How do you pressure cast?

[Chacal]

We are supposed to stir the resin and the RTV, right? If stirring gently does the trick, then why whip it? I stir my resin and RTV very nice and gentle, and I don't have THAT many bubbles. Of course, I use slower setting materials, but I'm willing to wait a day till my mold cures, then another till the other half does, then a coupla hours till the resin hardens at each copy.

[Sparky]

The bottle clearly says to shake well before using. And after the first time I had castings come out oily and weepy I made sure and thoroughly mix up each part before dispensing, and I mix with a badger paint mixer. the micro bubblers that form from this are easily driven out in pressure casting. Mixing like this mixes even the material on the walls of the cup. It's the bubbles that stick on the surface details of the mold you want to be sure and eliminate, if you have a big sheet of gribblies or a mold with lots of parts in it you won't have time to coax out the bubbles before the resin start to setup. You'll have to mold small portions and mix a little resin so you can work the resin into each part. It seems much easier to break down and get pressure pot.

If you get into coloring, you have to mix very thoroughly, which only takes 10 seconds with the paint stirer.

As the part A gets old you will find that white crystals form and sink to the bottom, these must be mixed up into the fluid or you'll have to toss the resin, it will not cure or will cure and weep for ever.

As Umi said, if you're making a few things for your self, then taking the time to carefully stir and carefully pour might be good enough.

But once you have the pot and vacuum setup, you'll see the number of rejections reduce, the cleanup time for 'good castings' reduce. You'll also start to enjoy the ability to mass cast a lot of parts in a short amount of time, and increase the complexity of items you cast.

It's like me resisting getting the laminator macfrank suggests for making brass etch. I finally got one and man it makes it so much easier to make circuit boards and brass etch. It gives quality and reproducible results.

As in all things the right tools for the job make all the difference.


Also remember that resin heat cures, so some bubbles are formed in the curing process, this is more apparent with fast set resins. I too use the 20 minuet potlife, there's just not time to mix and pour with the faster set stuff. Folks who get into casting on the 3 minuet stuff are either naturally born casters or bull headed. If I had started with that stuff I'd tossed the towel in right away.

Clearcast even when carefully mixing reveals even the tiniest bubbles, without pressure casting you'll never see the crystal clear casts they show on their website.

[Sparky]

BTW here's the method of how to make a bubble free mold with out a vacuum de-gasser/de-air er. . .

http://www.tapplastics.com/uploads/pdf/ ... ctions.pdf
and here's a link to part 2 of the how to video from tap plastics, it shows how even when mixing by hand you end up with air in the RTV, it shows vacuum degassing and the bombs-away method of degassing:
http://www.tapplastics.com/info/video_d ... quicktime&

Here's all the how to videos shinny (thanks to Less Than Super Ostrich for the linky):
http://www.tapplastics.com/info/video.php

[Antenociti]

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.

Actually you can pressure cast the mould. It works very well, you just need to be able to keep the pressure applied during the time it takes the mould to set. The result is a bubble-free mould. i.e. same result as degassing, just done a different way.

You can use a super-fast catalyst to set the RTV silicone in 2 hours or a normal 24 hour set time so long as you can keep the pressure applied the whole time (self-regulating compressor being ideal - soon as the pressure drops it turns itself back on and puts the pressure back up to where it should be).

The only drawback of pressure casting moulds is that the master inside the mould must be able to take the 40psi pressure also - so you cant use anything that will collapse under that pressure.

But, yes: youc an pressure cast the mould and then pressure cast the final piece in that mould.

Theoretically I see no reason why you can't pressure cast plaster other than the requirement of using a plaster that can "hold" the pressurised air-bubbles once it is set - Plaster of Paris would probably just puff apart as soon as you release the pressure - some of the stronger alpha-plasters should work though.

What you can't do is pressurise plaster to get the bubbles out and then think you can pour a bubble-free mix - the instant you release the pressure the bubbles will come back (so it will only work with a set material) - that's the advantage of vacuum de-gassing, the bubbles are gone for good.

[Sparky]

If you pressurize the mould during curing don't you have issues with pockets of bubbles that don't float out, expanding after decompression and distorting or destroying the mould? I guess a smaller bubble would exhibit greater buoyance tending to float free on its own (same reason a diver needs lead wieghts to nuteralized his compressed air tank).

BTW that venturi pump is on sale, I checked the add it says it needs a compressor that can produce 4.2 cfm but that will get you full vacuum in 2 minuets. Might be a good way to get in the door with a vacuum setup.

If your' in the Chicago area I can lend you mine to try out. I'd like to set it up some day again, now that I have a good pot and lid assembly, and try it out.

[Zen-Builder]

I found that a good parts layout and adding a few venting channels go a long way towards a bubble-free cast.

Normally I only cast for myself so the odd bubble in the cast is not worse than many of the kits that I pick up at events or off a shelf.

