Air bubbles in resin

[Guest petersm]

Hello everyone,

An old trick I learned but did not see posted yet is how to get rid of airbubbles in poured resin.

I tried this several times and it works fine.

The thing is to blow cigarette smoke over the mold immediately after pouring. It draws the bubbles to the surface and when you blow over the mould again you'll see the bubbles pop and disappear.

I don't know how or why but got this from a guy working with epoxy resins and large molds for many years.

Don't flame me for promoting smoking, that's an individual thing.

HTH, Marc

[KursadA]

Interesting. Not that I would use it, but I'd like to figure out if there is a scientific explanation to it.

The price of pressure pots is now low enough that I don't think any other method is worth using unless your budget is severely restricted. Harbor Freight sells a suitable pressure pot for $79.95, shipping included. Guaranteed to take out every single bubble, every single time.

[Kaan]

Maybe the trick is not in the cigarette smoke, maybe it is in the airflow.

As some of you know air, flowing over a surface creates a negative pressure area on top of it. Airplane wings generate lift by using that principle. It may be that negative pressure which draws the bubbles to the surface, not the smoke itself :P

[Matt Walker]

Why not make a vacuum chamber?

Using a container large enough to hold the mold, put this into a plastic bag. Pour the resin, put the mold in to the container, then gather the bag top around your vacuum cleaner nozzle, and turn it on. Voila, instant vacuum chamber!!!

Now, I haven't actually tried this myself, but intend to when I do a bit of casting in the not-too-distant future.......

thoughts?

Cheers

Matthew

[KursadA]

Matt,

A couple things why your scheme will not work:

1- You DO NOT want, or need vacuum to do casting. You need pressure. At or around 15-20PSI pressure, air bubbles in resin shrink to a point where they are practically invisible, and as the resin cures, they stay at that size. The idea is to pour the resin in the mold, put them in the pressure pot, seal the pot, hook it up to your airbrush compressor and bring the pressure up and keep it that way until the resin cures. You will have bubble-free castings.

2- Where you need vacuum is in de-airing the RTV mold rubber, so I think this is where your confusion is coming from. You want to get rid of the air in the RTV rubber so your mold will not have bubbles in it, and you can use a vacuum of 25-27in Hg for that. Problem is, you can never get that low with a vacuum cleaner, there simply is no way. Ask any physics teacher or A/C technician. The pathetic vacuum you can get with a vacuum cleaner is enough for pulling warm plastic to a surface for vacuforming, but not for pulling air bubbles from a liquid with the viscosity of RTV rubber. Try pulling in bubbles from a glass of Coke with a vacuum cleaner, even that will not work...

You need on oil-type real vacuum pump like the A/C technicians use, i.e. Robinair brand, etc. A fellow at Hyperscale suggested a hand-pump with a gauge that apparently is used by teachers for use in chemistry/physics experiments requiring moderate vacuum. They sell for $30 on Ebay, and I want to buy and experiment with one as I get ready for casting and selling resin 1/48 accessories here (Litening pod, etc.)

[Matt Walker]

Hey all,

Thanks for the thoughts KursadA. As I said in my post, I haven't actually tried this method, and was throwing it out to see what y'all thought.

Your knowledge is thankfully received.

matthew

[Guest rob clements]

Matt,

Not strictly true what KursadA tells you although what he posted is correct as far as he goes. The way to get NO bubbles is indeed to use a Vac chamber in conjuction with a pressure chamber. Especially in small very thin sections. See just by using a pressure system, you still can sometimes get a mass of tiny tiny bubbles that just may show if any sanding is done to the part in question. Doesent happen often, but believe me it does. Found that out to my cost a long time ago. Also using just a vac system. you can get voids in undercuts where the resin sometimes will just not flow. Best method is to degass the resin, pour into the mould, degass again and then whack it in a pressure pot. I used a custom designed version that had a working pressure of up to 150psi - bingo. NO bubbles. Ever. Anywhere. Still, shant worry about that any more as I have packed the business in. Anyone want a 2 foot 6 by 18 inch heated pressure chamber certified to 450psi?

Rob.

Aerodynamix Models

[KursadA]

Not strictly true what KursadA tells you although what he posted is correct as far as he goes.

Can you point out exactly where any of the information I posted is not "strictly true" to quote you? Depending on their viscosity level. many resins do not need de-airing. The brand that I use does not, for example, and I simply summarized my experience working with it.

[Guest rob clements]

KursadA, 'not strictly true' was perhaps not the correct wording I meant to get my point across. Been a long day. What I meant was you were correct as far as you go, but vac chambers do, and always will, have their uses in resin casting. No matter how low the viscosity of the resin - and I use the lowest viscosity there is anywhere as I have it formulated specifically for me only (it is EXPENSIVE and comes in 25kg drums!!!!) you will still get air entrainment. No matter how careful you are. Now, if you only pressurise, you can get away with the compressed 'bubbles' in the resin if you are using a 'fastcast' resin with a short demould time. However, if, like myself you use a longer demould time product it is POSSIBLE for the part-cured resin to allow those miniscule bubbles to expand again somewhat. I suppose my proceedures are way in advance of what most home-casters need, but I produced castings for the model kit and other non model industries with NO bubbles, tiny or otherwise by using both vacuum and pressure systems. Try using just pressure casting with optical resins and see just how many bubbles reamin visible. You might be surprised.

All the best,

Rob Clements.

Aerodynamix Models and Resin Tech Co.