The extrusion die, shapes the polymer melt from a circular entrance channel into the required extrudate shape. There are various types of extrusion dies like the film, sheet, annular and other profile extrusion dies. Having a uniform velocity profile at the exit face of the die is essential to obtain a high quality product.
After exiting the die, the semi-solid extrudate enters the calibration/cooling system. In this phase the extrudate is cooled below its melting temperature by a cooling medium (generally water), and also formed into the desired shape by applying an internal pressure or an external vacuum.
During this entire cycle the extrudate is constantly pulled by a set of rollers, which are called as the haul-off unit. The haul-off unit causes a decrease in the size of the extrudate, which is caused
due to the stretching action of the rollers. Draw down ratio, is the ratio of linear velocity of the haul-off system (v) to the velocity at which the extrudate emerges from the die.
The value of the draw down ratio depends on the rate of production and the quality of part desired. A higher draw down ratio enables faster production, whereas a lower draw down ratio
ensures better product tolerances. Finally, the sawing unit is used to cut the extrudate into lengths of the required dimensions.