Mechanically, the process of pelleting involves forcing soft feed through holes in a metal ring-type die. These holes may be round or square, tapered or non-tapered. Single or double rolls mounted inside the die ring on a cam or eccentric, turn on a rotating shaft as friction develops (due to the presence of feed between roll and die). Feed is forced through the die holes in increments so that dissection of a finished pellet shows tight layers of feed mixture. The die is driven by a motor and the rolls turn only as feed between rolls and die develops friction. To make dry feed particles pliable for close compression and to decrease friction and absorb mechanical heat, water is often added to the feed, either as the formula is mixed or in the conditioning chamber of the pellet mill. Water may be provided as liquid and/or vapour. If water is provided in the form of steam, two objectives are accomplished:
(a) a high volume of water vapour condenses on the surface of feed particles, wetting and softening them for better compression, and
(b) the high temperatures of this steam cause some gelatinization of raw starch present in all vegetative ingredients, providing the necessary adhesion for firm pellets.
If sufficient moisture cannot be added as steam, pretreatment with water may be used to gain the desired lubricating effect.