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As some materials are extruded or otherwise distorted during processing, the energy imparted during the process and friction with the die walls may be manifested in a significant increase in temperature. This phenomenon is known variously as viscous heating, shear heating and viscous dissipation. This can have unexpected effects on the mechanical and physical properties of the product and upon its appearance, and can lead to severe quality control problems. For example, rubber compounds may exhibit "scorch" or premature curing, some plastics may exhibit partial degradation, foods may start to cook, and active pharmaceutical ingredients may become denatured. As some materials are extruded or otherwise distorted during processing, the energy imparted during the process and friction with the die walls may be manifested in a significant increase in temperature. This phenomenon is known variously as viscous heating, shear heating and viscous dissipation. This can have unexpected effects on the mechanical and physical properties of the product and upon its appearance, and can lead to severe quality control problems. For example, rubber compounds may exhibit "scorch" or premature curing, some plastics may exhibit partial degradation, foods may start to cook, and active pharmaceutical ingredients may become denatured. The instrument will measure temperatures from below ambient to over 500°C to an accuracy of around 0.2°C. As with all infra-red thermometers, the measurement accuracy depends upon knowing the emmisivity of the sample surface, and our software provides a method for allowing for this. Multi-phase materials such as pastes and doughs may suffer from aeration, ie small bubbles caused by volatile ingredients boiling, and this can radically affect performance. Additionally, viscous heating may trigger wall slip by promoting the migration of low-molecular weight components to the die wall where they lubricate the flow. It has been known, in a material using a volatile solvent as a processing aid, that viscous heating has caused the solvent around the periphery of the extrudate to "flash off" directly after extrusion. When the product was subsequently dried, the core shrank away from the outer hardened skin, giving radial orientation around the outside of the product but with axial orientation, ie in the extrusion direction, along the central core of the profile. This orientation manifested in bizarre distortions when the material was heated and the locked-in stresses relaxed. Infra-red pyrometry relies upon the observation that bodies emit infra-red radiation at a wavelength related to their temperature. Such observations are used, for example, to estimate the temperatures of stars, and, more mundanely, in industrial processes to measure the temperature of molten metals and other materials by a non-contacting means. Note that the amplitude of the emitted infra-red radiation is a function of the emissivity of that particular material, which must be determined for accurate results. Developments
in semiconductor technology have provided sensors and associated circuitry
which will allow the temperature of materials to be measured by such
a technique at temperatures down to below room temperature. The CIRP
Non-Contacting Infra-Red Pyrometer uses such a sensor and is fitted
with a special close-up attachment to allow the temperature of a small
spot of diameter approximately 1 mm on the moving extrudate to be continuously
monitored from a distance of 6 millimetres. |
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We are always happy to quote for special modifications to suit your needs
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©
2008 Celsum Technologies Limited |
