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April 17, 2006
toxic cupcakes
by sven at 9:25 pm
During the past two months I've purchased several new-to-me art materials: ultra-cal 30, RTV silicone, polyurethane casting resin, glass microspheres, and "self-skinning flexible expanding urethane foam". On Saturday I finally had the opportunity to play with all this stuff. Doing small experiments in paper drinking cups, I now have a bunch of odd castings that look like... toxic cupcakes.
What do I plan to do with all these materials? Well, they'll allow me to make molds and castings with a precision that I've never achieved before. So as an artist, I'm interested in them just for the sake of having more techniques at my disposal. However, there are two projects that they may apply to immediately:
Project #1: I want to try making a stopmo puppet with a body cast in foam. Foam latex is the standard for this application. However, foam latex has several problems: it involves 4 chemical components; it requires a dedicated kitchen mixer and oven; it is very vulnerable to temperature/humidity/barometric pressure; and it starts to rot after a few years. ...Contrast this with urethane foam: it only has two components, cures at room temperature, and doesn't rot. --Worth a shot, don'tcha think?
Project #2: I like puppets with heads that are very large in proportion to their bodies. How do I make heads that are light enough so that the puppet won't tip over? Mike Brent suggested that I should try casting my puppet heads in resin. You can get a filler material (such as glass micro-spheres) that will lighten the casting significantly... And as an additional benefit: If you're doing castings, when a puppet breaks you can just cast another!
These materials give off toxic fumes, so I have to work with them outside. However, this is Oregon... It was rainy and windy on Saturday -- so I wound up working on the front porch, manipulating the chemicals inside of cardboard boxes that served as wind-breaks. It worked adequately; but boy do I wish we had a covered driveway!
Please refer to the photo above...
FRONT LEFT: RESIN
The front two "cupcakes" are polyurethane resin. This resin has you mixing part A and part B in a 1:1 ratio by volume, but allows for 5-10% error. For the cupcake on the left, I measured out part A and part B by weight (oops) on my new lab scale. It cured in roughly 20 minutes... It looked sort of like liquid lard congealing, a white clot appearing in the clear fluid. I was disappointed to see tiny bubbles over much of the casting's surface.
FRONT RIGHT: RESIN WITH MICROSPHERES
For the resin cupcake on the right, I tried eyeballing the measurements, and added in the glass microspheres as filler. The bottom-most part of the casting didn't seem as if it was going to cure -- which I attribute to the poor measurements... But the next day, I was surprised to find that the remaining material did finally harden. Conclusion: I prefer not to eyeball measurements. ...The microspheres definitely lightened the material -- and they also smoothed out the surface a great deal. Yay microspheres!
Next steps for exploring resin: The bottles "glug" when I pour them -- I need to see if I can find some screw-on spigots for more precise pouring. I tried painting the resin with acrylic -- but even dry, it wiped off easily. I suspect that resin requires enamel based paints. However, I may be able to use an enamel-based spray primer, over which I could paint acrylics. That, I believe, is how wargamers paint their plastic and metal miniatures -- so I'll have to pick some up at Bridgetown Hobby. ...I'm also interested in trying out some black pigment that gets mixed directly into the resin.
BACK FAR-LEFT: URETHANE FOAM
In the back row on the left is my first experiment with the expanding urethane foam. I was really surprised to discover that it cures in only about six minutes. There were gooey uncured areas at the bottom of the cup, however; I don't know what to make of that. The material feels like a Nerf football -- only a bit stiffer.
BACK MIDDLE-LEFT: URETHANE FOAM ON ULTRA-CAL
The next cupcake is an experiment I did to see if the urethane would stick to ultra-cal 30. I knew that urethane foam has a reputation for adhering to anything -- but I hoped against hope that the self-skinning variety would be different. No luck. This was my first batch of ultra-cal... I eyeballed how much water to add, and it turned out fine. It took a good deal longer to set-up than I expected, though -- around half an hour.
