Electrically-operated injection molding method

Abstract: A die compression injection molding method for an injection molding machine (A) for a precision molding thereof, comprises; (a) injecting weighed resin (3a) into a die cavity (2) of dies (1) in the course of moving in a die closing direction of a movable die (1b); (b) carrying out gate cutting when a specified amount of resin (3) is filled, continuously moving the movable die (1b) in the die closing direction to a specified position, (c) keeping the dies clamping with holding pressure at said specified position during the holding pressure process and the cooling process, (d) after said cooling process, removing the molding from the open dies (1).

Description: The present invention relates to a method of molding using an electrically-operated injection molding machine using a servo motor as a drive source of the injection molding process, and a more specifically to an electrically-operated injection molding machine for super-precision molding using servo motors for a product eject process and a molding removal process as well as an injection compression molding method using the relevant machine. BACKGROUND OF THE INVENTION Injection molding machines using servo motors for drive sources of each process known. However, even in such known usage, servo motors are used at most for drive applications, such as (1) injection into die cavities of weighed resin, (2) weighing of blended resin, (3) opening and closing of dies, (4) gate cutting, etc., and there has been no case in which servo motors are used in the product eject process. This is because as shown in FIG. 21, the location of screw mechanism for gate cutting is concentrated with an extremely large number of mechanisms, and an air cylinder has conventionally been used for driving the product eject pin though it can be barely contained in the space. In a typical known electrically-operated injection molding machine using servo motors, each servo motor is feedback controlled in each process based on the program data for servo control. This takes in the data from an encoder equipped to the relevant servo motor into CPU for arithmetic, and the position, speed, torque, etc. are feedback controlled as specified in the program. However, this kind of control method is an indirect method of control of the position, speed, torque, etc. via the encoder linked to the servo motor, and has been unable to directly detect injection the pressure, weighed resin pressure, and die clamping force, etc. actually applied to the screw and dies and to permit feedback control cased on the actually detected data. This is because it was not possible to find any place to install a pressure sensor for directly detecting the die clamping force applied to the dies, and so it was not possible to directly detect the die clamping force and carry out feedback control. It is to be understood that it is the screw ejection mechanism for removing moldings from dies and direct detection of resin filling pressure and clamping force to dies at the die portion that constitute bottlenecks in adopting servomechanisms. Even if pressure sensors are intended to be installed in the fixed die side for directly detecting resin filling pressure or die clamping force, there is a limitation in space, and even when the pressure sensor is installed on the movable die side, there is a restriction in that the screw mechanism for ejection is unable to be installed unless some special construction is adopted, and in addition, it is also a limit in space. The adoption of servomechanisms naturally means feedback control, but because in known machines, actual resin filling pressure and die clamping force were unable to be detected, feedback control using the data from the encoder attached to the servo motor, must be used. SUMMARY OF THE INVENTION The problem the present invention intends to solve is described as follows. Because the servo motor is used as a driving source, it is an object of this invention not only to achieve a higher cycle rate but also to directly detect weighed resin pressure and die clamping force exerted in dies in order to carry out feedback control. In particular, it is an object of this invention to enable direct detection of die clamping force on the die mechanism side, which has been considered impossible. Another problem to be solved is to develop an injection molding method in an electrically-operated injection molding machine using servo motors for simultaneously achieveing requirements for higher cycle of forming, improved transferability of fine protrusions and recessions formed on the dies, and stabilisation of the molding quality, and control of speed for injecting weighed resin to die cavity, control of gate cutting timing, die compression speed, and die compression pressure as well as positioned control of dies for achieving uniform molding thickness must be properly carried out to achieve this, and these requirements have never been achieved with the capabilities of conventional equipment which has employed a hydraulically-driven die compression forming method. Canadian Patent No. 1099065, which is considered to be the closest prior art relates to a method of die compression injection molding including injecting weighed resin into a die cavity while movable die is during, carrying outgate cutting when a specified amount of resin is filled and holding the pressure during cooling. There is disclosed in EP-890426 from which this application is divided, an injection molding machine used for a precision forming injection molding method, including an apparatus having servo motors used for driving sources in processes of weighing blended