Eliminating voids without a pressure pot?

admiralcag · Post by **admiralcag** » Fri Oct 14, 2011 9:41 pm

I am working on a 1/72nd scale XB-70 and need to make a replacement engine bulkhead. I have made a mold from a part Kylwell lent me and the mold turned out pretty good. I have brushed resin into the mold with varying results. I don't have access to a pressure pot. Not that it would do me much good since I didn't vacuum the RTV. Any ideas on how to best cast this or how to patch the voids? My best pull is the first image.

http://i226.photobucket.com/albums/dd85 ... image1.jpg
http://i226.photobucket.com/albums/dd85 ... image2.jpg
http://i226.photobucket.com/albums/dd85 ... image3.jpg

Let me know if you can't see the images.

Vern

Ziz · Post by **Ziz** » Fri Oct 14, 2011 9:57 pm

What kind of mold is it - two part, envelope, open face? What's the pot life of your resin?

As far as eliminating air bubbles, start by mixing your resin slowly so you don't introduce more air. Also try pouring it very slowly if it's a slow pot life. When I pour smaller parts, I hold the stir stick against the edge of the cup where the resin is flowing - that allows the resin to follow the stick down towards the mold opening rather than just hoping I'm aiming right. By directing the resin, I can make sure it fully fills the mold.

You could also try tapping or squeezing the mold part way through the pour to bring air bubbles to the surface, then pour some more. Again, this depends on the pot life of the resin you're using.

Lt. Z0mBe · Post by **Lt. Z0mBe** » Fri Oct 14, 2011 10:11 pm

Does baby powder brushed in the mold help at all for bubbles? Or, is that just what you pro casters use as a release sometimes?

Kenny

Ziz · Post by **Ziz** » Fri Oct 14, 2011 11:28 pm

I don't use any release coating but a lot of my parts are very small so they come out of the molds fairly easily.

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Sat Oct 15, 2011 12:39 am

You need to take a thin stick, stiff enough to work through each of the
engines and baffles and work the air bubbles out, and the resin in.
I do that with some of the thin water tight doors, where there is a
0.015 gap between the door and frame.

.

Stu Pidasso · Post by **Stu Pidasso** » Sat Oct 15, 2011 12:55 am

Okay, I'll do my best, without seeing what the mold looks like:

Mix and pour your resin in until the mold is 1/2 full. Then "flex" or bend the mold outward (like you're opening a book) to dislodge the air bubbles. Let them rise, then pour the rest of the resin into the cavity. Also, this might sound weird, but if you have something that... ahem... vibrates in the house, it won't hurt to place that against the mold, that will also bring bubbles to the surface.

Hope this helps!

admiralcag · Post by **admiralcag** » Sat Oct 15, 2011 7:05 am

I made a one-part mold. The inside of the bulkhead is inside the plenum chamber and not visible. I am using Alumilite. The pot life is only a couple of minutes and it is even shorter with the small batches I am mixing. A couple of the voids were caused by trying to get a little more resin in the mold after it had started to bloom. Do you guys think I will have enough time to work the resin?

Lots of good ideas. Jimi, if I can find my orbital sander, I can vibrate the mold. The way you said it made me laugh. Only you can create those mental images

Vern

skymods · Post by **skymods** » Sun Oct 16, 2011 9:07 pm

Coating the mold with baby powder will help quite a bit. It will break up the surface tension of the resin and make it flow better. Just make sure to either knock or blow out any excess powder. A fine coating is all you will need.

As already mentioned using something like a toothpick to pull out the bubbles will also help.

Another thing you might want to think about using a lower viscosity resin such as Smooth-On 320 or 300. It has a about a 3 minute work time before it blooms which should be plenty.

Dave

Tankmodeler · Post by **Tankmodeler** » Tue Oct 18, 2011 12:40 pm

Yeah, the Alumilite is probably going to make this really tricky. It just cooks off so darned fast.

The longer pot life Smooth-on products, like the 320, have significantly longer pot life. I usually get 4-6 minutes out of the 320 and that's plenty of time to agitate the bubbles out of the mould. There may still be a couple pin holes, but that may be unavoidable.

Paul

Grumpy Popeye · Post by **Grumpy Popeye** » Sun Oct 30, 2011 8:49 pm

Stu Pidasso wrote: Also, this might sound weird, but if you have something that... ahem... vibrates in the house, it won't hurt to place that against the mold, that will also bring bubbles to the surface

I've rested an old electric toothbrush against the side of a mold to good effect.

kenno3 · Post by **kenno3** » Wed Dec 28, 2011 9:34 pm

skymods wrote:Coating the mold with baby powder will help quite a bit. It will break up the surface tension of the resin and make it flow better. Just make sure to either knock or blow out any excess powder. A fine coating is all you will need.

For those who have used baby powder in molds, do you know if its usable with Clear epoxy resin (Clear Cast)? Or will the powder discolor or affect the clarity of the cured resin?

Tankmodeler · Post by **Tankmodeler** » Wed Dec 28, 2011 11:12 pm

kenno3 wrote:For those who have used baby powder in molds, do you know if its usable with Clear epoxy resin (Clear Cast)? Or will the powder discolor or affect the clarity of the cured resin?

