Vacuum Chambers and Pressure Pots... Need some Advice :)

Hi,

I wanted to pick peoples brains and feel free admins if this has been posted in the wrong forum to move it around. I couldn't find one that seemed to fit this subject.

Anyhow, I've been making and casting models for about two years using silicon rubber and easyflo120. The process was great for small kits (think the size of a micromachine - 2inchs) and now I've moved to models which are a little larger I'm starting to get pockets of air and the end result is producing a lot of flash (excess resin around the size or pockets of air) I've been scouring the internet for a remedy to the problem as I have a lot of kits ready to create moulds of, and I don't want to pay out for loads of resin and rubber only for the problems I'm currently faced with to persist.

The internet suggests using Vacuum Chambers and Pressure Pots to remove the o2 from the mixtures which is fine. But I started getting confused when people and youtube videos started using both methods to cast resin in rubber.

I don't know if people here have or know of the process and could enlighten me somewhat.

Essentially, I just need to know on the preferred process, a relatively cheap way of completing the process (I looked on eBay and £300+ for a chamber is crazy especially when you need to buy pumps and things on top) and just recommendations really. Just trying to understand it all.


The process I've read online (please feel free to correct if I'm wrong)


Make the rubber in a pot (which is approx. half full or rubber)
Place into a Vacuum Chamber and turn it on.
Air will be removed from the rubber (can cause it to expand or bubble for a time)
Pour rubber into premade mould.

Repeat the above for part B of the mould.

Place equal volume of Resin A and Resin B into Vacuum Chamber
Turn on and allow the air to be removed from both parts

Mix the two parts outside of the chamber until they're sufficiently mixed.

Place the mixture into a pressure chamber to remove any further bubbles.

Place resin into the mould and place resin filled mould into the pressure chamber.

Turn on until the resin has cured.

Remove the rubber part a and b to reveal the resin inside it.




The process I currently use is creating a latex mask and basically create a mould. Mix equal volume of easyflo120 into the mould and stand back for an hour. For smaller bits this is fine, as the air has escaped before the stuff starts setting, The problem as mentioned is with bigger pieces, it sets before the air can escape. Need to rectify the problem really as it's becoming frustrating lol

But yeah, just picking people's brains who are in the know really lol. Is the pressure chamber essencial if the air has been zapped out of the resin parts? would putting the mixed resin into the vacuum chamber work as uppose to a pressure chamber? I'm so confused


Adam,

Hi Adam
If you are making 2 part molds ? then yes, you need a Vacuum Chamber and a Pressure Pot.
I only use Mold Max 30 for making my molds, so I do not know what easyflo120 stuff is ??
Now, you will need to get the right measuring bucket that has the OZ and Quarts on it.
The 2 I use is the 5 Quart and the 2 1/2 Quart buckets.
Do not Pour rubber half full in a bucket as you will have a mess.
If you use a 5 Quart bucket you can pour up to 48oz tops.
You can make you own Vacuum Chamber for about $150.00 as I did.
When using a Pressure Pot this depends on what resin you have and cure time?
I use cast 300 so I have only 30 seconds to put it in the Pressure Pot to bring it to 35 to 40 psi.

My recent casting has been for non-model/prop type things, but I started casting about 20 years ago. I still follow the same basic process, though I sold my old equipment and now have newer stuff... I will try to remember to get some pictures tonight and post them here.

I use vacuum to de-air my RTV. My vacuum chamber is an thick-walled aluminum stock pot with an acrylic cover. I purchased it on eBay. For a vacuum source, I use a vacuum pump made for evacuating air conditioning units, also from eBay.
The mold-making process you describe is pretty much the one I follow.
When casting, I do not de-air the resins. The first time I cast resin, I de-aired it and it never set properly. I don't know if there was some necessary volatile that boiled off during the process, or if I mixed it wrong, or what, but I know that the same resin worked fine when it wasn't de-aired.
I pressure cast in a automobile painting pressure pot that I bought from a local tool supplier. As long as I put the mold under 30-60 PSI I don't seem to get any bubbles, so I don't think that de-airing the resin is necessary anyway.

In my experience, if you can't vacuum the mold material, putting the casting material under pressure won't help, because any bubbles inside the mold will collapse under the pressure and distort the casting. I made some samples showing the problem for my local makerspace (Novalabs, if you are in the Northern Virginia area), I'll see if I can find them and get some pictures of them as well.

Of course, that's just my experience, others will differ, I'm sure.