I was also taught to mix the components RTV and Resin slowly using glass swivel-sticks as this way you trap very little air in the liquid.
Same way when pouring the RTV/Resin you let the liquid run down the same glass swivel stick and this will avoid a LOT of bubbles in the liquid.

Add to that tapping the mold and I found most bubbles will disappear.

Once I start casting and moulding for other I might consider getting a pressure pot but for now not worth the investment.

HTH.

[Spacephrawg]

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.

Firstly, thanks very much for clearing this all up.
Secondly, please define "Reservoir". Thirdly, I've noticed I tend to get some bubbles with MM30 usually. especially around the most greeblie-ish areas. Theres always some.

Am I just doing it wrong? Do i just need a vibrating table? Am I vibrating? Am I not vibrating enough? Should I stop talking about my vibrating self? Yes...yes i should.

So whats involved in a vacume degasser? whats the average starter kit go for?

[Shinnentai]

please define "Reservoir".

This means a large chamber, usually funnel shaped, attached to or built into the mold's pour spout, and covering any vent sprue outlets as well. Typically the connection between the reservoir and main sprue/pour spout is designed so that either surface tension or syphon action will hold the resin in the reservoir with minimal leakadge into the mold cavity. When the vacuum is applied, the air in the mold cavity is evacuated through reservoir/sprue channels, so that when the surrounding pressure is restored, the resin in the reservoir is sucked into the mold cavity. It's a trick borrowed from metal casting (and is in fact what the term "vacuum casting" reffers to; vacuuming the rubber while making the molds is "vacuum degassing"), and from what I've read, works wonders on resin when combined with pressure casting.

The reservoir need not be built into the mold itself; it can be just a plastic kitchen funnel wedged into the sprue/pour hole. The sprue layout within the mold does have to be designed with this casting method in mind though.

So whats involved in a vacume degasser? whats the average starter kit go for?

Vacuum degassing= putting the mixed rubber into an airtight container, and using a vacuum pump to draw the air out of the chamber for a few moments. This will cause the air bubbles floating in the uncured rubber to boil off, leaving a solid, smooth body of rubber. Some people vaccum the rubber in a cup first, then pour it carefully into/over the mold pattern. Others preffer to pour the rubber first, then vacuum the whole thing together. There are trade offs either way. The former allows for simplified mold engineering, while the latter offers greater security Re: air bubbles attached/trapped against the master by the pouring process.

Vacuum setups are DIY for the most part, so cost can vary.

Do you have an air compressor already? If so, you may be able to get by with a venturi vacuum attachment (like the Harbor Freight one linked to by qc further up). It takes longer to use, and can't reach as deep a vacuum as an actual motorized vaccum pump, but it's dirt cheap, and can get a vacuum good enough for moldmaking/casting.

Most people opt to buy a full fledged vacuum pump, which cost around $100.. It's just like an air compressor, only it sucks intead of blows . These work faster, draw a harder vacuum, and can void a much larger chamber than a venturi compressor attachment.

For a beginner only looking to do the ocassional casting for their own purposes, a venturi attachment is just fine IMHO.

The vacuum chamber can be the same paint pressure pot used to do the pressure casting. Lots of people like to make clear acylic lids for their pressure pots (check your yellow pages under "plastics" for places that'll cut or mill acrylic to order) to use when vacuuming mold rubber, as it lets them monitor what's happening directly, in case something goes wrong or doesn't work. This isn't strictly necessary though. You'll want to buy a vacuum guage to add to the lid pipework in any case. Paint pots themselves cost $80-100.

My setup is (all prices IIRC):
* A small "Husky" compressor w/ four gallon pancake tank ($90.00)
* A paint pot with in-line vacuum gauge, pressure guage, and ball valve. No extra clear lid. ($90.00 for the pot, $20.00 gauges & assorted fittings)
* A Harbor Frieght venturi attachment, just like the one qc linked too, only red instead of blue ($15.00)

All with quick connector hose couplings so they can be swapped around easaly. It's not the most efficiant setup by far, and if I were going to be producing kits or parts for other people, I'd want something better. For the ocassional personal hobby casting here and there though, it's just fine.

[Antenociti]

If you pressurize the mould during curing don't you have issues with pockets of bubbles that don't float out, expanding after decompression and distorting or destroying the mould? I guess a smaller bubble would exhibit greater buoyance tending to float free on its own (same reason a diver needs lead wieghts to nuteralized his compressed air tank).

Depends on the strength of your mould material - RTV silicones can hold the bubbles without it expanding after decompression.

Ideally though you would still want to degass the RTV and then pressure cast - the pressure will then ensure not that the RTV is free of bubbles, but that there are no bubbles between the RTV and the Master - this then guarentess that all detail is picked up without any bubbles being trapped on the surface of the master-mould interface.

[Sparky]

The reservoir in this picture is on the bottom right. . .
http://www.kc6sye.com/images/images_04_ ... s_mold.jpg
Shinny, mega-bloks
http://www.kc6sye.com/images/images_04_ ... oxprts.jpg