① Mainly because the ground material contain large size particle, which can not be fully softened during the conditioning process, and cannot be well combined with other raw material when passing through the pellet die.
②The raw material after conditioning are mixed with steam bubbles. The steam bubbles lead to the feed produce air bubbles during the process of pressing them into pellets. When the pellets are extruded from the ring die, the bubbles burst due to the change of pressure and are generated on the surface of the pellets.
③ Raw material containing high fiber.
