In recent years, due to the increase of the screw speed […]
In recent years, due to the increase of the screw speed and the increase of the pressure in the barrel, it is mainly due to the addition of a large amount of filler during extrusion, which makes the wear resistance of the screw and barrel made of nitride steel appear to be relatively insufficient. Its failure is mainly caused by the following reasons: Since the depth of the nitrided layer is generally around 0.5mm, and the internal collective hardness is only about HV280, once the nitrided layer is worn away, the wear resistance is very poor. During nitriding, the nitriding layer becomes brittle due to improper nitriding process, and it is easy to peel off under high pressure, which destroys the wear resistance of the screw; in addition, because the collective hardness is too low, the thin nitride layer is compressed under high pressure. The sinking also destroys the stable work of the screw.
Although the corrosion resistance of nitrided steel has been enhanced due to nitriding, in general, the corrosion resistance is insufficient, especially when extruding rigid PVC and some engineering plastics.
Due to the above situation, we can think that nitrided steel is no longer an advanced material for the screw and barrel. The actual need for production is that people begin to seek better materials and processing techniques to meet the requirements of extrusion production. At present, the international community is developing in the following directions:
Hard chrome plating
This is an older chemical treatment method. The alloy screw material can be 40Cr or 45 steel. The screw plated with hard chromium has good surface finish and strong corrosion resistance, and the ability to resist wear is also good. However, this method has high requirements on the chromium plating process; if the plating layer is too thin, the structure is loose, and if it is too thick, it is easy to peel off. At present, it is generally taken as 0.05-0.1mm. The combination of the plating layer and the metal substrate should be very strong, and the peeled pits and the chromium layer will form the two poles of the chemical battery, which promotes the corrosion of the screw.