resin, for injecting weighed resin into a die cavity, of opening and closing dies for gate cutting after filling weighed resin into the die cavity, for ejecting a molding after forming, and for removing the molding, the injection molding machine. The present invention provides a method of molding suitable for the operation of such a machine. The reader of this specification is directed to EP 890426 for a better understanding of the method of the present invention. The present invention provides a die compression injection molding method of an injection machine for a precision molding comprising; (a) injecting weighed resin into a die cavity of dies while a movable die is moving in a die-closing direction; (b) carrying out gate cutting when a specified amount of resin is filled, continuously moving the movable die in the die closing direction to a specified position. (c) keeping the dies clamped with holding pressure at said specified position during holding pressure process and cooling process. (d) after said cooling process, removing the molding from the open dies, wherein the reaction force of said movable die, caused by the resin filled in the die cavity, is sensed by a sensor(7) placed between the movable die plate a(1b) and a housing (50) movable to urge said movable die plate in a die-closing direction, and at least either of the pressure control of the filling resin by the movable die or thickness control of the filled resin (3) by position control of the movable die (16) is carried out based on the output data from said pressure sensor. According to this configuration, because the movable die is continuously moved in the die closing direction from the start of injection filling of the weighed resin to the start of die clamping, the filled resin co-operates with the movement of the movable die to increase the relative speed of the filled resin with respect to the movable die, and as a result, the filled resin comes in contact with the inner side of the die cavity and flows more quickly, allowing the new resin inside to expose to the surface is hindered. As a result, the transferability of fine protrusions and recessions to the molding is remarkably improved. The filled resin of a volume exceeds the volume of the moulding and is compressed to the volume of the molding by compression molding, and forms a high-denisty substrate free of variation in density and contributes to the improvement in the quality. Preferably the operational timing of the gate cutting is controlled based on the output data from said pressure sensor. Desirably the speed of resin injection from an injection mechanism position (a) into the dies (1) is controlled based on the output data from said pressure sensor. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a sectional fragmentary schematic illustration showing a construction of an injection molding machine of the parent EP 890426 to enable the method of the invention to be more easily understood. FIG. 18 (1) - (6) are cross-sectional views showing the die operating condition in the whole injection process of this invention; FIG. 20 (1) - (6) are cross-sectional views showing the operating condition of dies in the whole injection process of this invention; and FIG. 21 is a sectional fragmentary schematic illustration of the partly omitted construction of a mechanism portion of a conventional (PRIOR ART) example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, an electrically-operated injection molding machine (A) as shown in Fig. 1 will be described briefly in order the better to understand the method. A first embodiment of the method of the injection molding machine according to this invention is shown in FIG. 18 and a second embodiment of the method in FIG. 20, respectively. The electrically-operated injection molding machine can be generally divided into an injection mechanism portion (a) and a die mechanism portion (b). The injection mechanism portion (a) comprises a drive mechanism portion (10) for forwarding and reversing the screw (4), a rotation servo motor (11) for rotating the screw (4), an injection servo motor (12) for forwarding and reversing the screw (4), the screw (4) for blending and injecting material resin, an injection cylinder (13) that houses the screw (4) free of forwarding and reversing and rotating, a heater (14) which is wrapped around the injection cylinder (13), an injection pressure sensor (15) placed between the screw (4) and the drive mechanism portion (10) and for detecting pressure applied to the screw (4), and pulse generators (11a), (12a) mounted on each of the servo motors (11), (12). Next description will be made on the die mechanism portion (b). The dies (1) comprise a movable and a stationary does (1a). The stationary die (1a) is mounted on the inner side of the stationary die plate (17), and the movable die (1b) is mounted on the inner side of the movable die plate (18). A housing (50) is mounted on the outer side of the movable die plate (18) via the pressure sensor (7) for detecting a resin pressure, which is fixed between the housing (50) and the movable die plate (18). At the center of the back surface of the housing (50) (that is, the opposite side of the movable die (1b)), a gate cutting screw mechanism (G) is mounted, and to the further back portion, an ejection screw mechanism (E) is mounted. Tie bars (19) are spanned between the stationary die plate (17) and a tail stock (20), and the movable die plate (18) is slidably installed to the tie bars (19). Next, the gate cutting screw mechanism (G1) will be described. The driven pulley (43) of the gate cutting screw mechanism (G1) is rotatbly placed to the housing (50) via a bearing, and a screw shaft (30a) is screwed through the center of the driven pulley (43) so that it forwards and reverses in accord with the normal and reverse rotation of the driven pulley (43). The gate cutting member (30) is formed by combining this screw shaft (30a) with the hollow gate cutting bar (30b) placed in the movable die (1b), but the screw shaft (30a) and hollow gate cutting bar (30b) may be integrated to form the gate cutting member (30). The driven pulley (43) is connected to a driving pulley (41) of a servo motor (40) via a timing belt (42), and the gate cutting member (30) rotated by pulley (43). Numeral (40a) is a pulse generator mounted on the servo motor (40). A detailed description of the workings of the mechanism may be found in EP 890426. Regarding the operation of pressure sensors (7) and (15), because the pressure sensor (7) is mounted between the movable die plate (18) and the housing (5), the die clamping force at the time of die clamping by a toggle mechanism (T) is directly applied to the pressure sensor (7), enabling the direct detection of the die clamping force. Similarly, because the pressure sensor (15) is placed in the drive mechanism portion (10), weighing resin pressure and injection pressure applied to the screw (4) can be directly detected by the pressure sensor (15). Referring now to FIGS 18 and 20, the invention will be described in detail. The method is described in terms of the movement of the dies (1). As shown in FIG 18(1), when die clamping is carried out the core (5b) of the stationary die (1a) fits into the die cavity (2) of the movable die (1b). However, at this point, the toggle (T) is not completely extended, and a small clearance (t) is provided between parting faces. Consequently, the die cavity (2) becomes wider than that at the die clamping time. Then, the injection servo motor (12) is worked for operating the screw mechanism portion (10), and the screw (4) is moved in the direction of the dies (1). The weighed and blended molten resin (3a) at the top of the cylinder (13) is injected into the die cavity (2) (see FIG 18(2)). The injection speed is optimally controlled by the controller (8). The molten weighed resin (3a) is injected and filled in the die cavity (2). When the resin (3a) comes in contact with the die cavity (2) of the movable die (1b) and pressurizes the dies (1), the filling pressure is detected by the pressure sensor (7). In this process, as described above, because the die cavity (2) is set slighlty wider, the molten blended resin (3b) is filled additionally in the amount equivalent to that. The above processes are controlled according to inputs from the pressure sensor (7). Because this injection process is carried out in such a manner to follow the setting value by the injection servo motor (12), filling is finished before thin resin film is formed on the filled resin (3). After that, gate cutting takes place, when the pressure sensor (7) indicates the specified value, and feed of resin (3a) to the die cavity (2) is immediately stopped. (see FIG. 18 (3)). That is, operating the servo motor (40) to rotate the driven pulley (43) with the dies (1) clamping underway advances the gate cutting member (30), and blocks the gate (1c) of the stationary die (1a) as its top end. By this operation, the die cavity (2) is completely isolated from the outside with extra filled resin (3), which is equivalent to the portion (or position) the movable die (1b) did not advance. Then, the die control servo motor (31) is operated again to further advance the cross head driving shaft (34a), and the cross head (35) is pressed to advance to further extend the toggle (T) so that the movable die plate (18) is pressed against the stationary die (1a) with a specified pressure. With this operation, the filled resin (3) is compressed with extremely strong pressure, and super-fine recessions and protrusions on the super-finely recessed and protruded inner surface (5) of the die cavity (2) are accurately transferred to the surface of the filled resin (3) that begins to solidify (see FIG. 18 [4]). Because the dies (1) are compressed while the resin pressure is directly detected with the pressure sensor (7), uniform die compression at high reproducibility can be carried out every time. Now, after that this condition, the movable die (1b) is held so that the resin pressure achieves the specified value with directed by the pressure sensor (7), till the filled resin (3) becomes solid body solidifies. By this operation, the molding (26) with high reproducibility and uniform thickness(s) can be achieved every time. Upon completion of solidification of the filled resin (3), the die control servo motor (31 is reversely worked to separate the movable die (1b) from the stationary die (1a) as shown in FIG. 18 [5]. In this event, the molding (26) moves together with the movable die (1b). Lastly, the operation moves to FIG. 18 [6], when the die opening by the servo motor (40) is finished to project the gate cutting member (30) from the die cavity (2), the molding (26) is separated from the movable does(1) and recovered. Now, a second embodiment of the die compression molding method according to this invention will be described in detail, this is an improvement of the first procedure described previously and the operation is described with reference to FIG. 20. As shown in FIG 20 [1], the movable die (1b) moves toward the stationary die (1a), and the core of the stationary die (1a) fits into the die cavity (2) of the movable die (1b), after that, the dies (1) is clamped. However, at this point, the toggle (T) is not completely extended and a wider clearance (t) than that at the time of gate cutting