Page 37 - Plastics News November 2018

FeAtures

as machine parameters can be adjusted to maintain Notice that the more viscous PE suffers less rate loss
productivity. When the radial clearance between the than does PP. When processing other shear-sensitive
screw flights and barrel wall increases, leakage flow is resins such as PVC and PC, minimal wear can adversely
inevitable. The operator will typically see a reduction affect quality due to the increase in melt temperature
in throughput and an increase in melt temperature, from excessive shear
forcing higher rpm and energy consumption to maintain As resin leaks over the screw flights, the material
the desired rate. With increased radial clearance, the experiences excessive shear rates. This raises melt
screw is unable to develop necessary pumping pressure temperature, which is only compounded by the higher
to maintain the intended output.
rpm needed to maintain output, as noted previously.
Obviously, the amount of radial clearance is a key variable Higher melt temperatures can lead to major quality
in leakage-flow calculations. A common “rule of thumb” concerns, as molded part physical properties are
considers a flight clearance four times the original greatly reduced at these excessive temperatures. When
tolerance to be the cutoff point for screw replacement, processing amorphous resins, these high shear forces
but it is important to consider the viscosity of the resin can easily burn and degrade the extrudate, leading to
processed. A resin with higher viscosity will provide less high scrap rates.
loss of output than will lower-viscosity resins with the Root wear generally creates pockets and areas for
same radial clearance. These rules of thumb should be material to hang up and degrade. This is observed as black
viewed only as guidelines.
specs in molded parts, and surging. Material degradation
To help better predict the effect of diametrical wear, can become a serious concern when processing resins
you can calculate leakage flow by considering overall like rigid PVC. PVC tends to generate hydrochloric acid
clearance, flight width, lead, head pressure, viscosity, at elevated temperatures, leading to extreme corrosive
and melt density. These calculations can become quite wear on equipment. In extreme cases of root wear, the
cumbersome when trying to accurately predict throughput; sides of the flights can become washed out, leaving little
but fairly accurate estimations can be generated that substrate to support the welded hardface inlay, as seen
predict potential rate loss. To help processors and in Fig. 1. With no support, the brittle hardfacing material
maintenance departments predict this loss of productivity, easily chips away, leaving rough edges, reduced flight
the author has developed a rate-loss calculator width, contaminated extrudate, and potential damage
that estimates the to downstream components.
potential loss of Replace or Rebuild?
rate due to screw
wear. This tool helps As many processors will agree, downtime is the enemy
conceptualize the of successful production. Many extrusion and injection
severity of wear molding plants rarely, if ever, pull screws to record
and helps teams wear. Tight production schedules and maintenance
develop a plan for complications can force companies to squeeze every
screw replacement ounce of life from existing equipment. The cliché, “If it
or repair. ain’t broke, don’t fix it,” comes to mind.

This tool was used to Trouble is, these types of practices can leave a processor
generate the results in limbo when screw wear can no longer be ignored.
shown in Tables 1 and Remember: Screw wear is a gradual process that can go
2, which reveal the unnoticed until scrap rates spike, energy consumption
estimated rate loss soars, or in extreme cases, catastrophic failure results.
for PP and PE under To save downtime and costs associated with buying
the same conditions. and installing new equipment, many processors spend

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