I checked The Prop Builder's Molding & Casting Handbook to see what mold-release it recommended for making urethane foam casts in a plaster mold. It said that PVA (white glue?) could work, but is not recommended. Instead, urethane foam should be cast in silicone molds. This leads us to the pretty blue cupcake...
BACK MIDDLE-RIGHT: SILICONE MOLD, PAINTED FOAM CASTING
The blue cupcake is made of silicone. Silicone is the gooiest stuff I've ever encountered: like wet peanut butter mixed with caulking. I wanted to actually try making a mold with this stuff, so I dangled a wooden sphere from a wire into the paper cup, and then poured in the silicone around it. I was worried that the cold would interfere with curing -- but it seemed to be cured after only six hours, which was listed as the minimum set time. I cut open the silicone with a razor, and bundled it back together with a rubber band. I used the hole where the wire had been as a pour hole, and made an impromptu paper funnel to help channel in the urethane...
The casting was OK, but not great. Air got trapped in the mold, so my casting wasn't perfectly spherical. I guess I need a second hole, so air can escape. The expanding foam was under pressure inside the mold, so it wound up being a good deal denser than I'd like. The surface is relatively smooth, but has pitting that looks like the pores in the skin of your nose. That pretty much terminates my fantasy of doing urethane foam puppets without latex skins. Drat.
One nice thing I'll say about the urethane though: It's really easy to paint. The sphere looks like a cherry because I painted it with red acrylic. The paint doesn't rub or peel off; and it's so flexible, I question whether you'd even need PAX paint if you were doing an all-urethane puppet.
BACK FAR-RIGHT: LIQUID LATEX ON UNTRA-CAL
The cupcake on the far right is another ultra-cal test. I did a quick google on the material, and discovered that 38 parts water for every 100 parts ultra-cal is recommended. This batch seemed to have a little water left over; now that I think about it, I wonder if it's because I measured by weight instead of volume? ...Even so, it set up. I tried painting some liquid latex on top; latex (unlike the urethane) comes off easily.
Conclusion: It looks like my next puppet will be made from flexible urethane foam with a latex skin, all cast in an ultra-cal 30 mold. The foam seems to be fluffiest when it has ample room to escape -- so I'll need to make vent holes in the puppet's feet. I have some Pro Adhesive (a Prosaide knock-off) for making PAX paint...
So I guess at this point I have everything I need to get started!!
posted by sven | April 17, 2006 9:25 PM | comments (5) | categories: sculpture, stopmo
Comments
Glad to see you messing with new toxic stuff! A few things I thought I should pass along....
Resin reacts badly to any moisture or cold, so you should cast it indoors if the whether is bad. Didn't you just get an arsenal of cleanroom devices for this very purpose? Shame to see them all go to waste while you work out on the porch and destroy all your casting attempts! Moisture will cause tiny bubbles all over the place, and cold will retard the cure time. In fact, I learned that you can make expanding resin foam by adding a little water to it. Not very controllable though. Also, for a 1:1 mix resin you don't want to mix by weight, you want to eyeball it. Just get two see-through cups of exactly the same dimensions/shape (plastic dixie cups) and get down so you can see the levels. It's never failed for me.
Resin must be cleaned before painting, to remove mold release residues. The resin contains mold release agents, which ooze out of the resin even after it's cured for a while (not sure how long). You need to clean them up with dish detergent or alcohol (or both). Acrylic paint works great on resin, better with primer of course, but for real toughness I use Alkyds, a synthetic oil-based paint that acts like oils but dries faster (not as fast as acrylics). Oils or enamels etc would also be much tougher than acrylics.
You poured urethane through a pour hole no bigger than a wire??!!! Insane! Nor5mally you'd build your mold with a built-in pour spout made from Kleen Klay or something, a funnel-shape to facilitate pouring and also big enough to let air escape through the same hole.
For mixing ultra-cal, you don't need to really weigh it out. Most people who use a gypsum material regularly (learned this from Nick among others) use the mud-flats method.... start with the water and slowly add powder until it comes up to the surface and forms something looking like mud flats. You can have one hand immersed and keep kneading out lumps/mixing as you go. You can tell when it's mixed right by sight/feel (gets easier with experience).
