What are the differences in the roles of screw bars in injection molding and extrusion?

1. Functional Differences:
The Screw Barrel in an injection molding machine is primarily responsible for forcing the melt into the mold and maintaining a seal under high pressure for rapid molding; while the Screw Barrel in an extruder focuses on continuous plasticizing and conveying, allowing the melt to form continuous products such as strips, tubes, or films at the die head.

2. Structural Design Focus:
Injection molding screws are typically equipped with special screw heads to feed the melt into the barrel storage chamber during the pre-plasticizing stage and seal the melt during injection to prevent backflow. Extrusion screws, on the other hand, have a longer length-to-diameter ratio (L/D) and multiple temperature control zones to provide sufficient plasticizing time and low shear heat, ensuring material uniformity during continuous extrusion.

3. Melt Strength Requirements:
After injection molding, the melt is immediately solidified by the mold, so the melt strength requirement is relatively low. However, after extrusion, the product is still in a free state and requires higher melt strength to prevent tensile fracture, especially in subsequent processes such as fiber stretching or blow molding.

4. Differences in Process Parameters:
Injection molding machines emphasize high back pressure, short holding time, and rapid cooling; extruders emphasize low back pressure, long holding time, and multi-stage temperature gradients to adapt to different product shapes and material properties.