> Very nice!! :thumbsup:


Thanks! Here it is with the parts together:



> From what I have seen, Neill is a cool guy - if you love models and making stuff, then he loves you.


I actually PM'd him a couple of times when he was still on general sites- he's SO nice!


> Are you going to be casting under pressure?


14.696 lbs psi.
Naw, no such equipment or funds to purchase. Smooth On makes stuff specially for small-timers like me. I already figured I'd be making more than one set of molds...


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## Chuck_P.R. (Jun 8, 2003)

Chrisisall said:


> Hi, I'm getting ready to mold & cast my scratch-built phaser, and I have a last minute question or two for the experts here: I'm going with Smooth On tin silicone for the mold, but is there any real difference between 30 & 40 shore hardness for my purposes?


Not an expert, but I have used a few of their products.

I've used Mold Max 40 and it picks up details at an incredible level.
Either will probably work well for you.

The biggest issue with getting a good cast is air bubbles.

If you have no vaccuming system you might want to consider
getting one.

A very cheap and easy way to get one without turning your work
area into a mad scientist lab is to get one that is made to work
with containers for kitchen use,

like this one.

http://www.walmart.com/ip/Foodsaver-Advanced-Vacuum-Packaging-Sealer/16628368

you can get an accessory for some of them like this:

http://www.walmart.com/ip/Foodsaver-Jar-Sealer-in-white/16628369

that will allow you to use standard mason jars.

You can reuse them for silicone, not as often for degassing resin though.

However a dozen mason jars aren't that expensive and are commonly available.

Just make sure you don't take forever mixing the rubber or resin,
not only will the RTV get less liquid as it get's closer to the max
working time, it stops having tact as a result too.

Degassing and pouring quickly is mondo important.

If you don't have the fundage for doing vacuuming,

I would highly recommend brushing on as many layers(three or four
at least)

of RTV before pouring the rest of the mold's worth on top of 
brushed on layers.

It takes time, but not having any bubbles next to the surface
of the mold is crucial.

If you take the time to brush on a couple of coats of RTV and
letting them set up between coats,

then pouring the rest over the set up coats(from the highest point,
letting the RTV run down to the lowest point - relatively slowly - 
don't just dump it)

will probably give you good results without having to worry as 
much about bubbles.

Heck, even if you get a vacuum system, I'd still recommend mixing
and pouring a couple of batches that are just a little bit more then
needed to cover the piece and bottom of the mold;

and pouring those first couple of batches in very thin layers with 
curing time between each batch.

Be prepared with a tootpick, brushes, etc to make sure the whole
surface is bubble free on those first couple of layers.

On the last big pour, you might want to buy a little cure accelerator,
as deep pours can take awhile to set up. I wouldn't use it on the 
first few thin layers, though.



Also, it was an issue with 40 so I'm sure it's just as much an
issue with the 30, but I would recommend using very sturdy-
sided boxes for your molds.

Boxes that you never have take them out of even if you want to 
stick them in an oven occasionally.

I recommend using heavy walled aluminum pans,
meatloaf pans might be suited to your subject.
You can get them in a surprising range of sizes on Amazon.com.

I would still recommend leaving a decent amount of RTV around
the part of the mold that will be poured into. Especially enough
so you can add small cones around the edges that you can use
later to mate the other side of your two-part molds.

Very flexible molding RTV like 30 and 40 is great for picking
exceptional detail.

But the downside to both is that unless you pour and waste
several inches of extra space all around the surfaces of the 
model,

if you pop 30 or 40 molds that don't have a lot of extra RTV
surrounding the surface to be molded,

just like jelly they'll distort on you.



I hope this advice helps a bit!

Good luck! :thumbsup:


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## Chrisisall (May 12, 2011)

Chuck_P.R. said:


> I hope this advice helps a bit!
> 
> Good luck! :thumbsup:


Dude, ANY advice is great!!

