Mounting Methods

Introduction

Mounting is the second step in metallographic sample preparation: the specimen, freshly cut from sectioning, is embedded in a resin so it can be ground, polished, and examined. The job of the mount is straightforward.

PACE compression presses, castable resins, and accessories cover the full mounting workflow.

Two methods cover almost all metallographic mounting: compression mounting, which uses heat and pressure to form a mount from resin powder or granules in a few minutes per cycle; and castable mounting, which pours a liquid two-part resin around the specimen at room temperature and cures it over minutes to hours. The two methods use different consumables, different equipment, and different parameter ranges. The choice between them determines edge retention, throughput, and the kinds of specimens you can mount safely.

Sample Planning

Three decisions matter before the resin touches the specimen: orientation, edge-retention strategy, and mount-to-sample ratio. Get these right and the mounting step earns its keep; get them wrong and grinding, polishing, and analysis all suffer.

Orientation

Mount the specimen so the surface you want to examine ends up perpendicular to the mount face. For cross-section analysis (welds, case-hardened parts, coatings, multilayer structures), that means orienting the cut surface from sectioning face-down on the mold (compression) or face-down with a small spacer (castable). For thin specimens or oddly shaped parts, use mounting clips to keep the specimen perpendicular to the examination plane.

Edge retention strategy

Edge retention is the biggest single quality difference between mounting methods and resins. If your analysis depends on the edge (coatings, surface treatments, case hardness profiles, thin films), pick the resin family that holds the edge tight to the specimen:

• Best edge retention: glass-filled epoxy or diallyl phthalate (DAP) compression mounting
• Good edge retention with castable: epoxy with the specimen-preheat trick (see Castable Mounting below)
• Lower edge retention: phenolic compression, fast-cure castable acrylics

Mount-to-sample ratio

Aim for the specimen to occupy roughly 50–70% of the mount diameter. Too small a specimen wastes resin and floats during pouring; too large a specimen leaves thin resin walls that fail at the edges during grinding. Standard mount sizes are 1″, 1.25″, 1.5″, and 2″ diameter.

Choosing Your Method

Compression and castable mounting are not interchangeable. Match the method to the specimen, the throughput target, and the equipment you have available.

The rest of this guide is split into two self-contained tracks. Read Compression Mounting if you have a press and routine specimens. Read Castable Mounting for delicate, porous, multi-specimen, or heat-sensitive work. Universal Best Practices and Troubleshooting apply to both.

Compression Mounting

Compression mounting uses heat and pressure to consolidate a thermoset or thermoplastic resin powder around the specimen inside a heated mold. Each cycle takes 10–15 minutes and produces a hard, dimensionally consistent mount with excellent edge retention. It is the default mounting method for routine metal samples and the only practical choice for production-scale labs.

How compression mounting works

The specimen sits on a hardened ram inside a cylindrical mold. Resin powder or granules are poured around the specimen, the mold is sealed, and the press applies heat (typically 150–200°C) and pressure (around 2,000–4,200 psi) for several minutes. The resin softens, flows around the specimen, and either cross-links (thermoset: phenolic, glass-filled epoxy, diallyl phthalate) or melts and re-solidifies (thermoplastic: acrylic). After the cure dwell, the mount cools under pressure, and the ram extracts the finished cylinder.

What to look for in a press

Two specifications separate good metallographic presses from inadequate ones:

• Heater capable of at least 250–300°C (480–575°F). Some resins (acrylics, DAP) molded at the high end of their temperature range produce cleaner, harder mounts.
• Integrated water cooling. Without active cooling, the cooling phase dominates total cycle time. The heater and cooler should be intimately connected to the mold assembly so heat transfers efficiently in both directions.

Recommended PACE compression presses

The PACE TERAPRESS line covers manual hydraulic, automated pneumatic, and recirculating cooling.

TP-7500S Hydraulic Press

Hydraulic force, heater plus water cooling, programmable temperature and pressure cycles. The workhorse for production labs.

TP-7100S Pneumatic Press

Pneumatic force, smaller footprint, lower cost of ownership. Right for moderate-volume labs that already have shop air.

View the full TERAPRESS lineup →

Compression mounting resin families

Five resin families cover almost all compression mounting needs. Pick by edge retention, color, hardness, and whether the lab needs SEM-conductive mounts.

Mount hotter than the minimum. Compression mounting at higher than the recommended minimum temperature generally improves the mount's properties: harder surface, fewer voids, better edge support. Stay within the resin's molding-temperature range and add a minute or two of dwell.

