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Rheology

The Importance Of Extensional Viscosity In Polymers

Extensional or elongational viscosity are two terms used to describe the same thing. This is the stress that pulls the sample in a straight line, so the force is elongated, without the shear force. Every polymer has an extensional viscosity that is consistent within a sample or the same type of polymer. The elongational viscosity is significant for most polymers because of their structure. These tend to be made up of chained molecules that are intertwined and wrapped around other chains. The more intertwining that occurs in a polymer, the greater the force that will be needed to create the stretching action. The more that the polymer resists stretching, the higher the viscosity will be for the polymer. How it is Measured Many polymers, in their room temperature state, cannot be stretched or elongated because of their molecular structure. However, by taking a small sample of the polymer and heating it to the point of melting or a molten state, the stretching can occur. This is not done by stretching the liquid on its own, but rather using a drawing stem of wheels, sometimes called a haul-off, to create filaments of the polymer. This can be done with multiple sets of wheels to create the optimal filament for testing. This is done at a precise speed, and the different result can accurately determine the polymer in the sample. In effect, three speeds have to be controlled including the linear pull-off speed, the acceleration, and the exponential speed. The force required to elongate the strand is measured, which provides measurements for extensional viscosity, elongational stress, draw ratio and the rate of elongation. Often extensional viscosity is measured in conjunction with other types of tests for a given polymer. When this is a factor, look for systems that can be integrated to streamline the testing...

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What are Laboratory Extruders For?

There’s no doubt about the fact that plastics and rubber are massive, and necessary in almost any major industry. It’s hard to come up with an efficient, functioning and contemporary design of any form of technology that doesn’t utilize plastics or rubber in its production. From the simplest of tires and grocery bags to far more complex and interesting uses for these two material types, such as more specialized engineering applications explored over the decades, like specialized rubber bearings as per a study on the subject, and the use of plastics in modern-day circuitry only goes to show how these two materials continue to shape our world, and will continue to do so for a long time yet. Entering the industry, however, means entering a field with lots of competition. And the one thing that determines who best gets the local deals in that competitive landscape is quality. Quality in plastics and rubbers is extremely important, even more so than pricing. What businesses truly value in a supplier is the ability to provide the exact parameters and compositional needs a business could require in their respective industries. Determining the quality and use of a specific material after it’s been sourced requires the use of a special laboratory extruder – and that’s where this device comes into play. What is a Laboratory Extruder? A laboratory extruder is a complex piece of machinery designed to tell the processability of a given rubber or polymer based on a number of on-site testing capabilities and device techniques. A quality laboratory device will be capable of measuring very fine powders, coarse materials and other forms of materials at relatively low sample sizes in order to determine the industrial viability of any given product, especially within the strict parameters set forth by companies today aiming for innovation in the production of their inventions and...

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Capillary Rheology: Measuring Success

The competitive nature of the plastics industry together with the invention of new and innovative materials and techniques can make success difficult. One mistake can result in an economic loss from which a company will have difficulty recovering. It makes it essential that companies utilize on their technology, instruments and skills to ensure the material they employ performs to expectations under the selected processing method. To help achieve this, many industries apply capillary rheology. While melt flow testing is another option, using a capillary rheometer can prove to provide more information on the behavior and various properties of a material such as a polymer and various plastics. Capillary Rheometers A capillary rheometer is an instrument companies employ to measure the changes in a material’s viscosity as they are relative to the shear rates. If the rheometer is controlled-stress and high-shear, its parts will consist of: A heated barrel – single or double bore are the two basic options. Double bores come into play if the technician wishes to conduct two tests under diverse conditions simultaneously. If a twin bore is combined with a “zero length die” this will allow the technician to determine both shear and extensional viscosity concurrently A piston A calibrated die – it is changed when the company requires the die to determine the rheological properties of the material under different conditions In this manner, the rheometer can measure not only the load, but also the piston speed and the die geometry. In addition, technicians can calculate the shear viscosity by knowing these three critical factors: Die dimensions Piston speed Pressure Technicians employ the capillary rheometer in various types of material processes. These include extrusion and injection molding where the rheometers track the flow of the plastic or polymer through the defined space to achieve a measure of true or absolute viscosity, something not achieved by torque rheometers. Why Employ Capillary Rheology? Several reasons lie behind the use of a capillary rheometer. By adopting this method, a technician can supply the company with valuable information including: Determination of the optimal working strictures for various processing methods including blow molding, extrusion and injection molding Examination of various processing concerns swifter and with less disruption Discovering which specific materials are the most suitable for long flow lengths or complex components Replication of the manufacturing strictures for various reasons including design, product or numerical simulations and troubleshooting While other reasons exist, including reducing the instances of lost time, wasted material and economic efficiency, capillary rheology is first and foremost about measuring true or absolute...

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