Common Metallographic Mounting Problems: Causes, Fixes and Resin Selection

A 25 mm phenolic mount comes out of the press with a clean, glossy face. Under the microscope, the case-hardened rim of the gear tooth inside it is rounded, and the hardness traverse reads low for the first 50 micrometres. The lab then spends a day chasing a heat-treatment problem that does not exist.

Mounting faults rarely stop a process. They move the error downstream into the results, and they cluster into five groups: edge rounding, porosity, cracking and shrinkage gaps, incomplete cure, and hot-mount pressure or temperature errors. Nearly all of them trace back to four variables - the resin, the mix ratio, the cure schedule and the way the mold was filled.

Mounting is not a waiting step. It decides whether the edge you cut survives to the microscope.

The Five Failure Modes at a Glance

Match the fault you can see to the cause that produces it before you change any procedure.

Table 1: Symptom-to-cause map for the faults reported most often at the mounting stage.
Symptom	Most likely cause	What to change
Rounded or torn edge	Resin softer than the specimen; edge far from the mold wall	Filled or edge-protection resin; at least 5 mm of resin around the edge
Bubbles on the face, voids around the specimen	Entrained air; high-viscosity resin; porous specimen	Mix by weight, pour slowly, use vacuum impregnation
Ring crack or a gap between resin and metal	Cure shrinkage and exotherm; wet or oily surface	Slow-cure epoxy, thinner pours, clean and dry the specimen
Soft, sticky or gummy block	Incomplete cure: wrong ratio, poor mixing, low room temperature	Re-weigh components, extend cure, hold the room at 20-25 °C
Specimen lifts during grinding	Thin resin coverage; no release agent; under-cured resin	Add resin above the face, apply release agent, verify cure first

Edge Rounding: Why the Perimeter Fails First

Edge retention is a hardness race between resin and specimen. A resin noticeably softer than the sample wears back during grinding, leaving the edge unsupported; the boundary then rounds or tears, hiding carburized cases, nitrided layers, coatings and weld interfaces exactly where the measurement matters.

Three conditions make it worse: a small specimen in a large mold, so the edge sits far from any support; an unfilled cold-mount resin used on hardened steel; and heavy pressure on a coarse disc.

Match the resin to the specimen. Filled phenolic and mineral-filled hot-mount resins sit at the hard end; unfilled acrylics sit well below them.
Keep at least 5 mm of resin between the specimen edge and the mold wall, and center the specimen rather than clustering pieces at one side.
Start on a finer grit than you would for a soft sample, and let the machine control the force.

For case-hardened, coated or plated parts, an edge-protection resin is usually the cheapest correction available.

Bubbles, Voids and a Porous Mount Face

Porosity appears in two places: a peppered face that never polishes clean, and voids that open around the specimen after grinding. Both come from air that was never given a way out.

The usual triggers are whipping the resin instead of folding it, measuring hardener by volume (a 100:10 epoxy mixed by eye can land at 100:7 or 100:13), pouring a viscous resin into a narrow cup, and mounting porous specimens - powders, sintered parts, graphite, thermal spray coatings, cracked or corroded material - where the air is inside the sample rather than the resin.

Mix by weight to within a few percent, and stir slowly for the full recommended time, scraping the cup walls and bottom.
Let the mix stand for one to two minutes before pouring, then pour down the side of the cup instead of into the middle.
For porous or multi-phase specimens, vacuum impregnation is the only reliable answer: pull a vacuum until bubbles stop rising, then release slowly so resin is pushed into the open pores.

A vacuum cold-mounting unit covers the cost of the first batch of powder or sintered samples.

Cracking, Shrinkage and Gaps Around the Specimen

A ring-shaped crack around the specimen, a curved crack across the block, or a fine dark line where resin meets metal are shrinkage and exotherm faults, not grinding faults.

Fast acrylics cure quickly because the reaction is strongly exothermic. In a large mold that heat cannot escape; the block expands, contracts on cooling, and something has to give. Epoxy cures more slowly and shrinks less, which is why it is the default for large blocks, dissimilar material pairs and fragile interfaces.

Keep fast acrylics to molds around 25-30 mm in diameter and switch to a slow epoxy above that.
Pour thick mounts in two layers so the exotherm peak is halved.
Clean and dry the specimen. Oil, coolant residue and moisture all become a gap during cure.
Cure at a stable room temperature; a cold bench slows epoxy and raises the final shrinkage stress.

Thin foils, coatings, sheet and wire are the hardest cases, because there is almost no bulk to hold the edge when the resin pulls on it.

