Injection unit of injection molding machine

The invention provides an injection molding machine injection unit that eliminates adverse effects on the load cell of belt tension due to belt drive. A load cell that detects resin pressure is mounted on a pusher plate and a rotation-transmitting member, on which an injection screw is fixedly mounted, is axially supported by an inner annular part of the load cell. The shaft of a pulley is axially supported on the pusher plate and coupled by splines to the rotation-transmitting member. A screw rotation motor is mounted on motor mounts of a housing mounted on the rear surface of the pusher plate. Tension on a belt run between the pulley and a pulley mounted on the motor output shaft concentrates a force at the base of the motor mounts. However, the housing is separate from the pusher plate and fixedly mounted on the pusher plate at a location other than that occupied by the base of the motor mounts. Therefore, the belt tension does not adversely affect the load cell, thus enabling high-accuracy resin pressure detection.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an injection molding machine, and more particularly, to an injection unit of an injection molding machine provided with a load detection unit for detecting resin pressure on the injection screw.

2. Description of the Related Art

In an injection unit of an injection molding machine, a screw-type injection unit (that is, a screw-in-line-type injection unit) melts and kneads resin inside a heating cylinder by rotating an injection screw, and further, retracts the injection screw while applying pressure on the resin with the injection screw and measures the melted resin at that retracted position. Thereafter, the injection unit injects melted resin into a mold by advancing the screw.

As a result, it is necessary that the injection unit be provided with a mechanism that rotates the injection screw and a mechanism for driving the injection screw in an axial direction and injecting the resin.

An injection mechanism is known that comprises an injection unit by providing a pusher plate to which is fitted an injection screw so as to be freely rotatable but axially unmovable, providing on such pusher plate a pulley that rotates the injection screw and a screw rotation motor that drives the pulley by a belt, and further, providing thrust force drive means for injecting the resin with the injection screw by driving such pusher plate in the injection screw axial direction, and further, has a load detection unit such as a load cell for detecting the pressure on the injection screw (for example, JP 2-16023A and JP 9-174628A).

A load detection unit such as a load cell detects the resin pressure inside the heating cylinder, and the resin pressure detected by the load detection unit is used in back pressure control during a measuring step. In addition, in injection and pressure holding steps, the detected resin pressure is used in injection pressure feedback control, pressure holding feedback control and the like. Hence, it is desirable that the resin pressure be detected with greater accuracy.

As the means for rotating the screw, in that which uses a pulley-and-belt transmission mechanism, the belt is run between a drive pulley provided on the motor and a driven pulley provided on the injection screw side. This belt exerts a force in a radial direction on the pulley provided on the injection screw side, creating moment on the shaft on which the pulley is mounted, causing the force of friction of the injection mechanism unit to fluctuate and affecting the resin pressure detected by the load cell. Inventions that prevent these things from happening are also known (for example, JP 2000-117789A and JP 2000-334789A).

In an injection unit in which, as the means for rotating the injection screw mounted so as to be freely rotatable and axially unmovable on the pusher plate, the screw rotation motor is mounted on the pusher plate and a belt such as a timing belt is run between the drive pulley mounted on the output shaft of the motor and the driven pulley mounted on the injection screw side for driving the injection screw, and such belt transmission mechanism is used to rotate the injection screw, the force exerted by the belt on the drive pulley in the radial direction of the drive pulley also acts on the pusher plate through the motor that is mounted on the pusher plate, adversely affecting resin pressure detection of the load cell or other such load detection unit provided on that pusher plate.

SUMMARY OF THE INVENTION

The present invention is configured so that the effect of the force generated by the belt from the motor side does not reach the load detection unit.