I've done *some* casting, so at least it's not ALL new to me...


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## robn1 (Nov 17, 2012)

I'd go with the 30A for the phaser. With all the little detail bits glued to it the 40 may be too firm to remove without tearing up your master.

Do you plan to mold it in one block and cut it open? Or will you embed it in clay to make a two part mold?

You can get by without vacuum, but I consider pressure a must. If you don't have either, then make a thin detail coat of rubber first. Apply it with a brush, make sure it's worked into all the recesses. Then use a few bursts from your compressor to pop any bubbles. Let this coat cure until it's firm but tacky, then apply more. Silicone will bond to itself so these multiple layers will stick OK.

Check Youtube for some instructional videos. Smooth-On, Brick In The Yard, and Freeman Supply all have great vids. The brand names mentioned don't matter, the techniques are useful for anything.


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## Chrisisall (May 12, 2011)

robn1 said:


> I'd go with the 30A for the phaser. With all the little detail bits glued to it the 40 may be too firm to remove without tearing up your master.


Thanks Rob!


> Do you plan to mold it in one block and cut it open? Or will you embed it in clay to make a two part mold?


 The latter. That way gravity lets the bubbles rise away from the master surface, if I understand the process correctly.


> You can get by without vacuum, but I consider pressure a must. If you don't have either, then make a thin detail coat of rubber first. Apply it with a brush, make sure it's worked into all the recesses. Then use a few bursts from your compressor to pop any bubbles. Let this coat cure until it's firm but tacky, then apply more. Silicone will bond to itself so these multiple layers will stick OK.


Thanks again, but I have no compressor... however I grok the process now (I think) and I won't be unhappy with a few problems here & there- I've worked on some pretty crappy pulls from the lame & uncaring retail market...:freak:


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## robn1 (Nov 17, 2012)

Use air duster cans then, it'll do the job.

And yes the bubbles will rise, if it's a low viscosity rubber with a long cure time. 

Make sure your clay doesn't contain sulfur, it will inhibit the cure of the rubber.


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## Chrisisall (May 12, 2011)

robn1 said:


> Use air duster cans then, it'll do the job.


Excellent!


> And yes the bubbles will rise, if it's a low viscosity rubber with a long cure time.


Yes, that's the plan!:thumbsup:


> Make sure your clay doesn't contain sulfur, it will inhibit the cure of the rubber.


Absolutely- all natural no sulfur! 
And a release agent in between the two halves of each mold so the silicone doesn't bond to itself, right?
I think I'll be ready to rock & roll pretty soon!
Thanks!!!


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## robn1 (Nov 17, 2012)

Sounds like you're all set. Good luck :thumbsup:


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## robn1 (Nov 17, 2012)

Sounds like you're all set. Good luck :thumbsup:


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## Chrisisall (May 12, 2011)

robn1 said:


> Sounds like you're all set. Good luck :thumbsup:


You are the master, I am but the learner, and I appreciate learning from you sir! I will post my progress in future threads here and at the 'other' sites we frequent!


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## Chrisisall (May 12, 2011)

Chuck_P.R. said:


> I would highly recommend brushing on as many layers(three or four
> at least)
> of RTV before pouring the rest of the mold's worth on top of
> brushed on layers.
> ...


Thanks Chuck, that's also great stuff, and I will make it so! You and Rob have made me confident!:thumbsup:


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## Chuck_P.R. (Jun 8, 2003)

Chrisisall said:


> Thanks Chuck, that's also great stuff, and I will make it so! You and Rob have made me confident!:thumbsup:


One last thing that no one has seemed to mention.

Maybe I'm stating the obvious here and if so please forgive me,

Mold Release!!!!


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## Chrisisall (May 12, 2011)

Chuck_P.R. said:


> Mold Release!!!!


I was under the impression that using tin silicone on non-porous masters required no release agent... however I'll lightly spray some on anyway.