Personally I never use a vacuum chamber, I only use a pressure pot. You can buy one from Harbor Freight for around $100. I use Smooth-on and most of their products are good at letting air naturally move out. I make sure I pour my rubber in a slow small stream into the mold, then place it in the pressure pot. It's a one step process and I have had great results. One draw back is the pots are not very big so it's not good if your mold won't fit into it. I also use the pressure pot for casting too, just place the empty mold into the pot, fill with casting material and close the lid.

Thank you so much for your feedback guys, means a lot.

So a vacuum chamber as such isn't really required?

Is there a kinda caster's choice when it comes to pressure pot? I'm on a budget of about £100-£150 and I'm going to be casting in moulds of around say 15cm squared by about an inch or so deep. Not sure if that helps lol.

Adam,

You can get away with just a pressure pot set up if you pressurize both your silicone and resin. The #1 most important thing to remember if you choose to pressurize your molds is that your pattern must be solid with no voids in it. If you pressurize a mold with a pattern in it that is hollow or has a void your run a considerable risk of crushing it like an empty beer can with the pressure, I did it once, just once and it was a hard lesson to learn. Also you need to keep pressure on the mold material thru the setting time or else you run the risk of any bubbles "re appearing" if you loose pressure overnight.

As others have said both grizzly and harbor freight have 2 1/2 gallon tanks for about $100, but I think both need some hardware to convert them from paint pots to pressure pots. When converting use caution and use appropriate hardware so you do not turn the pressure vessel into a projectile, or have it launch a projectile by using unsecured or under secured hardware.

Gattaca said:

... pour my rubber in a slow small stream into the mold, then place it in the pressure pot...

Click to expand...

Perfect example of differing experience. Way back when I first started casting I tried putting the mold under pressure while it cured. The rubber didn't release the air, the bubbles were just crushed during the material setting, and when I released the pressure I heard some little popping sounds. That was the air bubbles re-expanding and popping little holes in my mold. I never tried that technique again, but obviously it is a viable one. I was using RTV with a thixo additive, which may have been what caused my attempt to fail. I'm probably going to give it another try soon, because simplifying the process would be a welcome change. Plus I can just re-use the vacuum pump to build a vacuum former.

I'm going to assume it will work for me and thank Gattaca in advance.

The process I'm using at the moment is creating the part in 3D and have shapeways (or other 3D print companies depending on the cost) to print the 3D model. The model is broken into parts, and I then use latex rubber to create a 2 part mould. Then pour resin inside and create a replica part. If I need to buy chambers, and pots, and stuff then I'll have to work out viability. Got to look for a UK supplier lol.

So would I need to have the pressure maintained for the entire duration of the mould setting process? The concern I have is that the models I have printed have a 5mm wall the rest is hollow. The concern I have with a pressure tank is that the rubber will crush the model. :/


Adam,

Perhaps you can fill the hollow parts of the 3D print pieces with resin and let that set before attempting to pull the mold?

I think your hollow 3D prints might be crushed in the process unless you beef them up with a thicker wall. The next time I use my pressure pot i'll throw in a 3D printed part to see what happens. And yes, the mold or cast needs to stay in the pot at pressure for the full duration of cure time. Other wise like mentioned above the bubbles could expand, I opened the lid on a mold before it was cured to add another mold and ruined the first mold because of this.
If you get a pressure pot you will need to modify it slightly like mslz22 mentioned. It should come with a pressure guage and release valve, but you'll have to install a release valve and a air knipple to hook up to your air compressor (oh ya, you'll need an air compressor too, mine is just a 2 gallon home depot style compressor). Besure to use air fittings, nothing else will work and it could be dangerous. 60lbs of pressure is all you need, even 50 will work. The tanks are usually rated for 80, so stay below that and you'll be fine.

That's a good idea ekervina

When you say dangerous, you mean explosive in your face dangerous, or likely to crush whatever is inside dangerous? lol there any premade things does anyone know lol


Adam,

Dangerous as in a air fitting goes flying accross the room at 200 miles an hour. The lid is held on pretty good, but if you over pressurize it and you don't have a release valve you could have an exploding tank or pressure valve. Don't worry it's not that hard, a good supplier of industrial air fitting will know what you need. Or PM me when you get one.
there's a picture of mine part way down this post of mine:
http://www.therpf.com/f9/how-mold-cast-prop-part-221034/

I want to add my $0.02 because I don't think the reasons for each have been addressed:

Each item, vacuum chamber, and pressure pot, fill separate needs:

Vacuum chamber: degasses your RTV. When you mix the rubber, air is added by the mixing process, and because it is thick, many bubbles remain. This is usually not a problem unles you put you molds into a pressure pot for casting. When the vacuum is applied, all those bubbles grow to 10X their size, and escape the mix. This is the reason that you must use a larger container when degassing the RTV. Because the RTV expands 4X-5X its volume with all that expanding ai, and will spill over the container if it is under size.

