INTRODUCTION..
The mounting operation accomplishes three main functions;
- It protects the specimen edge and maintains the integrity of a materials surface
features
- It fills voids in porous materials
- Improves handling of irregular shaped samples, especially for automated specimen
preparation.
Methods of mounting for various specimen :
The two main techniques used are hot compression mounting and cold mounting
Depending upon the type of specimen and the number of samples required we can use different types of mounting. Cold mounting is used for specimens that
may be damaged by heat or pressure.Hot mountingis normally quicker and economical.
HOT MOUNTING :
Hot mounting of specimens requires the resin to be heated and cooled under pressure.They are easier to produce and takes only several minutes.The resins are sold at cheaper price than cold mounting resins.
There are two resins available for hot mounting:
a)THERMOSETTING RESINS
b)THERMOPLASTIC RESINS
Thermosetting resins cure at elevated temperatures and to avoid porous and non-uniformity, it is important that the pressure is always constant at the right level.The resin undergoes cross-linking reactions until almost all of the molecules are cross-linked to form three-dimensional networks.After setting,thermoset resins cannot be melted again,and the shape cannot be changed.Examples include the very familiar phenolic plastic,bakelite,diallyl phthalate, and the recently intoduced epoxies such as plastimet or Epomet.
Thermoplastic resins Thermoplastic resins soften at elevated temperatures and solidify again on cooling as no chemical bonding takes place.This characteristic allows thermoplastics to be remolded and recycled without negatively affecting the materials physical properties.Examples include transperent methyl methacrylate(known as lucite or transoptic),polystyrene,polyvinyl chloride (PVC),and polyvinyl formal(Formvar)
Thermoplastic mounting media can be melted again.
COLD MOUNTING :
Cold mounting are prefered for specimens that are sensitive to the heat or pressure.After encasing the sample,a catalyst is used to cure and harden the material.This method is best for laboratories with a small throughput but large quantities of samples and is neccessary for porous,brittle,or heat-sensitive samples.Once mounted,samples can be more easily handled by other metallographic instruments and maintain the integrity of the samples's edges.
There are three types of cold mounting namely:
a) EPOXY RESINS
b) ACRYLICS RESINS
c) POLYESTERS SYSTEMS
Epoxy resins are suitable for mounting of all types of materials and are especially recommended for vacuum impregnation. Epoxies have the lowest
shrinkage of all cold-mounting resins. The curing time is relatively long, but adhesion to most materials is excellent. They polymerize through a chemical reaction after
being mixed in the correct proportions. The hardened epoxy is duroplastic, and not affected by moderate heat or chemicals.Properties such as low vapor pressure, transparency, good adhesion, low viscosity, and no shrinkage are all specific to epoxies.
Acrylics resins are easy-to-use resins with short curing times, very limited shrinkage, and excellent mounting properties. They are well-suited for
both serial mounting of irregularly shaped specimens and for routine work or single specimens. Acrylics are available with and without a mineral filler. Dye can be used if
color coding is needed. When mixing acrylic resins, it is recommended to add powder to the liquid (hardener) – this will result in the most uniform resin mixture.
Polyesters systems Polyesters are an economical choice for labs with low sample volume and less concern about specimen edge retention. They provide an excellent way to make a mount quickly and easily. Their hardened specimen is duroplastic.
