Extrusion in a Magnetic Field
The viscosity of ferromagnetically filled thermoplastics can be increased by applying a magnetic field to the extrusion die. The cooler the melt, the stronger is the effect of the magnetic field.
Polymer-bonded magnets represent a growing market segment in the field of permanent magnets. These consist of a polymer matrix with magnetic fillers. Ferrite magnet products in the form of ribbons or sheet can be produced via extrusion.
Researchers at the Institute for Plastics Technology at the University of Paderborn (IKP) have now succeeded in demonstrating that the viscosity of magnetic compounds can be increased when a magnetic field is applied to the region of the extrusion die. in addition, a relationship between the magnetic field and the temperature has been observed. The stronger the magnetic field, the more temperature-sensitive is the viscosity of the compound. Conversely, at lower temperatures, the effect of the magnetic field on the viscosity is greater than at higher temperatures.
In the experimental arrangement, the magnetic field was applied crosswise with respect to the extrusion direction. The magnetic induction achieved a value of about 1 T (Tesla). Using this setup, the effect of the magnetic field on the flow behaviour of various mixtures of polyamide (nylon) 6 as the matrix component and strontium ferrite as the filler was investigated.
The amount of filler and the magnetic field strength were varied in increments. The compounds were processed at various throughput rates and melt temperatures. In all experiments, the magnetic field exhibited a strong influence on the viscosity of the compound. The magnetization results in orientation of the elemental magnets in the ferrite platelets.
In order to permit more precise statements, the entire set of experiments was subjected to multiple regression analysis. The model was calculated with a confidence level of 90% from 156 individual measurements with a degree of certainty of 0.8576 and can thus be considered statistically reliable.
As expected, the amount of filler has a direct and significant effect on the flow behaviour of the compound. Moreover, a relationship between the amount of filler and the temperature is clearly observable. At lower temperatures, the amount of filler has a stronger effect on the viscosity of the compound than at higher temperatures.
In the regression model, the magnetic flux is identified as a significant parameter. There is a primary effect as well as an interactive effect between the temperature and the magnetic flux. The lower the temperature, the more the viscosity is increased by the magnetic field.
In contrast, a direct dependence of the flow behaviour on the temperature without consideration of the interactive effects of magnetic flux and filler content is observable to only a slight degree. Similarly, only a slight effect of filler content and shear rate is noted.
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