An injection unit of the present invention rotates and axially moves an injection screw of an injection molding machine. The injection unit comprises: a first member arranged linearly movable and having a front face closer to the injection screw and a rear face remote from the injection screw; a pulley shaft rotatably supported by the first member and having a pulley fixed thereon on a rear side of the first member; a rotation-transmitting member connected with the pulley shaft such that a relative axial displacement in between is allowed, and connected with a rear end of the injection screw, for transmitting rotation of the pulley shaft to the injection screw; a load detection member mounted on the front face of the first member and having an inner annular portion supporting the rotation-transmitting member rotatably such that a relative axial displacement in between is inhibited, for measuring a resin pressure acting on the injection screw; a second member not integrally formed with the first member and attached to the rear face of the first member; a screw-rotation motor mounted on the second member for rotating the pulley fixed on the pulley shaft through a belt; and thrust force applying means for applying a thrust force to the second member so that the injection screw is axially moved.

The second member may have a main body and a mounting portion for mounting the screw-rotation motor to extend from the main body, and be fixed to the first member at positions remote from a proximal part of the mounting portion.

The screw rotation motor is drawn toward the second member by the tension of the belt run between the pulley shaft mounted on the first member and the screw rotation motor, causing the second member to deform. However, because the first member on which the load detection unit is mounted and the second member are configured as separate units, the first member is not affected by the tension of the belt, thus enabling high-accuracy resin pressure detection.

DETAILED DESCRIPTION

FIG. 1is a diagram illustrating an injection unit of a first embodiment of the present invention, showing a sectional view cut along a center line thereof.

An injection screw17is inserted into a heating cylinder18mounted on a front plate16, with a rear end shaft fixedly mounted on a rotation-transmitting member4. The rotation-transmitting member4is mounted through bearings5,6so as to rotate freely but is unable to move in an axial direction within an inner annular part3bof a load cell3operating as a load detection unit. An outer annular part3aof the load cell3is fixedly mounted on a pusher plate (first member)1. A pulley shaft7, on which is mounted a driven pulley9, is mounted so as to be freely rotatable but axially unmovable on the pusher plate1through bearings8. The pulley shaft7and the rotation-transmitting member4are connected by connecting means that limit only relative movement in a direction of rotation. In the present embodiment, the pulley shaft7and the rotation-transmitting member4are joined together by a spline coupling. Alternatively, instead of a spline coupling the pulley shaft7and the rotation-transmitting member4may be coupled by a key and key groove arrangement.

Further, a housing (second member)2is fixedly mounted on the pusher plate (the first member)1, and to the housing2is connected thrust force drive means for driving the injection screw17and the pusher plate1axially (left and right inFIG. 1) and injecting melted resin inside the heating cylinder18into a mold, not shown. In this first embodiment, the thrust force drive means is composed of a ball screw/nut mechanism13and a motor, in which the ball nut13bof the ball screw/nut mechanism13is fixed. In addition, a pair of motor mounts2athat mount a screw rotation motor12that drives the rotation of the injection screw17projects from both lateral sides of the housing2(that is, both lateral sides as seen from the axial direction of the injection screw; seeFIG. 2). The screw rotation motor12is mounted between the pair of motor mounts2a, and a timing belt10is run between a drive pulley11provided on the output shaft of the motor12and the driven pulley9.

A ball screw shaft13aof the ball screw/nut mechanism13is mounted by bearings14on an end plate15so as to rotate freely, and mounts on its front end a driven pulley19for rotating the ball screw shaft13a. A timing belt is run between the driven pulley19and a drive pulley provided on the output shaft of an injection motor, not shown, for driving the injection screw17in the axial direction. The injection motor drives the injection screw17, the pusher plate1and the like axially. It should be noted that the ball nut13bscrews onto a threaded part of the ball screw shaft13a. In addition, guide bars, not shown, that guide the pusher plate1are provided between the front plate16and the end plate15. The pusher plate1is guided by the guide bars so as to be movable in the direction of the axis of the injection screw17(to the left and right inFIG. 1). Alternatively, the means for guiding the pusher plate1may be a linear guide.

