17 Aug 2017

What is a pellet die? | Spectroscopy Guides

A pellet die is a simple tool used to form thin cylindrical pellets from a powder using a pellet press.

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How a pellet die works

At its simplest, it consists of a hollow cylindrical body capped at one end, which forms a blind tube into which a powder sample is poured. The cap usually forms the base of the die. A plunger is then inserted into the tube to complete the assembly. The die is then transferred to a press, which applies a load of several tons to the plunger, compressing the powder against the base and walls of the tube. 

Watch our webinar on sample preparation in the fields of FTIR, XRF and more. Will Campbell explores various techniques in this webinar, originally hosted live by Select Science.

If sufficient load is applied, the powder grains will bind to form a solid that can be removed by separating the base from the body and applying a small load to the plunger to push the pellet out from the die body.

Pellet dies are typically made from stainless steel, and of robust construction, with thick walls and bases to withstand the enormous pressures that develop inside the cavity as the powder is compressed. Nonetheless, it is often the case that the maximum safe load rating of the die is exceeded by the capabilities of the press itself, so care must be taken to avoid damage to the dies through overloading. 

Visit our Evacuable Pellet Die page for more information on our pellet die products and the chance to read customer reviews of the dies.

Laboratory pellet dies

In laboratories, pellet dies are typically used for the preparation of test specimens for analysis.

Because the powders involved may be particularly fine, small tolerances are required for the fit between the body, base, and plunger in order to prevent material migrating into the gaps between components. Such material can cause the die to jam solid and prevent separation of the pellet from the die, or lead to long-term wear of the die components.

Because of this, in practice the powder is usually compressed between a pair of internal metal pellets that are inserted at the time of sample loading. These allow the base and plunger to have looser fits for ease of assembly. An additional advantage is that these internal pellets are easily polished to give the final specimen a high surface finish, or replaced by another material such as tungsten carbide to prevent contamination of the sample with iron.

Pellet dies for spectroscopy

Pellet dies are usually involved with the preparation of samples for spectroscopic analysis techniques such as Fourier Transform Infrared (FTIR) spectroscopy and X-Ray Fluorescence (XRF) spectroscopy, among others.

For FTIR analysis, the sample is usually ground with an excess of potassium bromide (KBr) and pressed using a die of 13 mm internal diameter. Diameters of 5 mm, 10 mm, and 20 mm are also available.

For samples to be analysed with XRF, not all will compact easily to form a pellet, in which case a sample support cup may be required. This is a thin flexible dish made of aluminium that is placed inside the die prior to sample loading. When the powder is compacted, the support cup deforms around the bottom of the die and captures the sample, supporting it from the bottom and sides, and leaving only the top surface exposed for analysis. The use of wax binding additives can help the powder grains adhere to one another more easily.

In this video we demonstrate a slightly simplified version of how to prepare an XRF pellet of cocoa powder, using a Specac Manual Hydraulic Press and Evacuable Pellet Die.

Also, the nature of XRF analysis is that a larger sample surface is usually preferred – typically either 32 mm or 40 mm. For this Specac offer two types of die – one with internal pressing pellets for all sample types, and a second type to be used with aluminium sample cups that does not have internal pressing pellets and has a highly polished plunger face instead.

Finally, XRF spectrometers are often fitted automated sample handling systems. These require the pellet to be formed inside of a metallic ring that can be used to clasp the sample without damaging it. These are usually not compatible with standard designs of pellet die, so specialised designs are required to handle this.

For more information on sample preparation: