Feeder Screws in Polymer Compounding
The process of polymer compounding employs two methods i.e., volumetric and gravimetric feeding for achieving the accurate and efficient transfer of raw materials into the extruder while compounding. Volumetric feeding involves the measurement of materials based on their volume, typically using devices such as screw feeders or rotary valves. This method is suitable for bulk materials with consistent properties, but may prove to be less precise when dealing with variations in material density. On the other hand, gravimetric feeding relies on measuring the mass of materials, offering greater accuracy by accounting for density fluctuations. Gravimetric feeders, employing load cells or weighing systems, continuously monitor and adjust the material flow to maintain precise proportions in the compounding process. The choice between volumetric and gravimetric feeding depends on the specific requirements of the polymer compounding application, with gravimetric systems often preferred for applications demanding higher precision and consistency in material ratios.
In the delicate balance of material handling and conveying systems during polymer compounding, feeders hold immense importance. To exhibit the characteristics for which the polymer is being tailored for, it is important for the compounding extruder to precisely meter the required quantity of raw materials as given in the material recipe chart on a continuous basis. Of course, the properties are still dependent upon proper melt mixing inside the screw barrel assembly with a variety of factors involved such as: melt and zone temperatures, melt pressure, torque, etc. among others. In feeders, a very important component i.e., feeder screws play a pivotal role in the seamless movement of diverse substances such as minerals, polymers, glass, etc in various forms such as powders, granules, resin, etc. In this blog, we will delve into the feeder screw designs, dissecting the intricacies of spiral, concave, and auger configurations, unravelling their unique characteristics, and shedding light on the myriad applications that make them indispensable.
1. Spiral Feeder Screws: The spiral feeder screw is characterized by a continuous helical flight wrapped around a central shaft. The passageway is unobstructed for the ingredient to move unrestrictedly along the screw flights. The reduced surface area minimizes ingredient adhesion, altering the volumetric geometry of the screw. Material transfer occurs with minimal shear. However, a drawback is that aerated ingredients tend to overflow past the screw flights more easily. These screws are versatile and are used in conveyor systems, feeders, and material handling setups where horizontal or inclined transport of bulk materials in pellets or granule shapes is important.
2. Concave Feeder Twin Screws: The concave feeder twin screw introduces a unique twist with its trough or channel, specifically designed to cater to cohesive or sticky materials. Twin concave screws, co-rotating and self-wiping, minimize the adverse impact of adhesive powders as the material flows around their flights. These solid screws feature a lower volume in their flights compared to spiral and blade screws, resulting in high shear, particularly in the short screw tube designed to mitigate negative effects. Ideally suitable for powders with feed rates below 5 ft³/hr, twin concave screws have a wider cross-section than single screws, reducing the risk of bridging during the transition. Moreover, their higher screw speeds, facilitated by a larger inlet and lower flight volume, effectively diminish the drawbacks due to a pulsating flow.
3. Auger Feeder Screws: Auger feeder screws take on a more nuanced role with their tapered or conical shape, featuring a larger diameter at the material inlet and a smaller diameter at the discharge end. Renowned for precision, these screws find their niche in applications demanding controlled volumetric feeding and metering. Twin augers are employed for pellets and pellets/powder mixture whereas single auger screws are used for poorly flowing powders.
Twin screw arrangements: Concave, Auger and Spiral configurations
(Credits: https://link.springer.com/chapter/10.1007/978-3-030-90924-6_7)
The selection of a feeder screw for a particular application is no random decision. Careful consideration of material properties, flowability, abrasiveness, cohesion, and throughput requirements is imperative. The art of proper design and sizing is the key to unlocking the potential of these screws, ensuring efficient material handling, reliable feeding, and minimizing wear on equipment. The journey does not end with design considerations. The construction material of the feeder screw is a critical factor, considering the nature of the handled material and the environmental conditions. Durability and corrosion resistance become paramount for longevity and optimal performance.
Twin and single feeder screw configurations
(Credits: K Tron Feeders Brochure: http://vertassets.blob.core.windows.net/download/e6693cf0/e6693cf0-f7bd-49d2-ab41-f50d1cc78a50/k-tronlossinweightfeedersbrochure.pdf)
In the context of material handling, feeder screws, whether they are spiral, concave, or auger in design, emerge as indispensable components. Other factors such as agitators (flexible walled/external paddle or internally stirred) and screw trough also have their roles to play in furthering positive feeder performance. Understanding their unique features and applications empowers compounding industries to make informed decisions, ensuring not only seamless material transport but also achieving efficiency in compounding operations. As we navigate the expansive terrain of feeder screws, it becomes evident that choosing the right design is not just a technical consideration; it's the key to unlocking their full potential in diverse industrial applications.
If you have any other questions or would like to suggest topics for us to write about, please feel free to contact us at prashant.gupta@polymerupdateacademy.com
Author
Dr. Prashant Gupta
Faculty, Polymerupdate Academy
