Comparison of Parallel and Conical Twin-Screw Extruders

Ningbo Fangli Technology Co., Ltd. is a mechanical equipment manufacturer with nearly 30 years’ experiences of plastic pipe extrusion equipment, new environmental protection and new materials equipment. Since its establishment Fangli has been developed based on user’s demands. Through continuous improvement, independent R&D on the core technology and digestion & absorption of advanced technology and other means, we have developed PVC pipe extrusion line, PP-R pipe extrusion line, PE water supply / gas pipe extrusion line, which was recommended by the Chinese Ministry of Construction to replace imported products. We have gained the title of “First-class Brand in Zhejiang Province”.

Which is better, a parallel twin-screw extruder or a conical twin-screw extruder? This is a question often raised by users when purchasing a twin-screw extruder.

Classification of Twin-Screw Extruders

Based on the rotation direction of the twin screws, extruders can be divided into co-rotating and counter-rotating types. In co-rotating extruders, the two screws rotate in the same direction during operation, while in counter-rotating extruders, the two screws rotate in opposite directions.

Based on whether the axes of the twin screws are parallel or not, extruders can be divided into those with parallel axes and those with intersecting axes. Those with parallel axes are parallel twin-screw extruders, while those with intersecting axes are conical twin-screw extruders.

Twin-screw extruders can also be classified as intermeshing or non-intermeshing.

Similarities between Parallel and Conical Twin-Screw Extruders:

They share mechanisms for positively conveying plastic forward, good mixing/plasticizing capabilities, and dewatering ability. They have basically the same adaptability to materials and the forming processes for plastic products.

Differences between Parallel and Conical Twin-Screw Extruders

Diameter: The screw diameter is constant along the length in parallel twin-screws, whereas the diameter changes from small at the feed end to large at the discharge end in conical twin-screws.

Center Distance: The center distance between the two screws is constant in parallel twin-screws. For conical twin-screws, the two axes are at an angle, and the center distance changes along the length of the screws.

Length-to-Diameter Ratio (L/D): For parallel twin-screws, L/D refers to the ratio of the effective length of the screw to its outer diameter. For conical twin-screws, L/D refers to the ratio of the effective length of the screw to the average of the large-end and small-end diameters.

From the above, we can clearly see that the most obvious difference between parallel and conical twin-screw extruders is the different geometry of the screws and barrels, which leads to many differences in structure and performance. Although they have different characteristics, each has its own advantages.

Parallel Twin-Screw Extruder

Limited by the small center distance between the two screws, the space within the transmission gearbox for radial bearings, thrust bearings supporting the two output shafts, and the associated transmission gears is very limited. Despite designers' best efforts, they cannot overcome the realities of limited bearing load capacity, small gear module and diameter, and small tail diameters of the screws, resulting in relatively poor torque resistance. Small output torque and poor load-bearing capacity are the most obvious drawbacks of parallel twin-screw extruders. However, the adaptability of the length-to-diameter ratio (L/D) is an advantage of parallel twin-screws. The L/D can be increased or decreased according to different processing conditions to meet plastic processing requirements, which can expand the application range of parallel twin-screw extruders. This is something conical twin-screw extruders find difficult to achieve.

Conical Twin-Screw Extruder

The two conical screws are arranged horizontally, with their axes set at an angle inside the barrel. The center distance between the axes gradually increases from the small end to the large end. This allows for a larger center distance between the two output shafts in the transmission gearbox, providing more space for the gears, gear shafts, and the radial and thrust bearings that support them. This space can accommodate larger specifications of radial and thrust bearings, and the transmission shafts have sufficient diameter to transmit torque. Therefore, high working torque and high load-bearing capacity are major characteristics of conical twin-screw extruders. This is something parallel twin-screw extruders cannot match.

Thrust Bearings in Twin-Screw Extruders

During operation, the melt generates very high pressure at the screw head (die head pressure), typically around 14 MPa, sometimes even exceeding 30 MPa. This pressure creates a strong axial thrust force on the screws. Resisting this thrust is the function of the thrust (or "anti-backlash") bearings.

Parallel Twin-Screw Extruders: Limited by the small center distance between the screws, the load capacity of the thrust bearings is related to their diameter – larger diameter means higher capacity. Obviously, using large-diameter thrust bearings is impossible. This contradiction is usually resolved by using several smaller-diameter thrust bearings arranged in series to share the large axial force. The core issue with this method is ensuring that the load is evenly distributed across each thrust bearing. Otherwise, a bearing bearing a larger share will fail prematurely due to overload, transferring its load to the others and causing them to overload as well. The consequences of this cascading failure can be very serious. Thus, it can be seen that the transmission system of parallel twin-screw extruders is relatively complex. Compared to the transmission system of conical twin-screw extruders, the gearbox manufacturing cost is higher, and maintenance is more complicated.

Conical Twin-Screw Extruders: Because the screws are set at an angle, the transmission gearbox has a larger center distance between its output shafts. This allows for the installation of two larger, staggered thrust spherical roller bearings sufficient to withstand the axial force generated by the die head pressure. Their characteristics include high load capacity, lower gearbox manufacturing cost, and relatively convenient maintenance.

For users, the selection of a twin-screw extruder is very important. Different types of twin-screw extruders have different performances and application areas. Therefore, it is essential to understand the performance and application areas of various twin-screw extruders. For example:

· Intermeshing Co-rotating Twin-Screw Extruders are widely suitable for the modification of polymers not prone to thermal decomposition (e.g., blending, filling, fiber reinforcement) and reactive extrusion of materials, due to their high speed, high shear rate, and modular screw design.

· Intermeshing Counter-rotating Twin-Screw Extruders have good mixing and plasticizing functions, and their greatest feature is the direct processing of PVC powder. Although changing the screw geometry can allow for processing other materials, their strength still lies in PVC processing.

The required output should be determined based on the dimensions of the plastic profile, and then the extruder size should be selected based on this output. Under basically the same plastic processing conditions, conical twin-screw extruders can adapt to higher die head pressures, while parallel twin-screw extruders are suited for lower die head pressures.

If you need more information, Ningbo Fangli Technology Co., Ltd. welcomes you to contact for a detailed inquiry, we will provide you with professional technical guidance or equipment procurement suggestions.