Maya Ellison
Shear heating is an important concept in plastic extrusion, where the interaction between the screw and barrel creates friction and heat, melting the polymer. This process is known as viscous dissipation, and it is responsible for a significant portion of the heat generated during extrusion. The design of the extruder screw is critical, with factors such as the length-to-diameter ratio, compression ratio, and flight geometry influencing the melting and mixing efficiency, as well as the pressure and shear applied to the plastic. Understanding the location and magnitude of shear heating can help optimize the temperature profile and improve the stability and efficiency of the process. Excessive shear heating can lead to issues such as resin degradation, yellowing, and surface defects, so it is important to consider screw design and other factors to minimize these negative effects.
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Viscous dissipation
During viscous dissipation, the rotating screw shears the polymer, generating frictional heat through the interaction between the screw and the barrel. This heat contributes significantly to the overall thermal energy required for plasticising and heating the material. The highest shear stress and resulting polymer heating occur just inside the inner surface of the barrel. As the polymer traverses the barrel, its viscosity decreases, complicating the analysis of shear heating. However, by temporarily turning off barrel cooling and observing zone temperatures, operators can identify "hot spots" that indicate high mechanical energy conversion to heat.
The understanding of viscous dissipation is crucial for optimising screw performance and extruder control. By working with the equipment, operators can achieve more stable operations and efficient power usage. For example, adjusting the barrel temperature can lower melt viscosity and reduce viscous shear heating. Additionally, lowering screw revolutions per minute can decrease shear heating but may require a longer residence time, potentially leading to resin yellowing.
The concept of plastic dissipation is related to plastic deformation, where atomic bonds break, and energy is dissipated. This irreversible deformation results in materials retaining their new shape even after the removal of applied stress. The energy associated with plastic dissipation is the difference between the energy due to plastic deformation and the energy stored within the volume element as isotropic and kinematic hardening.
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Screw design
One of the key considerations in screw design is finding the "
The depth ratio between the feed flights and metering flights is a critical parameter in screw design. In small machines, the feed depth must be sufficient to allow smooth feeding while also preventing screw-shaft breakage. Shallower depths in the metering zone promote better mixing and control over output, while deeper depths increase output per turn but also increase sensitivity to high pressure. The length of the screw is also important, with longer screws providing more time for melting and typically resulting in higher output but at elevated melt temperatures.
The compression ratio of the screw was once a primary design parameter but has since been superseded by other considerations. While it provides some insight into performance, it does not capture the full complexity of screw design. Screw design is a delicate balance that requires careful consideration of numerous variables, and each screw is uniquely designed to match the specific requirements of the polymer, extruder, and process.
To enhance screw performance and efficiency, it is crucial to understand where shear heating occurs within the extruder. By temporarily disabling barrel cooling and observing changes in zone temperatures, operators can identify “hot spots" that indicate high mechanical energy conversion to heat. This knowledge, combined with an understanding of screw design, enables the prediction of high shear stress areas and facilitates troubleshooting and optimization of the screw's performance.
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Barrel cooling
The barrel is an important component of the extrusion process. It is made of alloy material, which is quenched, tempered, and desalinated to improve wear resistance. The pressure inside the extruder barrel during extrusion is high, with normal operating pressures ranging between 1,000 and 5,000 psi or 70 and 350 bar.
The rear-barrel temperature is critical as it controls pellet slip on barrel walls in the first zone(s) and, thus, controls the inpush/output rate. The first zone is usually water-cooled to prevent sticking and melting. With plain barrels, cooling may be necessary to avoid sticking in the feed entry passages.
Overall, barrel cooling plays a crucial role in maintaining temperature control during the extrusion process, ensuring the plastic does not get too hot and preventing potential issues with the final product.
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Backpressure
The pressure applied at the other end of the screw counters the pressure exerted by the melt in front of it, and this pressure is backpressure. Backpressure is essential in injection moulding and needs to be controlled for an efficient injection moulding run. It helps to expel volatiles from the melt, improving the consistency and quality of the final product.
Optimizing backpressure can be time-consuming, and it is just one of the many parameters that need to be controlled and monitored in the injection moulding process. Understanding how backpressure occurs is crucial for better process control. By increasing backpressure, the part weight also increases due to the increase in melt density.
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Compression ratio
The compression ratio is essential because it determines the shear rates throughout the screw. Shear heating is a crucial mechanism in the plastic extrusion process, where heat is generated by the plastic layers sliding against each other. This friction between the layers accounts for a significant portion of the total heat input in the process, often ranging from 60% to 80%. Therefore, the compression ratio directly impacts the amount of shear heating generated, influencing the overall melting efficiency.
While compression ratios have been standardised over the years, it is important to consider various variables to optimise the process. For instance, different polymers have distinct energy requirements, viscosities, specific heats, and ideal exit temperatures. Additionally, the bulk density, angle of repose, particle hardness, melt density, and hopper design can all influence the performance of the screw and the overall extrusion process.
The compression ratio also interacts with the barrel temperature. Increasing the barrel temperature in the compression zone can lead to earlier melting. However, this can also decrease the plastic's viscosity, reducing shear heating. Therefore, careful consideration of the compression ratio and barrel temperature is necessary to achieve the desired melting rate, especially when working with different polymers and recycling applications.
In summary, the compression ratio is a critical factor in plastic extrusion, particularly screw extrusion, as it influences the shear heating generated and, consequently, the melting efficiency of the process. By optimising the compression ratio and considering various variables, manufacturers can enhance the overall extrusion process and achieve the desired results.
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Frequently asked questions
Shear in plastic extrusion refers to the shearing or stretching of the polymer between the rotating screw and stationary barrel, causing heat to develop in the material.
Shear is created by the interaction between the screw and the barrel. As ingredients go through the barrel, screws and steamlocks, they are exposed to very high shear rates which create very high temperatures.
Shear heating can lead to resin degradation and yellowing in any layer within the structure. It can also cause flow issues, such as "shark skin" (surface roughness) or melt fracture.
Shear heating can be minimized by changing the screw design to a lower-shear screw in the melting and/or mixing sections. It can also be reduced by lowering the screw revolutions per minute and throughput rate in single-screw extruders.
