In a previous column (see Extrusion Know How, Sept. ’20), I explained how melting occurs in an extrusion (or injection) screw. It occurs by shearing or stretching the polymer with the rotation of the screw relative to the barrel. The resistance to turning the screw is how the drive energy is introduced to melt the polymer. The resistance to the screw rotation or drive power is proportional to the polymer viscosity in contact with the barrel. If you preheat the barrel before starting up, it will be coated initially with melt when screw rotation begins.
The viscosity of the polymer in contact with the barrel at a given location through which the screw turns changes continuously along the screw. The viscosity depends on the shear rate and the temperature of the polymer.
The shear rate is defined as:
γ = πDN/h, where
π = Pi,
D = diam. (in.),
N = Screw Speed (rev/sec),
h = Channel depth (in.)
The effect of the shear rate on the polymer viscosity for any particular polymer is defined by the power-law coefficient (n). The effect of temperature on the viscosity is defined by the consistency index (m). Power from shearing the polymer is termed “viscous dissipation” (em). It’s how most of the energy from the rotating screw changes the temperature of the polymer. Once screw rotation is started, very little energy typically enters the extruder from the barrel heaters. In fact, many extruders are run with all the heat/cool zones in cooling mode.
The calculation em = m(γ)1+n describes viscous dissipation at any point; however, it’s true only at that point and does not consider the accumulation of energy and the corresponding rise in polymer temperature along the entire length of the screw. To do so over the length of the screw is a complex and time-consuming calculation.
Read more: The Importance of Viscosity in Melting