How does the choice of screw geometry influence the melting, mixing, and conveying of materials in the extrusion process?

The choice of screw geometry has a significant impact on the melting, mixing, and conveying of materials in the extrusion process. Here's how:

Melting: Screw geometry profoundly influences the melting process by dictating the material's residence time within the extruder barrel and the effectiveness of heat transfer. A longer screw with an increased length-to-diameter ratio allows for more prolonged exposure of the material to the heating elements, facilitating comprehensive melting. This extended residence time is particularly beneficial for thermoplastics with high melting points or poor thermal conductivity. Specialized screw designs like barrier screws strategically introduce barriers or interruptions in the flow path, effectively prolonging the material's exposure to heat. These interruptions create zones of increased pressure and shear, promoting more thorough melting by breaking down polymer agglomerates and enhancing heat transfer efficiency.

Mixing: The screw geometry is instrumental in achieving homogenous mixing of materials in the extrusion process. Incorporating various mixing elements into the screw design, such as mixing sections, kneading blocks, or distributive mixing elements, enhances the blending of additives, colorants, or multiple polymer components. These elements disrupt the flow of material, inducing chaotic motion and facilitating intermolecular interactions. The arrangement, size, and configuration of these elements determine the degree of mixing achieved. For instance, a higher number of kneading blocks increases the intensity of mixing by imparting additional shear and elongational forces on the material, promoting dispersion and uniform distribution of additives or fillers.

Conveying: Screw geometry directly impacts the conveying efficiency of the extruder by influencing the volume of material transported per unit time and the pressure buildup along the screw. The helical flights of the screw act as a conveyor, pushing the material from the feed section towards the die. The depth, pitch, and shape of these flights play critical roles in determining conveying efficiency. A larger flight depth allows for a greater volume of material to be transported per revolution, while a higher pitch results in faster material flow. However, optimizing these parameters requires careful consideration to prevent excessive shearing or degradation of the material. The compression ratio of the screw, which represents the ratio of the feed zone depth to the metering zone depth, affects conveying efficiency. A higher compression ratio increases the pressure buildup along the screw, aiding in material transport but also impacting melting and mixing processes.

Conical twin screw