There are two types of extruders: single and twin screw extruders. Single screw extruders are primarily used for melting and conveying polymers to extrude them into continuous shapes, whereas twin screw extruders are used for melt-mixing polymers with additional materials (pigments, fillers, reinforcers, and APIs), and for devolatilization. In the production of pharmaceutical formulations, which require homo-geneous and consistent mixing of multiple formulation ingredients, a twin screw extruder is preferred because the rotation of the inter-meshing screws provides better mixing to produce a homogeneous solid containing finely dispersed API particles, or a solid-solution of API in polymer. This can improve the dissolution rate and bioavail-ability of poorly-water soluble API formulations. Uniformly distrib-uted API is also a pre-requisite for the production of drug-eluting devices with intra- and inter-batch reproducibility of drug-release ki-netics.
Melting is accomplished by fric-tional heating within the barrel, and for twin-screw extruders, as the materials undergo shearing between the rotating screws and between the screws and the wall of the barrel as they are conveyed. The barrel is also heated with heaters mounted on the barrel, or cooled with water. The barrel sec-tion temperatures are usually opti-mized so that the viscosity of the melt is low enough to allow con-veying down the barrel and proper mixing, while keeping tempera-tures low enough to avoid thermal degradation of the materials.
The screws of a twin screw extruder are usually to provide different types of mixing and conveying conditions at various zones in the barrel. During product development, modular screws with multiple ele-ments fitted on a common shaft, allow the tailoring and optimization of the screw design for each product. Sections of the screw can be designed to perform particle size reduction, mixing and conveying functions. The length of the screw in relation to the barrel diameter (the L/D ratio) is chosen to optimize the degree of mixing and the number of zones required to achieve the final product char-acteristics. An example of a complete modular screw. Single piece production screws may be built to the same design as the development screws, but are easier to clean for cGMP compliance.