Method of controlling hydraulic press

Abstract: A method of controlling a hydraulic press wherein changing over from velocity control to pressure control can be effected at an optimum point of time in accordance with a condition of material being filled. When a movable mold half of a metal mold is moved by a hydraulic cylinder to compression mold a work within a cavity defined in the metal mold, the velocity of movement of the mold half by the hydraulic cylinder is controlled until pressurization of the work is started, but after starting of the pressurization, the pressurizing force of the hydraulic cylinder is controlled. During the velocity control, a parameter which varies as the compression proceeds is successively detected during the velocity control, and changing over from the velocity control to the pressure control is effected at a point of time when the detected parameter coincides with a preset value, that is, when the work is fully filled in the cavity. A device for controlling a hydraulic press is also disclosed wherein optimal pressure control can be assured in accordance with a current condition of material for molding.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of controlling a hydraulic press for compression molding a work, according to the preamble of claim 1. 2. Description of the Prior Art Hydraulic presses are already known in which a slide is moved by a hydraulic cylinder to compression mold a work. Such hydraulic presses are divided into various types depending upon a material and a shape of a work to be worked. In order to compression mold a front panel, a body panel or a like part of an automobile using an SMC (Sheet Molding Compound) which is a thermosetting material in the form of a sheet, such a hydraulic press as disclosed, for example, in Japanese Patent Laid-Open NO. 60-15119 is conventionally used. The conventional hydraulic press disclosed includes a base, an upright mounted uprightly on the base, a crown provided at an upper portion of the upright, a hydraulic cylinder provided on the crown, and a slide supported at a lower end of a rod of the hydraulic cylinder for up and down movement under the guidance of the upright, wherein a lower metal mold half is secured to an upper face of the base while an upper metal mold half is secured to a lower face of the slide, and resin material is compression molded within a cavity defined between the upper and lower metal mold halves. In compression molding an SMC material with the conventional hydraulic press, the hydraulic cylinder is controlled conventionally in such a manner as illustrated in Fig. 16. In particular, referring to Fig. 16, velocity control for controlling the velocity of the hydraulic cylinder at a plurality of stages is adopted for a period of time (t₁, t₂ and t₃) until the slide is moved down from its top dead center to effect mold closing, and pressure control wherein the pressurizing force of the hydraulic cylinder is maintained fixed is adopted for another period of time (t₄ and t₅) until completion of compression molding after material has been filled into the cavity of the metal mold. Finally for a further period of time (t₆ and t₇) from completion of compression molding to mold opening, velocity control is adopted again. In compression molding of resin material, it is important to effect the pressure control of the resin material in accordance with a variation in condition of the resin material. This prior art, however, has the drawback that, because the pressurizing force of the press is fixed in the pressure control, the press cannot follow a variation of the condition of the resin material and accordingly good molded articles cannot be produced. In particular, the pressure of SMC material varies in such a manner as shown in Fig. 15. As seen in Fig. 15, the pressure rises suddenly at the instant when the metal mold half is contacted with the resin material so that the resin material is flowed in and fills up the cavity of the metal mold halves. After completion of such filling of the resin material, the SMC material is heated by an influence of the temperature of the metal mold halves and thus expanded in volume as seen in a zone denoted at a in Fig. 15. The SMC material then shrinks as seen in another zone denoted at b in Fig. 15, and finally it cures at a stage denoted at c in Fig. 15. A material which undergoes expansion, shrinkage and cure such as an SMC material thus varies in behavior and in pressure within a mold. With conventional presses, however, the pressure is constant and is not applied in accordance with a pressure of the resin material. Accordingly, optimal pressure control for different conditions cannot be attained, and if the pressure control is not appropriate, the molded articles will have defects such as sinks and cracks and cannot be produced with a high quality. Accordingly, in compression molding of a thermosetting resin material, appropriate transition from velocity control to pressure control and maintenance of an appropriate pressure in the pressure control are very important. The transition from velocity control to pressure control, however, is conventionally effected by changing over the velocity of movement of the slide at a plurality of stages in such a manner as shown in Fig. 17. With the manner of changing over of the velocity, the velocity presents a discontinuous variation at each of such changing over points. Consequently, smooth deceleration of the slide cannot be attained and the metal mold half may be contacted too fast or too slowly with the resin material or may be contacted with the resin material while being vibrated. As a result, the resin material may not appropriately be filled up in the metal mold halves, which will have a bad influence on the pressure control after then. Further, depending on a kind of the resin material, it is necessary to move down the slide at a high velocity and then decelerate the slide rapidly and smoothly at a final stage of the downward movement. Such movement, however, cannot be attained with such a conventional multi-stage deceleration pattern as described above. Accordingly, it is a problem of the conventional presses that optimum molding cannot be assured for each resin material. Changing over from velocity control to pressure control in the conventional hydraulic press described above is conventionally made by a setup of a timer. Such a conventional change-over of control by a timer has the following drawbacks. In particular, in order to obtain excellent results with a pressure control within a range denoted at t₅ in Fig. 16, it is necessary for the resin material (SMC) to be completely filled in a metal mold. According to the change-over of the control by a timer, however, the control is changed over to pressure control only in response to an external factor (timer) whether resin material is filled up in a metal mold or not. Consequently, satisfactory results are not obtained by the conventional change-over of the control by a timer. For example, where the periods of time t₃ and t₄ shown in Fig. 16 are set too short, the pressure control will be entered before the material is filled up in a metal mold. Consequently, the resin material in the metal mold will not be acted upon by a pressure, and in an extreme case, this will give rise to such problems as an occurrence of a short shot or a crack. On the contrary, where the periods of time t₃ and t₄ are set too long, the resin material is filled up into a metal mold and begins to be heated and cure before a pressure is applied to the resin material. Thus, when a pressure is actually applied to the resin material, it will not act upon the entire resin material because the resin material has partially cured already. Consequently, a molded article having a sufficient strength cannot sometimes be obtained. The change-over of the control by a timer described above thus depends upon an assumption that the resin material thrown into a metal mold is fully filled in the metal mold. In recent years, further uniform molding is demanded with minimized burr, and the supply of the resin material is not fixed for each molding. Thus, it is not known clearly whether the resin material is filled in a similar manner for each molding cycle, and consequently the assumption is not met satisfactorily. In particular, the control by a timer described above is a one-sided controlling method which ignores the fact that the filling conditions vary for each molding. When, for example, flat plates are to be molded, the resin material may be filled in different conditions in a metal mold or may be placed in different locations in a cavity of a metal mold. For example, the resin material may be placed to a rather leftwardly displaced location within a cavity of a metal mold as shown in Fig. 17, or to a rather rightwardly displaced location as shown in Fig. 18. Besides, the amount of the thus filled resin material may vary. Consequently, the molded articles may vary in thickness or density, and only molded articles which are different from each other in a strict sense are obtained by different molding cycles. As means for avoiding this to obtain molded articles as uniform as possible, a molding method is conventionally employed according to which an excessive amount of resin material is thrown into a metal mold so as to cause so-called burr to be produced on a molded article. The molding method, however, is inconsistent with the aforementioned requirement to minimize burr and thus deteriorates the productivity significantly. Also it is a problem of the prior art that, in the pressure control, the resin material is conventionally pressurized by a substantially fixed pressure as seen in Fig. 16 and, consequently, good molded articles cannot be produced with a resin material which exhibits a variation of the condition of expansion, shrinkage and cure. In this connection, the document KUNSTSTOFFE, Vol. 74, No. 9, September 1984, pages 527 - 530, Munich, DE; H. CHEREK et al.: "Einsatz adaptiver Regelungen beim Pressen von SMC" forming the preamble of claim 1 describes a method of controlling a hydraulic press which comprises a pair of metal mold halves which may be mounted to a slide and which are movable by a hydraulic cylinder to compression mold a work within a cavity defined between the metal mold halves. According to the described method, the velocity of movement of the metal mold half is controlled by the hydraulic cylinder until the pressurization of the work is started, and, after starting of the pressurization of the work, the pressurizing force of the hydraulic cylinder is controlled, wherein the changing over from the velocity control to the pressure control is performed in dependency on, e.g., the internal pressure of the hydraulic cylinder, the position of a member which is moved by the hydraulic cylinder, the internal pressure of the cavity, or the velocity of the metal mold half. Further, the document DE-C-3 401 203 discloses a hydraulic press for compression molding a workpiece (e.g., a sheet molding compound), comprising four pressurizing cylinders which are disposed at the corners of a bed and the pressures of which are detected, and a main cylinder for moving a pestle. The function of this press is as follows: Firstly, the workpiece is positioned in a cavity disposed in the space between the bed and the pestle, whereupon the pestle is downwardly moved at a relatively high speed to a position shortly above the contacting position of an upper mold half connected to the pestle, with the workpiece. Then, the pestle is fixedly connected to the piston rods of the pressurizing cylinders. Secondly, a velocity control mode takes place in which the pestle is continuously moved downwards until the workpiece completely fills the cavity, whereby the pressures of the pressurizing cylinders increase. Thirdly, the aforementioned velocity control mode is switched over to a pressure control mode as soon as the sum of the detected pressures of the pressurizing cylinders exceeds a predetermined pressure, wherein, during the pressure control mode, the pressure control is effected at a plurality of stages. Finally, after finishing of the pressure control mode, the pestle and the bed are