Next, a description is given of the operation of the injection unit. During measuring/kneading, the screw rotation motor12drives, the driven pulley9is driven by the drive pulley11and the timing belt10and rotates, and the pulley shaft7on which the driven pulley9is fixedly mounted, the rotation-transmitting member coupled to the pulley shaft7by splines and the injection screw17mounted on the rotation-transmitting member4all rotate. The rotation of the injection screw17melts the resin supplied to the interior of the heating cylinder18, and the pressure of the melted resin causes the injection screw17to retreat (that is, to move to the right inFIG. 1). The force of retraction of the retreating screw is transmitted to the rotation-transmitting member4, to the load cell3by the bearings5, and to the pusher plate1by the load cell3, causing the pusher plate1and the housing2to retreat.

At the same time, the driving of the injection motor creates back pressure. In other words, the injection motor is driven, creating a force corresponding to a set back pressure that causes the driven pulley19and the ball screw shaft13ato rotate, causing the ball nut13bto advance and the housing2and the pusher plate1to advance (that is, to move to the left inFIG. 1). However, when the resin pressure exceeds this set back pressure, the injection screw17, the rotation-transmitting member4, the load cell3, the pusher plate1and the housing2all retreat. During this time, the force of retraction of the injection screw17acts on the inner annular part3bof the load cell3through the rotation-transmitting member4, a distortion arises between the inner annular part3band the outer annular part3afixedly mounted on the pusher plate1, and the resin pressure is detected.

In addition, in the injection and the pressure holding steps, the driving of the screw rotation motor12is stopped, the injection motor is driven and the ball screw shaft13ais driven by the driven pulley19. As a result, because the ball nut13bengaged with threads of the ball screw shaft13ais fixedly mounted on the housing2it advances without rotating along the shaft of the ball screw shaft13a, the housing2, which is fixedly mounted on the ball nut13b, advances the pusher plate1on which the housing2is fixedly mounted, advancing the load cell3, which is fixedly mounted on the pusher plate1, the rotation-transmitting member4through the bearings5and the injection screw17fixedly mounted on the rotation-transmitting member4so as to inject resin into the mold. In addition, in the pressure holding step as well, the injection motor is driven so as to apply pressure on the resin at a set holding pressure by the injection screw17through the pusher plate1.

In these injection and pressure holding steps as well, a force generated by the driving of the injection motor that attempts to advance the pusher plate1is exerted on the outer annular part3aof the load cell3, the resin pressure exerted on the injection screw17acts on the inner annular part3bof the load cell3, a distortion between the load cell outer annular part3aand the inner annular part3barises and the resin pressure is detected.

The foregoing describes the operation of the injection unit of the present embodiment. However, as can be appreciated by those skilled in the art, the timing belt10is run between the drive pulley11provided on the output shaft of the screw rotation motor12and the driven pulley9, and this timing belt10exerts on the screw rotation motor12a force that pulls the screw rotation motor12toward the housing side, and exerts a force on the driven pulley9that pulls the driven pulley9toward the screw rotation motor12. Consequently, a moment acts on the pulley shaft7supported on the pusher plate1by the bearings8. However, the pulley shaft7and the rotation-transmitting member4are coupled by splines, and since the play in the bearings is smaller than the play in this spline coupling, the moment force acting on the pulley shaft7is received and supported by the bearings8.

At the same time, the tension of the timing belt10exerts on the housing2through the screw rotation motor12and the motor mounts2aa force that pulls the drive pulley11mounted on the motor shaft toward the housing2. The distinctive feature of the present invention is that the pusher plate1and the housing2on which the screw rotation motor12is mounted are not integrally formed and connected with each other, so that this force that is exerted on the housing2does not affect the load cell3through the pusher plate1.

FIG. 2is a diagram showing the interface between the pusher plate1and the housing2, as a sectional view cut along a direction vertical to the plane of the paper inFIG. 1(that is, as seen from the front side of the injection screw17).

