Highly effective single screw extruder-mixer for polymer composites production
Evgeniy Barmashin, director at research and production company ETO Ltd. (Ukraine)
Report points Nowadays the gaining new polymer materials with specific physical, mechanical and technological properties is largely performed with development of multiphase blending composites based on polymers with various modifying additives. Utilization of synthetic rubber as modifying additives provides wide opportunities in modification of thermoplastics functional properties. It allows to produce blending composites featuring enhanced plasticity, elasticity and frost tolerance. However realization of these opportunities is limited with absence of effective and at the same time simple designed and inexpensive continuous mixing equipment considering specific rubber technological properties and meeting production capabilities of small enterprises in terms of technical characteristics. The equipment package complying with the requirements stated above was developed and produced at research and production company ETO Ltd. (Kiev, Ukraine). Along with mixer the package includes extruder for base polymer melt preparation and feeding directly into spiral channel of mixer screw, volumetric dosing unit for rubber feeding into mixer loading zone, weighing dosing unit for bulky components as well as pelletizing head, cooling bath and milling pelletizer. The mixer itself was created basing on single screw extruder with operative parts (housing and screw) are divided in three sequential zones according to technological principle: loading zone, mixing zone and extruding zone. Mixer screw is driven by a motor with adjustable rotational rate. Loading zone screw is equipped with mixing device, representing tape screw consisting of coaxial contrary internal and external coils which are fixedly connected, and immobile mandrel produced as hollow cylinder with holes at the side face. Tape screw is fixed at the main screw and its internal coil encases the mandrel with minimal gap ensuring rotation. The mixer operates as following. Base polymer melt is fed from melting extruder through heated pipe fitting into mixer loading zone. Corresponding dosing devices feed other solid or liquid composite components inclusive of rubber here as well. Moving through screw spiral channel the material arrives at mixing zone formed by tape screw. Its internal and external coils interworking with internal cylinder surface and perforated external mandrel surface establish contrary melt flows within the circular working space. These flows are overlaid with direct flow formed by loading section of the screw. Contrary melt flows result in robust mixing of polymer blend within relative large volume and ensure preliminary components distribution and composite homogeneity at a level of agglomerates and large particles. The material being processed having no direct exit to unloading zone is extruded through holes into mandrel internal hollow. Thanks to that all melt discharge is processed within a small gap between internal coils of tape screw and mandrel (not greater than 0.1 mm). At relatively low rates of melt flow moving through mandrel holes climbing coils of tape screw repeatedly expose melt layer bordering on external mandrel surface to intensive shear. High shear rate gradients and corresponding shear stresses executed within the gap between the mandrel and the tape screw establish mixing effect contributing to agglomerates destruction, internal phase particles size reduction and thus achievement of high melt homogeneity. After mixing device melt arrives in unloading screw spiral channel, by means of which it is conveyed to exit from mixer and extruded through pelletizing head. Strands produced by the head are cooled with water and cut in pelletizing device. The main feature of the new extruder equipped with mixing device compared to well-known designs of single and twin screw continuous mixers is an opportunity to process material under conditions of intensive shear of material flowing through mixer working parts and continuously getting into small gap between rotating screw cog and immobile wall of perforated mandrel. Thanks to that it becomes possible to ensure high phases dispersity and homogeneity of blend even if it consists of technologically incompatible components. An effectiveness of single screw extruder-mixer, which is conditionally named «mixtruder» is proven by longstanding opearating experience in production of special polymer composites containing rubber, which were developed for products operating under low temperatures. The mixtruder can be supplied in various versions based on mixing principle described above in terms of screw design, mixing device position and size, designated for production of polymer composites with predefined properties made of various polymer and non-polymer materials with productivity from 10 to 1000 kg/h.