About Mounting

The purpose of embedding a specimen in resin is primarily to facilitate handling and improve preparation results. Specimens requiring perfect edge retention or those that are porous must be mounted. For the best possible results, specimens should be cleaned before mounting — the surface must be free of any grease and contaminants so that adhesion between the resin and the specimen is optimal.

There are two main mounting techniques: hot compression mounting (hot mounting) and cold mounting. Porous materials additionally require vacuum impregnation to ensure complete filling of the pores.

Hot Mounting

The specimen is placed in a hot mounting press, resin powder is added, and the resin is cured under high temperature and pressure. This method is ideal for high-throughput laboratory requirements: resin surface quality is high, specimen size and shape are highly consistent, and the cycle time per piece is short.

Thermosetting Resins

An irreversible cross-linking reaction occurs at high temperature, producing a duroplastic after curing that is unaffected by reheating. Excellent chemical stability, high hardness, suitable for the vast majority of metallographic preparation applications. Although curing can be completed at high temperature, it is recommended to cool under pressure to ensure minimum shrinkage and good adhesion between resin and specimen.

Thermoplastic Resins

Softens or melts at high temperature and hardens again upon cooling. Short cycle time, suitable for high-throughput applications; however, thermal stability is lower than thermosetting resins. If subsequent preparation requires chemical etching or elevated-temperature grinding, suitability must be carefully evaluated.

Cold Mounting

The specimen is placed in a mould. Two or three components are carefully measured by volume or weight, thoroughly mixed, and poured onto the specimen to cure at room temperature. No hot mounting press or high-pressure equipment is required. Suitable for processing a batch of specimens simultaneously, individual irregularly shaped specimens, or materials that are temperature-sensitive and cannot withstand hot mounting conditions.

Key points for cold mounting:

Component ratios must be strictly mixed according to manufacturer specifications — too much or too little hardener will affect the hardness and shrinkage of the finished mount.
Pouring must be completed within the specified pot life after mixing to avoid premature gelation.
The mould should be kept level to prevent edge bubbles; vacuum assistance can be used if needed.
Curing exotherm may cause local temperature rise in the specimen — low-exotherm formulations should be chosen for heat-sensitive materials.
In general, hot mounting resins are less expensive than cold mounting resins, but hot mounting requires equipment investment.

Resin Selection Guide

Cold Mounting Resin Comparison

Each of the three cold mounting resins has distinct characteristics. Choosing correctly is critical for edge retention, shrinkage control, and subsequent preparation steps.

Epoxy

✓ Lowest shrinkage
✓ Excellent adhesion to most materials
✓ Suitable for vacuum impregnation
✗ Longer curing time (several hours)
✗ Requires precise two-component measurement

Cures to a hard plastic; resistant to moderate heat and chemical attack. Polymerises by chemical reaction when mixed in the correct ratio.

Acrylic

✓ Fast cure (approx. 8–15 minutes)
✓ Slight shrinkage
✓ Easy to use, suitable for high throughput
✗ Lower chemical resistance than epoxy
✗ Higher exotherm, unsuitable for heat-sensitive materials

Cures to a thermoplastic. Self-polymerising system with catalyst liquid; resistant to chemical attack.

Polyester

✓ Short curing time
✓ Lower material cost
✗ Higher shrinkage than epoxy
✗ Strong odour — requires good ventilation
✗ Not suitable for vacuum impregnation

Cures to a hard plastic. Like acrylic, a catalytic system with a short curing time.

Special Application

Vacuum Impregnation

Porous materials (such as ceramics, thermal spray coatings, powder metallurgy parts, PCB cross-sections) — if ground directly without filling the pores — will have grinding debris packed into the open pores, creating structural artefacts and distorting analysis results.

Vacuum impregnation ensures that all pores connected to the surface are filled with resin. The mould and specimen are placed in a sealed chamber, evacuated to remove air from the pores, then atmospheric pressure is slowly released to force low-viscosity epoxy resin deep into the pores.

Once filled, the resin not only strengthens the fragile porous structure but also prevents pull-outs, cracks, and edge chipping during subsequent preparation.

Vacuum Impregnation Notes

Only epoxy resin is suitable — low viscosity, low vapour pressure, will not volatilise under vacuum.
EpoDye fluorescent dye can be added to the epoxy to make filled pores clearly visible under fluorescent light.
Vacuum level should not be lower than −0.09 MPa; hold time depends on pore depth and resin viscosity.
Pressure release must be slow — rapid pressure release may cause surface bubbling.

Struers CitoVac provides precise vacuum control with timed pressure release, used with EpoDye fluorescent epoxy for porous ceramics, thermal spray specimens, and electronic circuit cross-sections.