If straightness of the barrel tube is a
prerequisite for good performance and long wear life, the selection of
the bore lining material is equally as important. Clearly, the bore of
an unprotected plain steel barrel will deteriorate quickly when the
processor molds abrasive or corrosive plastic resins.
Nitriding the barrel is a common and cost-effective treatment of the
barrel bore if the resin to be processed is only mildly abrasive and not
corrosive. Not knowing the types of resins a processor may run, many
machine builders select nitrided barrels as their standard for their new
off-the-shelf equipment, only to exchange this type with a
higher-priced bimetallic barrel for superior wear and output performance
when the customer demands it or the processing application requires it.
However, high-performance barrels with superior output and long
service life require more than a thin nitrided bore surface. The
potential for contact between the rotating screw and the barrel bore and
the effect of heat, abrasion, and possibly corrosion, make it necessary
that the barrel bore is well protected. Bimetallic barrels feature a
super-hard or anti-corrosive protective layer in the bore that is
applied there through a complex process involving high heat and
centrifugal forces.
Hardly! As there are no specific
industry-standards for barrel and screw configurations, each machine
builder develops barrel configurations that suit his particular needs
for fitting the barrel to his extruder or injection molding machine.
There are thousands of different barrel designs in use today; many of
them have spawned further modifications at the user level to suit the
needs of particular processing applications.
Keeping track of all these different designs, and providing the buyer
with exactly what he/she wants is a daunting task for the barrel
manufacturer that requires an extensive technical drawing library and
dedicated, knowledgeable staff to research the information on behalf of
the buyer.
One of the most important aspects of a
high-quality and high-performance barrel is straightness. Nothing
affects barrel and screw wear as much as straightness, or the lack
thereof.
For example, to make a long and straight barrel is not an easy
undertaking for any manufacturer. Starting with different
interpretations by barrel makers of the requirement for straightness,
some will abide by the straightness standards.
Depending on the severity of the wear or
damage in the barrel bore, it may be cost-effective to have the barrel
repaired or re-sleeved (i.e. removing the worn bore section and
inserting a sleeve featuring the same protective bore lining material as
the original barrel).
As re-sleeving will be considered primarily for injection molding
barrels with their inherent high pressure operating conditions, the user
is well advised to have this work done by qualified shops specializing
in barrel repair.
Such reputable repair shops for barrels will advise the customer
about the pros and cons of repairing or re-sleeving his damaged or worn
barrel versus buying a new one.