_Anything_ you put on the surface of your mould will affect the surface of the Clear Cast. Spray mould release, when applied thinly, will have a minimum affect, especially of you don't disturb the surface of the coated mould when the resin is in place and can usually be removed after demoulding by washing with detergent or TSP. But some releases either apply heavily or have a powder base and still _may_ affect the clear cast.

Talcum powder (baby powder) will absolutely, positively bond to the surface of the part and will make the part, at best, translucent and, at worst, mostly opaque.

If you are planning to go into production of these parts, Clear Cast probably isn't your best choce of material (at least the polyester Clear Cast I remember wouldn't be). There are clear casting resins of other chemistries that would be much better. However, if you are only making a very few parts (say under 15-20) then an RTV Silicon mould can be poured without release and the parts will come out very clean. The mould will deteriorate much faster than if prepped with release, but it will certainly work for a reduced number of parts without release. Silicon is naturally resistant to _anything_ bonding to it. (Except other RTVs, which bond like crazy).

HTH

Paul

scratchy · Post by **scratchy** » Thu Dec 29, 2011 1:58 pm

I've punched some vent holes in a couple of my molds to eliminate this problem. I chose an area on the part with no detail and it's the highest point in the mold so that the air bubble works it's way out, so it's easy enough to trim and sand down once the part has cured.

I should let it be known, I don't have a pressure pot, and I've never had the opportunity to use one.

Antenociti · Post by **Antenociti** » Fri Dec 30, 2011 8:30 am

admiralcag wrote: I don't have access to a pressure pot. Not that it would do me much good since I didn't vacuum the RTV.
Vern

well, if you get a pressure pot you wont need to vacuum the RTV moulds, just pressure-cast them also - job done. (unless you are using a really soft silicone then pressure-cast RTV moulds will not distort after full-cure). I've gone down to shore 15 without any issues, just make sure you can keep the pressure constant overnight.

As other shave said talcum powder (aka French Chalk) works to help pull resin into small detail, ensure it is a dusting though and doesnt pile-up into small detail recesses. Graphite also works, but frnech chalk is easier to get hold of.

Wont work with clear-casts though (as others have already said). Mould-release agents will also tend to cloud the surface on clear-casts although sometimes the resultant partially-opaque parts can be desirable (such as for TO camouflaged figures).

In general for tricky parts that require time to ensure good casts, use a much longer pot-life resin with an extended cure-time to maximise your available working time...or get a pressure pot system which is very cheap and saves an awful lot of the hassle.

2nd-hand pressure-pots are available on eBay a lot, Harbour Freight in the USA do a nice one for $80, often on sale to $40, and a cheap compressor for DIY work is all you need to get a 40-50psi working pressure. Comp+pot is often under $100 and pays for itself in easier and better castings within hours.

admiralcag · Post by **admiralcag** » Fri Dec 30, 2011 11:10 am

Antenociti wrote:well, if you get a pressure pot you wont need to vacuum the RTV moulds, just pressure-cast them also - job done.

I was under the impression that if you pressure cast with a mold that hadn't been vacuumed, the mold would collapse. Is this not correct?

Vern

Tankmodeler · Post by **Tankmodeler** » Fri Dec 30, 2011 12:11 pm

admiralcag wrote:
Antenociti wrote:well, if you get a pressure pot you wont need to vacuum the RTV moulds, just pressure-cast them also - job done.
I was under the impression that if you pressure cast with a mold that hadn't been vacuumed, the mold would collapse. Is this not correct?

Vern

No, not necessarily. Any mould with interior bubbles near the resin surface (near but not necessarily breaking through) will have a soft spot that the pressurised resin will distort to an extent that is related to the size of the bublbe and the nearness to the resin surface.

Vacuuming the rubber is _a_ way to eliminate all or most of the bubbles (pretty much the best way), but careful pouring and a little agitation while the rubber slowly cures will remove almost all of the bubbles and a non-vacuumed rubber could be used in a pressure pot. Of course, if you have a pressure pot, you would probably find it no trouble to vacuum your rubber.

Paul

kenno3 · Post by **kenno3** » Sun Jan 01, 2012 9:13 pm

admiralcag wrote:
Antenociti wrote:well, if you get a pressure pot you wont need to vacuum the RTV moulds, just pressure-cast them also - job done.
I was under the impression that if you pressure cast with a mold that hadn't been vacuumed, the mold would collapse. Is this not correct?

Vern

Hi, I bought a pressure pot and used 39 psi on my silicone rubber molds (Oomoo 30). It worked at eliminating the bubbles, but because the rubber mold wasnt made with vacuumed rubber, there are tiny bumps on my cast piece. Even tho there were no visible bubbles near the surface and I poured the rubber from high up. Also the rubber mold is ruined because the cured resin left bump impressions (opposite of the bumps on the resin), and they didnt flatten out after demolding.

Perhaps the bubbles in resin could have been squished, and the bump marks from the mold could have prevented if you pressurize the resin for 10 minutes to break the bubbles, then release pressure so the bubbles in the rubber arent deformed, all before the resin cures. But someone else can try that.

Today I bought the Harbor Freight "2.5 CFM Vacuum Pump."
http://www.harborfreight.com/25-cfm-vac ... 98076.html
I hope that works at pulling 29 inches of mercury the next time I make rubber molds.

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Eliminating voids without a pressure pot?

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