The pusher plate1and the housing2are tightly connected and fixed in place by bolts or the like at an abutting portion2bindicated by cross-hatching inFIG. 2. However, the housing2and the pusher plate1are not tightly connected and are not fixed at proximal portions2a′of the pair of motor mounts2aeach of which extends from both lateral sides of the housing2so as to mount the screw rotation motor12. The force exerted on the housing2by the tension of the timing belt10through the drive pulley11, the screw rotation motor12and the motor mounts2aconcentrates at the proximal portions2a′and the housing2distorts around the proximal portions2a′of the motor mounts2a. However, the areas around the proximal portions2a′of the motor mounts2aare not connected to the pusher plate1, and consequently, since the housing2and the pusher plate1are coupled at a distance from the proximal portions2a′, the distortions that concentrate at the proximal portions2a′of the motor mounts2aare dispersed without being transmitted to the pusher plate1, and thus do not adversely affect the load cell3.

FIG. 3is a diagram illustrating an injection unit according to a second embodiment of the present invention, showing a sectional view cut along the central part thereof. In addition,FIG. 4, likeFIG. 2, is a diagram showing the interface between the pusher plate1and the housing2, as a sectional view cut along a direction vertical to the plane of the paper inFIG. 3(that is, as seen from the front side of the injection screw17).

In the second embodiment of the present invention, only the structure of the thrust force drive means that drives the injection screw17axially and the interface between the pusher plate1and the housing2are different from their counterparts in the first embodiment.

In the second embodiment, the ball nut13bof the ball screw/nut mechanism13that forms the thrust force drive means is fixedly mounted on the end plate15, and the threaded part of the ball screw shaft13ascrews into the ball nut13b. Then, the other end of the ball screw shaft13ais mounted on the housing2by bearings14so as to be rotatable but axially unmovable. In addition, the driven pulley19is mounted on the housing2side of the ball screw shaft13a, and a timing belt is run between the driven pulley19and a drive pulley mounted on the output shaft of an injection motor, not shown.

In addition, with respect to the housing2and the pusher plate1,2cand2dindicated by cross-hatching inFIG. 4form abutting portions, with the housing2and the pusher plate1joined at these abutting portions2c,2dand fixed in place by bolts or the like. The remaining structures are the same as those in the first embodiment. In addition, in the operation of this injection unit, the driven pulley19is driven by the injection motor and the ball screw shaft13arotates. The ball screw shaft13aengages the ball nut13bmounted on the end plate15, and therefore the ball screw shaft13arotates as well as moves axially, causing the housing2, on which is mounted the ball screw shaft13afixedly so as to be axially unmovable, and the pusher plate1, which is fixedly mounted on the housing2, to move axially, and moving the injection screw17axially to inject and the like. The operation of the thrust forcing drive means differs only slightly from that of the first embodiment, with the other operations identical to those of the first embodiment shown inFIG. 1.

At the same time, as shown inFIG. 4, the abutting portions2c,2dof the housing2and the pusher plate1are disposed at locations removed from the proximal portions2a′of the projecting motor mounts2a. As with the first embodiment, a force generated by the tension of the timing belt10is exerted on the drive pulley11, the screw rotation motor12, and, through the motor mounts2a, on the housing2, and concentrates at the proximal portions2a′of the mounts2a, causing the housing2to distort around the proximal portions2a′. The proximal portions2a′of the motor mounts2aand the abutting portions2c,2dare disposed at different locations, and therefore the distortion is diffused without being transmitted to the pusher plate1and without adversely affecting the load cell3mounted on the pusher plate1.

In each of the embodiments described above, the ball screw shaft rotates while the ball nut remains fixed. However, as can be understood by those skilled in the art, conversely, the ball screw shaft may be fixed while the ball nut rotates.

In addition, although a ball screw/nut mechanism13is used as the thrust force drive means, alternatively, a linear motor may be used instead.