PACE compression mounting consumables

View the full compression mounting consumables catalog →

Compression mounting procedure

1. Clean the specimen of cutting fluid, oxide, and handling residue. Oil contamination prevents polymerization at the specimen surface.
2. Remove all debris from the mold assembly and apply a thin coat of mold release.
3. Raise the ram, center the specimen on the ram face, and lower the ram into the mold cavity.
4. Pour a measured amount of resin around the specimen (use a calibrated scoop). Wipe excess resin from the mold thread area.
5. Lock the mold cover and bring the ram up to working position.
6. Apply the recommended temperature and pressure for the specified dwell time. Stay near the high end of the resin's temperature range for best properties.
7. Cool the mount under pressure to near room temperature. Slow cooling prevents cracking from internal stress.
8. Eject the mount, clean the mold and ram, and inspect the surface before grinding.

TIP: Preheating the resin powder and sample to 95°F (35°C) shortens the initial heat-up phase and improves throughput.

Edge retention strategy

For coatings, hardened-steel cases, plating layers, and other edge-critical work, the resin choice matters more than anything else you do at this step.

• Glass-filled epoxy or DAP directly against the specimen edge. The glass filler holds the resin tight to even very hard specimens (tungsten carbide, hardened tool steel) with no visible gap at the interface.
• Laminated mount technique to control cost. Glass-filled epoxy and DAP are 3–5× the cost of phenolic. Put a thin layer (enough to survive grinding) of glass-filled resin around the specimen, then back-fill the rest of the mount with red phenolic. The edge stays tight where it matters and the bulk of the mount is cheap powder. The two layers bond together during the press cycle.

Conductive mounts for SEM and electrolytic prep

Phenolic resin with a graphite or copper filler (PACE: CONDUCTO) is electrically conductive end-to-end through the mount. Two situations require this:

• Scanning electron microscopy. A non-conductive mount lets the specimen build up a surface charge under the electron beam, distorting the image. A conductive mount drains the charge to the SEM stage.
• Electrolytic etching or polishing. Current has to flow from the stage through the mount to the specimen surface. Non-conductive mounts break the circuit.

Castable Mounting

Castable mounting pours a liquid two-part resin around the specimen at room temperature and lets it cure into a hard mount. No press is required, the temperature stays low enough for heat-sensitive specimens, and a single pour can hold many specimens at once. The trade-off is cure time, which ranges from 8 minutes for a fast acrylic to several hours or overnight for a clear epoxy.

Castable resin families

Three resin chemistries cover most castable mounting work. Each one is built around a different trade-off between speed, clarity, hardness, and shrinkage.

Metallographic grinding requires the mount to reach at least Shore D80. All three resin families meet this; epoxy continues to harden over time toward Shore D90.

Epoxy variants

Standard epoxy works for routine mounting, but two specialty epoxies in the PACE line solve specific problems.

Low-viscosity epoxy is the right choice for porous and cracked specimens. It flows into voids, runs cooler so it does not stress the specimen, and shrinks less so edges stay intact. PACE's ULTRATHIN 2 is the low-viscosity option; EPOXY-ELITE is an environmentally friendly variant of the standard formulation.

PACE castable mounting consumables

View the full castable mounting consumables catalog →

Castable mounting procedure

1. Clean and completely dry the specimen. Cutting residue and moisture cause defects.
2. Remove debris from the molding cup or reusable mold and apply a thin coat of mold release.
3. Center the specimen in the mold. For thin specimens, use a clip to hold the specimen perpendicular to the examination plane.
4. Measure resin and hardener accurately by weight or volume (epoxy is strict on ratio; acrylic tolerates ±25%).
5. Mix gently with a folding motion to avoid pulling air into the resin.
6. Pour the resin around the specimen. For porous or cracked specimens, pour under vacuum (see below).
7. Let the mount cure at room temperature, or use the cure-cycle controls below to speed up or slow down the reaction.
8. Demold and inspect the mount before grinding.

TIP: Preheat resin, hardener, and specimen to 85°F (30°C) before mixing to shorten the cure cycle. This raises peak exotherm proportionally, so do not preheat if the specimen is heat-sensitive.

Edge retention via specimen preheating

Castable epoxy normally shrinks slightly as it cures, which pulls the resin AWAY from the specimen edge and leaves a microscopic gap. That gap is where edges round over during grinding.

The trick: preheat the specimen, not the resin. When the warm specimen contacts the room-temperature epoxy, the resin nearest the specimen surface cures first. As the rest of the mount cures and shrinks toward the already-set inner shell, the resin pulls TOWARD the specimen instead of away from it. The result is a tight, gap-free interface and noticeably better edge retention than a uniformly cured mount.

