How do you pressure cast?

[Darkson]

Some of you may have read in the star wars modeling forum that I plan on making a 1/2256 scale ISD kit. I want it to be as high quality as possible. To do this . I want to pressure cast it to prevent bubbles. How do you do this? Does it require any special equipment?

Any info would be great!!

[Schadenfreudian]

You need tons of equipment. First, a vacuum chamber, because if you pressure cast, your mould rubber must be absolutely free of bubbles, which can only be done with a vacuum chamber. This is because if your mould has sub-surface bubbles, the pressure will cause them to dimple, thus creating corresponding lumps all over the casting.

Usually, casters use what's called a pressure-pot, which is a steel vessel used by industrial painters:

http://www.hobbyengineering.com/H2253.html

Some people build their own:

http://www.lumicast.com/equipment.html

I think it's possible to use the same vessel for creating both a vacuum and, later, pressure-casting, though you'll need appropriate valves. In any case, either way you will also need a compressor, which can be reversed to pump a vacuum.

All in all, it all adds up to several hundred dollars, at least (and sizes are quite restricted). I noticed on ebay ages ago when looking into it that it's quite easy to get secondhand pressure-pots and compressors quite cheaply in the US, but anywhere else is damned expensive (secondhand pressure-pots in the UK seem to be as common as hen's teeth) - I don't know what it would be like in Australia. Oh, and the above prices are in US dollars, in the UK new equipment figures are similar - except they're in pounds rather than dollars!

[Darkson]

Well, looks like thats out!

Well, does anybody have any tips to create high quality, low bubble and flash molds and casts. I really need help. I've never cast anything this big.

Maybe I'll have to get someone to produce it for me.
Anyone capable of pressure casting interested?

Pleeeeeeeeeease!!

[Chacal]

When I cast resin parts (which is seldom), I get good results by mixing gently both the RTV and the resin. The secret, though, is to use a bit less "hardener" than suggested for the resin, because doing that gives a slightly longer cure time, and a similarly longer "work" time, and a longer time to stir thoroughly, so I dont need to "whip" the resin, I "stir" it. The RTV I use takes normally a long time to cure, so I can take my time stirring gently and pouring slowly over the master (both help to minimize the bubble problem). Same thing for the resin: pour slowly and consistently and you'll minimize bubbling.

[Schadenfreudian]

I was going to ask how large it would be, but having gone through the thread in the Star Wars section, I see 70.9cm long... I imagine it would be very wide - I don't know the dimensions of the ship in question, but I suppose somewhere in the order of 30-40cm width?

That's huge, I very much doubt anyone has a large enough pressure chamber, they tend not to be anywhere near that large. And if you could find one, I shudder to think what the price would be. Finally, I have a nasty feeling resin simply isn't up to such sizes. You might be better off producing a blank fibreglass hull, and producing cladding in small thin sections out of resin to coat it with, the cladding carrying all the surface detail. That way, the surface of the fibreglass blank wouldn't need to be perfect, whilst breaking up the detailed panels into manageably small pieces gives you the option of having them pressure cast.

However, those are just my thoughts and I have fairly limited experience in casting, so I'm probably wrong! There are, of course, many very experienced old hands on this board who will hopefully chip in at some point.

You may also want to subscribe (for free) to this yahoo group (I've found them a very helpful bunch in the past):

http://groups.yahoo.com/group/casting/

Finally, Chacal's advice is good - take care when mixing, don't get frantic, and everything should work out fine.

Good luck, and let us know how it goes!

[Darkson]

I have asked John Lester what his associates can do with it. I also asked him if I can sell through the SSM store.
Hopefully, I'll here from him soon.

[Schadenfreudian]

Well, good luck Darkson, I wish you well! I'm sure that one way or another you will attain your goal, which is always a jolly good thing in my book.

[modelnutz]

Have you considered Roto-Molding ?
Saves on resin and is likely the best way to produce something too large for pressure pots. When done correctly, it can produce a very nice casting.

Modelnutz 8)
"watch this space"

[Schadenfreudian]

Good call, Modelnutz - there's a good thread elsewhere here about it (where I misunderstood what was being discussed at first... ):

http://starshipmodeler.net/cgi-bin/phpB ... hp?t=36559

[Sparky]

Yep for very large models roto casting might be your best bet, I get casts that look as good as my pressure casts and weigh in much lighter. You may still want to vacuum your rubber.

Recently an article appeared in IPMS about using the venturi effect vacuum generator to degas RTV. The problem I had with this device is you need a lot of air to generate the vacuum. I got a medium sized compressor with a tank and it ran constantly when I tried to use the venturi pump thingy. Good news is that this venturi effect thing is $10.00 from Harbor Freight so it's not to much to test it out. You will need more than an airbrush compressor to work it though; it needs a lot of pressure and air flow to get any vacuum.