But I don't know much about it... not sure I got all this right, so best to seek out tutorials on plaster mixing. It's the same for pretty much any gypsum, ultra-cal or plaster.
Posted by: Darkstrider at April 18, 2006 2:50 PM
Advice form master plasterer Nick Hilligoss, pulled directly from the message board:
"I worked in a plaster factory as a mouldmaker for 18 months, and you don't really mix plaster by measuring. You put some water in a bucket (half full or less), then sprinkle in plaster untill it builds up an island above the water surface. Let it soak through a little, then mix it with your hand, squeezing out lumps until it's smooth and creamy. If you can see your skin colour through it on your hand, it's too thin, add some more right away. You're not really supposed to add the water to the plaster, but if it's too thick you can add some right away. (But not if it's thickening up later from chemical action, that's what it's supposed to do and adding more water will weaken it.) With practice you get a feel for it. There's a lot of leeway here on how thick it can be and still work, but it should still pour so it will get into all the details of the sculpt. (A long time ago, I couldn't get some plaster to set, I thought I was getting the mix wrong. It turned out the plaster was no good. Now I know it's quite difficult to get the mix so wrong it doesn't set at all!) Put a layer on to the sculpt. If there is no accellerator added, it will take about 15 minutes to set. Wait 10 minutes or until it shows signs of thickening, then mix some more plaster. If there is a little of the first batch left, you can add water and fresh plaster to it, and the old batch will speed up the setting of the new batch. Or, if you wash your hands in a bucket, keep some of the thin milky slurry that settles in the bottom, and add just a little (say a tablespoon to half a bucket of water)to the new plaster to speed it up. I keep some slurry, so I can speed up the first batch. The trick with layers is to add the new layer as the old one is thickening up but not quite set, that way they bond together and don't flake off. Fibreglass matting needs to be split into 2 or 3 layers so the plaster penetrates easily. Or you can tear it into bits and mix it into the plaster - but not in the first 2 layers, or you'll get hairy bits on the surface of the mould. RELEASE: Believe it or not, if you get a bar of plain soap and shave it into hot water, you get a good release agent for foam latex. Brush it on the mould, 2 or 3 coats the first time. Some will soak in, some will froth a little on the surface but settle down. But my GM foam kits come with a little GM release agent included!"
Posted by: Darkstrider at April 18, 2006 3:56 PM
'Nuther good thread on mixing gypsums, with emphasis on the ratio: Ultracal 30
Posted by: Darkstrider at April 18, 2006 4:02 PM
Ohmygosh!! ...Thanks for digging up the ultra-cal advice, Mike -- I really appreciate it!
I've worked with regular plaster in the past, and didn't remember it as being too fussy -- I wasn't sure with this other fancy gypsum product if I'd have to get precise. You confirm what I was sort of sensing: that ultra-cal really isn't very fussy either.
Regarding pouring the urethane foam through a wire-sized hole... You've caught me being sloppy. The hole was actually 1/4" wide. What I really did was drill a hole into the wooden sphere, then glue a dowel into that; I drilled a tiny hole at the other end of the dowel and put a piece of wire through. I basically hung this wooden lollipop from the wire cross-bar. ...I didn't think anyone would follow this explanation -- so I fudged and just said I dangled the sphere by a wire. My bad.
Thanks also for the info about resin's reactions with water and how it oozes out mold release! ...You're probably right that I can work with the stuff indoors. This being my first time, I didn't know how bad the fumes were going to be. Turns out they weren't bad at all -- so the Xytronic ventilation system ought to do just fine.
Posted by: sven at April 19, 2006 9:50 PM
Hiya
The PVA isn't glue it's a release agent in it's own right. Kinda threw me too when I first heard about it. Anyway googling reveals all http://www.google.co.uk/search?hl=en&safe=off&q=pva+release&btnG=Search&meta=
Posted by: ShadyCraig at October 17, 2006 4:14 PM