Controlling the cure cycle

Epoxy cure speed responds to temperature and gas exchange in both directions.

• To accelerate: preheat resin to ~120°F (50°C) before mixing, then cure at room temperature. An 8-hour resin can finish in 30–45 minutes this way. Peak exotherm rises in proportion, which can cause yellowing or cracking on large pours.
• To slow down: force air over curing mounts (fume hood or fan), submerge in a room-temperature water bath, or refrigerate. If the reaction stalls or the mount is too soft after cure, finish it by heating at 100–120°F for several hours. The mount hardens on cooling.

Acrylic and polyester can also be submerged in a water bath during cure to keep the exotherm down, reduce shrinkage at the specimen interface, and dampen the strong odor.

Vacuum and pressure impregnation

For porous specimens (thermal spray coatings, sintered metals, ceramics with closed porosity, cracked or weathered samples), vacuum impregnation forces resin into voids that would otherwise leave open pores in the polished face. The mechanism is gas-law arithmetic.

Recommended procedure

1. Place the mold and specimen in the impregnation chamber
2. Mix the castable resin (use low-viscosity epoxy for best penetration)
3. Seal the chamber and pull vacuum
4. Pour resin into the mold under vacuum
5. Slowly raise the pressure
6. Cure at room pressure, or apply external pressure for the cure cycle

TIP: Do not pull vacuum for more than 60 seconds before pouring. Extended vacuum makes dissolved gases in the resin itself bubble out (similar to opening a carbonated beverage), and those bubbles end up in your mount instead of in the specimen voids.

Recommended PACE castable mounting equipment

TeraVAC, TeraVAC Pro

Vacuum impregnation chambers. The base TeraVAC handles manual vacuum and pressure control; TeraVAC Pro adds programmable cycles and a built-in rotating table.

TeraCOMP Pressure Mounting

Pressure-only chamber for the cure phase. Compresses residual bubbles after a vacuum pour, useful for clear epoxy mounts where any bubble is unacceptable.

TeraUV UV Curing

UV chamber for fast-cure UV-curable castable resins. Cuts cure time from hours to minutes when the resin chemistry supports it.

View the full castable mounting equipment lineup →

Universal Best Practices

These habits apply to both compression and castable mounting.

Before the mount

• Clean the specimen with an appropriate solvent to remove cutting fluid, oxide, and handling residue. Oil and contamination prevent polymerization at the specimen surface.
• Dry the specimen completely. Trapped moisture vaporizes during cure and creates voids, especially in compression mounting and around copper-containing alloys.
• Plan orientation: which face needs to be examined? Mount it perpendicular to the mount surface.
• Confirm specimen size against mount diameter; target 50–70% specimen by area.
• For two-part castable systems, check the hardener expiration date. PACE hardeners typically have a one-year shelf life; degraded hardener is a common cause of soft, gummy mounts.

During the mount

• Measure two-part resins accurately. Epoxy ratios are unforgiving; acrylic ratios tolerate ±25% variation.
• Mix with a slow folding motion to avoid pulling air into the resin.
• Pour castable resin slowly and steadily to minimize bubble entrainment.
• Watch the cycle. A compression press that does not reach target temperature, or that cools too rapidly, produces a defective mount.

After the mount

• Let compression mounts cool fully under pressure before ejecting. Pulling a hot mount cracks the resin.
• Inspect every mount before sending it to grinding. Re-mounting is cheap; a hidden defect that fails during prep is expensive.
• Document the parameters (resin, temperature, pressure, dwell time, cool time) for the lot. Reproducibility starts with notes.
• Label the mount or use color-coding so it stays identified through the rest of prep.

Multi-specimen mounts

• Space specimens at least 2–3 mm apart so resin can flow between them.
• Keep all specimens at the same height in the mold so the ground face exposes all of them at the same plane.
• Use mounting clips to hold thin specimens perpendicular to the examination plane.

Troubleshooting

Most mounting problems trace back to specimen prep, resin chemistry, or cure-cycle parameters. The table below lists the symptoms PACE customers most often call about, with the cause and the fix. The "Method" column indicates whether the problem occurs in compression mounting (C), castable mounting (K), or both (C/K).

What's Next: Grinding

With the specimen mounted and the resin cured, the next step is grinding. Grinding removes the damage layer left by sectioning, flattens the mount face, and prepares a uniform surface for polishing. The grinding step also confirms whether your mounting choice was correct: a soft mount or a poorly-bonded edge shows up in the first 30 seconds against a SiC paper.

Continue to the Grinding Techniques Guide →