THE DISPLACEMENT OF GAS FROM POWDERS DURING COMPACTION

Abstract

A soap-bubble method was used to observe in detail the flow of gas out of powder masses during and immediately after compaction. The effects of powder material and size distribution, die size, pressing speed, and degree of compaction were investigated. The amount of gas trapped in the compact at completion of pressing varied from 12 to 83% of the initial amount of gas present in the spaces between the powder particles, over the range of compaction conditions studied. As expected, the amount trapped increased with increase in die size, with increase in pressing speed, and with decrease in particle size. Very little gas escaped during the later stages of compaction, even at slow pressing speeds. The effect of a small punch/die clearance was examined in typical cases, and shown to be minor, except with a coarse powder pressed at a high speed.

That trapped gas can produce cracks was demonstrated by making compacts containing varying amounts of trapped gas, other conditions remaining constant. With the particular powder used, once the amount of trapped gas had passed a certain level, the compacts tended to be fairly badly cracked. It appears, however, that cracks due to gas pressure alone tend to occur only when powders are rapidly compacted to very high densities.