Page 38 - Plastics News November 2018

FeAtures

a fortune on the hidden costs of increased energy Many reputable screw suppliers state that a screw can
consumption and material waste. When a screw is finally be rebuilt only three to four times. The size of a screw
pulled for inspection it may not be salvageable, forcing and a number of factors that contribute to the severity
longer downtime waiting for new equipment. of wear can greatly impact the life of a screw, but it is
not uncommon for an experienced screw manufacturer
With baseline data, wear-monitoring procedures,
and an understanding of what is happening to the to rebuild a screw many more times. With high-quality
welding, careful consideration of the metallurgical
aspects of screw manufacturing, and proper flight-profile
reconditioning, the author has seen screws being rebuilt
up to six to eight times.

After a screw is ground to a uniform undercut diameter,
the flights must be rewelded to utilize the higher-
hardness hardfacing alloys that provide maximum
wear resistance (Fig. 2). After hardfacing, the screw
OD must be ground to original tolerances (Fig. 3). The
leftover overhanging weld must be profile ground to
provide minimum flight-width loss. In many cases, poor
screw, maintenance and engineering departments can practices reduce the effective flight width during this
significantly reduce downtime and increase their bottom process, potentially affecting screw performance and
line. Replacing high-performance feedscrews can be complicating future rebuild potential. To help alleviate
expensive, but an experienced screw manufacturer can these complications, some screw manufactures opt
rebuild worn screws to like-new condition for a fraction to reduce the width of weld. When these screws are
of the cost in less time. ground to original tolerances, the welded hardface does
not encompass the full flight width. This negatively
When looking to rebuild a worn screw, ask yourself these
questions: impacts wear performance due to the exposed softer
base material.
1. How severe is the overall wear?
Develop a plan
2. What are the wear characteristics?
With proper baseline data and wear-monitoring
3. Can the current screw design be greatly improved? procedures in place, and an understanding of what is

4. Can the surface treatment be improved? happening to the screw, maintenance and engineering
departments can develop a robust maintenance program
If careful consideration of wear monitoring is ignored, to greatly reduce downtime and increase profitability. It
screw wear can be so extreme that it is difficult or is important to monitor screw wear before scrap rates
even impossible to repair reliably. Typically, the cost and energy consumption soar. Don’t be too quick to throw
of rebuilding a screw depends on the severity of wear out worn equipment. Consulting with an experienced
and the amount of work required. An experienced screw screw manufacturer on the potential of rebuilding screws
manufacturer can inspect worn equipment and processing can not only reduce downtime but the cost of rebuilding
performance to determine if the current design is worth tends to be a fraction of the cost of a new screw.
salvaging. In some cases, a new, optimized screw can
greatly increase overall profitability rather than relying Good rebuilding practices should allow for screws to
on the original, less-efficient design. Often the worn be salvaged many times, passing on major savings
screw can even be re-cut, improving the performance in maintenance cost. A robust planned maintenance
vs. scrapping the old screw. To improve abrasive and program rooted in baseline data, wear monitoring, and
corrosive wear resistance, hardfacing and surface a basic understanding of screw wear is far better than
treatments can be improved to extend the overall wear reactive maintenance.
life of a screw.

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