GB1059937A - Improvements in or relating to the manufacture of magnetic cores - Google Patents

Abstract

A method of making a magnetic core comprises forming a stack of ferromagnetic laminae having a curable adhesive between successive laminae, subjecting the stack to pressure in a pressing device to reduce it to the desired size, and curing the adhesive, wherein the adhesive is thixotropic and cures to form a resilient synthetic resin, and asbestos, rubber &c. layers are interposed between the stack and the pressing device to avoid tangential stresses, or these are avoided by using in the pressing device plates of a material having the same coefficient of expansion as the laminae. Abutments stop the press members when the stack has reached the desired size. The adhesive may be a mixture of one or more polyhydroxy compounds which are liquid at room temperature, and one or more crystallized polyisocyanates. The adhesive may be made thixotropic by brief heating, after application to the laminations, or by the addition of a filler, e.g. silica. The crystallized polyisocyanate may be dimerized toluylene-2:4-diisocyanate or the crystallized reactive initial adducts consisting of di- or polyisocyanates and di- or polyhydroxy compounds, which act as polyisocyanates. The polyhydroxy compounds may be branched polypropylene-glycols and castor oil. An accelerator for reducing the curing temperature may be organo tin compound. In an example the adhesive consists of a mixture of castor oil, crystallized, pulverous, dimerized toluylene-2:4-diisocyanate, in the approximate ratio of 2:1, 3 to 7% of highly dispersed silica, and from 0.2 to 1% of water to adjust the level of thixotropy. The adhesive is applied by rubber rolls and cured at 80 DEG C. for about 30 hours.