Hot vs Cold Mounting: How to Choose

Many metallographic specimens benefit from a mount before grinding and polishing, particularly when they are small, irregular, fragile, coated or edge-critical. Large, robust specimens may sometimes be prepared without mounting. Where mounting is appropriate, two process families produce the rigid, handleable block, and they get there by opposite means. Hot compression mounting forms the mount in minutes inside a heated press, under substantial pressure. Cold (castable) mounting pours a self-curing liquid resin around the specimen and lets chemistry finish the job at or near room temperature, with no press at all.

Neither family is better in the abstract. They trade speed and mount density against gentleness, geometry freedom and capital cost — and the right answer for a given laboratory is frequently "both, for different specimens". This page defines the two families, compares them criterion by criterion, and sets out where each one clearly wins.

The two families, defined

Hot compression mounting places the specimen in a cylindrical mould, surrounds it with thermosetting or thermoplastic resin powder, and applies heat and pressure together. Temperatures commonly sit in the 150–180 °C region, with an effective mounting pressure typically around 250 bar (150–300 bar is common practice). The heat-and-cure phase often takes 5–10 minutes, followed by cooling under pressure. The output is a dense, hard mount of standard diameter, and every mount from the same recipe looks like the last one. The process itself is covered in the hot mounting overview, and its two master parameters in the temperature and pressure guide.

Cold (castable) mounting dispenses with the press entirely. A liquid resin system — commonly a two-part epoxy, a powder-plus-liquid acrylic, or a catalysed polyester — is mixed and poured into a cup containing the specimen. The resin then cures chemically at or near room temperature. No external heat is imposed, and no mechanical pressure is applied at any point in the process.

How cold mounting works

A cold mounting station is modest: mounting cups (disposable plastic or reusable silicone), dosing aids for the mix ratio, stirring sticks and a ventilated bench. The workflow is mix, pour, wait.

The three common chemistries behave differently:

• Castable epoxies cure slowly — commonly several hours to overnight — with low shrinkage and a mild exotherm. They wet specimen surfaces well and are the standard choice for vacuum impregnation, where the filled cup is placed in a vacuum chamber so that the resin is drawn into pores, cracks and coating gaps before it cures.
• Castable acrylics cure fast, often within tens of minutes, which makes them the throughput option among castables. The trade-offs are a stronger exothermic peak during cure and generally more shrinkage than epoxy.
• Polyesters sit between the two in cure speed and are often supplied as clear systems where visibility of the specimen inside the mount matters.

The defining operational property of all three is parallelism: nothing limits how many cups cure at once. A technician can cast twenty mounts in a batch, walk away, and collect them cured — overnight, in the case of slow epoxies. No press time is consumed and no operator attention is needed during cure.

Cold mounting does have its own craft points. Mix ratios must be respected — off-ratio epoxy may never reach full hardness — and vigorous stirring entrains air, so gentle mixing or a brief vacuum de-gas gives cleaner mounts. Working time is finite once the components meet, and cure slows noticeably in a cold room. None of this is difficult, but it lives in operator technique in a way that a stored press recipe does not.

Decision criteria at a glance

Criterion	Hot compression mounting	Cold (castable) mounting
Equipment needed	Mounting press plus mould tooling; plant air (~6 bar) or hydraulics	Cups and mixing supplies; optionally a vacuum impregnation chamber
Cycle time per mount	Commonly 10–20 min including cooling	Minutes of preparation; cure from tens of minutes (acrylics) to overnight (epoxies)
Batch behaviour	Serial — one mount per cylinder per cycle	Parallel — many mounts cure simultaneously, unattended
Hardness / edge retention	High, especially with filled epoxy thermosets	Varies; castable epoxies respectable, acrylics generally softer
Shrinkage	Low when cooled under pressure	Epoxies low; acrylics more noticeable
Heat exposure of specimen	Commonly 150–180 °C	Ambient, plus the cure exotherm (mild for epoxy, higher for acrylics)
Pressure exposure of specimen	Commonly 150–300 bar	Essentially none
Vacuum impregnation of porous samples	Not applicable	Yes — a core strength of castable epoxies
Cost per mount (conceptually)	Capital cost up front; low consumable cost per mount at volume	Minimal capital; higher consumable cost per mount

Three rows deserve expansion.

Batch behaviour is the quiet decider. A press produces one mount per cylinder per cycle, so daily capacity is cycle time arithmetic — you can estimate yours with the mount throughput calculator. Cold mounting inverts this: each additional mount in a batch costs almost no extra labour, but every mount waits the full cure time. Hot wins on latency per specimen; cold wins on unattended batch volume.

Cost structure differs in kind, not just amount. Hot mounting concentrates cost in the machine and amortises it over thousands of cheap powder mounts. Cold mounting has almost no entry cost, but every mount consumes measured two-part resin and, usually, a disposable cup — so the per-mount cost stays flat while the hot-mounting per-mount cost falls with volume.

Pressure exposure is binary. If a specimen cannot tolerate being squeezed at hundreds of bar — thin-walled tubes, foams, fragile electronic packages, friable coatings — no press setting fixes that, and castable mounting is the answer rather than a compromise.

When hot mounting wins

Hot compression mounting earns its place when the work is routine and repetitive:

• Volume with low latency. A specimen goes from receipt to grindable mount in a quarter of an hour or so, cycle after cycle. For steady daily throughput, a press with one or two stations is hard to beat — see the single vs dual-cylinder guide for how station count scales capacity.
• Dense, hard mounts. Compression under pressure suppresses porosity and holds resin tight to the specimen, which is what good edge retention needs — particularly with filled epoxy thermoset powders.
• Repeatable standard sizes. Fixed mould diameters produce identical cylindrical mounts that fit holders, clamps and automatic polishing heads without adjustment.
• Consistency. A stored recipe (temperature, pressure, time) removes most operator-to-operator variation.

When cold mounting wins

Castable mounting is the right tool when the specimen, not the schedule, sets the constraints:

• Heat- or pressure-sensitive specimens. Soft solders, many polymers, delicate plated edges and some electronic components will not survive a typical hot cycle intact.
• Odd geometries. Anything too large, too tall or too irregular for a standard mould cavity can simply be cast in a suitable cup.
• Porous materials. Thermal-spray coatings, sintered parts, corrosion products and cracked samples benefit from vacuum impregnation with a low-viscosity epoxy — filling the porosity before it can smear or crumble during preparation.
• No press available. Field labs, satellite QC benches and very small laboratories can mount perfectly well with cups alone.
• Occasional mounting. If mounting happens a few times a week, press capital is difficult to justify.

Most laboratories end up with both

The comparison above rarely resolves to a single method for everything, because most laboratories see both kinds of work: a routine stream of production or QC specimens that suits the press, and a minority of delicate, porous or oddly shaped samples that suits the cup. A common working pattern is hot mounting by default, castable epoxy for the exceptions, and vacuum impregnation reserved for porous material.

If the routine share of your workload justifies a press, size it against your measured daily mounts rather than a hoped-for future — the configurator walks through station count, mould diameters and cooling options in that order. The resin side of the decision has its own trade-offs per family, covered in the mounting resins overview. Whichever way the split falls in your laboratory, the useful question is not "which method is better?" but "which of this week's specimens would each method ruin?" — answer that, and the equipment list writes itself.