spaced apart and the workpiece is removed from the cavity. Although the above described methods take the current condition of the resin material into account when the control is to be changed over from velocity control to pressure control, this state of the art suffers from the fact that the resin material may not fully be filled up in the cavity when the changing over of the control is performed. As a result, the molded articles may vary in thickness or density. SUMMARY OF THE INVENTION Accordingly, it is the object of the present invention to provide a method of controlling a hydraulic press according to which the changing over from velocity control to pressure control can be effected at an optimum point of time in accordance with a condition of the material being filled so that a uniform high quality of the molded articles can be attained. This object is solved by the features indicated in claim 1. According to the invention, there is provided a method of controlling a hydraulic press of the type wherein, when at least one of a pair of metal mold halves is moved by a hydraulic cylinder to compression mold a work within a cavity defined between the metal mold halves, the velocity of movement of the metal mold half by the hydraulic cylinder is controlled until pressurization of the work is started, and, after starting of the pressurization, the pressurizing force of the hydraulic cylinder is controlled, wherein an internal pressure of the cavity is detected at a predetermined stroke position of the hydraulic cylinder during the velocity control, and then the detected internal pressure of the cavity is compared with a preset value for the predetermined stroke position to obtain a difference there between, whereafter the control of the hydraulic cylinder is changed over from velocity control to pressure control at a point of time just when the hydraulic cylinder has been moved by an amount corresponding to the pressure difference from the predetermined stroke position. With the controlling method, changing over from velocity control to pressure control proceeds in the following procedure. At first, an internal pressure of the cavity is detected at a predetermined stroke position of the hydraulic cylinder during velocity control. The internal pressure of the cavity at the predetermined stroke position exhibits almost fixed values in accordance with filling conditions of a work in the cavity, and such values are obtained in advance through experiments or the like. Accordingly, if the detected internal pressure of the cavity is compared with a preset value for the predetermined stroke position, a current filling condition of the resin at the stroke position can be identified through such comparison. In other words, if the detected pressure and the preset pressure are compared with each other to obtain a difference between them, it can be estimated by what amount the metal mold half should be moved by the hydraulic cylinder after then until the resin is filled up in the cavity. Accordingly, if the hydraulic cylinder is operated by an amount corresponding to the difference, the resin will be filled up in the cavity. Then, if the control of the hydraulic cylinder is changed over from velocity control to pressure control at this point of time, a uniform molding can be attained. Consequently, the control can be changed over normally when the resin is filled up in the metal mold independently of an amount or manner of supply of the resin. According to the teaching of the dependent claim 2, there is provided a method of controlling a hydraulic press according to claim 1, in which the changing over from the velocity control to the pressure control is effected at a point of time when the hydraulic cylinder is moved to a position of a predetermined stroke value Z₀ and an internal pressure P of the cavity is detected upon such changing over and is compared with a predetermined preset internal pressure P₀ to obtain a stroke modifying amount δZ for modifying the internal pressure P of the cavity to the predetermined internal pressure P₀ in accordance with an equation δZ = α(P₀ - P) where α is a coefficient, and then a stroke value of the hydraulic cylinder at which the changing over from velocity control to pressure control is to be effected in a subsequent next molding cycle is modified to Z₀ + δZ, and wherein such modification is repeated for each molding cycle. With the controlling method, changing over from velocity control to pressure control is effected in response to a stroke position of the hydraulic cylinder detected by a suitable means. In particular, at first a stroke position Z₀ of the hydraulic cylinder at which it is estimated that a predetermined fixed amount of resin fills up the cavity of the metal mold and an internal pressure P₀ of the cavity 13 then are set. The preset values Z₀ and P₀ can be determined in advance through pre-molding, experiments or calculations. Then, in actual operation, at first in the first compression molding, the control is changed over from velocity control to pressure control when the stroke of the hydraulic cylinder reaches the preset value Z₀. Simultaneously, an internal pressure P of the cavity is detected. The detection value P is compared with the preset internal pressure value P₀. In case the detection value P is equal to the preset value P₀, it is determined that the resin is fully filled in the cavity as expected. Accordingly, also in a subsequent next molding cycle, the control is changed over when the hydraulic cylinder is moved to the preset stroke position Z₀. To the contrary, in case the detection value P is smaller than the preset value P₀, it is determined that the resin is not yet filled up in the cavity. Thus, in a subsequent next molding cycle, the point at which the control is to be changed over is corrected. The amount δZ for correction is calculated in accordance with the equation δZ = α(P₀ - P) . The coefficient α is determined through experiments or the like in advance. Accordingly, in the next molding cycle, the control is changed over from velocity control to pressure control when the stroke