Page 34 - Plastics News November 2018

FeAtures

What’s happening to your screw

Understanding causes, effects, and solutions for feed screw wear

eedscrew and barrel wear is a serious matter for increased pressure and temperature leads to increased
Fmaintenance and engineering departments in both friction. Since the transition section and last few turns
extrusion and injection molding, and rightfully so. of the metering section typically experience the highest
Excessive wear in a plasticating unit poses a litany of pressures and temperatures, we can predict that this
serious performance issues, naturally affecting cost and is where the bulk of OD wear occurs. This rings true
overall quality. empirically after inspecting and rebuilding thousands
of screws.
Now that we have some idea of what our extruders are
“telling us” from a processor’s point of view, the next Much like flight OD wear, root wear is typically most
step is to explore what is happening to the screw and severe in these high-pressure areas. Wear is most extreme
what can be done about it. In this article we will look on the forwarding side of the flight vs. the trailing side.
at how screw wear affects melt quality and at the cost- The severity of root wear is greatly impacted by the resin
effectiveness of rebuilding vs. replacing worn equipment. or resins processed. When processing abrasive fillers,
such as glass fibers or calcium carbonate, it is possible
to see severe wear in the feed section as pressure is
developed by the forwarding side of the flight, with
limited lubrication provided by the unmelted pellets.
The amount of filler present and particle shape strongly
influence the intensity of wear. Larger particles with
sharper corners tend to wear more than smaller, rounder
particles. Unfortunately, the desired physical properties
of the molded part dictate the filler to be used. Since
wear is more rapid (with fillers) in the solids-conveying
section of the screw, it is common for fillers to be added
Typical Feed screw wear downstream where the polymer is already molten to
allow for better lubrication.
Feedscrew wear in both extrusion and injection molding
is similar; the majority of wear occurs in areas of high Mixing sections and barrier flights also tend to wear,
pressure--in particular, the transition section and last few as their function requires resin to flow over their flight
turns of metering. Processors and suppliers commonly geometry. Dispersive-mixing designs incorporate some
theorize that wear in these areas is caused by metal-to- form of undercut geometry that imparts high shear
metal contact between the flight OD and barrel ID, due forces on the resin to promote a more isothermal melt
to screw flexion under these high pressures. This theory and uniform color. As the mixing design wears, there is a
is unlikely because the internal pressure is generally the loss of adequate dispersion. Much like dispersive-mixing
same all around the screw. designs, barrier flights are undercut to help separate
the melt pool from the solid bed. As the undercut flight
A pressure differential is needed to generate a force wears, barrier designs can lose melting capacity and
deflecting the screw, not a general high pressure. efficiency.
Metal-to-metal contact can occur, but it is a result of
mechanical conditions and is a symptom of potentially Performance issues
larger problems: misalignment, gearbox damage, bent Screw wear is generally a slow process that can go
screw, bent barrel, inadequate foundation, etc. Abrasion unnoticed until performance is greatly reduced. Minor
testing performed across many industries shows that wear will have little effect on overall performance,

Plastics News November 2018 34