Metallographic Mounting Resins: Phenolic, Epoxy, Conductive & Transparent

Choosing a hot-mounting resin is the first real process decision in metallographic mounting — before temperature, before pressure, before cycle time. The resin determines how hard the mount is, how well it supports the specimen edge, whether the mount conducts electricity, whether you can see through it, and how forgiving the press cycle will be.

In practice the choice reduces to four working families. This page describes each one — what it is chemically, when to choose it, how it behaves in the press, and what to watch for — followed by a comparison table, a note on castable (cold) resins, and the storage habit that prevents the most common resin-related defect.

How the families divide

All four families are supplied as powders or granules for hot compression mounting and processed in broadly the same way: heated commonly to 150–180 °C under an effective mounting pressure typically in the 150–300 bar region, held for often 5–10 minutes, then cooled under pressure. The chemistry underneath splits two ways:

• Thermosets (phenolics, hot-mounting epoxies, most conductive grades) cross-link irreversibly at temperature. Once cured, they do not re-soften, which is why they can often be ejected hot or warm.
• Thermoplastics (transparent acrylics) do not cross-link. They melt, consolidate, and solidify again only on cooling — which is why the cooling phase, not the heating phase, decides their quality.

Within each family, suppliers also offer different granule sizes. Finer grades are commonly preferred for delicate features and thin edges, because small granules flow around fine geometry more faithfully — at the cost of more trapped air to expel during consolidation, which is one more reason consolidation pressure matters.

What temperature and pressure each actually do inside the mould is covered in the temperature and pressure guide; here the focus is on how the families differ.

Phenolic (Bakelite-type) resins

What it is. A phenol–formaldehyde thermoset, usually compounded with wood-flour or mineral filler, supplied as an opaque coloured powder — black, red and green are common, and some laboratories use the colour itself to code sample batches.

When to choose it. Phenolic is the routine workhorse: economical, widely available, and good enough for the large majority of general microstructure work. If the task is "mount this steel so we can look at it", phenolic is usually the default.

Mounting behaviour. The most forgiving family. Phenolics typically cure near the middle of the common temperature range, tolerate modest parameter drift, and can commonly be ejected while still warm, which keeps cycles short.

What to watch for. Edge retention is moderate rather than excellent — the resin is somewhat softer than filled epoxy and can polish back slightly faster than a hard specimen, gently rounding the edge. Shrinkage gaps appear if pressure is released while the mount is hot, so cooling under pressure remains good practice. And like all mounting powders, phenolic picks up moisture in storage (see the storage section below).

Epoxy thermoset hot-mounting resins

What it is. An epoxy-based thermoset powder, very commonly filled with mineral or glass fibre. The filler is the point: it raises hardness and stiffness and lowers cure shrinkage.

When to choose it. Whenever the specimen edge is the measurement. Coating thickness, case-depth on carburised or nitrided steels, plating evaluation and surface-defect work all depend on the resin supporting the specimen edge right to the polished surface — this family is the standard answer. The low shrinkage also means fewer edge gaps on difficult geometries.

Mounting behaviour. Filled epoxies generally reward a disciplined cycle: full recommended pressure maintained through cooling, and cure times honoured rather than shortened. Their abrasion resistance — the property you bought them for — also means grinding and polishing consumables can wear somewhat faster than with phenolic mounts.

What to watch for. Cost per mount is higher than phenolic, so many laboratories reserve filled epoxy for edge-critical work rather than making it the default. An under-cured filled epoxy can look acceptable and still polish poorly, so resist trimming the cure time.

A common economy where only the specimen face matters: place a layer of filled epoxy against the surface to be polished and back-fill the remainder of the mould with ordinary phenolic. The premium resin does the metallurgical work at the edge; the phenolic provides the bulk. Many laboratories run this two-layer fill as their standard recipe for coating and case-depth work.

Why fillers improve edge retention

Edge rounding during polishing happens when the resin abrades faster than the specimen: the resin recedes, the abrasive cloth wraps over the unsupported edge, and the edge rounds off. Mineral and glass fillers raise the resin's hardness and abrasion resistance closer to that of hard specimens, so resin and metal polish back at more similar rates and the interface stays in one plane. The same fillers also reduce cure shrinkage, simply because filler occupies volume that would otherwise be shrinking polymer — which is why the filled families both hold edges better and gap less.

