Compression Mounting Medias.

A.      Thermosetting ( Compression Molding)

B.      Thermoplastic ( Compression Molding)

Thermosetting: Bakelite (powder, pelletized, preformed), Diallyl-Phthalate, Epoxy, require heat and pressure during the molding cycle and may be ejected at maximum molding temperature.

Pressure, time and molding temperature are the variables involved in molding specimens.
The pressure and molding temperature may be held constant by equipment design; thus time is only the variable.
Bakelite’s are also available in premolded condition. Premolded materials may be used when the section being mounted will not be damaged by pressure initially applied. Advantages of material in this form are convenience in handling and shortening of the molding cycle. Since premolds may be heated to a point just below the melting temperature before inserted into the mold.

With more fragile specimens, powders must be used. Powdered material should be initially started in molds at room temperature .This is necessary as material in this form has more exposed surface area and would exhibit a marked tendency for individual grains to cure immediately without complete fusion as they contact the heated molds. The bakelites are generally accepted and used for most metallographic mounting. Slight hardness differences are the only variation in these materials. Selection of Bakelite color is mostly personal preference.

The latest material, diallyl-phthalate, has properties more suited for metallographic use. It has a lower shrinkage factor than Bakelite and, therefore will cling tighter to the specimen. One form of diallyl-phthalate is filled with glass fibers producing a harder mount.

The latest and by far the best mounting material is glass filled epoxy. This resin shrinks less than other compression molding material and can be molded with pressures as low as 100 psi. Molding times are much shorter than other materials.

Procedures:

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