I have been attempting to cast figures that I am scratchbuilding from putty and have trouble casting resin after I have made the molds. Particularly when dealing with smaller apendages such as hands and feet. It looks like there are air pocketst trapped after I break them out of the mold.
I heat up the molds and pour the resin while they are still very warm. Then I squeeze the sides of the molds to force out the air bubbles but there are always some left in the smaller areas.
Usually the molds are poured into Dixie Cups.....3oz. Should I use a more narrow vessel to make the molds? I have tried to drill holes in the bottom of the molds but still no luck.
Large portions such as upper torsos are no problem. Just a few hints please. Again these are very small figures.......54mm.
Thanks, Rob

The issue is surface tension and the thickness of the resin, not the size of the molds. To remedy it, you have to use pressure casting. This forces the resin into the smallest nooks and crannies and forces the air out of the resin. The key is a pressure vessel. Some use old pressure cookers or paint pots. Pour your resin into the mold then place the mold in the pressure vessel. Seal and pressurize the vessel to about 30psi and you should have no further issues.

Good luck.

Thank you for that advice. I am not familiar with pressure vessels. Where would I look to purchase something like this?

Check out paint supply stores or commercial compressor parts suppliers. Some people also modify old pressure cooker pots with an air fitting and a gauge and use them as well.

Please excuse my ignorance on this procedure. What iam trying to picture is the basics of this. So I put the mold and all inside the vessel? And then the vessel will pull a vacuum or apply pressure......again sorry for being so dense here.
Is there a website that may have this information as well? Robert

I don't know of any websites on it. The vessel places pressure on the mold and forces the resin to fill it fully, it also forces the air out of the mold.

http://www.finishsystems.com/resincastingpressurepots.html

http://www.lumicast.com/equipment.html

A friend sent the above links if anyone else is interested in this type of thing. Thanks for the input. This has been a learning experience. And I thought I knew about making molds and casting........man, was I mistaken. rob

Hi Rob,
Your problem could be a number of things. Without seeing your mounted
figure, mold, and results, it's hard to tell. I'll take some shots in the
dark. I appologize if I say things you already know.

Are your figures mounted so as the volume fills with liquid resin the
air can escape or are there areas where bubbles can be trapped? Imagine pouring
water slowly down your sprue line. If you have pockets, you should be adding
more sprue lines so the air has a way to escape from the mold. Another option
is to separate the figure into parts (although that may not be an option for
a scratch built figure).

If you're using a resin with a short working time, it could be partially
setting before it flows into all the nooks and crannies. As it sets, it flows
more like a thick oil than water.
The alumilite kits in hobby stores have a working time of under three
minutes which is ridiculously short in my opinion. Smooth-on has resins with a
working time of nearly 20 minutes and very low viscosity. It flows like water
just as advertised. Other vendors should too.
These links have tables which are nice for comparing working time,
viscosity, strength, etc.
http://www.reynoldsam.com/index.php?cPath=5_1120_1209
http://www.reynoldsam.com/index.php?cPath=5_1120_1157

You can use the handles from your old hobby paintbrushes to force the
air out of the nooks and crannies. They won't fit into individual fingers but
are more than small enough to force the air out of a 54mm figure hand or boot.
Here is where a long working time helps. You'll need a pair of sticks to
stretch the mold and let air escape. Some rubber gloves will also be needed to
keep your fingers clean. Make sure the resin is poured deep enough so that any
openings down in the mold fill with resin rather than air when the sticks are
pulled out.

The recommended approach to making the mold is to use a vaccuum to
remove the bubbles from the silicone. Unfortunately, vaccuum pumps are
expensive. Without this, you can get fair to good results but probably not
great or consistent results.

As far as a pressure pot, please don't use an old pressure cooker. I've
read about people using them but I've also read that it's VERY dangerous.
They're designed for relatively low pressure. I think it's better to spend a
little extra for a container with guaranteed ratings for safety than to spend
a lot on medical bills if the thing explodes in your face. One example of a
pressure pot is a paint tank.
http://reviews.sears.com/2328/00916102000P/reviews.htm
Essentially, you fill your mold with resin and then place it in the
container and pressurize it while the resin cures.
The pressure doesn't force the bubbles out of the resin. It just
squeezes the bubbles in your resin so they are smaller. Once the resin has
hardened, the bubbles aren't allowed to return to their normal size. In terms
of how much smaller, I think the size is cut in half for every 30 psi of
pressure. Look up scuba diving information as it's the same thing that happens
to the air in your lungs when you dive.
One other thing to note about using a pressure pot. The air bubbles
in your silicone are subjected to the same pressure as the ones in the resin.
I've read articles that say this can cause deformation of the mold and gives a
cottage cheese effect. I don't know if curing your mold in the pressure pot is
an effective way of getting around this problem.

I did a google search for "resin casting". There are sites which may
help you.
The alumilite web site has some how-to's including one for pressure
casting.
http://www.alumilite.com/howtos.cfm

The starship modeler web site has a bunch of articles on molding
and casting. Start here and choose "Molds and Casting" from the Search for
tips on selector:
http://www.starshipmodeler.com/projects/jw_resin.htm
http://www.starshipmodeler.com/tech/techmain.htm

Good luck :-).