Conductive resins

What it is. A thermoset matrix — commonly phenolic or epoxy — heavily filled with conductive particles, typically graphite or a metal powder such as copper or iron.

When to choose it. Two applications dominate. First, electron microscopy: SEM imaging, EDS analysis and EBSD mapping all need charge dissipated from the specimen surface, and a conductive mount grounds the specimen through the stage without sputter-coating the whole block. Second, electrolytic polishing and etching, where current must pass through the mount to the specimen.

Mounting behaviour. The heavy filler loading makes the molten resin flow more stiffly than unfilled powders. In practice this family rewards the upper end of its recommended pressure range: consolidation that happens easily with phenolic needs more encouragement here, and residual porosity harms both the mount and its conductivity.

What to watch for. Conductive grades cost noticeably more than phenolic, so they are usually reserved for the instruments that need them. After polishing, it is worth verifying that specimen-to-mount continuity is adequate for your instrument before a long SEM session, since a porous or contaminated interface can degrade the ground path. For EDS work, remember that the filler itself — carbon, copper or iron — appears in any spectrum that samples the mount, so choose a filler that does not overlap the elements you are analysing for.

Transparent acrylic resins

What it is. A thermoplastic acrylic (methyl-methacrylate-type) powder that consolidates into a clear, glassy mount.

When to choose it. Whenever you need to see the specimen inside the mount: locating a specific feature while grinding down to it, controlling how much material has been removed, orienting wires and small components, or simply documenting what was mounted.

Mounting behaviour. Being a thermoplastic, the resin melts and consolidates under pressure but only solidifies — and only solidifies clear — during cooling. Cooling under full pressure is therefore mandatory, and slower controlled cooling typically gives the best optical clarity. Cycles run longer than phenolic cycles for exactly this reason.

What to watch for. Released hot, an acrylic mount can cloud, blister or distort — the signature failure of this family. Acrylics are also softer than the thermosets, so edge retention is modest; if a job needs both visibility and a supported edge, it may take two mounts or a compromise. Persistent cloudiness at correct settings is worth a look through the troubleshooting guide.

Comparison at a glance

Family	Chemistry	Hardness / edge retention	Shrinkage	See-through	Conductive	Typical role
Phenolic (Bakelite)	Phenol–formaldehyde thermoset, filled	Moderate	Moderate	No (opaque colours)	No	Routine, economical default
Epoxy (filled)	Epoxy thermoset, mineral/glass filled	High — best routine edge retention	Low	No	No	Coatings, case-depth, edge-critical work
Conductive	Phenolic/epoxy matrix + graphite or metal filler	Typically good	Moderate	No	Yes	SEM/EDS/EBSD, electrolytic polishing & etching
Transparent acrylic	Thermoplastic acrylic	Modest	Higher; managed by cooling under pressure	Yes	No	Locating features, visible specimens

The table describes tendencies of each family, not guaranteed properties of any product — formulations differ meaningfully between suppliers.

Cold-mounting resins exist too

Everything above is a hot-mounting powder, processed in a press. A parallel world of castable resins — two-part epoxies, acrylics and polyesters — cures at or near room temperature in cups, with no press, no imposed heat and no pressure. They are the answer for heat- or pressure-sensitive specimens, odd geometries and vacuum impregnation of porous samples. How the two approaches compare, and when each wins, is covered in hot vs cold mounting.

Storage: keep resin dry

The most common resin-related defect has nothing to do with the resin choice at all.

Sensible habits are cheap: keep containers sealed and reseal them promptly, store resin cool and dry rather than next to the press heater, prefer smaller containers that get used up faster, and rotate stock so the oldest powder is used first. Some suppliers describe a drying procedure for powder that has taken up moisture — follow their guidance rather than improvising. If mounts from a fresh, dry batch still show porosity, work through the hot-mounting troubleshooting guide before changing resin family: temperature, time and pressure each leave a different fingerprint on the defect.