position of the hydraulic cylinder reaches Z₀ + δZ. On the contrary, in case the detection value P is greater than the preset value P₀, it is considered that the resin is filled by an excessive amount in the cavity. Also in this instance, the stroke position at which the control is to be changed over is similarly corrected to Z₀ + δZ for a subsequent next molding cycle. Then, also in the second molding cycle, an internal pressure P of the cavity is detected and compared with the preset value P₀ to effect similar correction of the stroke value. As such correction is repeated in successive molding cycles, an appropriate stroke position at which the control is to be changed over can be found out. With the controlling method, a timing at which the control of the hydraulic cylinder is to be changed over is modified by the machine itself in accordance with a parameter of the same and the machine itself finds out an optimal molding condition. Accordingly, modifying operations which are conventionally effected by hand can be reduced significantly, resulting in improvement in production efficiency. Besides, the changing over from velocity control to pressure control can be effected appropriately, and in the pressure control after then, the pressurizing force of the hydraulic cylinder can act sufficiently upon a work. Accordingly, the molded articles thus obtained can be improved in strength, accuracy in dimension and so on. According to the dependent claim 3 the pressure control is effected at a plurality of stages in accordance with a change of the current condition of the work. With the controlling method, the pressure control after changing over from velocity control is effected at a plurality of stages in accordance with a change of the condition of the work. Accordingly, during compression molding, the pressurizing force will be maintained at an optimum level. A change of the condition of the work is obtained in advance through experiments or the like, and a variation of the condition in accordance with the passage of time is grasped. The multi-stage control is thus effected in accordance with a preset time schedule, as set forth in claim 4. A change of the condition of a work may be detected from a pressure of the work or a hydraulic pressure of the hydraulic cylinder, as set forth in claim 5 and 6, respectively. According to the teaching of the dependent claim 8, in a deceleration control of the velocity control before the changing over from velocity control to pressure control, the velocity V of the movement of the hydraulic cylinder is controlled in accordance with an expression where Z₁ is a stroke position of the hydraulic cylinder at a point of time when the deceleration control is started, Z2 another stroke position of the hydraulic cylinder upon the changing over from the velocity control to the pressure control, α a time constant, e a base of natural logarithm, and t an interval of time after starting of the deceleration control. With the controlling method, the velocity control of the hydraulic cylinder is effected in accordance with an exponential function. Accordingly, the deceleration can be made smoothly and a uniform material flow can be attained. Further, optimum deceleration patterns for various resin materials can be obtained by suitably changing the time constant or the position at which the deceleration is to be started. Consequently, the pressure control after then can be maintained in optimal molding conditions. According to the teaching of the dependent claim 9, the pressure control proceeds such that the pressure P of the hydraulic cylinder is maintained at a fixed value P₀ for a fixed interval of time T₀ after starting of the pressure control, and, after then, the pressure P is controlled in accordance with an expression where P₁ is a pressure of the hydraulic cylinder upon completion of the compression molding, β a time constant, e a base of natural logarithm, and t an interval of time after lapse of a fixed interval of time T₀. With the controlling method, the pressurizing force is controlled in an exponential function, and the control based on an exponential function conforms well to the actual behavior of the internal pressure of the mold. Thus, the pressurizing force is controlled in conformity with a variation of the condition of expansion, shrinkage and cure which takes place during molding of the resin material. Accordingly, any resin material can be pressurized in an optimal condition, and, consequently, molded articles of a high quality can be attained. Further advantageously developed embodiments of the present invention are subject-matter of the remaining dependent claims. The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an operational diagram illustrating a method of controlling a hydraulic press according to the present invention; Fig. 2 ia a partial vertical sectional view of a hydraulic press to which the present invention is applied; Fig. 3 is a circuit diagram of a hydraulic circuit for a main cylinder of the hydraulic press shown in Fig. 2; Fig. 4 is a circuit diagram of a hydraulic circuit for a leveling cylinder of the hydraulic press shown in Fig. 2; Fig. 5 is a flow diagram illustrating different stages of operation of the hydraulic press of Fig. 2; Fig. 6 is a graph illustrating a variation of the internal pressure of the main cylinder shown in Fig. 2; Fig. 7 is a graph showing a relationship between a stroke of a slide and an internal pressure of a metal mold; Fig. 8 is an operational diagram illustrating a modification to the controlling method illustrated in Fig. 1; Fig. 9 is a flow chart illustrating a further modification to the controlling method illustrated in Fig. 1; Fig. 10 is an operational diagram illustrating a still further modification to the method illustrated in Fig. 1; Fig. 11 is a graph illustrating a relationship between a position of the slide and a time in the velocity control in the modified method illustrated in Fig. 10; Fig. 12 is a circuit diagram showing a controlling circuit to which the method illustrated in Fig. 10 is applied; Fig. 13 is a flow chart illustrating the operation of the controlling circuit shown in Fig. 12; Fig. 14 is a graph illustrating a relationship between a pressure and a time in the pressure control according to the method illustrated in Fig. 10; Fig. 15 is a pressure - time diagram illustrating internal pressures of a metal mold; Fig. 16 is a stroke - time and pressure - time diagram illustrating a conventional method of controlling a hydraulic press; Fig. 17 is a stroke - time diagram illustrating a conventional multi-stage deceleration control; and Figs. 18 and 19 are schematic sectional views showing different conditions of a resin material put in a metal mold. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to Fig. 2, there is shown a hydraulic press for an SMC material. The hydraulic press shown includes a bed 1 secured to a floor, an upright 2 provided uprightly at each of four corners of the bed 1, and a crown 3 securely connected to upper end portions of the uprights 2. A main cylinder 4 of the single action type is mounted at a central location of the crown 3 and includes a piston rod 5 which extends downwardly through and below the crown 3, and a slide 6 is connected to a lower end of the piston rod 5. The slide 6 is supported for up and down movement under the guidance of the four uprights 2. A pair of sub-cylinders 7 of the double action type are provided on the left and right opposite sides of the crown 3 and each includes a piston rod 8 connected to the slide 6. A leveling cylinder 9 is provided at each of the four corners of the bed 1 and includes a piston rod 10 having an upper end face located for engagement with a lower face of the slide 6. An upper metal mold half 11 is removably mounted on a lower face of the slide 6 while a lower metal mold half 12 is removably mounted on an upper face of the bed 1. The upper and lower metal mold halves 11 and 12 are designed such that a cavity 13 may be defined between them when they are in a mutually closed position. A mold pressure sensor 14 for detecting the internal pressure of the cavity 13 is installed in the lower metal mold half 12. A rotary encoder 15 is mounted on a side face of the bed 1, and a sprocket wheel 16 is mounted on an input shaft of the rotary encoder 15. A chain 18 extends between and around the sprocket wheel 16 and another sprocket wheel 17 which is mounted for rotation on a side face of the crown 3. The opposite ends of the chain 18 are secured to a bracket 19 mounted on the slide 6. The rotary encoder 15 is thus provided to detect a position and a velocity of movement of the slide 6. An oil tank 20 is placed on the crown 3 and hydraulically connected to the main cylinder 4 via a full oil valve 21. The main cylinder 4 is further connected to a hydraulic unit 25 for the pressurizing cylinders 4 and 7 by way of hydraulic pipes 22, 23 and 24. Meanwhile, the leveling cylinders 9 are connected to a leveling hydraulic unit 27 by way of a hydraulic pipe 26. The pressurizing hydraulic unit 25, the leveling hydraulic unit 27, the mold pressure sensor 14 and the rotary encoder 15 are electrically connected to a controlling means 28. A table 29 for carrying a metal mold into and out of the hydraulic press is provided adjacent the hydraulic press. Referring now to Fig. 3, there is shown a hydraulic circuit for the pressurizing hydraulic unit 25. The hydraulic circuit shown includes an oil tank 30, and first and second hydraulic pump driving electric motors M₁ and M₂. The first motor M₁ is connected to drive first and second velocity controlling servo pumps P₁ and P₂, while the second motor M₂ is connected to drive third and fourth hydraulic pumps P₃ and P₄ and a servoing hydraulic pump P₅. Delivery ports of the first to fourth pumps P₁ to P₄ are connected to first to fourth hydraulic oil lines 31, 32, 33 and 34, respectively. The first to fourth hydraulic oil lines 31, 32, 33 and 34 are joined to a fifth hydraulic oil line 35 which branches into sixth and seventh hydraulic oil lines 36 and 37. The sixth hydraulic oil line 36 branches into the hydraulic pipe 22 connected to the main cylinder 4 and the hydraulic pipe 23 connected to the head sides of the sub-cylinders 7. The seventh hydraulic oil line 37 is connected to the hydraulic pipe 24 connected to the rod sides of the sub-cylinders 7. A pilot hydraulic pipe 38 is connected to the delivery port of the servoing hydraulic pump P₅ and shown in a broken line in Fig. 3. Relief valves 39, 40, 41 and 42 for remote operation are interposed in the first to fourth hydraulic oil lines 31, 32, 33 and 34, respectively, and connected commonly to a drain line 43. A cooler 44 is interposed in the drain line 43. First to third on-off valves 45, 46 and 47 are interposed in the sixth hydraulic oil line 36. The first on-off valve 45 is provided to control the sixth hydraulic oil line 36 to be opened or closed. The second on-off valve 46 is provided to control the sixth hydraulic oil line 36 to be opened to the oil tank 30. The third on-off valve 47 is provided to control the hydraulic pipe 22 for the main cylinder 4 to be opened or closed. The first to third on-off valves 45, 46 and 47 are operated by the pressure oil of the pilot hydraulic pipe 38 in response to an instruction from the controlling means 28. A pressure controlling valve 48 is interposed in the sixth hydraulic oil line 36 between the first on-off valve 45 and the third on-off valve 47. The pressure controlling valve 48 is provided to control the pressure in the hydraulic oil line 36 infinitely or stepwise in response to an instruction from the controlling means 28. Fourth to sixth on-off valves 49, 50 and 51 are interposed in the seventh hydraulic oil line 37. Those on-off valves 49, 50 and 51 are also operated to open or close in response to an instruction from the controlling means 28. A check valve 52 is installed in the full oil valve 21 and operated to open or close by the pressure oil of the pilot hydraulic pipe 38 in response to an instruction from the controlling means 28. Referring now to Fig. 4, there is shown a hydraulic circuit for the leveling hydraulic unit 27. The hydraulic circuit shown