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## Chuck_P.R. (Jun 8, 2003)

Chrisisall said:


> I was under the impression that using tin silicone on non-porous masters required no release agent... however I'll lightly spray some on anyway.


I believe they sell one in a spray can that works well with silicone.
It may be overkill, but I've read it can prolong the life of the mold,
something about chemicals from the resin seeping into the silicone
over time, not just a matter of helping with the physical separation.

I've never had problems with or without mold release using the 40,
but I read the mold release would prolong the life of the RTV by
not letting chemicals gradually seep into the RTV.


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## robn1 (Nov 17, 2012)

Mold release is good to use for casting, it will prolong the mold life by facilitating release and by preventing chemical contamination from the resin. Silicone can absorb chemicals from the resin, which can be seen as a colored stain. This can also weaken the rubber over time. It's also a MUST for two part molds so the rubber won't bond to itself.

But it's not really needed on the master part. The styrene should release fine without it. But it helps on masters made from other materials, some releases are sealers as well.


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## Chrisisall (May 12, 2011)

robn1 said:


> Mold release is good to use for casting, it will prolong the mold life by facilitating release and by preventing chemical contamination from the resin. Silicone can absorb chemicals from the resin, which can be seen as a colored stain. This can also weaken the rubber over time. It's also a MUST for two part molds so the rubber won't bond to itself.


Thanks yet again, Rob!!
You guys have schooled me well.:thumbsup:


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## Chrisisall (May 12, 2011)

Bugging you again here, Rob, but is 8lbs of silicone enough to mold my phaser (2 large in 2 parts; 2 small in 2 parts) do you think? Or do I have to go for 20?


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## robn1 (Nov 17, 2012)

That's hard to say, I really suck at figuring that stuff. You can use rice (the cheap kind) to find out the amount of rubber you need. Pour rice into your mold box, then pour it out into a measuring cup to see how much there is. Use just a bit more rubber as a safety.


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## robn1 (Nov 17, 2012)

These pages may help, I've never used them though.
http://www.smooth-on.com/tools.php


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## CaptCBoard (Aug 3, 2002)

I'll poke my nose in here to correct something said early in this thread.

When making a mold, NEVER use pressure. Pressure should only be used when resin is curing.

Air trapped in rubber will shrink under pressure, but re-expand when the pressure is released. This can produce surface bulges in the mold-- the opposite of what you want to happen.

After mixing rubber, it needs to go into a vacuum chamber and the air removed. During this process, air trapped in the rubber will expand and pop because they exist under normal room pressure in the mix. This is why rubber expands during de-airing and rises like cake batter as the pressure drops. When the trapped air escapes, the mixed rubber falls back into the container (which should always be large enough to contain the expanding rubber so nothing overflows).

If a vacuum chamber is not available, it is possible to remove most trapped air by holding the mixed rubber 4 to 3 feet above the mold box and pouring it into the box in the thinnest stream possible. This will make just about every bubble pop that was mixed into the rubber, but will not defeat any air trapped in an undercut. Brushing a 'skin' of rubber over the master and into undercuts in advance of pouring the bulk of the mold will take care of that.

Pressure is only used during the casting process. Most surface bubbles can be defeated by giving the mold a coat of talcum powder. Pour powder in, pour it out and blow out the remaining pockets. What naturally sticks to the mold is all that is needed. Trust me, it makes a real difference. If the part has next to no surface detail, you can skip the talc.

Undercuts need to be vented, so the air can go somewhere when pressure is applied. 

One thing to note is resin should never be subjected to a vacuum. The gel-time is way too fast for this to help. The only exception is a really slow-cure resin, like a clear resin. A vacuum is required for some clear resins, to get the air out of the mix. These resins usually have a 24-hour cure, so you do have the time to de-air the mix and pour it into a mold before it gels.

This is a basic review of what needs to be done when making molds and castings. If anyone needs more info, please contact me by email, or post here.

