Degassing Smoothcast 300

Etewaf

Etewaf

Active Member
Hey guys. I'm having a bit of trouble. I recently built a vacuum chamber in order to try and make bubble free casts of the Lady Gillette used for the Jedi Comms in the Prequel Trilogy. If anyone has tried to mold and cast one of these before, you know those little grips are a pain.

I guess I'm having trouble finding out what steps should be taken and in what order. Should I be degassing the resin before pouring it into the mold? I've heard of people doing it both ways, before and after. I know if you degass while it's in the mold it will spill over, but I'm sure this could be combated with building up a retaining wall around the pour spout. And I know if you pour it into the mold after degassing, you will still trap bubbles during the pour. Should I bump up to one of the smoothcasts with a longer working time to degass both before and after?

How long should it be left in the vacuum? Until it cures? Should the vacuum be released at some point? I've done probably ten or so of these things with varying degrees of success depending on what I've tried. So I'm curious what you guys have tried, and I would be appreciative of any nudges in the right direction.

Thanks,
~Spencer
 
Unless you're pouring clear or translucent resin, you can easily get away without degassing resin. in fact, if anything, you go the opposite direction with resin and use a pressure pot to compress the air in the resin.

Your time would be far better spent powdering up the mold prior to pouring. I actually took on this EXACT same project many years ago, so I know what kind of trouble all those little wavy patterns on the grip can be. But the trick to getting great pours all the time is baby powder, and just moving the mold around in your hands for a few seconds while the resin is still new in the mold.

hope that helps.
 
Do I have this correct: you are trying to mix resin and pour it into the mold, put it into a vaccum chamber to remove the bubbles from mixing?

If I am correct, the Pot-Life Smooth-Cast 300 is 3 minutes. That is very little time to mix, pour, and place into a vacuum chamber to degas. Even with a very powerful pump that removes the air in 30 seconds, this doesn't sound right.


In my experience, a vacuum chamber is used to degas viscous rubbers, and a pressure chamber is used to compress bubbles in resin.


If you want to put resin into a vacuum chamber, then you need a slow setting resin, one with a long pot life. Then you can put the mixed resin into the chamber and draw out the bubbles before it sets. If you want to place the mold into the vacuum chamber, there needs to be a reservoir for the expanding resin. That is something that is built into the mold from the start.

Generally, they formulate resin to be very low viscosity, so as not to trap air. As anyone who has ever poured resin knows, you always get some type of pinholes, especially in detailed patterns. Sometimes, Baby Powder brushed into the mold prior to pouring the resin will help. The best castings come from Pressure Casting, not a vacuum chamber. Once the resin is poured, the mold is placed into a pressure pot, and it is pressurized to 60-70 lbs of pressure. This shrinks the size of existing bubbles, and keeps new ones from forming. Here's the kicker: Pressure will also compress any air bubbles in the silicone mold, and distort the mold. So if you are going to pressure-cast, you need to degas your silicone in a vacuum chamber for a bubble free mold.
 
MattMunson said:
But the trick to getting great pours all the time is baby powder...
Click to expand...

I agree but I'd suggest cornstarch baby powder and NOT talcum baby powder. Talcum can have a cumulative effect to your lungs as it never goes away so obviously that's a hazard. Nurses working in hospital nursuries have not used talcum in decades. I was surprised when I began work at McFarlane Toys that they didn't use the cornstarch baby powder when casting the 2-up protos so I suggested they switch over.
 
Clonesix makes a good point! If you need to force out the bubbles, you want a pressure pot for this type of casting material.

Do you have any bleeders in your mold? The best bet would be to lightly coat with baby powder and blow out all the excess with an air compressor or can of compressed air and then pour a little bit of the resin in, rotate the mold for a second and then pour in the rest. Very much like Matt said.

If this doesn't work, get a small gauge of brass tube at a hobby store and slowly turn it as you press through the silicone in the problem areas all the way through the rubber creating a bleeder that will allow the air to escape. It seems like a bleeder would solve this problem. It's some minor sanding/clean up in the final casting but it will literally be a few seconds as opposed to god knows how long to fill and sand and fill air bubbles...
 
Hey guys, thanks for the advice.

Matt, what exactly does the powder do? Does it just help the resin soak into the crevasses? I'm definitely going to give that a shot. It took me two attempts just to mold the thing, both times I painted on detail layers to avoid grip problems. Second time was the charm. It isn't a pretty mold, but I don't get any warping or seam issues, so it does the trick. I usually put two pieces of cardboard on either side of it, rubber band it, and stand it upright in a small mixing bucket. I've tried slushing with a little resin and then pouring in the bulk, but no luck. I actually may go test out the baby powder technique in a few minutes. Too curious to wait until tomorrow. :)

Clonesix, the reason I didn't go the pressure pot route is because, quite frankly, I'm paranoid about the thing blowing up. :lol Worst comes to worst, I'll get the smoothcast with the 20 minute working time.

Anywho, thanks again for the advice. Other than the grips, the piece turns out wonderfully. I'm OCD about these things, so I'm sure I'll get it eventually. I'll mess around with this thing a bit more this weekend, and I'll let you guys know how it went.
 
Spencer said:
.....

Clonesix, the reason I didn't go the pressure pot route is because, quite frankly, I'm paranoid about the thing blowing up. :lol Worst comes to worst, I'll get the smoothcast with the 20 minute working time.
....
Click to expand...

