Variable Geometry Dies for Polymer Extrusion

Abstract

This paper presents the development of variable geometry dies that enable the extrusion of plastic parts with a varying cross section. Extrusion accounts for 40% of all manufactured plastic parts because it is a relatively low-cost and high-production-rate process. Conventional polymer extrusion technology, however, is limited to fixed dies that produce continuous plastic products of constant cross section defined by the die exit profile. A shape-changing die allows the cross section of the extruded part to change over its length, thereby introducing the capacity to manufacture plastic faster and with lower tooling costs than injection molding. This paper discusses design guidelines that were developed for movable die features including revolute and prismatic joint details, land length, and the management of die leakage. To assess these guidelines, multiple dies have been designed and constructed to include an arbitrary four-sided exit profile where changes were made to the internal angles and length of sides as the extruder was operating. Experimental studies were conducted by using different extruder line settings and time between die movements. Test results are presented that include shape repeatability and the relationship between extrudate profile and die exit geometry.

Design and Experimental Assessment of Variable-Geometry Dies for Polymer Extrusion

“…For general shape-change, the joints connecting the die land components must allow relative movement, including pure rotation (revolute joints) and pure translation (prismatic joints, potentially curved). The following joint designs have been developed and utilized in experimental dies [10].…”

“…4(c), a set of fixed-end RR-type profiles have two stationary endpoints, hence it can be approximated by a chain connected to the ground with two fixed pivots (revolute joints). Since straight and curved prismatic (crescent) joints are preferred in variable-geometry dies [10], implementing fixed-end profiles with end links being prismatic joints is a necessity. Die design with crescent joints requires two additional types of fixed-end profiles: PP-type with both ends being prismatic joints with one fixed end ( Fig.…”

“…Stationary dies cannot produce complex parts that require changes to the sectional profile. Giaier et al [10] described design aspects of dies that have the ability to alter their exit geometry. Dies that exhibit gross changes to the exit orifice would allow the production of more complex extruded parts, thereby leveraging the associated cost and process time savings.…”

“…Established theory places a priority on revolute joints [12][13][14]. Concepts for variable-geometry die design have shown that revolute joints are difficult to implement, whereas prismatic joints (straight or curved) have fewer mechanical design challenges [10]. To accommodate die design, shape-changing mechanism theory is extended to produce designs that place a priority on prismatic joints and to eliminate or reduce revolute joints [15].…”

Control of Final Part Dimensions in Polymer Extrusion Using a Variable-Geometry Die

“…A true variable-geometry die might have an integrated actuator that could adjust the die dimensions in real time during the extrusion process to produce distributionready products more quickly. Some work has been done in designing [9] and prototyping [10] such dies, though these existing variable-geometry dies lack integrated actuation and control.…”