Conical twin screw and parallel twin screw: What are the key differences in mixing efficiency?

1. Differences in structural design and torque bearing capacity

Conical twin screw:
The two screw axes are at an angle, the diameter gradually changes from the large end to the small end, the transmission gearbox has a large center distance, and larger bearings (such as tandem thrust spherical ball bearings) can be installed. It has a higher torque bearing capacity (the working torque can reach more than 1.5 times that of the parallel twin screw), which is suitable for high head pressure (14-30MPa) scenarios (such as PVC hard material processing).

Parallel twin screw:
Due to the small center distance, the transmission system needs to be connected in series with a small diameter stop bearing, and the torque resistance is weak, but the aspect ratio can be flexibly adjusted (22:1 to 36:1), which is suitable for processes that require long residence time (such as modified blending).

2. Comparison of mixing mechanism and efficiency

Dispersive mixing (shear-dominated):
Conical twin screws (especially co-rotating ones) can provide stronger tensile stress and shearing due to their large compression ratio (3:1 or more) and gradual groove design, and the particle separation scale is smaller, and the dispersive mixing efficiency is significantly better than that of parallel twin screws (numerical simulation shows that its dispersive mixing capacity is the largest).

Distributive mixing (flow-dominated):
Parallel twin screws have a longer aspect ratio and uniform flow channels, and the particle residence time distribution range is wider, so the distributive mixing capacity is slightly better than that of conical twin screws.

Shear rate control:
The shear rate of conical twin screws is low, which is suitable for the processing of heat-sensitive materials (such as PVC) to reduce the risk of degradation; the shear rate of parallel twin screws is high, which is suitable for blending and modification requiring high dispersion (such as glass fiber reinforcement).

3. Process adaptability and application scenarios

Conical twin screw:
The advantage scenario is direct extrusion of high-viscosity powder (such as UPVC pipes), with a large compression ratio (above 5:1), which can quickly compact the material, with small melt pressure fluctuations (high stability), but a short aspect ratio (usually <24:1), and limited plasticization uniformity.

Parallel twin screw:
The aspect ratio can be expanded to 36:1, and the plasticization path is optimized by combining segmented thread elements. It is suitable for complex formula processing (such as high filling, reactive extrusion), with low energy consumption and more controllable screw wear.

4. Energy consumption and maintenance cost

Energy consumption: Due to the long aspect ratio and high shear efficiency, the energy consumption per unit output of parallel twin screws is usually lower than that of conical twin screws (for example, the energy consumption of 36 aspect ratio models is 15-20% lower than that of conical twin screws).
Maintenance: The split barrel design of the conical twin screw is easy to clean quickly, but it is difficult to repair the screw after wear; the parallel twin screw adopts a building block screw combination, which has better maintenance flexibility