PURPOSE:To control a leak of magnetic flux by flowing the magnetic flux smoothly, by providing in a fixed state a side yoke composed of a magnetic material which fits in a tie bar on a mold side of a stationary platen or a movable platen and either approaches or comes into contact with the mold. CONSTITUTION:Side yokes 16, 17 which fit over a tie bar 6 and either approach or come into contact with mold members 2, 7 at fine clearances are fixed on facing surfaces of a stationary platen 1 and movable platen 4 and a solenoid coil 18 is arranged by fitting over the tie bar 6 between the side yokes. When the solenoid coil 18 is electrified, magnetic flux passes through a mold cavity 9 by making use of the side yokes 16, 17, molds 2, 7 and tie bar 6 as magnetic paths as shown by an arrow. As the magnetic paths 2, 6, 7, 16, 17 are short and parts 1, 4, 5 whose permeability is poor are excluded from the magnetic paths, the magnetic flux flows smoothly, a leak of the same is little and a magnetic field can be formed at favorable electric power efficiency.
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