Pressure pot: Compresses small bubbles, and prevents new bubbles from forming during the curing process. Curing resin, under pressure will not allow gassious bubbles to form, thus giving you a cleaner casting.

But wait, there's more!

RTV silicone that has NOT been degassed by a vacuum chamber, has lots of tiny bubble beneath the surface. As I said above, this is NOT a problem unless you use a pressure pot for casting. WHY? Because under pressure, all those bubbles compress, and leave dents in the mold surface. The resulting cast will have a acne-like surface.

So, for your best casting: Cast into rubber that has been degassed, and pressurize the mold duing curing.

Vacuum chambers can be made from thick-walled aluminum stock pots.

Do NOT make you own pressure pot. things under pressure become explosive devices. The purchase of a 2-gal paint pressure pot should work for small molds. These are built to withstand pressure, and you only need 60 psi.

I hope that addresses your questions.

All good points here, a good rule is to carefully read the fact sheet for the product you're using. It should say weather degassing is needed. Different products will require different methods to use them properly.

I use a Grizzly pressure pot which has very beefy lid clamps, I have a thread on it here http://www.therpf.com/showthread.php?t=184777
I operate it at 60psi and I get perfect results.

I also don't bother with vacuum, I pressurize my molds. I mix my rubber well but I'm not too careful about it, and just dump it into the mold box. It bubbles and burps like crazy but the pressure takes care of it. I've even sliced open old worn out molds and there are no bubbles in them at all, they're smooth and solid all the way through.

I also pressurize my resin. Living in a humid environment I have to, it will foam up otherwise.

One other issue you mentioned is that your resin is curing before it has time to completely fill the mold. I'm not familiar with the resin you mentioned, but I'm assuming that it must have a pretty fast gel/cure time. Most resin manufacturers make a line of resins with a variety of cure times. Some may cure in 60 seconds and some may take 16 hours. The resins also may come in different viscosities (how watery the are).

You may want to check with your supplier if they make either a runnier resin (to make it flow into the mold faster) or a resin with a longer cure time (to give it more time to flow into your mold). I've been very happy with Smooth-On's products.

In case you were wondering "why would anyone want a resin that took 16 hours to cure?", well, there are applications when you need exactly that. One example would be a very large, thick casting. When resin cures, it creates heat (an endothermic reaction). Fast curing resins in a large casting create a lot of heat which may cause shrinkage, distortion, bubbles or even a fire! (I've seen it happen!) Slow curing resins create less heat, but spread it over a longer period of time. Kinda like slow cooking versus flash frying. They both work, but each one is suited better for different applications.

Thanks for the advice and the explanation between vacuum and pressure.

So the norm as such is vacuum the rubber to ensure there are no air holes, then use a pressure pot to cast the resin AND let it cure inside the pressure pot.


Smooth-On has been recommended by more than one caster and the fact that you've all mentioned it would seem to indicate that it's the caster's choice as such so I'll invest into getting a trial pack and see what happens with that. Once modeller on another board said to avoid needing a vacuum chamber, to layer the rubber like sedimentary rock. That way the air has more chance to escape as you build thin layers (about 10mm) it would take a while, but would save money potentially (depending on the scale of the operation I guess).

Has anyone ever tried that technique?

Easyflo120 usually starts to turn into a gel at around 60 seconds of mixing. If I'm going to use a pressure tank, this Smooth-On stuff would need to give me a minute or so to get it in there, prepped and everything lol.


Adam

You shouldn't need to mix your easyflo for 60 second, you should be able to mix it with 10-15 seconds, just enough to get the 2 chemicals evenly mixed. Or you need to find something with a longer pot life. How much are you mixing at a time, a cup full? more?

It varies on the mould, usually between 20-100g mixed. I mix for 40 seconds, then pour into the mould. After probably 30 seconds it starts setting and will be warm for between 10-20 minutes before setting. I usually remove them after an hour so it's had enough time to harden.


Adam,

If it's getting warm while mixing then you're mixing for too long, the reaction has already started and it's getting too late to pour. Try a test, mix for 5 seconds in a small cup, then let it set up to see what happens.