Operation and Use of Micro-Compounders

Mixing and compounding form an essential part of the polymer industry. At industrial level, these operations are generally carried out using twin-screw extruders, capable of delivering outputs of up to tens of tons per hour. However, to develop a product on a laboratory scale, smaller machines are required. Twin-screw extruders with a minimum diameter of 11 mm are now available on the market, enabling manufacturers to achieve throughputs of up to 20 g/hour. However, for expensive materials or those available in very small quantities, throughputs of a few grams per hour would be desirable, at least to define appropriate formulations. This is why, at the end of the 1990s, a new category of machines appeared: micro-compounders. They are based on the same idea: coupling a small twin-screw extruder (co- or counter-rotating) with a recirculation channel. In this way, the material can be processed during the time required for its elaboration, but under real extrusion conditions. Once this thermomechanical treatment has been completed, a valve allows the material to be purged by extruding it through a die. On some machines, the design of the recirculation channel also enables an apparent viscosity to be deduced from the measurement of the local pressure drop.

Micro-compounders are mainly used in laboratories or research and development (R&D) units. However, despite their growing use, little is known about how they work, and there is little literature on the process itself. In this article, after explaining the basic principles of micro-compounder operation, we will describe in detail their geometry, both in co- and counter-rotating configuration, and then present a number of experimental results. We will then introduce some flow modelling concepts and show the practical results that can be obtained using these models.