Screw extruders enable the continuous manufacture of polymeric products and the preparation of highly uniform polymer melts at high rates. Single-screw extruders remain the primary form of extruder due to their ease of production, reduced equipment costs, and ability to handle high torques. Twin-screw extruders are mainly used for compounding, in which high-quality dispersive mixing and well-defined residence times are required for devolatisation, temperature-sensitive polymers, and reactive extrusion.
Film extrusion has been developing as one of the most important polymer processing technologies, with applications emerging in electronics, photovoltaics and the optical industry due to the convenience of adopting flexible substrates with added functionalities and improved performance.
Extrusion is usually performed as a continuous 24-hour operation, which means that production costs are increased by machines that start and stop frequently. Because extrusion is basically a conversion industry, direct materials also represent one of the greatest costs. Extruders are therefore interested in reducing raw material costs and ensuring that all the material that makes it through the extruder is converted into a top-quality product.
Modern plastics processing operations are longer and more complex, thus making automation and control key factors for running a smooth process, maximising output rates and delivering extruded products of high quality with minimal material losses.
Extruder machines now feature several sensor systems (e.g. thermocouples and transducers) that display the main processing parameters for better data collection and acquisition, which enables users to record any event that causes defects, automatically send warnings to the production team, and carry out automated diagnoses. Extruders can then intervene, help prevent defects, and increase quality and productivity.
Key process variables
Barrel heaters are not the primary source of heat during operation. The extrusion process converts thermoplastic pellets into a homogeneous melt at sufficient pressure to allow the melt to flow through a die. Frictional heat generated inside the barrel is by far the most important source of heat. Measuring melt temperature (at the tip of the extruder) provides consistent information on polymer degradation due to overheating, cooling, sizing and appearance problems, and the effects of additives.
Pressure at the extruder end reflects the resistance of the breaker plate, filter screens and the die itself. It not only depends on the geometry of these components but on system temperatures, which affect resin viscosity and throughput rate. Process stability can be evaluated by monitoring melt pressure variations as an indication of output consistency.
Motor load may be expressed in amperes or as a percentage of the full load. It indicates the actual motor power used and thus expresses efficiency as output (kg/h) per unit of power. Monitoring motor load is important because it warns of rapidly rising pressure, such as in a clogged screen pack, as well as an indirect measure of melt viscosity.
Aimplas
Control with it
The interface between operator and machine is a critical factor in maintaining control of the extrusion process. While many processes are automatic, others must be carefully controlled. Close monitoring helps reduce scrap, increase uptime and improve material yield. It also eliminates product giveaway and short length claims, thus saving time and money.