includes an oil tank 53 from which hydraulic oil is supplied by way of a hydraulic pump P₆ and an eighth hydraulic oil line 54 to the hydraulic pipe 26 connected to the leveling cylinders 9. Four servo valves 55 are interposed in the eighth hydraulic oil line 54 and connected to be operated in response to an instruction from the controlling means 28. The four servo valves 55 are provided corresponding to the leveling cylinders 9. Fig. 5 illustrates a flow of operations of the hydraulic press described above, and Fig. 1 is an operational diagram of the same. Referring to Figs. 1 and 5, STEP 1 illustrates the hydraulic press at a starting point of time at which the slide 6 is positioned at its top dead center. In the position of the hydraulic press shown, the SMC material 56 is filled into the cavity 13 of the lower metal mold half 12. Subsequently, STEP 2 shows the hydraulic press in a condition in which the slide 6 is being moved down at a high velocity. Thereupon, hydraulic oil from the first to fourth pumps P₁, P₂, P₃ and P₄ shown in the hydraulic circuit diagram of Fig. 3 is supplied to the main cylinder 4 and the sub-cylinders 7 by way of the first to fourth hydraulic oil lines 31, 32, 33 and 34 and the fifth hydraulic oil line 35 and then by way of the sixth hydraulic oil line 36 and the hydraulic pipes 22 and 23 for the main cylinder 4 and the sub-cylinders 7, respectively. In this instance, the first, third, fifth and sixth on-off valves 45, 47, 50 and 51 are open while the second and fourth on-off valves 46 and 49 are closed. The hydraulic oil on the rod sides of the sub-cylinders 7 is thus returned to the tank 30 by way of the fifth on-off valve 50. STEP 3 illustrates the slide 6 moving down at an intermediate velocity. In this instance, the delivery amounts of the velocity controlling servo pumps P₁ and P₂ are controlled by the controlling means 28 to move down the slide 6 at an intermediate velocity. The changing over from STEP 2 to STEP 3 is effected in response to the detection of a specific position of the slide 6 by the rotary encoder 15. At the subsequent STEP 4, the velocity control and the leveling control are effected in a simultaneous relationship. In particular, the changing over from STEP 3 to STEP 4 is effected in response to the detection of another specific position of the slide 6 by the rotary encoder 15 such that the lower face of the slide 6 may be contacted softly with the top end faces of the rods 10 of the leveling cylinders 9. After then, the slide 6 is moved down while pressing against the rods 10 of the leveling cylinders 9. In this instance, the servo valves 55 are controlled individually so that the leveling cylinders 9 may be positioned at a same level in order to attain a horizontal position of the slide 6 with a high degree of accuracy. At STEP 4, the velocity controlling servo pumps P₁ and P₂ are controlled so as to move down the slide 6 at a plurality of stages. At the end of STEP 4, the mold closing of the upper and lower metal mold halves 11 and 12 is completed and the SMC material 56 fills up the cavity 13. As the SMC material 56 fills up the cavity 13, the internal pressure of the cavity 13 exhibits its peak value as seen in Fig. 15. The internal pressure is successively or continuously detected by the mold pressure sensor 14. In response to such a detection value, or, more particularly, when such a detection value coincides with a preset value set in advance in the controlling means 28, the control of the movement of the slide 6 or the upper metal mold half 11 is changed over from the velocity control to the subsequent pressure control. Such changing over of the control is illustrated at STEP 5. Such changing over of the control can naturally depend on the detection of any other parameter of the hydraulic press which varies as the velocity control proceeds. The parameter may be a position of the slide 6 or the upper metal mold half 11. In such an instance, the position at which the internal pressure of the cavity 13 exhibits its peak value is found out in advance through experiments, and the position is set in advance in the controlling means 28. Thus, at a point of time when a position detected by the rotary encoder 15 coincides with the preset value, the control of the movement of the slide 6 is changed over from the velocity control to the pressure control. The parameter may otherwise be a pressure of hydraulic oil within the main cylinder 4 or any of the sub-cylinders 7 because the pressure exhibits its peak value when the internal pressure of the cavity 13 exhibits its peak value. In this instance, a pair of pressure detecting means 22a and 23a may be interposed in the hydraulic pipes 22 and 23, respectively, and connected to the controlling means 28 as shown in phantom in Figs. 2 and 3. Thus, as the SMC material 56 is filled up in the cavity 13, the internal pressures of the main cylinder 4 and the sub-cylinders 7 exhibit their peak values as seen in Fig. 6, and at a point of time when a pressure detected by the pressure detecting means 22a or 23a coincides with a preset value set in advance in the controlling means 28, the control of the movement of the slide 6 is changed over from the velocity control to the pressure control. The velocity of the downward movement of the upper metal mold half 11 or the slide 6 may also be adopted as the parameter. In particular, as the SMC material 56 is filled up in the cavity 13, the velocity of the downward movement of the upper metal mold half 11 or the slide 6 is gradually reduced to zero or a value near zero as seen in the stroke - time diagram of Fig. 1 while the internal pressure of the cavity 13 approaches its peak value. Thus, the velocity of the downward movement of the upper metal mold half 11 or the slide 6 is successively detected by means of the rotary encoder 15, and when the detected velocity value coincides with a preset value, the changing over from the velocity control to the pressure control is effected. STEP 6 illustrates the hydraulic press in which the pressure control and the leveling control are effected in a simultaneous