Scott
[email protected]


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## robn1 (Nov 17, 2012)

CaptCBoard said:


> ...When making a mold, NEVER use pressure. Pressure should only be used when resin is curing.
> 
> Air trapped in rubber will shrink under pressure, but re-expand when the pressure is released. This can produce surface bulges in the mold-- the opposite of what you want to happen...


I've heard opposing opinions on this so I did a test. I made a mold and cured it under 60psi. It came out perfect. I even sliced it open and there were NO bubbles inside, it was smooth and solid all the way through. No vacuum, just pressure. I've made a few more this way and they were all perfect. 

And I've heard that a mold intended for pressure casting _should _be made under pressure, to ensure there are no voids.

I'll never make a mold without it.


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## Chuck_P.R. (Jun 8, 2003)

CaptCBoard said:


> I'll poke my nose in here to correct something said early in this thread.
> 
> When making a mold, NEVER use pressure. Pressure should only be used when resin is curing.
> 
> ...


I entirely forgot to mention making sure to leave extra room for the expanding rubber.

I'm glad you did Scott.

I also never bought a pressure chamber, so I knew nothing either way as to whether or not it was supposed to be used with RTV. I hadn't heard it was supposed to be used with RTV either.
But since I don't own one nor have used one I didn't feel comfortable commenting on them.

Good heads up on that one!

On the resin issue, after first buying some stuff that cured in less then 4 minutes, 
I quickly switched to slow cure resin and also use an electric hobby
mixer that has a small mixer on the bottom of a stainless steel shaft.

I'd recommend going with the stuff that has a 20 minute pot life.
Even if you don't use clear resin, the more working time the better
I've found.

There's also stuff that sets in about 7 minutes that might work okay.

As far as I know there isn't a downside to using the stuff with extended
pot(mixing) life. 

The tensile strength may be a hair lower, 2600 vs 3000.
But you can get the 7 minute stuff in 3000psi strength.
But I doubt it would make a difference unless it were a 
piece that had large thin sections - like the upper saucer
half of a TOS E.


Chris, have you seen this link on Smooth-On's site?

It shows step by step how to cast and reproduce a TOS Klingon Disrupter.

http://www.smooth-on.com/gallery.php?galleryid=378

I would not recommend the one piece mold then cut method,
but other then that it might be helpful info.

Also, as CaptCBoard pointed out, not only do you want to pour from
the highest point and let it pour downwards from there,
pour the RTV from a very good height above the piece so 
gravity will help break the bubbles.


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## JediPuju (Oct 12, 2009)

I checked out the smooth-on demo with the disruptor. 
Something about this photo gives me a chill :


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## Chrisisall (May 12, 2011)

_*Thanks so much, my friends!! *_I ask a couple of questions and get a WHOLE & COMPLETE tutorial!!!:woohoo:
And yeah, two part molds for sure; I _expect_ some clean up, and am prepared to be 'ceative' with where the seam will be to make that clean up a bit easier on certain detailed areas.


JediPuju said:


> I checked out the smooth-on demo with the disruptor.
> Something about this photo gives me a chill :


Yikes, potential modeler suicide!:freak:


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## Chuck_P.R. (Jun 8, 2003)

robn1 said:


> I've heard opposing opinions on this so I did a test. I made a mold and cured it under 60psi. It came out perfect. I even sliced it open and there were NO bubbles inside, it was smooth and solid all the way through. No vacuum, just pressure. I've made a few more this way and they were all perfect.
> 
> And I've heard that a mold intended for pressure casting _should _be made under pressure, to ensure there are no voids.
> 
> I'll never make a mold without it.



I don't know enough about the pressuring issue to comment on that.
So I won't.

However, either way I would recommend that Chris pour a couple of thin
coats on the model before filling the mold.

A thin enough coat will be translucent to enough of a degree that you should 
be able to see any potential problems in the first couple of coats.