David

Hey David
Thanks for taking the time to fill us in on some of the ins and outs of casting... I for one found it extremely interesting. I have been sculpting 1:35 figures for a while, and having a casting kit at home this has really given me the impetus (I think! )
Brad

Yep, I use the Smooth On that has the potlife of about 7 min..... It's a good medium between pouring, getting it in the pot & sealed and pressurized... and not having a huge full cure time.

I have Read that some folks use the HF pressure pot also.

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93119

$54.00 as compared to $89.00.

I guess as it can take the same pressure what is the difference except $35.00

Incredible information!! Great job guys!!
I did not know that Alumilite set so fast. When I am done with my current supply I will def use Smoothon from now on. Alum does indeed set way too fast.
I have some mold I made for things such as barrels and other resin stowage that are okay with shorter setting times but they too show some bubbling on the surface.
One thing to remember is that the mold should be put in the microwave for one minute to heat up before the resin is poured into it. This helps alot but I have just ordered a pressure vessel off ebay and cannot wait to try it out.
For those of you who are interested in finding one on the internet......look up "dental supply" on ebay or as a general search word. It seems that these items are used all the time by dental crown labs and so forth. Robert

would vibraion get the resin to "sit" in the mold easier , like holding the mold and rest it on a running airbrush compressor ?

I myself do not de-air my RTV. I pour mine verrry slowly which will result in a mold with no air bubbles. Agitating the mold on a vibrating table also helps with the bubble issue.
As far as resins go (such as Alumilite) it helps to mix it cold. I usually refrigerate the "A"
bottle, which helps extend its working time to about 4.5 minutes. I only use this material
when making parts such as rifles and blast shields due to its thickness.
I use a resin from a company called Vagabond (36xxx) for more detailed items. It pours like water and has a fairly good working time of about 4-5 minutes. I also use a pressure pot
to debubble my poured resin and help the resin cure quicker.

Well, the pressure pot came in and I made new molds with Smoothon this time. And I bought the Smoothon casting resin as well.
First of all, the pot life of Smoothon on is much less than seven minutes. It is more like two,......at the most. Anyway, using Smoothon for both mold and resin the results were only marginally better than Alumilite. The surface of the cured resin WAS smoother with Smoothon. There is not doubt about that.....either using a pressure pot or not. In fact, I would venture to say from my exper that the pressure vessel was not the better of the two.
However, when casting heads (35th scale) and limbs....no dice. Not really any noticeable improvement when using a pressure pot. Still had air in the extremities. Arms would form all the way down to the end of the fist and no farther. Used small rods plunged down into the cavities and still had air.
One thing that I did notice,....Smoothon has the tendency to be less consistent when compared with Alumilite regarding state of solidification. Probably one out of every four pourings were not completely solidified and exuded a "gummy, sticky" texture after curing with Smoothon. Alumilite, to me, is more dependable in that area but again, Smoothon definitely has a smoother surface.

(Keeping mind that your cast will only be as good as your mold)

I have been experiencing similar problems with bubbles in my castings. As soon as I bought a pressure pot, the problems were eliminated. I run it up to 35-40 lbs and the castings come out perfect every time. This particular project I'm currently doing has very fine details and thin walls. I'm using MicroMark's CR-600 resin (www.micromark.com).

A gummy, sticky texture can often mean moisture in the air (do you have a moisture trap on your air compressor, or is it humid where you live?). The other possibility is that too small a volume of resin is being mixed. When I finally read the directions that came with the resin (!) I learned why my castings were sticky! This is apparently because an inadequate amount of resin will not generate a sufficient amount of heat (I'm no chemist, I'm just parroting other sources). Another cause of gumminess is pouring into cold molds, as this will also inhibit the necessary heat reaction. I learned all of these the hard way! You can easily preheat your molds with a small electric heating pad.

Can we see a photo of your molds? Maybe the bubble issue is in how they're laid out, and where the vents are attached.

Cordially-
Mark

Temperature can play havoc with your pour ups. My workshop averages 72 degrees year round. My molds and resins are used at room temperature (unless its Alumilite) and I don't have any issues unless I get lazy with my technique. As far as the resin goes, how do you pour it into the mold? I use a monoject syringe, and that works way better than pouring it from a cup. Using a mold release or debubblelizer may work, but I don't bother with it myself. Using a stick or rod may help, but sometimes it just creates more bubbles. vents may help, but are unnecessary. my pressure pot is pressurized to 25 psi and that seems to work fine. I also noticed you live in Texas. too dry or too much humidty. a double edged sword there.

Dust your molds with talcum powder, I use plain old baby powder, to break surface tension and help the resin flow into small details.

Sticky parts!
Your mold may not be fully cured. Some resins are not compatable with certain rubbers unless you bake the molds to fully cure them. I use a tin can and 200watt bulb as an oven to cure new molds. I use to wait 24 hrs after removing the master before using it. Now I just bake the molds for a few hours at 140+ degrees before casting parts.

Poor or uneven resin mixes can be a cause of sticky or soft parts. Shake each resin part well before mixing together. Stir well including scraping the bottom and sides of the mixing cup.

Or maybe the resin has just gotten to old.



Joe

Thank you for that. Great photos and information!!! One question I have is regarding the syringe.....I take it that you can only use them once? I am sure that the resin solidifies making them useless with each pouring? Also, your point about the Talc....I have been curious about using it......you are the first one to mention it I believe.
Amazing setup there as well!