Here's my vacuum/pressure casting setup:
http://www.kc6sye.com/techmages_10_10_05.html

You might note a female quick connect on the other spout of the paint pot. This is because I daisy chain two pots, one mounted horizontally for casting sheets of parts. The seals are pretty good but I've come back to a pot and found it very low on pressure (resin flash or drippings stick to the pot's lip and rubber seal). Lately I've been casting itty bitty things and even small bubbles blow away those parts, so I leave both pots hooked up to the compressor. It's the kind that automatically kicks on to keep it's tank pressurized.

Here's a quick look at the horizontally mounted pot, its just a setup to allow a shelf style pressure chamber, Neil once again did the work. He cut and fabricate the stand. I added two supper big water tube/pipe clamps to clamp the pot down to the stand. I have to watch the level so that open face molds without backings, don't end up sitting at an angle and leaking their resin out. This also a source of the resin that interfears with the lids ability to seat well. . .Its not that difficult but I like to get all the issues I've found well documented.
http://www.kc6sye.com/advancedcasting_c.html

Really you can hack this thing together in a weekend if you have the compressor and vacuum pump. You will be in fornt of the home depot brass fittings/adapters bins for a few hours though. . .

[jack wendt]

if you make you parts where they can be cast as open face molds then the bubbles will float away from the surface. this requires more assembly than just pouring the rubber around the blank. also you can use your home washer or dryer to vibrate bubbles away from the surface.

i use the dryer's vibration for getting the bubbles out of oomoo and to the surface.

[Cmdr.Bubbles]

In Kitbuilders #53 there is an article on using one of those huge wooden spools for roto-casting that you may want to look at.

[modelnutz]

Darkson, you have a PM

Modelnutz 8)
"the name sez it all"

[Shinnentai]

Recently an article appeared in IPMS about using the venturi effect vacuum generator to degas RTV. The problem I had with this device is you need a lot of air to generate the vacuum. I got a medium sized compressor with a tank and it ran constantly when I tried to use the venturi pump thingy. Good news is that this venturi effect thing is $10.00 from Harbor Freight so it's not to much to test it out. You will need more than an airbrush compressor to work it though; it needs a lot of pressure and air flow to get any vacuum.

I have a small hardware store compressor with a 4-gallon "pancake" tank. I found the trick is not to run the compressor constantly straight through the venturi pump (takes forever, as the compressor cant keep up), but instead to pressurize the tank, then bleed it through the pump. By doing this twice in a row with my four gallon tank, I can get one paint can (similar to the one you have) up to about 24 HG. If I spend the extra $20 for a larger external tank, I could do it in one go. In theory (based on my tests with the four gallon tank), it would take about two minutes to fully pressurize the tank, but once it's set, I can evacuate the paint can in just a few seconds.

Dunno if that helps.

[Sparky]

A got the comresoor with the preasurized tank too, you can do it with the venturi but all the lines have to be well sealed and tight. I jsut found it easier to get the vacuum pump and be sure I got a good vacuum every time.

[JimPV]

I have a vacuum "desiccator" (chamber) from Scienceware:

http://www.bestlabdeals.com/Vacuum_Desi ... _s/457.htm

One just went on eBay for under $10:

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 237&fkxs=1

Get one of these, a venturi pump, and a compressor with a decent sized tank (I've got a compressor with an 11 gal. tank that works well) and you're in business to de-air your rubber. De-airing silicone really uses the stuff to it's utmost advantage. You have no fear of any (any) air bubbles in your mold.

Though I've not pressure cast anything, I think it's quite a bit easier than even de-airing silicone (which itself ain't hard). Just get a paint pressure-pot and hook it up to your compressor.

In all of this, the "big" expenditure is the compressor, which you can get for probably $125 from any home improvement store.

[superfloo]

Is this the method the big resin companies (True Details for example) use? I actually had no idea casting was this involved. All this time I thought it was just simply pour and wait for it to harden.

[JimPV]

Is this the method the big resin companies (True Details for example) use? I actually had no idea casting was this involved. All this time I thought it was just simply pour and wait for it to harden.

You certainly can "just pour and wait for it to harden". But you always run the risk of air bubbles if'n you don't pressure cast, particularly with casts of highly detailed items, of which garage kits certainly are.

[superfloo]

Hmm.. I think I would have to 'think small' since we live in an apartment. Certainly no room for a compressor with a tank.

[Sparky]

You don't need a compressor with a tank, even an airbrush compressor will work. You're only going to 40 psi, the tank just helps you get to 40 psi pretty fast. Tony's pancake tank w/ compressor is pretty small.

Using the slower set resin helps when you add all the time to mix, pook out big bubbles, put in pot, and pressurize.

