FDM Ultra Low Shear Technology

Fluid Division Mixing (FDM) is a patented rotor-stator mixing technology that behaves as a high shear batch or inline mixer when operated at high speed and as an inline ultra low shear mixer when run at low speed.

In low speed (ultra-low shear) mode, FDM machines use the principle of inter-cavity transfer to gently cut and fold material in multiple planes for highly effective distributive mixing (blending).  For viscous fluids, rubbers, plastics, pastes and doughs, the large rotor-stator gaps, low speeds and laminar flow regime means that very little shear is imparted to the material.  This makes FDM an excellent solution for blending delicate or shear-sensitive materials such as long-chain polymers, adhesives and structured food products.  Features of the FDM in ultra-low shear mode include:

  • multiple rotor and stator rows to ensure complete blending along the mixer length
  • simple, low-cost geometry allowing cost-effective, reliable machines
  • multiple addition points for additive streams are possible
  • highly effective temperature control is possible (heating and cooling)
  • retrofit onto existing extruders is possible
  • large rotor and stator gaps mean very low pressure drop from inlet to outlet

Geometry & Operation

FDM is a concentric rotor-stator system with indentations or cavities machined into the rotor and stator surfaces.  The cavities in the rotor and stator are typically spherical segments which are open on both vertical and horizontal faces.  The rotor and stator are based on a stepped conical form so that the diameter of the cone increases from inlet to outlet.  Injectors can be placed on the outer surface of the stator to allow direct injection of fluid additive streams into the cavities.  Thermal jacketing of the stator is possible and the rotor can also be heated or cooled using circulating liquids.

Whereas in high speed mode FDM machines can self-pump, in low speed mode with high viscosity fluids, external pumping is normally required.  Fluid is forced into the mixer and travels along its length, being cut and folded many times before exiting.  Fluids can be dosed into the injector ports on the stator for direct inclusion in the blend.  The mixer is typically driven using a geared electric motor although other drive mechanisms are available.

Performance

There are very few competitors to FDM in ultra-low shear mode for inline blending.  Static mixers, although commonly used, are not ideally suited to high viscosity fluids and do impart significant shear.  Their blending performance at high viscosities is generally poor.  Pin mixers are sometimes employed, but again these devices exhibit higher shear and poor blending.  The original “cavity transfer mixer” developed by RAPRA in the 1970s and 80s is perhaps the best known alternative technology to FDM but, as it uses the same principles and suffers a number of technical and commercial drawbacks (cost, cleanability and short-circuiting), FDM is the more attractive approach.