How to Choose the Twin Screw Extruder?

Is a parallel twin screw extruder better or a conical twin screw extruder better? This is a common question for users when purchasing twin screw extruders.

Classification of twin screw extruders

The extruder can be divided into two types according to the rotation direction of the twin screws: co-rotating and counter-rotating. Co-rotating extruders have two screws rotating in the same direction, while counter-rotating extruders have two screws rotating in opposite directions.

The extruder can also be divided into two types according to whether the axes of the twin screws are parallel or intersecting: parallel and conical twin screw extruders. Parallel twin screw extruders have parallel axes, while conical twin screw extruders have intersecting axes.

Similarities and differences between parallel and conical twin screw extruders

Parallel twin screw extruder

Due to the limitation of the distance between the two screw centers, the space for supporting the radial bearings, thrust bearings, and related gears that support the two output shafts in the transmission gearbox is very limited. Despite the designer's efforts, it is difficult to solve the reality of the small bearing capacity, small gear modulus, small diameter, and small tail diameter of the two screws, resulting in poor torsion resistance. The small output torque and poor load-bearing capacity are significant shortcomings of parallel twin screw extruders.

However, the flexibility of the length-to-diameter ratio is the advantage of parallel twin screws. It can increase or decrease the length-to-diameter ratio according to the differences in molding conditions to adapt to plastic processing technology requirements, which can expand the application range of parallel twin screws. However, this is difficult to achieve with conical twin screw extruders.

Both types of extruders have a forced forward conveyor mechanism for plastic, good mixing and plasticizing ability, and dehydration ability, and have basic adaptability to material and plastic product molding technology.

Conical twin screw extruder

Two horizontal conical screws are arranged, and the two axes are installed into the barrel at an angle. The center distance between the two axes gradually increases from the small end to the large end, making the center distance of the two output shafts in the transmission gearbox larger. The gears and gear shafts in these transmission systems and the radial bearings and thrust bearings supporting these gear shafts have larger installation space, which can install larger radial bearings and thrust bearings, and each drive shaft has a sufficient shaft diameter to satisfy the torque transmission. Therefore, the large working torque and load-bearing capacity are the main characteristics of conical twin screw extruders.

Centers

The centers of parallel twin screws are the same, while the two axes of conical twin screws are at an angle, and the size of the center distance changes along the axis.

Diameter

The diameter of parallel twin screws is the same, while the diameter of the small and large ends of conical twin screws is different.

Length-to-diameter ratio

The length-to-diameter ratio (L/D) of parallel twin screws refers to the ratio of the effective length of the screw's effective part to the outside diameter of the screw. The length-to-diameter ratio (L/D) of conical twin screws refers to the ratio of the effective length of the screw's effective part to the average value of the large and small end diameters.

From the above descriptions, we can clearly see that the difference between parallel and conical twin screw extruders lies in the different geometric shapes of the screw barrel, which leads to many differences in structure and performance. Although the characteristics of the two are different, they have their own advantages.

Stop thrust bearing of twin screw extruders

When the twin screw extruder is working, the melt will produce a very high pressure (head pressure) at the screw head. The pressure is usually around 14 MPa, and sometimes even up to 30 MPa. This pressure will form a strong axial thrust on the screw, and the role of the stop thrust bearing is to resist this thrust.

Because the two screws of the conical twin screw extruder are arranged at an angle, the center distance between the two output shafts in the transmission gearbox is large. Two thrust spherical roller bearings that are larger and arranged alternately front and rear are installed in the gearbox to prevent the axial force generated by the head pressure. Its characteristics are large bearing capacity, low manufacturing cost of the gearbox, and convenient maintenance.

The parallel twin screw extruder is limited by the small distance between the centers of the two screws, and the bearing capacity of the stop thrust bearing is related to its diameter. It is impossible to use a large-diameter stop thrust bearing. This dilemma is usually solved by connecting several small-diameter stop thrust bearings in series to bear the tremendous axial force together. The core problem of using this method is that the load borne by each thrust bearing must be uniform and the same. Otherwise, the thrust bearings that bear the load will be overloaded and prematurely destroyed, and the load they should bear will be added to other bearings, causing them to be overloaded. This continuous damage has serious consequences.

Therefore, it can be seen that the transmission system structure of the parallel twin screw extruder is relatively complicated compared with that of the conical twin screw extruder, and the manufacturing cost of the gearbox is higher, and maintenance is more complicated.