Chloe Zhang
Shear rate is a crucial concept in understanding the behaviour of plastics during processing, particularly in injection moulding. It refers to the rate of change in velocity as a fluid undergoes progressive shearing between adjacent layers. In the context of plastics, shear rate influences the viscosity or fluidity of the material, impacting its processability and final part quality. By manipulating the shear rate, manufacturers can control the viscosity of plastics to ensure consistent and efficient production. This understanding of shear rate is essential for optimising processes like injection moulding, where the fill time and injection speed directly affect the shear rate and, consequently, the viscosity of the plastic.
| Characteristics | Values |
|---|---|
| Definition of shear rate | The rate of change of velocity with progressive shearing that takes place between the adjacent layers of fluid. |
| Shear rate in relation to viscosity | The viscosity of plastics is influenced by shear rate, temperature, and pressure. |
| Shear rate in relation to velocity | Shear rate is like the velocity of the plastic divided by the gap it is flowing through. |
| Shear rate in relation to fill time | Fill time affects how much shear thinning the plastic experiences, which in turn affects the material's viscosity. |
| Critical shear rate | The point where the slope changes from shallow to steep on the viscosity curve. |
| Shear rate in relation to injection molding | The shear rates during injection molding are high and can be 1000–10,000 s−1. |
| Shear rate in relation to shear heating | The highest shear rates are in the outer laminates, adjacent to the stationary wall, and drop down to zero in the center of the channel. |
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What You'll Learn
Shear rate and viscosity
Shear rate is a critical concept in understanding the flow behaviour of plastic materials. It describes the rate at which adjacent layers of a material move relative to each other during deformation. In simpler terms, it quantifies how quickly layers of plastic are shearing past each other when a force is applied. This force could be due to processing conditions like extrusion or injection moulding, or it could be a result of applied stresses during use. The shear rate is typically denoted by the symbol $\gamma$ (gamma) and is measured in inverse seconds (s$^{-1}$) or radians per second (rad/s).
Mathematically, the shear rate is defined as the velocity gradient perpendicular to the direction of flow. Imagine a thin layer of plastic material being stretched or deformed. The velocity of this deformation varies across the layer's thickness, and the shear rate quantifies how much it changes per unit distance. It is calculated by taking the velocity difference between two adjacent layers and dividing it by the distance between them. A higher shear rate indicates a more rapid change in velocity, while a lower shear rate suggests a more gradual deformation.
Shear rate plays a significant role in determining the viscosity, or fluidity, of plastic materials. Viscosity is a measure of a fluid's resistance to flow and is often referred to as a fluid's thickness or internal friction. In the context of plastics, viscosity determines how easily the material can be processed and how it responds to applied forces. By controlling the shear rate, manufacturers can influence the viscosity of the plastic melt and, consequently, the final properties of the moulded part.
The relationship between shear rate and viscosity is described by the rheological behaviour of the plastic material. Rheology is the study of how materials deform and flow under applied forces. Different plastics exhibit distinct rheological behaviours, and this plays a crucial role in processing and end-use applications. For example, some plastics may show shear-thinning behaviour, where viscosity decreases with increasing shear rate. This behaviour is advantageous in processing as it allows for easier flow during injection moulding but maintains dimensional stability once the part is formed.
On the other hand, some plastics may exhibit shear-thickening behaviour, where viscosity increases with increasing shear rate. This behaviour is less common but can be beneficial in specific applications, such as impact resistance or vibration damping. Understanding the rheological properties of a plastic material, including its response to different shear rates, is essential for optimizing processing conditions, predicting flow behaviour, and ensuring the final product meets the desired specifications.
The shear rate also influences other critical aspects of plastic behaviour, such as melt elasticity and shear-induced crystallization. Melt elasticity refers to the ability of a plastic melt to store and release energy elastically during deformation. This property is influenced by the shear rate and can impact the processability and final properties of the material. Shear-induced crystallization is another important phenomenon where the application of shear can induce or enhance the crystallization process in semi-crystalline polymers. This has implications for the mechanical, optical, and barrier properties of the final plastic product.
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Injection moulding
Shear rate is a critical factor in injection moulding, a process used to create plastic parts. It refers to the change in velocity of parallel planes in a flowing fluid, which can be thought of as the water flow in a river, where the water moves differently near the shore compared to the centre.
In injection moulding, the plastic melt is injected at high pressure, and the shear rate is influenced by factors such as the screw rotational speed, gate size, cavity volume, and fill rate. The fill time, or injection rate, is a crucial parameter to establish as it affects the shear heating, shear thinning, viscosity, pressure, and temperature of the plastic inside the cavities, ultimately impacting the overall part quality.
To optimise the process, the Relative Viscosity (RV) test is employed. This involves adjusting the shot size to achieve a 95% full part at maximum injection velocity, recording the fill time and pressure, and then repeating the process at reduced injection velocities. The data is then used to calculate relative shear rates and viscosities, with the ideal fill time selected on the "flat" portion of the resulting curve.
It is important to note that every material has a critical shear rate limit, beyond which the polymer melt fractures and the molecular weight decreases, degrading the material's properties. This can lead to issues such as melt fracture, where the plastic experiences excessive shear resulting in aesthetic and mechanical deterioration. Therefore, understanding the shear rate and its impact on viscosity is essential for successful injection moulding, allowing for the production of consistent and high-quality plastic parts.
