Several important geometric parameters of screws
- Screw diameter (D) a.Related to the required injection volume Ejection volume=1/4 × π × D ²× S (ejection stroke) × 0.85 b.Generally speaking, the diameter D of the screw is inversely proportional to the maximum injection pressure and directly proportional to the plasticizing ability.
- Conveying section
a. Responsible for the transportation, extrusion, and preheating of plastics, ensuring preheating to the melting point;
b.Crystalline plastics should be longer (such as POM, PA), followed by amorphous materials (such as PS, PU, ABS), and thermosensitive materials should be the shortest (such as PVC). - Compression Section
a.Responsible for plastic compounding, compression and pressurized exhaust. By the end of this section, almost all of the raw material has melted, although it may not be uniformly mixed.
b.In this area, the plastic gradually melts, and the volume of the screw groove must correspondingly decrease to match the decrease in the geometric volume of the plastic. Otherwise, the material will not be compressed, heat transfer will be slow, and exhaust will be poor.
c.Generally, it accounts for more than 25% of the working length of the screw, but for nylon (crystalline material) screws, the compression section accounts for about 15% of the screw’s working length. For high viscosity, fire-resistant, low conductivity, high additive content and other plastic screws, it accounts for 40%-50% of the screw’s working length. PVC screws can account for 100% of the screw’s working length to avoid intense shear heat. - Metering Section
a.Generally, it accounts for 20%-25% of the screw’s working length, ensuring that the plastic is completely melted and the temperature is uniform, and the compounding is uniform.
b.A longer metering section yields better compounding results, but if it is too long, the molten material may stay for too long and undergo thermal decomposition. If it is too short, the temperature may be uneven.
c.For thermosensitive plastics such as PVC, the residence time should not be too long to avoid thermal decomposition. A shorter metering section or no metering section should be used. - Feed Screw Depth and Metering Screw Depth
a.The deeper the feed screw groove depth, the greater the conveying capacity, but the strength of the screw must be considered. The shallower the metering screw groove depth, the higher the plasticizing heat generation and mixing performance index. However, if the metering screw groove depth is too shallow, the shear heat and self-heating will increase, causing the plastic to discolor or burn. This is particularly disadvantageous for thermosensitive plastics.
b.The metering screw depth KD=(0.03-0.07)*D, where D is the screw diameter. As D increases, choose a smaller value of K.