[Sparky]

Here's a pic of tony's pancake compressor/tank setup:
http://www.kc6sye.com/images/images_03_ ... _setup.jpg

[superfloo]

Yeah, that IS small! Only a little bigger than my current airbrush compressor. Thanks!

[qc]

pardon my ignorance on the issue..but could i get a ...lay-person set of instructions?

I'm interested in having my own pressure AND vacuum pot...
I already have an iwata sprint jet..
What else do I need?

What is this venturi valve/compressor (remember venturi from my enginnering classes, but forget the concept)?
Can I rig the Iwata to pull a vacuum or do I need to purchase another vacuum pump? Will I need to purchase a vacuum regulator...I'm asuming the current one I use for my airbrush only reads positive pressure? How much pressure and vacuum (psi, mercury, etc) do/should I be aiming for (saw a little vacuum pump at Harbor Freight http://www.harborfreight.com/cpi/ctaf/d ... mber=92475 ....that good enuf)?

I read/reviewed Sparky's pics and set ups...but didn't quite follow...seemed to assume a bit of knowledge on the reader's behalf?

Thanks in advance

[Chacal]

The venturi vacuum pump works because of the Venturi effect (very similar to the Bernouilli principle): when a fluid, flowing through a pipe, passes by a constriction, the flow rate must remain the same, therefore the velocity shall increase and the local pressure shall decrease. The use of the Venturi effect to pull a vacuum is to run compressed air through a lenght of tube with a constriction. On that constriction there is the mouth of another tube (both tubes make a "t" joint). The second tube, because of its position on the constriction, has its end at the low pressure area, and that turns it into a "sucking" pipe, or a vacuum pump.

There is however a whole lot of calculations needed to see if a particular compressor/tube diameter/constriction diameter/vacuum pipe diameter will give you enough negative pressure for what you need.

[Sparky]

I tried to write it all from the starter perspective.

I like using the paint pot for both vacuuming and pressure casting to save a little room.

What you need to get started is:
Paint pot.
Compressor.
Various brass fittings.
A vacuum gauge.
Venturi pump or vacuum pump.

Now you won't be seeing full vacuum with the venturi pump thing. I have one and it is real finicky to get hooked up, takes a lot of air flow and pressure to get vacuum.

For a compressor you have 2 ratings. How much air it can move CFM, and max pressure it can achieve. Base model airbrush pumps can't go very high and don't move much air (they don't need to).

For pressure casting you rig up a means to hook up the compressor to the paint pot. That's it. Put your mould inside and pressurize it 40 psi. Little bubbles will be eliminated, if you have big ones left it means you didn't layout your mould well (there is an art to moulding masters).

For vacuuming you rig up a means to hook up a vacuum line to the pot. Put in a bucket of mixed rtv and vacuum it down until it has risen and collapsed back down (it will still be bubbling, it’s actually boiling). Then let the vacuum out, and pour this into your mould box.

If you are worried about surface bubbles or pockets (pockets will collapse under pressure destroying the mould) you can vacuum again, it won't take as long as the first degassing, you'll see the big air pockets coming out, and the RTV start to boil. Let the pressure out and set the mould aside to cure (on level surface trust me, crooked surface moulds are a pain to pour into, one side ends up short shot, while resin runs off the other side).

There are some neat tricks to making a mould with out degassing; something called the bombs-away-method. I can look for the link to the video clip if you like.



BTW please let me know what part of the progress pages wasn't clear or seemed to have missing background information. I'd like to have a more complete write up. I use it as a bases for the casting class at wonderfest.

[Antenociti]

You could always pressure cast the original mould to erradicate bubbles also - doesn't have to be vacuum'd.

Just need to be able to keep the mould under pressure whilst its setting. The stop-motion aniamtion guys who worked on "Wallace & Grommit" use a fast set (2 hour) RTV silicone and pressure cast that.

Of course your master needs to be able to take the 40psi also otherwise....

[Spacephrawg]

You could always pressure cast the original mould to erradicate bubbles also - doesn't have to be vacuum'd.

Just need to be able to keep the mould under pressure whilst its setting. The stop-motion aniamtion guys who worked on "Wallace & Grommit" use a fast set (2 hour) RTV silicone and pressure cast that.

Of course your master needs to be able to take the 40psi also otherwise....

So if I pressure cast the mold itself...wouldnt i still want to pressure cast anything cast in it to eradicate bubbles too?(N00B here)

Could one conceivably use a pressure pot for getting bubbles out of other things, like freshly mixed plaster, for instance?

How does the whole thing work? I was actually under the impression that the way to go was a vacuume chamber. I didnt know there were otehr ways of doing it. What are the pros and cons of Pressure Pots vs. Vacuum chamgerss?


