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According to the change of thread lifts and depths of screw groove, the conventional three-stage screw can be divided into three forms:

I. Isobathic deepening screw
The change of the depth of the screw from the groove can be divided into two forms:

1. Isometric changing screw: A screw with its depth of the last screw groove from the beginning of the charging section to the homogenization section being gradually shallower. On the longer melting section, the depth of the screw groove is gradually shallower.

2. Isometric mutation screw: that is, the depth of the screw groove in the charging section and the homogenization section is unchanged but the depth of the screw groove will suddenly becomes shallow at the melting section.

II. Isobathic varying-pitch screw

The isobathic varying-pitch screw means that the depth of the pitch is constant and the pitch is narrowed from the beginning of the first slot in the charging section to the end of the homogenization section.

III. Deepening and varying-pitch screw

The deepening and varying-pitch screw means the groove and the angle of thread rise are gradually changed from the beginning of charging section to the end of homogenization, that is, it's a extruder screw with the lifts of thread gradually becomes narrowed and the depth of screw groove gradually becomes shallow. The screw has the characteristics of the above two kinds of screw, but it's difficult to produce, so it's less used.

Extruder can generally be divided into screw repair and extruder barrel repair.

Extruder screw repair methods:

1. According to the actual inner diameter of the exreuder barrel, the broken screw should be made by the gap of the barrel and the deviation compare to the new screw.

2. The worn screw with a reduced diameter can be added the thermal spraying abrasion-resistant and then sharp it to fit the size after some processes have been done. This method generally has the specialized spraying factory to do the repair and the expense is also relatively low;

3.Repair screw can also be chromeplate on its surface. Chromium is also a kind of wear-resistant and corrosion-resistant metal, however, hard chromium layer is easier to fall off.

Repair method of extruder barrel:

1. With the barrel with increased diameter due to wearing, if there is a certain nitrogen layer, a new screw can be made by add an inner hole in the barrel and ground it to a new diameter size.

2. The diameter of an extruder barrel being processed and remolded with alloy in the thickness from1~2mm then do the finish machining of the size.

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