What is your favorite "brand" of Casting Resin?

[Major Weenie]

Oh,

There's always "Hooverite" casting medium.  The guy who 'invented' it had the last name of Hoover.  Years ago he was talking with the, very old & experienced, guy at Silpak and the subject of Bondo came up.  For those outside of the US, Bondo is a trade name for a brand of automobile body putty.  Anyway, Lee was incensed that Bondo was selling a thick mixture of ceramic powder and fiber glass resin as if it was some sort of miracle product.  We asked it we could thin out Bondo with fiber glass resin, and he said sure.

So if you have a simple open-faced, drop mould, and the detail isn't too tiny... You might try casting up your product in Hooverite.

It sets more slowly and depending upon  how much Bondo/ceramic is in your mix, and can be sanded.

Buy a can of Bondo, and pour in a pint or so of fiber glass resin.  Stir it a bit, then let is set for a day or two.  When you want to use it, spoon it out with a paper cup (so you can throw the cup away after wards), pour it into another paper cup (so the sides are clean), squirt in the fiber glass catalyst and a bit of the Bondo cream hardener and mix/stir with a art stick. (One of those wooden sticks used in American Popsicles.  I the UK they used to be called ice lollies.)  The catalyst is what makes it set up, the cream hardener is what lets you see if you've mixed it thoroughly.  The catalyst is clear, but the cream hardener is red.  When the mixture is a light pink it's ready to pour.

After you've filled the mould it sets up faster in a warm location, out of UV (sunlight).

The product can be solid, or if you over catalyze you can swirl it around in the mould until it kicks.  This leaves a relatively ugly, but open, center on the piece.

Hope this helps,
MW

[Connectamabob]

The trouble with vac chambers and urathane is the short pot life. I cant seem to get decent casts with urathane, its either goes off too quick or else I dont vac it for long enough.

I use polyester resin. Much cheaper and kinder on the moulds, but apparently more toxic (while curing). It has a 20 min pot life and 1 to 2 hour demould time. Its any colour you want when you add dye to it.

Who's your supplier, and do you have any other suppliers available? I ask because it's strange to hear pot life being cited as a categorical disadvantage to polyurethane: most resin companies here in the US (like Smooth-On, Tap Plastics, Alumilite, etc.) make wide ranges of different urethane formulas with different properties, including widely varying pot lives. All these companies also make dyes for their resins as a standard accessory product.

Polyurethanes are also available in much stronger formulas, and cure with less shrinkage than polyesters. Can't argue with the cost angle though: polyester is definitely cheaper.

The only stuff that has brick-and-mortar LHS distributorship around here is Alumilite's quick setting formula, which has a (uselessly short IMO) 90 second pot life, but if I skip the LHS and go over to a plastics supplier instead, I can get a whole range of different Silpak resins (though the sample castings they have on display do not speak well of that brands quality). Skipping brick and mortar altogether and going on line gets me all kinds of options.

I dunno what the UK supplier market is like though. I've heard hints that it's more limited, but not specifics.

Also, when you talk about vacuuming the resin, are you talking about vacuuming prior to pressurizing, or instead of pressurizing? The former should only be necessary with small, thin parts (like figures), provided proper sprue placement. The latter shouldn't really be done at all.

[Connectamabob]

I can remember some time ago that they had two cunning plans for vacuum chambers.  One had to do with using one of those 'vacuum pack your frozen food' products sold in mega-membership stores.  The other was to use compression, rather than vacuum.  Compression would supposedly drive the air bubbles into the center of the product, where they would not interfere with the surface of the model.

The former wouldn't work, as those food vacuum packers can't pull enough mercury to get the bubbles out. Basic rule of thumb is, if it can't boil water at room temperature, it can't degass rubber or resin. Unfortunately this also rules out most of the cheaper options like venturi devices and the like. You're pretty much stuck with having to invest in a proper vacuum pump if you wanna do this kind of casting.

You can, however, use any of those to infuse fruit slices with gin or vodka, provided their flesh is porous (so apples and melons yay, grapes and citrus nay... unless they've been freeze dried first). Heck, you can probably DIY a good water aspirator for less than ten bucks. Science party, whoohoo!

