How many tonnes should I press at?

Hydraulic presses designed by Specac for use in laboratories have maximum load ratings spanning a range from 2 to 40 tonnes (20 to 400 kN). What tonnage range will you need? It all depends on what you are trying to achieve. In particular, you should consider the type of materials you are pressing and the molds or dies that you are using to contain them.

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Pressing powder samples with a pellet die

The purpose of pressing powder samples is to create a solid material which does not fall apart after the removal of the load. The powder grains are pushed into closer contact, closing the gaps between them and forcing them to flow and rearrange themselves into a closer packing. Once all available volume has been reduced, flow of particles ceases, and they then begin to undergo a range of plastic and elastic deformations which result in them bonding to one another [1].

The load actually required to achieve a fully bonded pellet depends greatly upon the materials involved. Hard and brittle materials generally have a harder time binding to each other than softer materials. Powders which do not flow easily in the die will also make for more difficult samples, since they cannot achieve sufficient packing for adhesion to occur. The required pressure for compaction can also depend on things like moisture content, particle size and distribution, and overall homogeneity of the materials [2].

If your lab will predominantly be pressing KBr pellets for FT-IR, for example, the material is predominantly KBr salt with a small percentage of the analysis material added in. The two materials should be well mixed without allowing too much moisture to be absorbed. A load of 10 tonnes applied through a 13 mm diameter pellet die is usually more than adequate for the task, and this is often treated as a ‘rule of thumb’ for KBr pellet making.

To press pellets for XRF analysis, pellet diameters of 32 mm or 40 mm are common, which ensures that there will be enough space on the pellet surface for the X-ray beam.

Materials studied by XRF can range from pharmaceutical compounds to powdered rocks, sands, and other geological materials. Since geological samples are often full of very hard, brittle minerals, their ability to form pellets is quite poor. They are usually ground to a very fine powder and mixed with a cellulose or boric acid binding agent before proceeding. These agents lubricate the flow of particles in the die and help them to adhere.

Most samples will form a suitable pellet at a load of 10-20 tonnes to form pellets in a 40 mm die. However, loads of up to 40 tonnes are occasionally needed for very difficult samples.

Food and plant-based materials will often contain oils that dissociate and seep out under high pressure. Unless you actually want to collect the oil, these materials should be pressed at lower tonnages (4 tonnes or less).

Molding polymer films with a film maker

Analysis of polymers using spectroscopic techniques often requires a thin film to be produced. This can be achieved with heat and pressure using a heated platen and a film-making mold to set the thickness exactly.

To achieve this, the plastic needs to be melted so that it can flow within the mold and then just a small amount of pressure is needed to squeeze it into shape. The film maker equipment is usually rated to around 2 tonnes, but only 1 tonne or less is usually needed.

What is the pressure on the sample?

In research applications the relevant parameter is more often the actual compaction pressure, in which case this can be controlled not only via the load, but also by reducing the size of the sample.

The press applies a fixed load (force) to the work between the platens. The pressure that is experienced by the work depends on the area over which the force is applied.

When using pellet dies for the pressing of powders, there is a trade-off between the size of the required pellet and the compaction pressure that can be achieved, as can be seen in the figure below.

Pellet dies usually have a maximum rated load that limits the pressure to under 1000 MPa, but it can still be noted that a pressure of, say, 250 MPa can be achieved with as little as 0.5 tonnes in a pellet die of 5 mm. To achieve the same pressure in a 40 mm die, we need over 30 tonnes of load.

Here are some typical pressures seen for different applications:

Application	Diameter (mm)	Sample area (mm²)	Typical load (tonnes)	Resulting pressure (MPa)
Molding thin films	29	660	2.0	30
Pelletising milk powder	32	804	4.0	31
Densification of powders, pastes	13	137	0.5	37
Cement samples	32	804	20	256
KBr mini pellets	7	38.5	2.0	500
KBr pellets	13	137	10	739
Powder metallurgy	20	314	25	780