The use of castable resins, or cold mounting plastics, has been increasing its popularity for more than 20 years now. These resins offer certain advantages over the force molding resins, and have properties, which expand the mounting capabilities of some laboratory because this is very important for Metallographic analysis.
These resins are usually two component systems, liquid to liquid, liquid to solid, or solid to solid in which must be melted so it can be used. One section is the resin and the other factor is variously known as the hardener, activator, or catalyst. Resins available for cold mounting applications include polyesters, polystyrenes, acrylics, and epoxies.
The abovementioned resins are usually viewed under metallographic microscopes for this purpose.
The castable resins are being ready for use by carefully mixing the required amount of resin and hardener. Pot life of the mixture will vary from a few minutes to many hours, depending on resin type and temperature of storage if the resin is not to be use immediately. Mounts are prepared by placing the sample into a re-usable mold, and then filling the mold with the resin. The mount is cured at room temperature or slightly above. This kind of experiment is commonly done in a laboratory and always being done with the metallographic microscopes available.
A lot of choices for the materials are used for molds, including glass, aluminum, brass, steel, Teflon (used for kitchen ware), and silicone rubber and the surface of the materials can be checked under metallographic microscopes. Mold release agents such as vacuum grease or silicone oil are often required, depending on the mold material and the type of resin. Epoxides stick to most materials, so epoxy mounts are surely the most difficult to remove from the mold. On the other hand, silicone rubber molds have proved very handy to use with the epoxies as well as the other castable resin. Even if molds are usually made in the same variety of standard sizes used for pressure mounts, the size and shape of the mold and resulting mount are limited only by the capabilities of the grinding and polishing tools on hand.
The castable resins have a broad range of usage in situations where pressure mounting is impossible, not convenient, or unproductive. The mounting of very easily broken or weak samples is easily accomplished with castable resins. Vacuum impregnation is often used to improve mount quality for these specimens. Several authors have reported on apparatus and techniques to accomplish vacuum impregnation, but the simplest method is probably to use a small chamber such as an oven connected to a mechanical vacuum pump. Doing it again after successive grinding steps is sometimes necessary to assure specimen reliability or to get rid of scratches or empty spaces on the mount surface.
Somewhat low curing stresses for the castable resins are a benefit in mounting delicate specimens, if you want to check delicate specimens, you can always view it with your metallographic microscopes. On the other hand, some specimens, because of their geometry, may be destroyed by the curing stresses that are present. Some Metallurgists Crouse and Gray investigated brazed T-joints which cracked after being mounted in castable resins. These Metallurgists were able to duplicate the cracks using a number of different resins, and attributed the defects to the curing stresses. To alleviate cracking in the joint, the specimens were nickel-plated prior to mounting. The joints did not crack, but the nickel plating was separated from the specimens, leaving an undesirable gap, the gaps can be seen clearly when you view it under metallographic microscopes. The addition of palletized alumina to the resins was effective in dropping the stress on the sample while maintaining reliability of the mount-to-specimen edge.
Castable resins have found wide use in the nuclear field, where the simple casting techniques are easily adaptable to hot cell or glovebox work. Some Authors have reported a method for minimizing the quantity of resin which must be cured inside a glovebox. They make a pre mold outside the glovebox which leaves a small conical or pyramidal cavity which is consequently filled inside the glovebox with the sample and the small amount of uncured resin. This removes the use of dies inside the glovebox and lessens contamination of the box atmosphere by gas released during curing.
The castable resins are also convenient to use when it is desired to alter the mount properties by addition of a filler material. Ground glass has been reported as effective filler and recent interest has centered on various grades of palletized alumina. Fillers are very effective in improving abrasion and polishing characteristics, which are not especially good for the cold mounting plastics.
The epoxides show signs of many characteristics which make them excellent resins for use in castable mounts. They stick well to most other materials, therefore, they need not rely on differential shrinkage to grip the specimen. Of the castable resins, they have the lowest shrinkage factor. Some epoxies cure rapidly, but others require a much longer curing time than the other castable resins. However, this is a definite advantage when vacuum impregnation is necessary.
Curing can be made very fast by heating to about 60-70 degree Celsius. They can also use a radio frequency oven to speed up curing has also been reported. Some epoxies can be stored under refrigeration or cold temperature for several weeks after hardener and resin have been mixed. The excellent bonding characteristic of the epoxies makes them practical to use for double mounting or re-impregnation techniques. Some epoxies show excellent transparency. Chemical resistance of the epoxies is very good except in concentrated nitric or glacial acetic acid. The epoxies are sensitive to slight variations in the resin-to-hardener ratio. The resin and hardener can be purchased in pre-measured packets to minimize mixing problems. Some systems are toxic or may cause skin irritation, so adequate ventilation is required and skin contact should be avoided.
The main advantage of the acrylics is their rapid cure rate. As a result, they do posses a relatively high exotherm, which may be disagreeable. The curing exotherm may be controlled somewhat by mount thickness. Also because of their short curing time, they are not very applicable to vacuum impregnation. They show the highest shrinkage of the castable resins. Acrylic mounts may be clear to opaque. The acrylics are attacked by strong acids and are somewhat soluble in some organic solvents. They are probably less sensitive than the epoxies to variations in the mixture ratio. Breathing of the vapors or prolonged skin contact with the resins should be avoided.
The polyester resins exhibit less volume shrinkage than the acrylics, but they do rely on shrinkage to grip the specimen. The polyesters cure fairly rapidly, but generally have longer curing times than the acrylics. They should therefore be more applicable to vacuum impregnation than the acrylics. They are not very sensitive to slight variations in the mixture. They show good chemical resistance to the normal metallographic reagents.
Probably the biggest advantage of the various castable resins is their versatility. By use of appropriate filler materials, the mount can be customized to the sample. Also, a wider variety of specimens, especially fragile or delicate items, can be mounted with castable resins than can be with the pressure-mounting techniques. On the other hand, the casting plastics must be measured fairly accurately, and generally require a longer curing time than the pressure-molding resins. They also exhibit relatively high shrinkage, which must be compensated for when mounting some types of specimens. The castable resins probably present a greater health hazard due to their vapors and possible skin irritant effects. On the other hand, these dangers are not great if you will just be very careful in handling these specimens.