Application of Single Screw Extruder in Pipe Production

The traditional single-screw extruder design is mostly three-stage, and its main function is to plasticize, homogenize, and continuously press and send granular or powdered plastic to the extruder head. The plastic undergoes different physical state changes during the extrusion process, but no chemical reaction occurs.

Ⅰ. The main defects of single screw extruder process

In the process of the silane cross-linked polyethylene pipe extrusion line, the raw material undergoes the above physical process in the extruder, but also completes the polyethylene-silane grafting reaction. The traditional three-stage single-screw extruder process obviously cannot be fully adapted The main drawbacks of reactive extrusion molding requirements are that the residence time distribution of the materials is too wide and the relatively short length of the complete melting zone of the materials. At the same time, there are also problems such as low conveying efficiency of the solid-liquid mixed system by the solid conveying section.

Ⅱ. The specific requirements of single screw extruder process reaction extrusion

1. Raw material drying system

The single-screw extruder process requires that the water content of the resin is not greater than 200mg/kg. If the drying requirements cannot be met with only an ordinary hopper dryer, for this reason, a raw material drying system with air dehumidification device should be equipped.

2. Metering and feeding system

The single-screw extruder process is equipped with an advanced weighing and feeding system. The solid and liquid are measured separately. The proportion is controlled by a computer, and the accuracy can reach 0.25%.

Since vinyl silane can easily absorb moisture in the air and turn it into silanol, the metering and feeding system has adopted strict moisture-proof measures. At the same time, the flash point of vinyl silane is low, the metering pump should be equipped with an explosion-proof motor, and the equipment and containers in contact with the silane must have a reliable ground to prevent electrostatic sparks from detonating the silane.

3. Plasticized extrusion parts 

As mentioned above, the single-screw extruder process not only plasticizes the plastic, but also ensures the smooth grafting of polyethylene and silane. Therefore, the basic principles for designing the screw of the single-screw extruder are:

① There must be a sufficiently long feeding section;

②The screw runner should avoid dead corners, and at the same time improve the surface finish of the screw and barrel to prevent the accumulation of reaction materials and shorten the continuous operation cycle of the extruder;

③The groove depth of the screw metering section should not be too shallow to avoid excessive shearing of the graft body;

④The screw length to diameter ratio can be appropriately longer;

⑤ Properly extend the residence time of the melt in the extruder, and make the residence time of the melt as uniform as possible.

However, theoretical and experimental studies have shown that simply increasing the length of the screw cannot effectively improve the efficiency of the grafting reaction. At present, a four-stage single-screw extruder, through actual production and operation, has proved that it can significantly improve the efficiency of silane grafting reaction and increase the output of a single machine.