Not necessary to worry. Go on ebay and buy a pressure painter, it's the Exact same as what all the molding sites but it it only $100. Hook up an air compressor that has a bigger tank then the pressure pot's tank is. and mold it with silicone. People argue that it doesn't do anything, there wrong! it crushes the little bubbles until the don't exist, or are so small you can't see them with a microscope. leave under pressure until cured. then you cast the same way, and have an extremely nice piece. Make sure to make the mold with a recess cavity for extra resin, as the resin you pour in will have bubbles that will be crushed creating more space. I believe these are very safe as long as you follow the rules, mine was able to take 100psi but it's release valve was set to go at 50psi. in order to obtain enough pressure for molding/casting you just need 40psi. Hope this helps!!
 
Spencer, the baby powder releases surface tension, allowing the resin to flow into where the air would normally trap bubbles.

I see it's a two part mold. Why not mix up a small batch of resin and pour or even brush it into the problem area and then carefully put the mold together so as not to dislodge it. Pour up the rest of it and the resin will bond to itself.
 
I just poured a few of these, and boy did that powder work. Near flawless on the grips. I've attempted to brush it on before, and then join the two halves, but I always end up dislodging it, making the final piece look off-set. I tried to snap a few pictures, but with the white resin it's hard to see. I'll prime one tomorrow to show off. :) Thanks again guys.
 
With Smoothcast 320 (10 minute kick) if you get in the groove you can vac degas, pour and than pressure cast, but there is little room for error...

I do this regularly with certain castings that require being bubble free but it's a hassle...

It goes like this when I do this, mix resin, run heat gun across surface, place in vacuum tank for 30-40 seconds (actual high pull vacuum is only say 10 seconds as most of that time is pull down) remove from vacuum, lightly 'swirl' the mixture vs mix it, I swirl right and than left... I find swirling introduces less bubbles and simply gives a final mix... Fill the mold and place in the pressure tank at 60 PSI...

As I said there is little room for error with the 10 minute kick, but once you get in the groove it works fine...

But, generally I have found that if you pour out the two parts into paper cups let sit (covered) for about 5 minutes, run a heat gun across the surface of the cup to knock down the surface bubbles... Pour into a shallow mixing container and again run a heat gun over the surface, mix and yet again run a heat gun over the surface... At this point if you didn't stir like a mad man a vast majority of the air is gone, carefully pour into the mold... A little massaging of the mold will help and surface bubbles... As will dusting the mold with baby powder before pouring...

I dust every mold now as I find it's the easiest step in getting clean castings, worth the effort every time...
 
let me ask you guys - if you use baby powder in your molds do you also use a release agent? Or does the powder act as one?
 
I've never used a release agent when working with silicon. Only ever used it to keep the halves of the mold from sticking together during construction.
 
Hecubus114 said:
let me ask you guys - if you use baby powder in your molds do you also use a release agent? Or does the powder act as one?
Click to expand...

Depends on the mold and how long I expect it to last, generally the only time I use release on silicone molds in near the end of their life right before they are retired to the garbage can to extend their life just that much more... The talc isn't so much a release as it used ti simply break surface tension between the mold and resin...
 
The easiest solution to get good casts would be a pressurepot and a slower kicking resin. Smoothcast 300 kicks to fast for this.

I use 305 for most of my castings and it gives me enough time. You could even go 310 which has an even longer kicktime.

Make sure the molds you make are either from degassed silicone on let them cure under pressure (which is wat i usually do). I use silica free baby powder to dust the molds and just pour resin in the molds and let them cure under 5 bar of pressure. The babypowder 'draws' the resin into the tiny areas and the pressure takes care of the rest
 
QFT. Best way to do this type of casting, exactly how I do mine. :cool

LeMarchand said:
The easiest solution to get good casts would be a pressurepot and a slower kicking resin. Smoothcast 300 kicks to fast for this.

I use 305 for most of my castings and it gives me enough time. You could even go 310 which has an even longer kicktime.

Make sure the molds you make are either from degassed silicone on let them cure under pressure (which is wat i usually do). I use silica free baby powder to dust the molds and just pour resin in the molds and let them cure under 5 bar of pressure. The babypowder 'draws' the resin into the tiny areas and the pressure takes care of the rest
Click to expand...
 
I love you guys. I've had this problem for years and never knew who to ask about it. :D <3

Spencer, I'm the same way about air bubbles!
If you want to see the pressure pot in action here's a video:
TOY LORDS Of CHINATOWN: Episode 1 on Vimeo It's at about 1:15 . If you don't know who Suckadelic is, check out his website suckadelic website . Anyway, it looks like he's using SmoothCast 325, some pigment, and what I thought was a vacuum chamber, but you're right. It's a pressure chamber, just like: Pressure Chamber - 5 Gallon | Reynolds Advanced Materials | Everything for Mold Making & Casting

Now, this is great for silicone molds and molds that can fit in pressure chamber, but what about urethane rubber molds, or urethane rubber molds that are too big to put in the pot?
 
Also check ebay for your pot, I have picked 4 off there for little to nothing, 2 were 3 gallon for abut 30 bucks, a 40 gallon pick up only for 99.00 and a 50 gallon selfstanding autocalve for 100.00 that weights in at 300 pounds LOL
 
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