I've cast some parts using MicroMark CR-600 but the end product is non-sandable. Any suggestions?

From the MM website:

"Our water-thin urethane casting resins mix and pour easily, resulting in perfectly formed, bubble-free castings. Just mix equal volumes of A and B together; the resin is odorless, gives excellent reproduction quality, and can be painted, sanded, sawn, tapped, milled, filed and drilled easily. Cures to white color. Compatible with all mold materials, including latex, clay and RTV silicone. Also acts as a super-strong glue for bonding metal, softwood and plastic. Resists fuels, acids and most chemicals when cured, is not tacky, and has no oily coating after curing like other casting resins. Includes 16 fl. oz. each of parts A and B in dispenser bottles and complete instructions."

http://www.micromark.com/CR-600-CASTING-RESIN-32-FL-OZ,7879.html

Try using Micro Mark CR-300, or Alumilite resin. Both work very well for me.

Jesse

Thanks. When I try and sand the 600 either nothing happens or it gets mushy.

Sounds as if the chemical reaction isn't kicking off properly. This can happen if you're mixing tiny amounts at a time. If so, it won't generate sufficient heat. The usual result of this is mushy or gummy parts. Try mixing larger amounts. Another possibility is that the temperature you are working in is too cold.

I'd second Mark's comments. Sounds like the ratio was off slighly. Using graduated cups and a scale helps quite a bit when trying to mix resins.

Hi Patience,

I've made a casting or two in my time so I thought I'd offer my advice as well. First off, everyone has very good suggestions. If you are mixing under an ounce of resin I usually prefer to use a small disposable hobby paintbrush for very good mixing. These are usually a couple dollars for 30 at Wal-Mart.

If you have tried multiple types of resin and still get mushy castings then it could also be the mold itself. Silicones are bad for picking up moisture in the air and the result is an incomplete cure where the resin and rubber meet. Smooth-on has some excellent advice on "post curing" (baking slightly at low temp) to remove any moisture and cure this type of problem. Look under Casting and Mold making sections on the Smooth-On website for exact details.

Despite the very low humidity of Alaska in general I have to post cure all my molds due to creating them in lower than ideal temperatures. This may be what you are experiencing.

If you or anyone else has any questions on casting please feel free to ask. I've already made every mistake imaginable and have probably invented a few by now.

Jim
Large Scale Armory