relationship. In particular, the pressure of the hydraulic circuit is controlled by the pressure controlling valve 48 while the delivery amounts from the hydraulic pumps are held fixed. In the pressure control, a change of the condition of the SMC material 56 within the cavity 13 is detected by the mold pressure sensor 14, and the pressurizing force is controlled at a plurality of stages (including infinite control) in response to the change of the condition. Such a controlling instruction is developed from the controlling means 28. Such multi-stage pressure control can be achieved otherwise, where a change of the condition of the SMC material 56 within the cavity 13 in accordance with passage of time is known in advance, by controlling the pressurizing force stepwise to a selective one of preset values after each lapse of a predetermined interval of time. In the pressure control, the leveling cylinders 9 are controlled in response to a change of the pressure in the main cylinder 4 or the sub-cylinders 7 so as to maintain the slide 6 in a horizontal position. After completion of the compression molding, the slide 6 is moved up a little by way of a pressure relieving step in order to effect in-mold coating. The step is shown at STEP 7. At STEP 7, the velocity control is effected by the servo valves 55. For such upward movement of the slide 6, the second, third and fifth on-off valves 46, 47 and 50 are closed while the first, fourth and sixth on-off valves 45, 49 and 51 are opened. Hydraulic oil is thus supplied to the cylinder sides and the rod sides of the sub-cylinders 7 by way of the fifth hydraulic oil line 35 and the sixth and seventh hydraulic oil lines 36 and 37 so that the sub-cylinders 7 are brought into a free condition. The oil in the main cylinder 4 is thus enabled to flow into the oil tank 20 via the full oil valve 21 as the check valve 52 is operated by the pilot pressure. The leveling cylinders 9 are thus moved up by means of the servo valves 55 thereof and thus move up the slide 6. Also upon such upward movement of the slide 6, the leveling cylinders 9 are controlled so as to maintain a horizontal position of the slide 6. A velocity of the upward movement and a position of the slide 6 are detected by the rotary encoder 15 and fed back for the velocity control. After completion of the in-mold coating, the slide 6 is moved down again, and when a predetermined position is reached, the control of the movement of the slide 6 is changed over from velocity control to pressure control. The step is shown at STEP 8. At STEP 8, the multi-stage pressure control is effected in a similar manner as at STEP 6 described above, thereby completing the compression molding including the in-mold coating. Then, the pressure relief is effected, and then, the slide 6 is moved up back to its original top dead center. The upward movement is illustrated at STEP 9 to STEP 12, and after STEP 9, the velocity control is effected. For the upward movement of the slide 6 at and after STEP 9, the first, third and fifth on-off valves 45, 47 and 50 are closed while the second, fourth and sixth on-off valves 46, 49 and 51 are opened. Thus, hydraulic oil is supplied to the rod sides of the sub-cylinders 7 by way of the fifth hydraulic oil line 35 and the seventh hydraulic oil line 37. The oil on the rod sides of the sub-cylinders 7 is returned to the tank 30 via the second on-off valve 46 of the sixth hydraulic oil line 36, while the oil in the main cylinder 4 is returned to the oil tank 20 via the full oil valve 21 as the check valve 52 is operated by the pilot pressure. All of the steps of the hydraulic press are thus completed. The method of controlling a hydraulic press of the embodiment described above can be modified so as to attain more accurate control of a hydraulic press to produce molded articles with a higher degree of accuracy on the hydraulic press. Figs. 7 and 8 illustrate such a modification to the method of the embodiment described above, according to the present invention. Referring first to Fig. 8, according to the modified method, an internal pressure of the cavity 13 at a point of time before a point A of time at which the control is to be changed over from velocity control to pressure control is detected by the mold pressure sensor 14 and compared with a preset pressure to correct the point A at STEP 5 so as to cause the changing over from velocity control to pressure control to be effected at an optimum point of time in accordance with a condition of the resin material. Fig. 7 shows a relationship between a position of the slide 6 and an internal pressure of the cavity 13 in a velocity control region denoted at B in Fig. 8. Referring to Fig. 7, the reference symbol S₀ denotes a point of time at which the downward movement of the slide 6 for pressurization is started, S₁ a point at which the velocity of the pressurizing downward movement is changed over from a first velocity to a second velocity, S₃ to S₄ denote each a point at which the velocity is changed over similarly, and S₅ denotes a point at which the control of the downward movement of the slide 6 is changed over from velocity control to pressure control. The point S₅ coincides with the point A shown at Fig. 8 and thus is a point at which the SMC material 56 is filled up fully in the cavity 13. Referring to Fig. 7, an internal pressure P₀ of the cavity 13 at the fixed point S₄ before the changing over from velocity control to pressure control is set in advance. The preset pressure P₀ is a pressure at the point S₄ when the SMC material 56 is filled in an ideal condition. A difference ΔS₀ = S₅ - S₄ in such an ideal condition is also set in advance. Further, internal pressures P at the point S₄ in various extraordinary conditions and differences ΔS = S₅ - S₄ then are also found out in advance through experiments or the like fin this instance, the point S₄ is a fixed point while the point S₅ is a variable point). A difference ΔP between P₀ and P and the difference ΔS then are thus stored in the controlling means 28. In the actual operation of the hydraulic press, when the SMC material is filled by