Screw-rotating/injection mechanism of injection molding machine

A screw-rotating/injection mechanism of an injection molding machine has a screw shaft (1) having a rear portion thereof connected to a drive shaft (1') which is rotatably fixed to a pressure plate (4). A nut (6) which is spline-coupled to a spline shaft (5) constituting a rear portion of the drive shaft (1') is driven by a screw rotating motor (M1) through a first transmission unit (9, 10, 13, 15) to rotate the screw shaft (1). A ball screw (8, 8') threadedly engaged with a ball nut (7, 7'), fixed to the pressure plate (4), is driven by an injection servo motor (M2) through a second transmission unit (11, 11', 14, 16) to move the screw shaft (1) forward, thereby performing injection.

TECHNICAL FIELD 
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. 
DISCLOSURE OF INVENTION 
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.

BEST MODE OF CARRYING OUT THE 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.