Predicting Sharkskin Instability in Extrusion Additive Manufacturing of Reinforced Thermoplastics

The development of large scale extrusion additive manufacturing systems such as the Big Area Additive Manufacturing (BAAM) system has enabled faster printing with throughput as high as 50 kg/h and the use of a variety of thermoplastics and composites with filler loading as high as 50%. The combination of high throughput and heavy reinforcements can give rise to a phenomenon known as “sharkskin” instability, which refers to extrudate surface distortions typically in the form of roughness or mattness, and is commonly observed in traditional extrusion processes. The onset of this instability depends upon the viscoelastic properties of the material and processing parameters such as throughput, shear rate, extruder die geometry, and temperature. For printed parts, such instabilities are undesirable and detrimental to mechanical properties. This work examines the effect of process parameters on the rheological properties of BAAM thermoplastics and composites to predict the occurrence of sharkskin during printing.