In commercial molding operations, whether of plastics, metals, or other materials, releasing agents are often necessary to effect the easy removal of the product from the mold. This is the case with XRF sample disc preparation, but releasing agents are not widely used because of concerns about sample contamination. Even samples which are not inherently sticky can become somewhat adhesive when forced against a steel plate by 25 tons of ram pressure, and in general it is difficult to remove a pressed sample disc from a die by hand pressure alone. The use of the press together with the die’s knock-out ring is common practice, and as long as the disc does not bind to the die bore or the polished pellets, the knock-out ring approach works well enough. However, the use of a lubricant in the bore of the die does not have to contaminate the face of the sample if that lubricant is carefully applied, and the sample disc is carefully handled during and after removal from the die. For example, there is a fluoropolymer spray which can be squirted into the bore of a die before the die is loaded. After application, the volatile carrier evaporates, leaving a thin coating of fluoropolymer on the die bore. When the die is then loaded and the sample pressed, the sample disc and the polished pellets should have lubricant only along their edges, and should be easy to remove from the die. Even so, any such lubricant should be thoroughly tested before it is incorporated in a pressing procedure. Every XRF analyst knows the anguish of having a sample disc emerge from the die firmly attached to the polished pellet; at best the disc is ruined. This may be due to the pellet being damaged or poorly cleaned, but it is more likely that the sample is at fault. While spraying the polished pellet with lubricant will prevent sample adhesion, it will also contaminate the face of the sample disc with lubricant. The simplest non-contaminating approach is to place a piece of thin plastic film
between the polished pellet and the sample before pressing. SPEX SamplePrep 3516 Mylar window film is recommended because of its 0.12 mil thinness. After the sample powder is added to the die and leveled, a piece of film can be placed over the bore, and the upper
polished pellet pushed down on top of it. (A more elegant approach is to cut circles of film the size of the die bore. When the pressed sample disc has been removed from the die, the film is peeled off and discarded.)