Metallographic Mounting Pressure Explained

Ask three people what pressure a mounting press runs at and you may get three honest, numerically different answers: "6 bar", "about 18 kilonewtons" and "250 bar". All three can describe the same machine at the same moment. The word pressure is doing too much work in hot mounting conversations, and the resulting confusion causes real problems — under-specified machines, porous mounts, and data-sheet settings that cannot be reproduced.

This guide defines the four quantities people conflate, shows how they connect through the force-amplification chain, and sets out how to read machine gauges and write specifications that cannot be misread.

Four quantities that share one word

Plant or line supply pressure

This is the compressed-air line into the building or laboratory — commonly around 6 bar nominal, sometimes a little more or less depending on the compressor installation. On a pneumatic press it is the raw input the machine works with, adjusted at the machine's regulator. It says nothing, by itself, about what the resin experiences.

Hydraulic system pressure

On a hydraulic press the working fluid is oil, pressurised by a pump rather than drawn from a plant utility. Hydraulic circuit pressures are typically far higher than plant air, which is why hydraulic actuators can be physically smaller for the same force — a comparison explored in the pneumatic vs hydraulic guide. But the circuit pressure is still an internal machine quantity. It is not what the resin data sheet is asking for.

Actuator force — not a pressure at all

The cylinder converts its supply pressure into a force: supply pressure multiplied by piston area. Force is measured in kilonewtons (kN), not bar, and this distinction is worth being pedantic about. Force is the honest, mould-independent statement of what a press can do — the same actuator delivers the same force whether you fit a 25 mm or a 50 mm mould. Many specification problems dissolve the moment both parties switch to talking in kN.

Effective mounting pressure at the mould face

Divide the actuator force by the mould's cross-sectional area and you get the pressure actually applied to the resin. This is the number resin data sheets mean. It is commonly in the region of 150–300 bar, with around 250 bar a typical starting point for routine phenolic work. Because it depends on mould area, it changes every time the mould diameter changes — even though the machine itself has not changed at all.

Why "Pressure: ___ bar" is dangerously ambiguous

A specification sheet or purchase order that says only "Pressure: 250 bar" can be read at least three ways: as a required air supply (implausible but seen), as a hydraulic circuit rating (plausible for a hydraulic press), or as the intended effective pressure at the mould face (probably what was meant — but at which diameter?). Each reading implies a completely different machine.

The failure modes are not hypothetical:

• A press quoted against a line-pressure reading of the requirement arrives hopelessly undersized, because 250 bar of mould-face pressure needs only a modest air supply driving a large cylinder — not a 250 bar utility.
• A press specified as "250 bar" without a mould diameter may meet that figure at 25 mm and fall well short at 50 mm, since required force rises with the square of diameter — see the mould diameter guide.
• Laboratory recipes written as "250 bar" without recording the diameter cannot be transferred between machines or moulds and silently drift when tooling changes.

The force-amplification chain

A mounting press is a force amplifier. Modest supply pressure acts on a large piston to make a large force; that force is then concentrated onto the small mould cross-section, multiplying the pressure.

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

The last line is the one to remember: the pressure gain is the square of the diameter ratio between cylinder and mould. Doubling the cylinder diameter quadruples the mould-face pressure; halving the mould diameter also quadruples it. This is why the mould diameter belongs in every pressure statement.

Run your own numbers in the mounting pressure calculator, which converts in both directions, and the cylinder force calculator for the force a given bore delivers at your line pressure.

What the pressure does at the specimen

Effective mould-face pressure is the number that matters because it is the one the resin feels. Its three jobs, in brief:

1. Consolidation — squeezing dissolved gas and inter-granular voids out of the molten resin, preventing porosity.
2. Edge contact — holding molten resin tight against the specimen so it cures in intimate contact, which is what produces good edge retention.
3. Shrinkage compensation — continuing to feed material against the specimen as the resin cools and contracts, resisting edge gaps.

Too little pressure shows up as porosity and edge gaps; more pressure beyond the resin's recommended range buys little and risks delicate specimens. The full treatment — including the interaction with temperature and cooling — is in the temperature and pressure guide.

Reading the gauge

Here is the uncomfortable part: the gauge or display on a mounting press does not necessarily show effective mounting pressure. Depending on the machine generation and configuration, it may show:

• Line or circuit pressure — a simple mechanical gauge teed into the air or hydraulic supply. Honest, but it reads ~6 bar (pneumatic) or the hydraulic circuit value while the resin sees hundreds of bar. Operators who copy this number into a recipe sheet record something nearly meaningless for process transfer.
• A pre-scaled dial — a gauge relabelled in mould-face pressure, valid for one mould diameter only. Fit a different mould and the dial silently lies.
• A calculated display — an HMI that takes the measured supply or circuit pressure, applies the known cylinder geometry and the currently selected mould diameter, and displays the computed mould-face pressure.

The calculated display is the configuration worth asking for whenever a press will run more than one mould diameter, because it keeps the displayed number tied to the quantity the resin data sheet specifies. If the operator changes the mould and updates the diameter selection, the display stays truthful.

Specifying pressure without ambiguity

When writing a requirement or an RFQ for a mounting press, any of these three forms is unambiguous:

1. Effective mould-face pressure plus diameter(s): "≥ 250 bar effective pressure at Ø40 mm" — and list every diameter you intend to run, because the largest one sets the force budget.
2. Actuator force: "≥ 31 kN ram force" — mould-independent and directly checkable against the machine design.
3. Both, with the conversion shown — the safest option, since each figure cross-checks the other.

State your available plant air pressure too if the press is pneumatic; the machine has to achieve the force from the supply you actually have, not a nominal one. Whether pneumatic or hydraulic actuation suits your force target and utilities better is a separate decision — weighed in the pneumatic vs hydraulic comparison — but the specification language above works identically for both.

The configurator asks for exactly these quantities — target effective pressure, mould diameters and available utilities — so the force budget is computed rather than assumed.