Screws reparing

A weared screw can gives several problems such as poor quality of melt, poor flow. In such cases it may be convenient, instead of replacing it with a new one, to repair it. This operation is possible in most cases and leads to very advantageous results.

Restoring the nominal diameter

The revision of a screw is made by welding special alloy on crest of flight on all the helical portion of the screw in order to restore the nominal diameter.
Special alloy have different composition and hardness depending on processed material. In some cases a new nitriding treatment has held. In this way you get a screw that can have life characteristics even higher as it was originally and with considerably lower costs.

On the crest of the flight, in fact, you get a higher thickness of alloy than the thickness that would have a hard nitrided screw, thus achieving a greater wear resistance.
Not always, however, this operation is possible; the welding operation makes it possible to restore the outside diameter of the screw to the original nominal size, but does not change the core diameter of the screw.

Consumption of the root diameter

If the problem of malfunctioning, which led to replace the screw, it was not due to the consumption of the outside diameter, but the consumption of the root diameter (and hence changing the compression ratio), the revision cannot solve it.
In some cases, you can also repair the root, but only for a few pitches, otherwise the repair is not optimal and not very cost-efficient.

Alloy for screws

Type of alloy	Apply	Alloy base	Hardness (HRC)	Wear resistance	Corrosion resistance
Stellite 12	Plasma – TIG	Co	45-49	Good	Good
Colmonoy 56	Plasma – TIG	Ni	52-56	Good	Very good
TIG 5	Plasma – TIG	Fe	58-62	Very good	Good
WC	HVOF	WC	56-60	Excellent	Excellent

Cylinders reparing

The weared plasticizing unit is carefully checked visually and then we proceed with checking the internal diameter of the cylinder along all its length and to fill out a report where the measurements taken are shown as a graph. A careful evaluation of the type of wear, its location, the materials of construction of screw and barrel, allows to make a economically advantageous proposal for revision.

Wear concentrated in the terminal part of the cylinder

If the wear is concentrated in the terminal part of the cylinder is possible to realize a sleeve, of adequate length, and insert it into the barrel with a procedure that guarantees the maintenance of the nominal diameter. The sleeve is usually constructed of hardened steel but can also be a bimetallic sleeve.

Wear distributed along the length of the cylinder

If the difference between the nominal diameter and the actual diameter is not too high (comparing also to the dimensions of the cylinder), it can perform the repair enlarging the inner bore of the need to eliminate the defects, and then rerunning the nitriding treatment (if the cylinder was nitrided in origin). Consequently also the outer diameter of the screw will be changed in an appropriate manner and also special valves are supplied.

Wear distributed along the length of the cylinder

In case of excessive wear that does not allow the widening without compromising compression ratios or other, you can proceed with the total sleeve of the cylinder, but this opportunity is evaluated from time to time because often it can be inconvenient in economic terms and of correct functionality.