Surface Engineered Polymer Bonded Magnets
Alfred Sidambe, Peter Logan, Jeffrey Alcock, David Stephenson
Advanced Materials, School of Industrial and Manufacturing Science, Cranfield University
United Kingdom

Keywords: Co-injection , Bonded, Magnets

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The feasibility of the manufacture of surface engineered polymer bonded magnets using the co-injection moulding process has been investigated. This is a technique for producing bonded magnets where the magnetic powder is confined to the skin of a component. The use of co-injection moulding is expected to have economic benefits, because of reduced powder usage and the reduction of concomitant processing - for example to add fixings and shafts to magnetic components. Many parts only require surface magnetism, for example multi-pole magnets self demagnetise at depths of approximately 3mm. In this study, co-injected polymer bonded magnets were made with two different combinations of materials.

The first mouldings were made with a Nylon 6 (Grilon A28GM) core and a Strontium Ferrite powder (SrFe12O19) / Nylon 6 compound skin. The second were made with a Nylon 12 (Grilamid L16) core and a Neodymium-Iron-Boron powder (Nd2Fe14B) / Nylon 12 compound skin. An APV Baker MPF19 twin-screw extruder was used for feedstock preparation of 50% volume fraction of SrFe powder in PA6 and 30% volume fraction of NdFeB in PA12. A Dasset Twin Barrel machine was used to co-inject a PA6 core in SrFe/PA6 mix skin and a PA12 core in NdFeB/PA12 mix skin.

All the main processing routes of such magnets were investigated including the mixing/compounding techniques for preparation of powder and polymer. Homogeneity studies were carried out using torque measurements, optical and burn-off analysis. The co-injected samples were finally magnetised and results showed that the values of Br (remanence) were approximately a third of standard commercial values of magnets which do not have a polymer core.