The SPEX SamplePrep evacuable pellet die is a precision tool which must be handled carefully and diligently maintained for proper operation. Although in design and function such dies are very simple, precise fit of the working parts is absolutely necessary, and easily jeopardized. A pellet die in good condition will produce thousands of sample discs without difficulty; a damaged or heavily worn die is likely to produce frustration, delays, and chipped or broken sample discs.

Any damage to the polished pellets or the bore of a pellet die should be corrected immediately.

In a pellet die in proper condition, the polished pellets should pass smoothly through the die bore without binding, but their fit should be so precise that the pulverized sample will not “leak” around the pellet edges. A good test is to assemble the die bore and base, seal the evacuation port with a fingertip, and place a polished pellet (polished side up) in the bore. It should remain at or near the top of the bore, and spring back when pushed down lightly, due to compression of the air inside the die. When the evacuation port is unsealed, the pellet should drop smoothly to the bottom of the bore. If the polished pellet sinks immediately with the evacuation port
sealed, it is either a loose fit, or the seal between the die body and base is damaged.

Polished pellets are made with close tolerances and sharp edges so that a sample powder will not “feather” into the gap between the pellet and the bore. (When this happens, the sample disc may have a raised, crumbly lip and the disc and upper polished pellet may be difficult to remove from the die.) The edges of these pellets are by far the most vulnerable part of a die, as they can be dented or chipped by dropping them even a short distance onto a hard surface. Such dents are extremely dangerous to the integrity of the die. Not only can they cause the pellets to bind in the die, but also, under the high pressures of pelletizing, the stressed edge of the pellet can spall off. The resulting chip can be dragged through the bore, scarring it deeply, and potentially jamming the plunger and ruining the die.

Minor damage to the polished pellets and die bore should be immediately corrected with a fine-grained (e.g. 600 grit) emery paper. If after this the pellet will still pass smoothly through the die bore, and its leading edge is not significantly chipped, the pellet and die can continue in service. (A badly chipped pellet should be retired, as sample can wedge into the space left by the chip, making it difficult to extract the pellet from the die. In addition, further chipping is likely to occur under pressure.) A lightly scarred die bore, properly smoothed, can continue in use.

Damage to the polished face of the pellet should also be avoided, but will probably not affect the functioning of the die. Analytical accuracy is what suffers. Scratches on the order of 20-30 microns can cause shielding effects in the sample disc, and overall abrasion of the polished pellet face can very slightly change the geometry and distance in the critical relationship between X-ray tube, sample disc, and detector. Obviously if analytical results are being distorted because of the condition of the polished pellets, it is time to replace them, but the degree to which such distortion is tolerable will vary considerably from user to user. A simple way to check polished pellets is to press two sample discs of identical material, one with a pristine polished pellet and the other with the worn pellet, and compare the analytical data.

In handling the pellet die, some simple rules should be kept in mind: keep the die clean, and always treat it as the precision tool it is. Pellet dies should be cleaned after every use, to avoid both sample cross-contamination and the possibility of disc jamming or sticking from sample build-up. In cleaning the polished pellets, treat them like glass; in other words, use the same cleaning technique you would for a glass lens or mirror. Steel has a hardness similar to glass, and it is important to avoid scratching the polished surface.

Remember that the polished pellets are the most critical parts of the die, and the most easily damaged. When inserting the polished pellets into the bore of the die, take extra care that they do not jam; the fit is so precise that a very slight tilt will cause them to stick. When this happens, free the pellet gently. Above all, do not push it down further and make the situation worse, as this can cause the pellet to chip, and quite possibly ruin the die. A simple technique for inserting the polished pellet into the die bore is to hold the trailing edge of the pellet lightly with the finger-tips, and rotate it gently inthe mouth of the bore to make sure it is properly lined up. When a
polished pellet is placed in the die bore, it should move freely; if it does not, careful corrective action should be taken immediately.