Rheometer - A357
| Rheometer | |
|---|---|
| |
| Rheometer, Anton Paar, MCR 502 | |
| Document Type | Article |
| Document Identifier | 357 |
| Themes | |
| Relevant Class |
Equipment |
| Tags | |
| Factory Cells | |
Introduction[edit | edit source]
A rheometer is an instrument used to study material flow and deformation under external loads. Rheology, the 'study of flow behaviour' is not only about studying the flow behavior of liquids but also about deformation and rate of deformation of solids. All types of shear response stands somewhere between flow of an ideal viscous liquid and deformation of an elastic solid. Therefore, the material's response is a blend of both a viscous and elastic response, and is referred to as viscoelastic. Viscosity and elasticity are essential characteristics of material behavior, which can be quantified by applying a rotational and oscillatory load on a sample of the material.
Rheometry is the technology for measuring rheological properties of materials. To evaluate viscous behaviour, rotational tests are performed. For characterizing viscoelastic behaviour, creep, relaxation and oscillatory tests are performed.
Features[edit | edit source]
Rheometers can control the applied stress or strain and measure the resulting strain or stress, respectively. In a parallel plate configuration, the rheometer comprises two parallel plates with the sample positioned between them. The upper plate rotates while the lower plate remains stationary. The temperature of the plates can be controlled to maintain a constant temperature or gradually increased to reach a specific point.
A wide range of measuring geometries are available, depending on the characteristics of material being tested and information required. For instance, disc spindles prove effective in assessing the viscosity of a diverse range of liquids. On the other hand, cylindrical spindles are well-suited for examining the shear stress, shear rate, and viscosity of non-Newtonian fluids, while vane spindles are adept at measuring the viscosity of gels. Additionally, Couette cells show excellent sensitivity when dealing with low-viscosity fluids and fluid suspensions. In contrast, parallel plate, cone, and plate geometries are common for analyzing polymeric materials.
Uses and Test Types[edit | edit source]
Simple instruments, used to measure the viscosity values of a sample via speed-controlled rotational tests used to be called rotational viscometers. Instruments are termed as rotational rheometers when further rheological tests are possible such as torque- or shear stress-controlled rotational tests. With most rheometers, also the viscoelastic behavior of samples can be characterized by performing creep tests, relaxation tests, and oscillatory test. The rheometer can be used to model the behavior of a wide variety of liquids over time and at different temperatures. Rotational tests conducted with a rheometer can operate in one of two modes. In the first mode, velocity is applied through either rotational speed or shear rate, known as controlled shear rate (CSR or CR). This mode effectively replicates processes dependent on flow velocity or volume flow rate, such as coating with a brush or spraying. In contrast, the second mode involves applying force through torque or shear stress, referred to as controlled shear stress (CSS or CS). Tests conducted in this mode simulates force-dependent applications, such as the force required to pump liquid, to extrude sealing materials from a cartridge, or to dispense paste from a tube. In oscillatory tests, the frequency of oscillation and strain can also be varied.
Analysis of Results[edit | edit source]
For rotational tests, shear stress vs shear rate or viscosity vs shear rate is usually plotted. In the latter case, the log of both variables is usually plotted to better visualize the relative changes in viscosity response. In oscillation tests, shear lag and shear strain are plotted to determine the phase shift, or how much one variable lags behind the other. From that, we can then determine the storage and loss moduli vs frequency or strain, which describes the viscoelastic behavior of the sample.
Frequency sweeps generally serve the purpose of describing the time-dependent behavior of a sample in the non-destructive deformation range. High frequencies are used to simulate fast motion on short timescales, whereas low frequencies simulate slow motion on long timescales or at rest.
Equipment Demonstration[edit | edit source]
The video below provides an introduction to the Rheometer. It outlines it's uses, parts of the instrument, different geometries, and tests that can be conducted with a Rheometer. In the second half of the video, tests are performed on two different resin systems and results are analyzed and discussed.
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Welcome
Welcome to the CKN Knowledge in Practice Centre (KPC). The KPC is a resource for learning and applying scientific knowledge to the practice of composites manufacturing. As you navigate around the KPC, refer back to the information on this right-hand pane as a resource for understanding the intricacies of composites processing and why the KPC is laid out in the way that it is. The following video explains the KPC approach:
Understanding Composites Processing
The Knowledge in Practice Centre (KPC) is centered around a structured method of thinking about composite material manufacturing. From the top down, the heirarchy consists of:
- The factory
- Factory cells and/or the factory layout
- Process steps (embodied in the factory process flow) consisting of:
The way that the material, shape, tooling & consumables and equipment (abbreviated as MSTE) interact with each other during a process step is critical to the outcome of the manufacturing step, and ultimately critical to the quality of the finished part. The interactions between MSTE during a process step can be numerous and complex, but the Knowledge in Practice Centre aims to make you aware of these interactions, understand how one parameter affects another, and understand how to analyze the problem using a systems based approach. Using this approach, the factory can then be developed with a complete understanding and control of all interactions.
Interrelationship of Function, Shape, Material & Process
Design for manufacturing is critical to ensuring the producibility of a part. Trouble arises when it is considered too late or not at all in the design process. Conversely, process design (controlling the interactions between shape, material, tooling & consumables and equipment to achieve a desired outcome) must always consider the shape and material of the part. Ashby has developed and popularized the approach linking design (function) to the choice of material and shape, which influence the process selected and vice versa, as shown below:
Within the Knowledge in Practice Centre the same methodology is applied but the process is more fully defined by also explicitly calling out the equipment and tooling & consumables. Note that in common usage, a process which consists of many steps can be arbitrarily defined by just one step, e.g. "spray-up". Though convenient, this can be misleading.
Workflows
The KPC's Practice and Case Study volumes consist of three types of workflows:
- Development - Analyzing the interactions between MSTE in the process steps to make decisions on processing parameters and understanding how the process steps and factory cells fit within the factory.
- Troubleshooting - Guiding you to possible causes of processing issues affecting either cost, rate or quality and directing you to the most appropriate development workflow to improve the process
- Optimization - An expansion on the development workflows where a larger number of options are considered to achieve the best mixture of cost, rate & quality for your application.
