Introduction
Over the last three decades industrial adaptability has allowed hot-melt extrusion (HME) to gain wide acceptance where it has established its place in the broad spectrum of manufacturing operations and pharmaceutical formulation development. HME has been demonstrated as a robust, novel technique to make solid dispersions in order to provide 1.) time-controlled, 2.) modified, 3.) extended, or 4.) targeted drug delivery, resulting in improved bioavailability as well as taste masking of bitter active pharmaceutical ingredients (APIs). Hot melt extrusion applies heat and pressure to melt a polymer and force it through an extruder in a continuous process. The extruder generally consists of one or two rotating screws (either co-rotating or counter rotating) inside a stationary cylindrical barrel. Regardless
of type and complexity of the function and process, the extruder must be capable of rotating the screws at a selected, predetermined speed, while compensating for the torque and shear generated from both the material being extruded and the screws being used. Extruders come in various sizes, so an 11mm, 16mm, or 24mm extruder refers to the diameter of the twin screws being used in the respective instrument. In addition, throughput will vary depending on the size of the extruder. For example, a 16mm extruder might be applicable for a throughput rate of up to 5kg/h (Figure 1), ideal for small-scale production quantities of an API/excipient formulation. But, when a formulation is developed at an R&D scale—such as on an 11mm extruder—making the jump to production-scale levels is not an easy proposition. One way to accomplish this successfully is by developing a process design that takes many variables into consideration, including speed, torque, volume, and temperature. Here we describe a scientific approach to developing the scale-up of an HME process across geometrically similar twin-screw extruders.