I am looking to get something/anything for the purpose of removing bubbles from rubber molds and resin and plaster casts. Which device would be best for the purpose: vacuum or pressure chamber?Also, how much would either one cost and whats involved, like what kind of compressor or vacuum system or whatsit? Most importantly, which of the two is more sensible for a lone guy like me casting for non commercial use?

THanks very much and thanks for tollerating this slight deviation from the thread topic.

[Umi_Ryuzuki]

I have a friend, that does not pressure cast, and de-airs his molds by leaving them on top of the running clothes dryer with a couple of tennis shoes bouncing around in the drum.

He says it works well to de air his molds, and since he doesn't pressure cast, any small bubbles left in the mold are inconsequential.

Pressure casting isn't necessary, but if you have complex molds, or are casting for customers, it "helps" to reduce, if not eliminate, any air that does get trapped in the resin when casting.

If all you are doing is a few personal castings, there is no need to invest in the pots, the compressor, the fittings, the gauges, the safety relief valves, or the vacuum pump.
Just take your time and pour your rtv slowly, vibrate out any bubbles in the mold, and don't stir violently when mixing resin. If you take your time, you can come out with great castings without all the equipment.

[Spacephrawg]

I *love* the bouncing balls in the dryer option. I've heard of ways of de-bubbling the rubber of the mold, like using a compressor to blow the bubbles of out of the initial "milk coat" that goes onto the master.

Maybe I should just get an all purpose small compressor for the job... in the old days of rubber mold making, i'm told, they would use, get this, straws that the studio crew would all blow through. Imagine a team of people all getting up close to a toxic rubber-clad sculpture in some basement somewhere w/ no ventilation and they're all blowing on straws and one after the other getting dizzy and falling down.

THe pressure pot question was moreso a speculation on howcouldidothismoresimply kinda thing. The compressor might just be a better idea.

Man, i can't get over that dryer idea still... damn. Wow this forum's great ideas are going to reduce my vocabulary to "damn" and "man" by the end of the month, i swear.

[Sparky]

You could always pressure cast the original mould to erradicate bubbles also - doesn't have to be vacuum'd.

Just need to be able to keep the mould under pressure whilst its setting. The stop-motion aniamtion guys who worked on "Wallace & Grommit" use a fast set (2 hour) RTV silicone and pressure cast that.

Of course your master needs to be able to take the 40psi also otherwise....

So if I pressure cast the mold itself...wouldnt i still want to pressure cast anything cast in it to eradicate bubbles too?(N00B here)

Could one conceivably use a pressure pot for getting bubbles out of other things, like freshly mixed plaster, for instance?

How does the whole thing work? I was actually under the impression that the way to go was a vacuume chamber. I didnt know there were otehr ways of doing it. What are the pros and cons of Pressure Pots vs. Vacuum chamgerss?


I am looking to get something/anything for the purpose of removing bubbles from rubber molds and resin and plaster casts. Which device would be best for the purpose: vacuum or pressure chamber?Also, how much would either one cost and whats involved, like what kind of compressor or vacuum system or whatsit? Most importantly, which of the two is more sensible for a lone guy like me casting for non commercial use?

THanks very much and thanks for tollerating this slight deviation from the thread topic.

Ok Isee where the confusion is: you don't pressure cast the mold, you vacuum degas it. If the mold has any bubbles init then you cannot pressure cast resin in it later. The action of compression and decompression will give the mold the bends it will die. I had a mold that experienced this but I tossed it before taking pictures.

Vacuum degassing is used in making molds*.
Pressure casting is used in making resin casts.


* There is an advanced technique Ken aka Valkyrie uses to get the sweetest thin small parts to come out. This is a process of vacuum casting and pressure casting. When making his molds he leaves oversized reservoirs. Then when he casts he fills the mold and the reservoir, then vacuums the mold just long enough to get it the vacuum done to 25" or so, then lets out the pressure slowly. This pulls out the air collapsing the mold, and then as the pattern's pocket re-expanses it draws the resin from the reservoir. If the mold is complex he will do another 25" cycle. Then he pressure casts.


Back to the normal process:

The resin will have some amount of bubbles init. The bottles say to shake thoroughly before mixing, this always leaves me with bubbles in the thicker part (B I think). Also if you ever cast in clear you will need to pressure cast, those micro bubbles will show up outstandingly.

As far as whether or not you need a vacuum setup to get good molds, if you use oomoo 25/30 yes. This stuff is so thick when mixed I don't see a vibrating table technique as a means for getting all the air out. I ended up with lots of little bubbles right on the surface detail of the defense towers we made for the space station. The sharper the edges on the little gribbles the more bubbles tend to stick in there. And of corse these detail spots on the casting are the most time consuming to fix if not impossible.

Now mold max 30, has a stronger mold rating and yet it mixes much nicer, it is very runny just after mixing, and a vibrating table method might be all you need to agitate out bubbles, even surface ones.