in Fig. 20 [3] is provided between cavity faces clearance between cavity faces is shown with (T). Under this condition, injection filling takes place, as shown in FIG.20 [2], the weighed resin (3a) is injection-filled while the movable die (1b) moves in the die closing direction from (T) to (t). In this event, the skin layer tries to be generated on the resin (3) surface from the moment of the injection, but since the movable die (1b) is moving in the die closing direction, the resin (3) flows in the inner surface (5) of the die cavity (2) of the movable die (1b) at higher speed, and the formation or growth of the skin is suppressed, and consequently, the generation of super-fine air pools is suppressed, and the resin (3) faithfully adheres to the inner surface (5) where super-fine recessions and protrusions are formed. Successively, gate cutting takes place (see FIG. 20 [3] at such timing (PI) that the gate cutting member (30) operates when the pressure sensor (7) indicates a specified value and protrudes from the movable die (1b) under travelling to carry out gate cutting, and shuts up the die cavity (2) from the gate (1c). The clearance between cavity surfaces of the movable die (1b) and the stationary die (1a) at this moment is (t), Because gate cutting is carried out every time at the same resin pressure, the resin (3) of the same amount is stably filled in the die cavity (2) every time (however, since the width (t) is greater than the width (s) of the molding (26), the filled volume becomes greater that the volume of the molding (26)). Because (t) is greater than the final thickness (S) of the molding (26), the resin (3) is filled in the die cavity (2) more than the volume of the molding (26). Gate cutting is carried out by operating the servo motor (40) with the movable die (1b) under travelling for die clamping to rotate the driven pulley (43), and allowing the gate cutting member (30) to advance from the movable die (1b) under travelling, and blocking the gate (1c) of the stationary die (1a) at the top end, and the movable die (1b) is completely isolated from the outside with resin (3) additionally filled: The above process is controlled by inputs from the pressure sensor (7). This injection filling process is carried out in such a manner to follow the setting valve by the injection servo motor (12). In this case because the movable die (1b) is traveling during the resin (3) injected to the die cavity (2) the filled resin (3) increases the relative speed to the movable die (1b) in cooperation with the motion of the movable die (1b), and quickly flows in contact with the inner surface (5) of the die cavity (2) while constantly exposing the new inside resin to the surface. As a result, filling is finished before this resin film is formed on the filled resin (3), and this hinders generation of super-fine air pools that impairs transferability. Consequently, super-fine recessions and protrusions formed on the die cavity (2) of side can be transferred to the molding (26) with remarkably excellent transferability. Then, the operation moves from the pressure-holding process to the moving process, but the pressure-holding process and removing process are same as those of the first embodiment, and the description will be omitted. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restricitive of the method, the scope of the invention being indicated by the appended claims than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein Because a pressure sensor (7) is disposed between the movable die plate and the housing, directly detecting the die clamping force actually applied to the dies, which has been assumed to be difficult, has been made easy with this invention.

Claims: A die compression injection molding method for an injection molding machine (A), for the manufacture of a precision molding, comprising; (a) injecting weighed resin (3a) into a die cavity (2) of dies (1) in the course of moving a movable die (1b) in a die closing direction, (b) carrying out gate cutting when a specified amount of resin (3) is filled, continuously moving the movable die (1b) in the die closing direction to a specified position, (c) keeping the dies clamped with holding pressure at said specified position during a holding pressure process and a cooling process, (d) after said cooling process, removing the molding from the opened dies(1), wherein the reaction force of said movable die, caused by the resin filled in the die cavity, is sensed by a sensor(7) placed between the movable die plate and a housing movable to urge said movable die plate in a die closing direction, and at least either of the pressure control of the filling resin by the movable die or thickness control of the filled resin (3) by position control of the movable die (16) is carried out based on the output data from said pressure sensor (7). 2. A die compression injection molding method as claimed in claim 1 wherein the operational timing of the gate cutting is controlled based on the output data from said pressure sensor (7). 3. A die compression injection molding method as claimed in claim 2 wherein the speed of resin injection from an injection mechanism position (a) into dies (1) is controlled based on the output data from said pressure sensor.

Inventors: HARA YOSHIAKI, HIRAYAMA NOBUYUKI, TANIGUCHI YOSHIYA, HARA, YOSHIAKI, HIRAYAMA, NOBUYUKI, TANIGUCHI, YOSHIYA, Hara, Yoshiaki, Sony Music Entertainment Inc., Hirayama, Nobuyuki, c/o Sony Disc Technology Inc., Taniguchi, Yoshiya, Toyo Machinery &amp; Metal Co.,Ltd
Applicants: TOYO MACHINERY & METAL CO LTD, TOYO MACHINERY & METAL CO. LTD.
Application year: 1998
Main classification: B29C45
IPC codes: B29C45