Cold cast resin prop casting issues

[Capn_Jack_Savvy]

So I made an excellent mold for my small prop and started the process of cold casting using Smooth On 325 resin, small amount of yellow dye, and powdered brass. Add powdered brass and color to Part B, stir, add Part A stir some more and pour. Everything goes great until I start moving the resin around in the mold to get into all the nooks and crannies before it sets up. The resin starts to "effervesce" and I end up with a lightweight item because it is littered with air bubbles. I have to chip off the back where the resin has bubbled up. The front of the prop looks great but the back looks like a pimply faced pre-teen with pock marks everywhere.

What causes the reaction between the resin and the powdered brass to effervesce so much? I am using plastic containers and metal stirring rod to avoid air bubbles. Is the brass reacting with the metal stirring rod? :confused :confused

Resin casting newbie needs ta know!!! I'm at work so I will post pics of what happens tonight!!!!

 

[PaulEscolar]

really recomend a pressure pot, you ca throw one together with an air compressor and a paint tank (with some modding).

From my experience anytime you put additives into smooth on resin it creates unpredictable reactions...even with adding certain pigments, more so when adding calcium, brass, atomized steel, etc. It's pretty much trial an error to get your "recipe" right for whatever you're doing, make sure you mix the dry/wet ingredients well, this might be causing problems- a pressure pot will def help with the bubbles and random reactions.

oh and room temperature, cold/hot weather has messed up some casts for me before.

whatcha makin? =p

 

[robstyle]

try this trick, replace the material your using with clear fiberglass gelcoat.

With the materials your using, the yellow "dye" or pigment may be your problem. There is only so much the material can take before its "contaminated". If this is the problem, the dye your using could be water based?.
BUT,
this however sounds like heat causing the issue. The brass powder is metal, the material cures and makes its own heat. The metal powder will get hot.... Make sense?'

To cure air bubbles on teh open end of a mold, try dusting the exposed liquid material before its cured with mold release.

For some reason ultra fine mesh brass powder isnt the easiest item to find. This is a test I did for a brass finish on a pistol. Its fine mesh brass powder brushed into the mold and also mixed into the plastic. The plastic was clear and just poured into the mold. You see one with a dull finish which is strait from the mold. The other has been gone over quickly with steel wool. You will get a better result if using clear fiberglass resin or gel coat though.

 

[Capn_Jack_Savvy]

I'm using the Smooth On brass powder. It is very fine....literally. It does make sense that the individual brass powder pieces is amplifying the heat from the chemical reaction and thus create air pockets in the resin.

I have tried adding a light powder coat to the mold first and pouring but the air pockets seem to get larger. I have also tried waiting a bit more for the resin to cure and get to a thicker viscous fluid and brush it into the mold but the moment it starts to get thicker it starts bubbling up like champagne!

I guess I need to experiment more. I am using Smooth Ons pigments.

Those pieces look really good. My viewed surface looks perfect but the underside is very pock marked. Like a small moon! That's no moon.....it's a space station.............sorry!

 

[exoray]

To me it sounds like moisture contamination, be it from the pigment or the air...

Is this a one side mold with a flat top?

If so get a sheet of Plexiglas the appropriate size spray on a release agent to one side of the Plexiglas, or simply wax it up with some Carnauba car wax 3 or 4 coats will work well... Place the mold on a some paper or what not as you now want to overfill the mold and get the surface tension bubble to form above the mold, and then starting from one side slowly lower/tilt the Plexiglas down the overfill bubble will move forward as you lower the Plexiglas filling the additional areas... The Plexiglas serves two purposes, it creates a smooth back surface and also creates a moisture barrier...

If you still have bubble issues then you likely have moisture contamination and additional steps will be necessary depending on the source of the contamination... If you have a vacuum chamber it's easy enough to put the pigmented and brass infused side (likely source of the contamination) into a container 5 times it's size and boil it under vacuum for 20-30 minutes before mixing with the other half and pouring...

 

[modelnutz]

Absolutly..sounds like moisture contanimation.
Either your resin has been exposed to the air for too long or the powder has picked up moisture.
Water is the foaming agent for urethanes.
It really doesn't take much moisture either.

I might suggest putting some un-mixed resin in small glass containers ( like a baby food jar ) and, along with some of the powder...bake them in an oven at 180 deg. for an hour or 2.
That should drive off much of the moisture ( if that is in fact the issue )...and...once the materials have cooled...try a new casting.
Also, bake your molds in the same way to drive off any surface moisture they may have....just be sure to apply a good mold release before you try to cast something in them...if you don't, the now dry surface of the mold may bond with the resin...destroying the mold.

 

[exoray]

bake them in an oven at 180 deg. for an hour or 2.
That should drive off much of the moisture

You need 212°+ to boil out the moister From what I have read the resins themselves can handle 250° without issue, and thus can be boiled in a pot as long as you mind the temp and don't exceed the 250°...

I would suggest a disposable pot (or at least one you don't use for food) on a tabletop hotplate vs your oven that you cook food in...

 

[Ozymandius]

Brass powder shouldn't cause any bubbling at all. You might want to double check and make sure that you didn't get the Cast Magic Brass powder by accident as that is a totally different product and would cause bubbling if mixed with the resin. Cast Magic powders are only meant to be brushed onto the surface of the mold.

You can tell the difference as Cast Magic powders have the consistency of talcum and will stick to your fingers and silicone and be shiny. Real brass powder should be dull and greenish looking and will dust off easily from your hands and silicone.

 

[bookface]

Has the resin been opened for a while, or otherwise exposed to the air for some time before mixing? I've noticed with smooth on's stuff, it goes 'off' and will bubble up just like you're describing, even without any additives. The actual front of the cast will look fine, but the back will be riddled with tiny air bubbles and the whole thing will be extremely light.

 

[Utinni]

I'd say pour a straight resin cast without any additives and see what happens. If it does the same thing it's the resin. Post pix, that will help us see what it looks like!

- Jeff