If you pour a thin coat of RTV, making sure there are no bubbles, popping
any areas of trapped air or bubbles with a toothpick as you go,

you'll have far less to worry about.

Doing that and then pouring the rest of the RTV has never caused a problem for me.

But I would still recommend mixing and pouring in small batches though after the first couple of layers. The batches don't need to be as small as the ones for the first couple of layers, but I wouldn't just dump all the rest at once either.


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## Chrisisall (May 12, 2011)

Chuck_P.R. said:


> However, either way I would recommend that Chris pour a couple of thin
> coats on the model before filling the mold.


I will make it so! :thumbsup: Dag, I gotta print this thread up!!!


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## CaptCBoard (Aug 3, 2002)

I won't dispute what others report as their experience. I've been doing this for a really long time and have never made a mold under pressure. I have made thousands of castings under pressure with almost 100% success. My point is, if you don't have a pressure chamber, its no big deal. 

I should also say that all of my experience is with molding and casting a wide range of kits. Well over 500 copies of several of my products have been sold. Having said that, anyone not making a kit (or more than 10 or 20 parts out of a single mold) won't have to worry about how their mold performs over time. In my business, mold optimization is important and what I report here is first-hand information, gained over the last 15 years or so. If anyone has questions about molding/casting, let me know-- here or by email (please-- no PMs).

Scott


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## Chuck_P.R. (Jun 8, 2003)

CaptCBoard said:


> I won't dispute what others report as their experience. I've been doing this for a really long time and have never made a mold under pressure. I have made thousands of castings under pressure with almost 100% success. My point is, if you don't have a pressure chamber, its no big deal.
> 
> I should also say that all of my experience is with molding and casting a wide range of kits. Well over 500 copies of several of my products have been sold. Having said that, anyone not making a kit (or more than 10 or 20 parts out of a single mold) won't have to worry about how their mold performs over time. In my business, mold optimization is important and what I report here is first-hand information, gained over the last 15 years or so. If anyone has questions about molding/casting, let me know-- here or by email (please-- no PMs).
> 
> Scott


Do you have any links, info on how to build your own pressure chamber?
I don't know, but I'm guessing anything commercially made of a larger size
is probably mondo expensive.


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## robn1 (Nov 17, 2012)

I recently converted a Grizzly paint pot for pressure casting, I detailed it here http://www.therpf.com/f9/pressure-casting-grizzly-paint-pot-conversion-184777/


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## CaptCBoard (Aug 3, 2002)

A lot of people use paint pots for pressure casting. The only drawback to that is the time it takes to dog down the lid. In my situation, that doesn't work. But it works great for the average hobbits.

I do have a design for a pressure chamber that works very well, but it can not be built by just anyone. It takes someone with a professional knowledge of welding because of the pressures involved. Pressure chambers are potentially very dangerous. They need to be built to withstand the highest pressure generated by your average compressor sold by Sears, which is about 160 psi. I don't use more than 80 psi, which is about half of what can be delivered by my compressor.

I'd like to point out the difference in danger between a pressure chamber and a vacuum chamber. A vacuum chamber has to withstand the pressure difference between a complete vacuum and normal atmosphere. This is known as 1 atmosphere. This is the most any vacuum chamber has to deal with. A pressure chamber has to withstand almost 11 atmospheres at 160 psi. At 80 psi, the chamber is under 5.4 atmospheres. If a weld fails, there is a potential for a catastrophic failure-- what amounts to an explosion.

This is why it is okay to build your own vacuum chamber-- there's no danger of catastrophic failure since it only has to hold up under 14.7 psi. But building a pressure chamber is completely the opposite-- not to mention expensive. If closing a paint pot was easier, I'd probably have a stack of them!

Scott


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## robn1 (Nov 17, 2012)

There are welding and fabrication services that specialize in making pressure vessels. I read a post on another forum where someone consulted one of these places about making a pressure casting chamber, with a hinged lid and a single flip down clamp, but their estimate was several thousand dollars! 

