The compression-molding process
Compression molding is a technique in which the NdFeB magnetic powder is mixed or encapsulated with the smallest amount of resin possible and pressed into a green (uncured) compact with the highest practical pressure to achieve the highest possible density: the best compression-molded magnets have a density of ~79% of the theoretical density of 7.65 g/cm3 for the NdFeB magnetic powder. The principle advantage of compression molding is that the metal-loading factor is the highest of any of the processing methods and magnets with high tolerance, except in the press direction, can be rapidly produced. This is particularly true of thin-walled, ring-shaped magnets that are now commonly used in a variety of spindle and stepper motors. Ring-shaped magnets are almost impossible to produce from sintered Nd magnets because of warping or cracking during the sintering process. Although ring-shaped magnets can and are produced by the hot deformation of melt-spun ribbon, the back extrusion process used is more expensive and complicated compared to the compression-molding process. These hot-deformed ring magnets are discussed in Chapter 6, Hot-deformed NdFeB permanent magnets. The major disadvantage of compression molding is that complex shapes are difficult to produce. Also, the magnets are friable in the green state and are still somewhat brittle even after curing.
Compression molding is a well-established method of rubber part manufacturing that utilizes heat and pressure to bond the rubber compound into shape leaving minimal parting lines (seams). The process consists of compressing a piece of uncured rubber between heated mold plates placed into a hydraulic press. The heat and pressure molds the rubber to the shape of the cavity and vulcanization occurs after a prescribed time. Once the part is removed from the mold, any excess rubber material can be trimmed off or ‘deflashed’. Rubber compression molding is a manufacturing technique which has proven to be a cost-effective manufacturing solution for many types of molded rubber parts.
Compression molding is used for manufacturing electrical wall receptacles, brush and mirror handles, meter cases, trays, circuit breakers, cookware knobs, clothes dryer blower fan blades, electronic and cooking utensils, milling machine adjustment wheels, automotive parts, water testing equipment buttons, television cabinets, dinnerware, appliance housings, radio cases, aircraft main power terminal housing, hoods, pot handles, spoilers, electric plugs and sockets, fenders, dinnerware plates, and scoops.
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