Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA

Polymer melts are often formed into simple shapes or "extrudates", such as cylindrical pellets, flat sheets, or pipe, using an extruder.¹ Polyolefins are among the most common extrudable polymers. Extrusion involves transporting and melting solid feed, which is sometimes mixed with non-polymeric materials, and the pressure build-up and transport of the melt or mixture. It is applied to thermoplastic polymers, which deform when heated and resume their earlier "no-flow" properties when cooled.

Using a simple lab extruder, the effect of operating conditions on polymer output and pressure drop can be examined and the resulting data can be correlated using the "Power Law" model for flow of polymer melts and solutions. This model is used to scale up the process to more complex extruders. The relationship between operating conditions and the deviations from theoretical displacement behavior ("slippage") and extrudate shape ("die swell") can be determined.

In this experiment, a typical thermoplastic polymer, such as a high-density polyethylene (HDPE) copolymer (of ethylene + a longer chain olefin) will be used. The operating temperature for the die and zones depend on the material. The flow rate can be determined by weighing the die output at timed intervals. All other necessary data (screw speed, zone temperatures, pressure entering the die) can be read from the instrument panel.

For this experiment, a typical thermoplastic copolymer (ExxonMobil Paxon BA50, melt temperature ~204 °C) of high-density polyethylene (HDPE) plus a longer chain olefin will be extruded through a cylindrical die.

1. Initialize the Extruder

1. Turn the exhaust "ON" when you are ready to power up the extruder.
2. Fill the hopper and extruder with polymer pellets.
3. Make sure that the motor switch is "OFF". Then turn the main switch "ON".

The Q vs. ΔP relationship was calculated using the Power Law model, and ir takes on a simple form for flow in a conduit of simple geometry, which in this case is the die. From the flow, speed, and temperature measurements, the Power Law constants and other quantities, such as shear rate, shear stress, and degree of slippage were calculated. Representative data and a fit to Equation 2 by linear regression are shown in Figure 2. The data s

Polymer extrusion begins by melting polymer resins that enter the extruder through the hopper. The flow of the molten polymer depends on the viscosity (ratio of shear stress to shear rate) behavior of the substance. The polymer leaves through the die, and is shaped to desired dimensions. The flow of polymer is expected to follow the Power Law model.

In this experiment, the mechanics of the Power Law model, including how it is used in conjunction with the z-direction equation of motion to analy

1. Principles of Polymer Processing, Z. Tadmor and C.G. Gogos, Wiley Intersicence, Hoboken, 2006 (Ch. 3, 4, 6, 9-10); Analyzing and Troubleshooting Single Screw Extruders, G. Campbell and M.A. Spalding, Carl Hanser, Munich, 2013 (Ch. 1, 3, A3).
2. Transport Phenomena by R.B. Bird, W.E. Stewart, and E.N. Lightfoot, John Wiley, New York, 1960 (Ch. 2-3) and Process Fluid Mechanics by M.M. Denn, Prentice-Hall, Englewood Cliffs, 1980 (Ch. 2, 8, 19)