Most of the casting chambers I've seen, from CA Technologies and others, start around $300. But I noticed that the Grizzly paint pot looked nearly identical, with the same clamp design. For $140, plus another $30 or so for extra parts, I was able to make a comparable casting pot. It does have a slow leak, but by leaving my air supply connected it will hold pressure long enough to cure.

It only takes a couple of minutes to tighten the clamps, hand tight is enough. The Harbor Freight pot I used before had to be tightened with a wrench. But the poor clamp design damaged the lid, and it started to scare me :freak:


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## Chuck_P.R. (Jun 8, 2003)

Sounds like it's much simpler to be really careful to vent all the undercuts(which should be done anyway of course) and thoroughly degass slow set resin.

Unless like the two of you, someone needs to make a large number of pulls.

Thanks for the advice!


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## robn1 (Nov 17, 2012)

Yes, careful venting and a slower resin can make the difference. But I have a problem with humidity where I live, and urethane doesn't play well with moisture. The long set resins can suck water right out of the air, and the reaction makes it foam up and rise like bread. 

For small simple parts I use a fast resin, 2.5 minute pot life. I also use it for slush casting. But larger more complex pours need something slower, and pressure will prevent the foaming reaction.


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## Chuck_P.R. (Jun 8, 2003)

robn1 said:


> Yes, careful venting and a slower resin can make the difference. But I have a problem with humidity where I live, and urethane doesn't play well with moisture. The long set resins can suck water right out of the air, and the reaction makes it foam up and rise like bread.
> 
> For small simple parts I use a fast resin, 2.5 minute pot life. I also use it for slush casting. But larger more complex pours need something slower, and pressure will prevent the foaming reaction.


Are you working in an un-airconditioned workshop perhaps?

Here in New Orleans we are virtually an island, surrounded by the
Mississsippi and lake Ponchartrain on either side, and vast swamps 
to the north and south, 

so 90% humidity is pretty much the norm. Really sucks when it's over 80 
as your sweat never evaporates or cools you, just keeps pouring out and doing little good as far as cooling you!!! 

It must be a pain to not be able to use slower setting resin.
I can't imagine not having the option to use the slower stuff. 
At least anything that sets up in less then 7 minutes. 

But then again you and CaptCBoard are producing lots and lots
of pulls, so I guess it's worth it and after awhile you have become
used to it.

That's one of many things that seperate amateurs like myself and pros
like you guys.


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## Rondo (Jul 23, 2010)

Here is what I used to make a pressure chamber:

http://www.discovervalve.com/100057/brassbv.html

The 4" valve is the "door" of the chamber. A few bucks for 4" pipe and fittings to make the rest of the chamber, and some 5" muffler clamps give it feet to stand on. I believe that this is the same company I dealt with but I think the cost was $150 at the time for the valve. If one were to snoop around, he might find a surplus/used valve and perhaps a larger size as well. 4" can be a real limitation. It opens and closes instantly. To depressurize, I unhook the QD supply hose and use that smaller ball valve to let off pressure.

I made a small version first with IIRC a 1 1/2" (correction, 2") valve from Lowes to try the concept. It worked fine and would be good for very small molds.

I started with a Harbor Freight paint pot like others but it was a bit slow to get sealed. In fact it really never did seal that well under pressure. Now it is my vacuum chamber for mold rubber. If you're going to pressure cast, you MUST vacuum your rubber, at least with the types of rubber I've used.


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## Rondo (Jul 23, 2010)

Took a couple of pics today since I was thinking about it. Probably going to do some casting soon.





You may notice some spilled resin around the valve. It seems to shrug it off with no effect. I usually use a styrene tray to slide the mold into the chamber since the rubber mold is hard to slide in smoothly. I should clean this thing up and give it a coat of paint. I believe it is rated for 2000 psi so 150 is not a threat at all.


