Extruding Very High-Flow Polymers

When the melt flow is very high, that means the melt viscosity is very low. The screw design usually has to be changed to accommodate that difference. As I’ve noted in previous columns, melting in a single-screw extruder occurs primarily as a result of viscous dissipation (or shearing) of the polymer (see illustration). In both a conventional flighted screw or a barrier screw, the melting principle is essentially the same. The unmelted polymer gets dragged against the barrel wall by the rotation of the screw, developing shear stress in the solid resin. This heats it up to begin melting. Continued melting is facilitated by the compression or declining channel depth of the screw, which brings the solids nearer the barrel wall to create a renewed region of viscous polymer, where shear changes the mechanical or rotational energy into heat in the polymer.

There are many other interactions going on in the screw channel simultaneously, but let’s keep things simple and stay with the basic melting process. Once melting starts, there is always a film of melted polymer against the barrel wall. All of the shearing force or drive torque must then be transferred through that melt film. The less viscous the film, the lower the resistance to screw rotation, and the lower the power entering the system from the drive. The result is that it is harder to transfer the energy necessary to raise high-melt-flow polymers to their processing temperature. Yet high-flow resins require a similar, if not identical, amount of energy per pound to reach their processing temperature—the same as low-flow polymers, in fact, since their specific heat is largely unrelated to the melt flow rate (MFR).