Rate-Limited Extrusion Processes

Often an improvement in one section of a line or plant will cause the rate-limiting section to shift to either an upstream or downstream process. These types of projects often have considerable value associated with them because of the multiple bottlenecks that exist. 

Extrusion processes are often rate limited by motor power or torque, discharge temperature, or the melting capacity of the screw. Other root causes associated with the design of the screw can limit rates as shown in previous sections. The problems, however, are typically associated with other defects such as flow surging or resin degradation. Chapters 11 and 12 discuss process defects associated with resin degradation and flow surging, respectively. Rate limitations due to inadequate motor power and torque are common problems for commercial plants. Two case studies are presented in the next sections that show rate limitations due to the lack of torque and motor power. 

Extrusion processes can be rate limited by the maximum allowable temperature of the discharge. For thermally sensitive resins, the extrudate will be required to 

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Table 13.1 Screw Channei Dimensions for the 114.3 mm Diameter Screw for the Specialty TPU Resin Extrusion Process. This Screw was Rate Limited by an Opticai Defect...

In polymer processing practice, we need to ensure that the particulate gravitational mass flow rate of the hopper exceeds, over the complete operating range, the extruder open discharge rate (i.e., the rate without any die restriction). That is, hoppers must not be the production-rate limiting factor. Second, and more importantly, it is necessary for stable extrusion operations and extruded product quality that the flow be steady and free of instabilities of the particulate flow emerging from the hoppers. Finally, as we will see in Chapter 9, we need to know the pressure under the hopper in order to determine the pressure profile in a SSE. 

These phenomena have been the subject of intensive study during the last 50 years and still represent major problems in polymer rheology. From a processing point of view they are very important, since melt fracture represents an upper limit to the rate of extrusion, and swelling and the large pressure drops must be accounted for in product considerations and in the design of the die and processing equipment. 

The rubber-elastic deformation of the liquid stream can reach such high values that the liquid breaks. Though the remaining fragments will stick together in a further part of the die channel, this melt fracture forms a notorious limitation in the rate of production with extrusion processes. The only remedy is to increase the time scale of deformation, so that the elastic behaviour is less outspoken. 

During the extrusion of polymers different defects and flow instabilities occur at very low Reynolds numbers. The commonly known ones are sharkskin, melt fracture, slip at the wall and cork flow. These defects are of commercial importance, since they often limit the production rate in polymer processing. Many researchers have been interested in the subject, and thorough reviews on flow stability and melt fracture have been written in the last 30 years [1-4]. More recently, two review papers deahng with viscoelastic fluid mechanics and flow stability, were published by Denn [5] and Larson [6]. However, although much work has been done in the field of extrusion distortions, controversy still exists regarding the site of initiation and physical mechanisms of the instabilities. 

Consideration should be given to the availability of small-scale equipment, which is vital for development work prior to scale-up on pilot- or production-scale machines. Equipment choice is not necessarily based on maximum throughput rate, since the subsequent processing stages (e.g., cutting, spheronization, and drying) are batch processes and are therefore, a rate-limiting factor in production. Since extrusion is a continuous process, it allows adequate production rates for most purposes with any of the above mentioned extruder types. 

Although the extrusion step is a continuous process with a very high throughput, the subsequent steps (spheronization, drying, and sizing) are batch processes, and thus are rate limiting. As a result, the overall extrusion-spheronization process is a batch rather... 

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