Metallographic Mounting Temperature & Pressure Guide

Hot compression mounting has two master parameters. Temperature drives the resin's flow and cure chemistry. Pressure consolidates the resin around the specimen and suppresses voids. Nearly every common mount defect — porosity, edge gaps, dull surfaces, incomplete cure — traces back to one of these two being outside its working window, or to the cooling phase that links them.

This guide explains what each parameter physically does, what happens at the extremes, how they interact with resin choice, mould diameter and cooling, and where a laboratory should start.

What temperature does

In hot mounting the specimen sits in a heated cylindrical mould filled with resin powder or granules. As the mould heats:

1. The resin softens and flows. Granules melt into a continuous mass that wets the specimen surface and fills the cavity.
2. The cure reaction proceeds. Thermosetting resins (phenolics, mounting epoxies) cross-link at temperature; the reaction rate rises steeply with temperature.
3. Thermoplastic resins consolidate. Acrylic-type transparent resins do not cross-link — they melt, consolidate under pressure, and must then cool under pressure to solidify clear.

Temperature too low

• The resin never reaches proper flow, so it cannot wet the specimen — expect a grainy, friable mount and visible unmelted particles at the surface.
• Thermosets cure slowly or incompletely. An under-cured mount may look intact but feels soft, smells strongly on ejection, and can crumble during grinding.
• Cycle time stretches, because operators compensate with longer holds.

Temperature too high

• Resin can scorch or discolour — burnt phenolic shows dark rings or a mottled surface.
• Heat-sensitive specimens are put at risk: tempering of hardened steels begins well below typical mounting temperatures if holds are long, and solders, polymer coatings and electronic components can be damaged outright.
• Thermal shrinkage on cooling grows with the peak temperature, encouraging edge gaps around the specimen.

What pressure does

Pressure acts on the molten resin throughout heating and, critically, during cooling:

1. Consolidation. Pressure squeezes dissolved gases and inter-granular voids out of the melt, producing a dense, pore-free mount.
2. Specimen contact. Pressure holds molten resin tight against the specimen while it cures, which is what produces good edge retention — the resin supporting the specimen edge right to the surface being polished.
3. Shrinkage compensation. As the resin cools it contracts. Maintaining pressure during cooling keeps feeding material against the specimen, resisting the formation of a shrinkage gap.

Pressure too low

• Porosity — the signature defect: scattered pores or a spongy core where gas was never driven out of the melt.
• Edge gaps — resin pulls away from the specimen during cooling because nothing compensated the shrinkage.
• Dull, under-dense surfaces against the mould wall.

Pressure too high

• Little additional benefit once the resin's recommended range is reached — the resin is already consolidated.
• Risk of deforming soft or thin specimens, crushing porous materials, or cracking brittle ones (some ceramics, glass-sealed components).
• Higher stress on mould tooling and seals for no metallurgical gain.

Line pressure is not mounting pressure

The single most common specification error in mounting-press discussions is mixing up three different pressures:

The machine is a force amplifier: modest line pressure acting on a large piston produces a large force, which is concentrated onto the small mould area.

Cylinder area   A_cyl   = π × D_cyl²  / 4
Actuator force  F       = P_line × A_cyl
Mould area      A_mould = π × D_mould² / 4
Mould pressure  P_mould = F / A_mould
                      = P_line × (D_cyl / D_mould)²

Run your own numbers in the mounting pressure calculator — it computes both directions: pressure from a given cylinder, or required cylinder size from a target pressure.

The mould diameter interaction

Because pressure is force divided by area, the force needed to hold a given pressure grows with the square of mould diameter:

A press sized for 250 bar on a 30 mm mould delivers proportionally less pressure on a 50 mm mould unless the actuator was sized for it. If you need interchangeable diameters, say so when configuring the machine — it changes the force budget. More in the mould diameter guide.

Cooling under pressure

Cooling is not an afterthought; it is the third act of the cycle.

• Thermosets (phenolic, mounting epoxy) benefit from cooling under pressure to counteract shrinkage — releasing pressure while the mount is hot invites edge gaps.
• Thermoplastics (transparent acrylics) require cooling under pressure: released hot, they can cloud, blister or distort.
• Cooling rate matters for surface quality: many laboratories find slower, controlled cooling gives glossier mounts and better dimensional consistency, while aggressive quenching saves time on robust routine work.

Resin-specific behaviour

Different resin families shift the working window:

• Phenolic / Bakelite — the routine workhorse: forgiving window, cures hot, ejected hot or warm. Most tolerant of parameter drift.
• Mounting epoxy (thermoset) — harder, lower shrinkage, better edge retention for coatings and case-hardened layers; often benefits from full pressure maintained through cooling.
• Conductive resins — filled with graphite or metal; flow is stiffer, so they reward being held at the upper end of their recommended pressure.
• Transparent acrylics — need melt, consolidation and clear solidification: controlled cooling under pressure is mandatory for optical clarity.

Details per family in the mounting resins overview.

Practical starting points

For orientation only — your resin's data sheet governs:

A sensible commissioning sequence for a new resin:

1. Start at the supplier's recommended temperature, pressure and time.
2. Mount a representative specimen and inspect: porosity, edge gap, surface.
3. Adjust one parameter at a time; porosity → check resin condition and temperature before pushing pressure beyond the recommended range.
4. Record the working recipe. If your press supports stored recipes, save it per resin/specimen combination.

When to defer to specifics

Treat the numbers above as scaffolding, not specification, whenever you have:

• resin supplier data for your exact product — it always wins;
• heat- or pressure-sensitive specimens — reduce first, then re-validate;
• unusual mount diameters — recompute force, don't assume the machine keeps up;
• application-critical requirements (edge retention for coating measurement, conductivity for SEM) — pick the resin first, then set parameters around it.