Soft, Sticky or Gummy Mounts: Incomplete Cure

If the block takes a fingernail impression at the edge, or smears grey paste onto the first disc, stop. Grinding a soft mount wastes paper, rounds the edge and pushes resin into the surface you are trying to examine.

Four causes cover most cases:

Wrong mix ratio - usually volume measurement. A few grams of error in a 100:10 system is a large percentage error.
Poor mixing - unmixed hardener in the bottom of the cup leaves a tacky patch that never sets.
Low temperature - epoxy cure time roughly doubles for every 10 °C drop.
Moisture - amine-cured epoxies react with humidity to form a sticky surface film.

8-15 minFast acrylic, 20-25 °C

1-2 hFast cold-mount epoxy

12-24 hSlow epoxy, low shrinkage

Treat those figures as minimums at 20-25 °C. Below that, add hours rather than minutes.

Hot Mounting: Pressure, Temperature and Cooling Errors

Hot mounting fails differently - the resin is fine, the cycle is wrong.

Too little pressure leaves voids around the specimen and a spongy edge. Typical phenolic and epoxy hot-mount cycles run at roughly 15-30 MPa, held through melting and curing.
Too much pressure crushes thin sheet, hollow parts and brittle coatings.
Too much temperature alters the material you are about to examine. Aluminum alloys, solders, tempered steels and polymer-bearing samples are sensitive above roughly 130-150 °C, so cold mounting is often the safer route for them.
Releasing pressure during cooling is the classic cause of gaps and cracks. Hold the load until the block is cool enough to handle, then eject.

Preheat the press so the cycle is not spent warming the mold, and log resin, temperature, pressure and hold time so a good mount can be repeated.

Resin Selection Cheat Sheet

When mounting problems keep coming back, the resin choice is usually the reason.

Table 2: Cold-mount and hot-mount resins compared against the problems described above.
Mount type	Typical cure	Hardness	Edge retention	Best suited to
Acrylic cold mount	8-15 min	Medium	Fair	Routine mounts, small specimens
Epoxy cold mount, fast	1-2 h	Medium	Good	General metallography, mixed materials
Low-viscosity epoxy	4-12 h	Medium	Very good	Foils, coatings, porous and fragile samples
UV-curing resin	1-3 min under UV	Medium	Fair to good	Transparent mounts, small parts
Phenolic hot mount	5-10 min cycle	High	Good	High-volume production, hard specimens
Filled epoxy or melamine hot mount	5-10 min cycle	High	Very good	Case-hardened and coated edges, weld interfaces

A Ten-Minute Check Before You Grind

CureCoverageVoids

Run through this list before the first disc touches the block.

Fingernail test at the block edge - no impression means the cure is complete.
Faces flat and parallel, with no taper from a mold that was not clamped evenly.
At least 5 mm of resin around the specimen, with the specimen face below the resin surface.
No voids: check the face under a stereo microscope, or tap the block and listen for a change in tone.
Specimen identity and orientation recorded on the block or in the log.

Five minutes here saves a re-mount, a re-cut and often a re-test.

Frequently Asked Questions

Q1: Why is my cold-mount epoxy still tacky after 24 hours?

Check three things in order. If only the surface is sticky, moisture reacted with the amine hardener and you can scrape the film off. If the whole block is soft, the mix ratio is wrong - extra curing time will not rescue it. If one patch is soft and the rest is hard, the hardener was never mixed in.

Q2: Can I grind a mount that is only slightly soft?

No. Soft resin smears within seconds, loads the grinding paper, rounds the edge and contaminates the surface with polymer. Re-mount the specimen; it is faster than cleaning up the failure.

Q3: What causes a crack that circles the specimen?

Differential shrinkage. The resin contracts more than the metal, and a fast exothermic cure in a large mold concentrates the stress at the interface. Use a slow epoxy, pour in thinner layers and let the block cool gradually.

Q4: How much resin should surround the specimen?

At least 5 mm on all sides, and closer to 10 mm when the edge is the measurement of interest or the material is very hard. A specimen sitting 1 mm from the mold wall has almost no support during grinding.

Getting the Mount Right the First Time

Most mounting problems are decided before the resin is poured: choose the resin for the specimen, measure by weight, control the cure temperature and give the edge enough material around it. When a fault does appear, work backwards from the symptom instead of changing three variables at once.

Our sample preparation section follows the full cutting, mounting, grinding and polishing sequence for metals, superalloys, precious metals and hard brittle materials such as silicon carbide wafers. The download center holds the 2025 mounting, cutting and grinding and polishing application guides, with recommended cycles and consumable pairings you can put straight onto the bench.