C) EpoxyResin.
Are you using Tap Epoxy?
1) —I've used Tap Plastic's 4-to-1 epoxy resin for casting small stuff, and the casts have been very detailed and durable. TAP have renamed it as Super Hard Four-to-One, and it's at http://www.tapplastics.com/shop/product.php?pid=29&. They claim that shrinkage is "close to zero;" does anyone have any actual numbers or experiences to the contrary?
As for casting, again since the specimens are small and detail is important, I'm looking to avoid polyesters that shrink. In the past, I don't know what the shrinkage is.
2) For making casts, better to go with an epoxy resin. We use is "4 to 1 Epoxy Resin" from TAP Plastics, although there are many suitable epoxy resins available.
3) —Hello! TAP still markets their 4-1 epoxy, we use it all the time for casting up pretty much everything from latex and silicone molds in our lab at SDSM&T. We've had great results with it too. It beats the squeezable stuff in a syringe and the way-too-expensive stuff from the hobby shops any day. Good luck!
4)— I use either epoxy resin (although this is not the brand that I use, they have a lot of information on their website) https://www.polytek.com/store/merchant.mvc?Screen=CTGY&Category_Code=008 or polyester resin, either can be made opaque. Or you can get polyurethane resins which are often filled and so opaque. https://www.polytek.com/store/merchant.mvc?Screen=CTGY&Category_Code=006.
Lamb is right on the money here. The polyester resins are much nastier, and do tend to off-gas perpetually. We like the two-part urethane resin (Insta-cast) from Douglass & Sturgis also. It is easy to use, cuts and cleans up well, and can be easily pigmented if you like.
Hobby Resins.
"Clear casting resins" are probably not the best thing for your application. They are usually hobby-grade polyester resins intended for making paperweight embedments, etc. Polyester resin has considerable shrinkage which obviously can adversely affect cast dimensions. Shrinkage can also affect surface detail since polyester resin may cure and shrink internally before the surface against the mold cures, especially on large, solid casts.
3) Another consideration besides the already noted severe shrinkage, is that the clear hobby shop casting resins, being polyesters (the stuff used in “fiberglass” boat hulls) off gas some fairly noxious fumes, and they do this for months. Fumes even from the uncured resin are not liver and kidney friendly. If you get too much catalyst in your mix, the shrinkage is even more pronounced, and I have seen the cast begin shrinking before cure is complete, so it draws away from the mold before it has all the detail recorded in the cast.
Urethane. You also might consider using a two-part urethane casting plastic. Equal volumes part A and B, sets up in about 10 minutes. Comes in an off-white color and is opaque., and hard to screw up. It’s also available from Douglas and Sturgess. The urethanes off gas cyanoacrylate fumes, but only for a few minutes and then they are done, and pretty much odorless forever after. They do set up heat during cure, but nearly as much as the epoxies. The only rub is that silicon doesn’t always hold up well when casting urethanes into them. If you’re only making one or two casts that isn’t a problem. There is a company in North Carolina that sells a brown casting urethane – I can find the address if you’re interested.
(On “urethane rubber” not casting plastic) But I know that the last guy who posted to the list suggested urethane rubber. Our experience here is “Don’t use urethane rubber”. My tech used it for some footprint moulds from a series of prints. Inside a year, the moulds were wet with whatever was leaching out of the stuff, making the moulds totally useless. A look at the fine print doesn’t tell you that the shelf life of the mixed product isn’t what it should be. We threw the moulds out, only to find that the company (Smooth-On) had changed the formulation because the shelf life was poor [understatement]. Other urethane stuff we used some time ago hasn’t gone soupy, but it doesn’t retain its shape without having something to fill out the mould, such as an extra cast.
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