The mixing of different compounds is a key step of the extrusion process. Twin-screw extruders are often used to distribute and disperse additives into polymers. The quality of the final products is mainly dependent on the dispersive and distributive provided by the kneading blocks. A kneading block is a discontinuous unit mainly composed of several discs and is an important functional part of the co-rotating twin screw extruders.
The melting process is generally carried out in the kneading blocks. Standard kneading blocks are characterized by the length, number of discs and the offset angle between the discs. In zones where there should be a greater distribution kneading block with narrow discs are used.
Coperion Company cover its ZSK series twin screw extruder with 5A kneading blocks. Coperion claims its 5A kneading block prevent torsional vibration resonance that may lead to cracked shafts and extend life of large machines.
Twin-screw compounding machine builder Steer introduced at NPE2015 a new series of screw elements that reportedly are suited to replace kneading blocks in certain applications.
Called Melt Formation Elements (MFEs), they will be used on Steer’s Mega and Omega co-rotating twin-screw platforms to reportedly provide compounders with improved reliability, reduced wear and increased uniformity of melting and mixing.
Steers says the elements are designed to combat a series of problems faced by compounders of masterbatch, engineering plastics and difficult-to-process materials, notably: high wear; degradation during melting; uncontrolled breakdown in the process and transmission section; and improper material flow causing pressure peaks, which in turn leads to shear peaks that create torque instability and re-agglomeration,
As Steer explains it, while conventional kneading blocks (right handed, left handed or neutral) are effective at dispersive mixing, they are too harsh for many applications. This is because they present a perpendicular face to the flow causing melt stagnation and large pressure and shear peaks during melting.
Notes Dr. Babu Padmanabhan, Steer’s managing director & chief knowledge officer, “The MFEs are designed to create turbulence to the melt flow without stagnation. They can replace conventional kneading blocks that suffer from lack of shear uniformity completely removing any right angled face to the melt flow.”