Over molding magnets:
Typically, the magnet or magnet assembly is inserted into the cavity of an injection mold, and the molten plastic material flows around it after closing, completely covering the magnet or magnet assembly or covering other than the magnetized surface.
Magnets Over molding points:
1. During the molding process, the temperature of thermoplastics is high, so usually unmagnetized permanent magnets are put into the mold for injection molding to avoid magnetic attenuation caused by high temperature injection molding of magnetic devices.
2. During the injection molding process, the hot-melt plastic usually exerts impact and high pressure on the magnet. Therefore, the fixation of the magnet in the mold and the design of the mold clamping tolerance are very critical, so as to avoid the magnet from breaking, overflowing glue (burrs) or not full injection.
Purpose of Over molding:
1. For pumps, valves, etc., which are in contact with liquids or solvents, in order to avoid corrosion or failure of the magnet, it is necessary to coat the surface of the magnet to improve the environmental resistance of the magnet.
2. Auto parts, electric tools, switches, etc. are often opened, have large vibrations, and require high tensile and compressive strength. Selecting magnets for secondary injection molding can improve the strength of the magnets and prevent the magnets from cracking or cracking during work. Break release.
Overmolding magnets Material Selection:
Injection molded plastics are typically overmolded with layers of polymers such as POM, Nylon, PPS, etc., using polymer binders such as Nylon 6, 11 and 12, and PPS (polyphenylene sulfide). Binder evaluation is an ongoing activity in the development laboratory to improve the physical strength, maximum recommended service temperature, corrosion resistance, dimensional stability in the presence of water and solvents, and ease of processing of rare earth magnetic materials.
Magnetic powder includes ferrite and neodymium iron boron (NdFeB) or samarium iron nitrogen (SmFeN) magnetic powder. Ferrite and NdFeB and Samarium Iron Nitride (SmFeN) are available in a variety of grades offering a wide variety of magnetic properties.
Overmolding magnets Temperature characteristics
The maximum service temperature is a function of the magnetic alloy powder and binder. For example, ferrite powders in nylon 6 or PPS were tested satisfactorily at 150°C and 180°C, respectively. Nylon 12, on the other hand, melts above 170°C and is not recommended for use above 150°C. Magnetic materials, such as the highest energy NdFeB grade, with an intrinsic coercivity of 9,500 Oersted (760 kA/m), suffer significant irreversible magnetic losses above 120°C regardless of the binder used . The maximum operating temperature is listed on each product's specification sheet.
Overmolding magnets Tolerance
An important feature of injection molded magnets is that they fit dimensionally into precisely machined mold cavities so the finished product has tight tolerances. The small dimensional change is due to the shrinkage of the polymer during cooling. It depends on the thickness and shape of the part. Typical tolerance is +/-0.03 mm.
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