Tools Pressurized Casting, my current adventure.

Yesterday I did my first pressurized cast and to celebrate this milestone, I thought I would share some of this here.

This is not a tutorial, I am new to this equipment and I am still figuring it out. Don't trust me on anything without verifying it independently and please correct me if I get things wrong. Advice from experienced casters are especially welcome!

How I got here and why it's taken so long is a long story. I started this process years ago.
But the short version of why I took this step now is that my old resin supplier went out of business and I have not been able to find any substitute that will cast as well without pressure as that resin did.
Many false starts, shipping issues, products arriving damaged and miscellaneous other misadventures later, I finally have my air pump, vacuum chamber, compressor and pressure pot set up and working!
I got my vacuum set up done a couple of months ago and I have sort of familiarized myself with it. As it uses negative pressure, it's not as scary as the positive pressure equipment. At least, that's how it feels to me.
I don't have much experience with air compressors and to be perfectly honest, I am terrified.
Hopefully this will wear off, as I get used to it.

Here is my current pressure setup.

You have no idea how big of a victory this is to me. It's exciting, but also terrifying.
The last few days can be summarized like this:


Yesterday I made my very first test cast using the new equipment.

The book on the right is cast straight without pressure at an ambient humidity of about 27%. (my old resin could have handled that and I am still bitter) and you can clearly see the microbubbles. But look at the left book, not a single bubble! And this is despite a much more saturated color. (I am really trying to not get ahead of myself here, I know I still have some way to go before casting full dolls in this thing, but I can't help but seeing the possibilities of much more interesting skin colors in the future. Oh, I hope I hope I hope! )
The small nubby imperfections you see on it is simply because this mold was made before I had my vacuum chamber for degassing the silicone, so there are some bubbles in there. This is less on an issue when casting small parts without pressure.
But those little nubbs can be shaved off and there will still be a perfectly usable cast underneath, so this mold will live out it's natural life making casts all the same, and next time I remake the mold it has a very good potential to make flawless casts!

There are still a lot of things to figure out and problems to solve.
* The pressure chamber is easy to use, but it is rather small, so I may need to plan my future molds so that I can fit as many as possible in the chamber at once.
* I may need to cast smaller batches with longer wait times between, compared to what I am used to, so some new routines for mixing pigments and general work flow may need to be worked out. I am hoping the pressure can negate the effects of the longer exposure to air that the resin will be subjected to.
* A good substitute for the drinking straws I've been using as channel rods needs to be figured out. Without pressure they work spectacularly well, but with pressure I can't have anything hollow in the mold. So I need to find something that can still be easily cut to measure, doesn't interfere with the curing and that can be easily slid out of the cast afterwards, preferable without the need for a release agent as that always introduces a lot of extra work. Single use or reusable doesn't matter as long as there isn't too much prep work to set up each cast. there are many parts to a doll and it adds up fast Any suggestions are most welcome!

Then starts the slow, expensive and tedious process of remaking my doll molds with the new equipment in mind. There is a lot of work ahead, but this step needed to be taken sooner or later and I am very happy to have come this far, even if I wish some of the circumstanced would have been different.

Wish me luck, guys. I'm going to need it.

 

Wonderful. I'd be happy to see what you make if you feel like making a project journal here, when you get started!

As a friend used to say; "I rather risk a dumb question than a dumb mistake." Never feel you need to apologize for asking something.
The simple answer is that the pressure pot is there to crush out the bubbles as they form while the resin is curing. The air inside the straw is basically one big bubble. The straw will collapse and the air inside will be compressed, or possibly escape through a vent, depending on how fast things cure. Anyway, the straw will not leave the perfect smooth channel that it is meant to.

A hollow steel tube will probably not collapse, but you would still have two problems to solve.
First is that there still would be an air pocket inside the mold and the pressure would, if not collapse the tube, then at least compress the air and force some of the resin into the tube and you may end up with a place holder that is very difficult to remove and possibly a starved cast.
The second problem is that steel contains sulfur, that can inhibit the curing of the resin. You could possibly get around this by a heavy coating of release wax, but that would add a significant bit to you set up time, since it would need redoing every single time you used it.

A solid plastic rod may work, but could possibly get stuck. And you'd have to make sure it's not a thermoplastic, because the resin can get quite warm while curing and if it's a plastic with a low melting point it may get permanently fused.
If you feel like experimenting, please let us know how it goes. New methods are always interesting.

I am not an expert by any means, but so far I've gotten the best and most reliable results by using solid brass rods with a diameter that just fits inside the plastic straw. I've tried with just the rods, but they are very difficult to remove, no matter what release agent I've used. So by putting the rod inside a straw I only have to push the rod out of the straw and then I can collapse and pull the straw out with pliers, the same way I would when not using pressure. Even if a little bit of resin has seeped in between the rod and the straw, it can be easily pealed off once the rod is out. So far it seem to be working fine, even without any release on the rods.
The rods can be reused over and over and set up is not much slower compared to using only straws.

Take this for what it's worth. I'm still a beginner an many ways, solving things as I get to them.

 

Hehe... Polyurethane, while having some wonderful properties when cured, is very temperamental in it's resin form. There are so many things it just doesn't like.
But as a doll material it's absolutely wonderful, so what can you do? At least it won't burst into flames if mixed improperly, like some resins might. We have to take our victories where we can find them.

 

@Leenah
I look forward to see what you make! A gargoyle would be fabulous.

And thank you for your kind words. I'm in a bit of a creative slump right now and it means a lot to hear that something I've done is appreciated! Thank you.