Melt fracture" in polymer extrusion: a visco-elastic instability
In the plastics industry, there are essentially two ways of making a plastic. Injection molding refers to the process in which a polymer melt is injected into a mold of the object one wants to make. The other process is extrusion. Extrusion is essentially the process in which a plastic wire or sheet is made by pumping a polymer melt through a pipe or slit, and having it solidify.
One of the basic problems that has plagued polymer industry since long is that when the polymer exits too fast from an extruder, the the surface starts to exhibits undulations or irregularities. The behavior is illustrated in the figure below, which shows five photographs of a polymer flowing out of a syringe, with the flow rate increasing to the right. For slow flow rates (the two pictures on the left), the flow is smooth. The middle picture is near the onset of the undulations, while the two rightmost pictures show that the undulations become progressively stronger as the flowrate increases. At even higher flow rates, the distortions can become so severe that they cause the extrudate to break