The latter is SOP for production resin casting, both garage and industrial, and has been for a long time. Pressurizing the resin while in the mold forces larger gas bubbles to be expelled, and smaller ones to be crushed. Vacuuming resin is usually only done with small, thin parts where the potential air pockets may be too big relative to the fluid volume to be eliminated by compression alone. In those cases, the resin is first vacuumed while in the mold, then pressurized. In all cases, pressure (about 2 atmospheres or 35 PSI is all you really need) is maintained until the resin has cured enough to demold, to ensure that those crushed bubbles stay crushed forever.

Vacuuming by itself can be done, but pressurizing produces better quality casings, and allows for the use of shorter pot life resins for faster turnaround.

You want to make sure your rubber was vacuum degassed (not pressure degassed) when you were making the molds, otherwise air bubbles in the rubber will either crush or expand to create pits or warts on the castings.

The Bondo/fiberglassing resin mix is great for making "mother mold" shells for large molds. It's thicker & therefore heavier than fiberglass laminate, but it's much stronger, lighter, faster, and easier to use (toxicity precautions aside) than plaster.

[Rick]

I cast resin in the uk and have found that if you go to a specialist company (such as http://www.tomps.com/shop/mould-making-c-1.html), they have a variety of different types available and are full of handy tips and tricks to help you get the most out of casting. One trick they recently told me about, which isn't for the faint-hearted, is to add about 5% high purity (about 90% pure) acetone to your resin before you pour - this apparently makes the resin less brittle and you can actually bounce a resin figure off the floor! This is what the new Citadel Finecast resin figures are made with to give them their supposedly "unique properties". I am seriously considering using this idea when I cast! 

[Ramshackle_Curtis]

Who's your supplier, and do you have any other suppliers available? I ask because it's strange to hear pot life being cited as a categorical disadvantage to polyurethane: most resin companies here in the US (like Smooth-On, Tap Plastics, Alumilite, etc.) make wide ranges of different urethane formulas with different properties, including widely varying pot lives. All these companies also make dyes for their resins as a standard accessory product.

Well, my vac chamber only really gets to pressure after about 2 mins. Factor in the mixing/pouring time and you hit the 5 min wall. Combine with a hot day and the whole thing is just a nightmare for me! Id be happy to cast with urathane if it had a 20 min pot life, but I cant seem to.

I do actually have a pressure pot but no pump. Im thinking of investing in one. I have heard the ones for pressuring paint for spraying are ok....

[Connectamabob]

Anything that will get you up to 35psi quickly will do. Check the compressor's CFM (Cubic Feet per Minute) ratings against the volume of your casting chamber to get an idea of how fast it can fill it up. Just make sure you don't get an "oil-less" model: those are made for weekend warrior DIY-ers, and will burn out really quickly under the kind of everyday use you'd be putting yours through.

This goes for vacuum pumps too. If your pump isn't pulling quickly enough (two minutes is pretty bad) that means you got a pump with too low a CFM rating for your chamber size.

Bad news is each step up the CFM ladder will more or less double the price of the pump. Good news is you can work around this by buying a spare pressure tank to use as a buffer.

Get a tank that's twice the volume of your casting chamber, hook it up in-line between the casting chamber and the compressor, with valves at both ends. Charge the tank up to 70psi, then bleed it into the chamber 'till the chamber's pressure gauge reads 35psi. Charging the tank will still take however long is dictated by the compressor's CFM limit vs. the tank volume, but once that's done, the casting chamber can be pressurized in mere seconds from the tank.

Same trick works for vacuuming, sort of. For that you'd want the tank and the chamber hooked up to the pump in parallel rather than in series. Using the tank to void the chamber will only reduce the pressure in the chamber by the volume ratio between the two, so you have to then close the valve between them and use the pump directly as a second stage to suck out the rest. Not as effective a fix as the same trick with compression, but you can at least halve the time it takes to vacuum your chamber for a fraction of the cost of buying a new pump with twice the CFM.

Hope that helps.

[Connectamabob]

Here's a nifty trick I've been wanting to try, but haven't gotten around to yet:
http://scihighmodels.com/howto_shrink.html

Imagine the possibilities!