GENERAL CONCEPTS OF DIE DESIGN

An extrusion die is a shaped orifice made in a block of steel or other suitable structural material that is wear- and corrosion-resistant. The function of the die is to receive the melt stream emerging from the extruder screw and to reshape it to the required form. In some instances，this is the final shaping operation, and in others, post-die shaping equipment completes the shape to the required form. An example of post-die forming is the three-roll stack used to size and cool the plastic sheet coming from the die.

Designing dies requires an accurate knowledge of flow characteristics of the particular plastic material to be processed. The designer must determine the precise shape of the flow channel(s) in the die, and the exact shape of the exit orifice. Unfortunately, the amount of data available to a designer is quite limited. In fact, much of it is still considered proprietary information by die-makers or by extruder operators. Fortunately, the authors have persuaded some of these craftsmen to permit publication, for the first time, of many of the basic principles developed over many years. The information in general availability is flow data for the material to be extruded through the die. Inasmuch as many errors result from “overlooking the obvious,” we recommend that each designer collect all possible information concerning the various plastics materials for which die designs may be required.

Further, the designer must glean knowledge of how plastic flows under pressure. From these two pieces of information—the characteristics of material to be processed and how it flows—the die passage design can be tentatively generated so that the relative flow from each portion of the die lips (orifice) corresponds to the extruded dimension required at that particular point.

In order to accomplish the exact dimension, it becomes necessary that the pressure drop along the flow path(s) in the die be equalized. The result of a “correct” design will be precise relative rates of flow from all regions of the orifice of the die.