Do You Have Screw Wear?

Rate of heat into the film= Km/α(TB-Tm) +[μ (Vj)2/2α]

The first part of the equation is the heat conducted from the barrel into the film. But once the film temperature approaches the barrel temperature, it no longer contributes to melting, leaving only the viscous dissipation term:

[μ (Vj)2/2α]

In that term, μ is the viscosity, which is dropping as the film gets hotter, and α is the melt-film thickness, which is increasing. (Vj)2 is the square of the solid-bed velocity in the down-channel and barrel direction. Solid-bed velocity is divided by 2α, or half of the melt-film thickness. So even if the solid-bed velocity remains the same, the viscous dissipation or melting decreases by the lowered viscosity and the increasing melt-film thickness.

This wear in the melting section of the screw is often not so easily detected because it can be corrected by very small increases in screw speed over time. Eventually the most telling effect is the deterioration of the melt quality to a degree that it causes quality problems in the extruded product. Since wear usually happens over an extended period of time, melt-quality issues are often attributed to many other causes, losing valuable time in making the proper corrections. Moreover, if a screw design originally had excess melting capacity due to its length, depths, and other design features, or if it contains a high- shear dispersive-mixing section, wear may not be detected in the performance for long after the screw would be considered worn by most standards.