Screw-Rotating/Injection Mechanism of Injection Molding Machine

The present invention relates to a screw-rotating/injection mechanism of an injection molding machine which rotates a screw and performs injection through forward movement of the screw. Background Art

In a conventional injection molding machine, an electric motor or a hydraulic motor is used as a drive unit for rotating a screw, and a hydraulic unit is usually used as a drive unit for moving the screw forward to inject a molding material into a mold. Another conventional injection molding machine has also been developed which uses a motor as the injection drive unit.

It is an object of the present invention to provide a screw-rotating/injection mechanism of an injection molding machine using a motor as a drive unit for rotating a screw and as an injection drive unit for forward movement of the screw, wherein injection control can be easily performed.

In order to achieve the above object, the present invention comprises a drive shaft fixed to a rear portion of a screw shaft and rotatably fixed to a pressure plate, a nut spline-coupled to a spline shaft which is formed on the drive shaft at a rear side of the drive shaft with respect to the pressure plate, a first transmission unit coupled to the nut, a screw rotating motor for driving the nut through the first transmission unit, a plurality of ball nuts fixed to the pressure plate, a plurality of ball screws threadedly engaged with corresponding ones of the plurality of ball nuts, a second transmission unit coupled to the plurality of ball screws, and an injection servo motor for driving the plurality of ball screws simultaneously through the second transmission unit.

In this manner, according to the present invention, when the screw is rotated the screw rotating motor is driven, and when the screw is moved forward, the injection motor is driven to rotate the plurality of ball screws simultaneously, the rotation of which moves the pressure plate forward so that the screw fixed to the pressure plate is drivingly separated from the screw rotating mechanism by the spline shaft, thereby moving only the screw forward. To perform injection, the screw rotating mechanism is first drivingly separated and a servo motor M2 for injection then drives only a second transmission mechanism for moving the screw forward and the screw. Since no unnecessary load mechanism is included, injection can be performed with a good response characteristic, and the pressure plate can be moved smoothly forward, thereby performing excellent injection control.

The accompanying drawing is a partially sectional side view of an embodiment of the present invention.

The accompanying drawing shows an embodiment of the present invention. In the drawing, a screw shaft 1 has a screw at its distal end. A cylinder 2 storing the screw extends though a central hole 17a of a front base 17. A drive shaft 1' is coupled to be stationary to the screw shaft 1 at the rear portion of the screw shaft 1, but is rotatably fixed to a pressure plate 4 through thrust bearings 3 and 3', disposed at large-diameter portions at two ends of a central stepped hole 4a in the pressure plate 4. A rear portion of the drive shaft 1', away from the pressure plate 4, constitutes a spline shaft 4 having a spline engaging portion on an outer surface thereof. The spline shaft 5 extends through a central hole 17'a in a rear base 17' and through a spline coupling nut 6 disposed in the central hole 17'a, and engages with the spline coupling nut 6 through a spline. The nut 6 is fixedly fitted in an inner hole 10a in a shaft 10 of a timing belt gear 9. A timing belt 15 extends between the timing belt gear 9 and a timing belt gear 13 fixed to the motor shaft of a servo motor M1. Ball nuts 7 and 7' are fixed to the pressure plate 4. Ball screws 8 and 8' threadedly engaged with the ball nuts 7 and 7' extend through insertion holes (not shown) in a peripheral portion of the pressure plate 4, and are parallel to the screw shaft 1 between the front base 17 and the opposing rear base 17'. The two ball screws 8 and 8' are symmetrically arranged to each other with respect to the screw shaft 1. The plurality of ball screws 8 and 8' are thus arranged around a circumference of the screw shaft 1 in this manner, at the same angular intervals, and drive the pressure plate 4 to allow its smooth forward movement. Two ends of each of the ball screws 8 and 8' are pivotally supported by bearings arranged in through holes 17b and 17'b in the bases 17 and 17'. Timing belt gears 11 and 11' are fixed to the rear ends, respectively, of the ball screws 8 and 8'. A timing belt 16 extends between the timing belt gears 11 and 11' and a timing belt gear 14 fixed to the motor shaft of a servo motor M2. Note that a thrust bearing 12 is provided and a hopper 18 supplies the molding material into the cylinder 2.

When the screw is rotated by the above arrangement, the servo motor M1 is driven to rotate the spline shaft 5 and the screw shaft 1 through the timing belt gear 13, the timing belt 15, the timing belt gear 9, the shaft 10 and the spline coupling nut 6. In this case, the drive shaft 1', to which the screw shaft 1 is fixed, and the pressure plate 4 are fixed by the bearings 3 and 3' to be rotatable relative to each other and axially immovable relative to each other. Thus, when the molding material is melted by rotation of the screw and the screw shaft 1 is moved backward (to the right in the drawing) by the increased pressure, the pressure plate 4 is moved backward together with the screw shaft 1, thereby rotating the ball screws 8 and 8' threadedly engaging with the ball nuts 7 and 7', integral with the plate 4. As a result, the timing belt gears 11 and 11', the timing belt 16 and the timing belt gear 14 are rotated, thereby rotating the motor shaft of the servo motor M2. At the same time, the drive current of the servo motor M2 is controlled by a servo circuit (not shown), thereby controlling the back pressure to the screw by the servo motor M2.

When injection is performed, driving of the servo motor M1 is stopped at a metering point to stop rotation of the screw shaft 1, and the servo motor M2 is then driven to rotate the ball screws 8 and 8' simultaneously in the same direction through the timing belt gear 14, the timing belt 16 and the timing belt gears 11 and 11', so that the pressure plate 4 is moved forward (to the left in the drawing) by the ball nuts 7 and 7' engaged with the ball screws 8 and 8'. Thus, the screw shaft 1 slides between the spline shaft 5 and the spline coupling nut 6 and is separated from the screw rotating mechanism consisting of the spline shaft 5 and the servo motor M1. As a result, only the screw shaft 1 is urged forward by the pressure plate 4.