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Shear heating
During injection molding, the pressure-driven flow consists of both internal and kinetic energy, where mechanical energy is converted into thermal energy (heat). The combination of high viscosity and high shear rates in plastic flowing through a runner can result in significant viscous dissipation. As flow is laminar during injection molding, the highest shear rates are in the outer laminates, adjacent to the stationary wall, and decrease to zero in the center of the channel. This results in shear heating being concentrated in the outer laminates, where shear rates are highest.
Studies have shown that shear heating in the outer laminates of a melt flowing through a runner can easily exceed 100°F, leading to significantly non-homogeneous melt temperature distributions during molding. These variations are often underappreciated, yet they create one of the biggest variables in injection molding today, influencing mold filling imbalances, filling patterns, polymer and additive orientation, part shrinkage, warp, and residual stress development.
Understanding the relationship between shear rate and viscosity is crucial to comprehending how materials, such as plastics, process and flow once molten. Shear rate is defined as the rate of change of velocity with progressive shearing between adjacent layers of fluid. It is influenced by the temperature of the plastic, the pressure in the molding machine, and the shear rate itself. The viscosity of plastics changes with variations in shear rate, and in some cases, shear rate influences viscosity more than temperature.
In the context of shear heating, viscous dissipation refers to the shearing or stretching of polymers between a rotating screw and a stationary barrel, causing heat development in the material. This process can account for up to 90% of the drive power in single screws. By understanding where shear heating occurs in an extruder, operators can optimize the temperature profile and improve the efficiency of power usage.
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Non-Newtonian plastics
Shear rate is the rate of change of velocity with progressive shearing that takes place between the adjacent layers of fluid. It is defined as the quotient of the change in velocity and the width of the channel through which the fluid is moving. The viscosity of plastics depends on temperature, pressure, and shear rate.
Plastics are non-Newtonian, meaning they change viscosity with changes in shear rate. At lower shear rates, plastics are non-Newtonian, but as the shear rate increases, plastics tend to exhibit Newtonian behaviour. This is because, with increasing shear rate, polymer molecules start to untangle from each other and align themselves in the direction of flow.
The viscosity of non-Newtonian fluids can change when subjected to force. For example, ketchup becomes runnier when shaken and is thus a non-Newtonian fluid. Many salt solutions and molten polymers are non-Newtonian fluids, as are many commonly found substances such as custard, toothpaste, starch suspensions, paint, blood, melted butter, and shampoo. Chilled caramel ice cream topping is also a non-Newtonian fluid if it incorporates hydrocolloids such as carrageenan and gellan gum.
Non-Newtonian fluids typically exhibit either shear thinning or shear thickening behaviour. Shear thinning fluids have a decrease in viscosity with increasing shear stress. Shaking an inverted bottle of ketchup can cause it to transition to a lower viscosity through shear thinning, making it easier to pour. On the other hand, shear thickening fluids have an increase in viscosity with increasing shear stress. For example, corn starch suspended in water (also known as "oobleck") appears milky when stirred slowly but feels like a very viscous liquid when stirred vigorously.
There are several models used to characterise non-Newtonian fluids, including the Herschel-Bulkley, Power Law, Casson, and Bingham Plastic models. A Bingham plastic fluid exhibits a yield stress, meaning it behaves as a fluid above the yield stress and like a solid below this stress level.
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Rheology and processing parameters
Rheology is the study of the deformation and flow of matter. In the context of plastics, rheology is concerned with how plastics behave when they are subjected to stress and deformation. The processing parameters of a plastic refer to the conditions under which the plastic is processed, such as temperature, pressure, and shear rate.
Shear rate is a critical factor in determining the rheological behaviour of plastics. It is defined as the rate of change of velocity with progressive shearing that takes place between adjacent layers of fluid. In other words, it is the velocity of the plastic divided by the gap it is flowing through. For example, a fast-moving plastic flowing through a large gap would have a low shear rate, while a slow-moving plastic flowing through a small gap would have a high shear rate.
The viscosity of plastics is highly dependent on the shear rate. As the shear rate increases, the viscosity of plastics decreases. This is because the molecules in the plastic are forced to align in a more parallel fashion, allowing them to flow more easily. This relationship between shear rate and viscosity is described by the viscosity curve, which plots the viscosity of a polymer against the shear rate.
The viscosity curve is a useful tool for understanding the rheology of plastics and optimising processing parameters. By generating a viscosity curve for a particular process, it is possible to determine the critical shear rate, which is the minimum speed required to produce consistent parts. This information can then be used to set the ideal fill time and injection speed to ensure process stability and consistency in the final product.
Additionally, shear rate also influences other aspects of plastic processing, such as shear heating. Shear heating is the increase in temperature that occurs due to the friction and deformation of the plastic during processing. It is highest in the outer laminates, adjacent to the stationary wall, and decreases towards the centre of the channel. Shear heating can impact the quality of the moulded part, and thus it is important to consider when designing processing parameters.
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Frequently asked questions
Shear rate is the rate of change of velocity with progressive shearing that takes place between the adjacent layers of fluid.
Shear rate influences the viscosity of plastics. As the shear rate increases, the viscosity of plastics decreases.
The viscosity of plastics is influenced by temperature, pressure, and shear rate. For some plastics, shear rate has a more significant impact on viscosity than temperature.
The shear rate in injection molding can be controlled by adjusting the injection speed and fill time. By keeping the fill time consistent, the shear rate can be maintained at a desired level.
The critical shear rate is the point on the viscosity curve where the slope changes from shallow to steep. Operating within a specific range of shear rates ensures process stability and consistency in the final product.