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## Chrisisall (May 12, 2011)

I'd like to get my hands on that ample nacelle!


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## Rondo (Jul 23, 2010)

LOL. Aye, she's a right bonnie lass and would likely develope significant thrust if the door were opened under full pressure. Warp nine Mr. Sulu!


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## robn1 (Nov 17, 2012)

Holey smokes! That's quite a chamber, and a clever use of the ball valve. It looks like an air cannon. Fill it to full pressure, open the valve and BOOM! 

Seriously though, I've read a lot of posts across the web and there are a lot of folks who cure molds under pressure. In fact they insist that molds intended for pressure casting must be pressure cured. I've only made a few molds this way, but as a test I used little care in mixing it. It was full of bubbles, I just dumped it into the mold box, and put it in the pressure pot. Pressure did all the work, no bubbles or voids. And the silicone feels a little more firm when it's solid all the way through with no voids.


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## Rondo (Jul 23, 2010)

I've not tried curing molds under pressure but it should work as long as you cast at the same pressure the mold was cured at. 

If you've ever tried to pressure cast in a non-vacuum/pressure treated mold though, you know how the pressure differential affects the shape of the mold. You get little pimples all over your cast parts where bubbles in the mold collapse under pressure. With a pressure cured mold, the affect would be reversed. The mold would be distorted whenever the mold is NOT under pressure. Could be a long term problem. On the plus side, it uses the same pressure equipment you already have.

OTOH, a vacuumed mold is about as pressure inert as possible. It will behave the same from <1 atmosphere to any level we are likely to produce. Setting up to vacuum rubber is one more expense but can be done reasonably. A pump is not cheap but HF vacuum units which run on compressed air are very cheap. You need a healthy compressor to run one though. With a small vac chamber, a small compressor might well get the job done. I've thought about making one from a large Mason jar but worry about the jar imploding at some point. Would still need a bigger chamber to vacuum the poured molds though.


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## robn1 (Nov 17, 2012)

Rondo said:


> ...With a pressure cured mold, the affect would be reversed. The mold would be distorted whenever the mold is NOT under pressure. Could be a long term problem...


This is the part that had me concerned, which is why I wanted to test it. As I said earlier, I made a pressure cured mold and sliced it open, there were no voids inside. The bubbles do not re-expand, the mold is not distorted in any way. Whatever form the rubber takes when it's curing, it will retain that form once cured. It does not change when pressure is released.


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## Rondo (Jul 23, 2010)

That is interesting. The bubbles may be crushed small enough that even at a high internal pressure they don't have enough surface area to overcome the strength of the rubber. If a larger bubble ever did become trapped though, I believe the effect would be seen. Likewise, resin has enough strength to contain small bubbles but I've had a few big ones pop as the part came out of the mold.

I suppose that the ultimate technique would be to vacuum the rubber before and after pouring, holding it under vacuum for as long as it remained liquid then completing the cure at full casting pressure to crush any potential bubbles to nothing. As long as the vac unit is working well though, I haven't seen any room for improvement over vacuuming alone.



Here are some parts from a mold where the vac was "sick" (big piece of debris in the air pressure inlet). The vacuum did not reach normal levels but I hoped for the best and got icky parts. Curing the rubber under pressure could have been a pretty good alternative at the time. Doh!


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## robn1 (Nov 17, 2012)

My experience with it has been very positive. Of course I'm still new to this, I've made only a few molds and cast a few dozen parts. 

As long as my master part can withstand pressure, I'll always pressure cure the rubber.


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## Trek Ace (Jul 8, 2001)

Rondo said:


> Took a couple of pics today since I was thinking about it. Probably going to do some casting soon.


That's one big flash light!


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## Rondo (Jul 23, 2010)

Or as she calls it, the One-eyed monster. And she IS reffering to the pressure chamber.


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