an excessively large amount, if an internal pressure P₁ of the cavity 13 at the fixed point S₄ is detected, then P₁ > P₀, and a difference ΔP' = P₀ - P₁ is calculated. Then, a value ΔS' corresponding to this ΔP' is found out, and at a position where the stroke of the slide 6 coincides with S₅ = S₄ + ΔS' , the control of the downward movement of the slide 6 is changed over from the velocity control to the pressure control. On the contrary, when the SMC material is filled by an excessively small amount, an internal pressure P₂ of the cavity 13 at the fixed point S₄ is P₂ < P₀, and, thus, a difference ΔP'' = P₀ - P₂ is calculated. Then, a value ΔS'' corresponding to this ΔP'' is found out, and at a position where the stroke of the slide 6 coincides with S₅ = S₄ + ΔS'' , the control is changed over from the velocity control to the pressure control. According to the modified method, a fluctuation of a filling condition originating from a difference in the manner in which the SMC material is thrown into a metal mold is detected from an internal pressure of the metal mold in order to correct a point of time at which the filling of the material is to be completed. Consequently, the changing over from velocity control to pressure control takes place accurately at a point of time at which the material is fully filled up in the metal mold, which makes the compression molding after then uniform. Fig. 9 illustrates another modification to the method of the embodiment described above. In the modified method illustrated in Fig. 9, a stroke position Z₀ of the slide 6 for the changing over of the control and an internal pressure P₀ of the cavity 13 then are set in advance. The preset values Z₀ and P₀ are values of a position of the slide 6 and an internal pressure of the cavity 13, respectively, when a predetermined amount of the SMC material 56 is filled until it is filled up fully in the cavity 13, and they are obtained in advance through experiments or the like. Thus, at first in the first compression molding, the control is changed over at STEP 5 when the slide 6 reaches the preset position Z₀. The timing of such changing over is determined by a position of the slide 6 detected by way of the encoder 15. Simultaneously, an internal pressure P of the cavity 13 is detected by the mold pressure sensor 14. The detection value P is then compared with the preset internal pressure P₀. In the case of P = P₀, it is determined that the internal pressure of the cavity 13 at the control changing over position Z₀ exhibits an appropriate value, and, accordingly, the position of the slide 6 at which the control is to be changed over at STEP 5 in a subsequent next compression molding cycle is left as the initially set value Z₀. To the contrary, in the case of P ≠ P₀, it is determined that the internal pressure of the cavity 13 at the control changing over position Z₀ is either greater or smaller than an appropriate value, and, accordingly, the position of the slide 6 at which the control is to be changed over at STEP 5 in a subsequent next compression molding cycle must be corrected. The amount δZ for the correction is calculated in accordance with an equation δZ = α(P₀ - P) . The coefficient α is determined through experiments or the like in advance. Thus, in a second compression molding cycle, the stroke position of the slide 6 at which the control is to be changed over is changed from Z₀ to Z₀ + δZ. In any compression molding cycle after then, a similar correction is repeated to find out an optimum position of the slide 6 at which the control is to be changed over. The controlling means 28 has such a learning function of finding out an optimum position. It is to be noted that where the stroke position at which the control is to be changed over for the first time after starting of the hydraulic press is the preset position Z₀ itself, the correction amount δZ may appear with a positive sign (+) or a negative sign (-) and the number of times necessary for the correction amount δZ to converge to δZ -> 0 may be too great. Thus, the stroke position for the first changing over of the control is set to a value a little smaller than the preset value Z₀ so that the correction value δZ may converge to zero only from the positive (+) side in order to reduce the number of times required for the correction. On the other hand, when the detected value P of the internal pressure of the cavity 13 is greater than the preset value P₀, the bad influence on the products is considerably small comparing with that when the detected value P is smaller than the preset value P₀. Accordingly, for practical use, the correction of a stroke position of the slide 6 may be effected only in the case of P < P₀. While the velocity control of the slide 6 is effected in a stepwise manner in any of the methods of the embodiment and the modifications to the embodiment described hereinabove, it may otherwise be effected partially or entirely in an infinite manner. Figs. 10 to 14 illustrate such a modified method wherein the velocity control includes an exponential function control. In particular, the controlling means 28 has the following initial preset values stored therein in advance: V₀ : velocity of the movement of the hydraulic cylinder or cylinders or the slide before the exponential function control is entered (fixed velocity at STEP 3) Z₁ : position of the slide at which deceleration is to be started (position at which the changing over from STEP 3 to STEP 4 is to be effected) Z₂ : position of the slide at which the pressure control is to be started (position at which the changing over from STEP 4 to STEP 5 is to be effected) α : time constant V : minimum controlling velocity of the velocity controlling pumps P₁ and P₂ Then, the position Z of the slide 6 is successively detected by way of the rotary encoder 15, and such a detected value Z of the position is compared with the deceleration starting position Z₁. In case the detected value Z is greater than the deceleration starting position Z₁, the slide 6 is controlled to move down at the fixed velocity V₀ by means of the velocity controlling pumps P₁ and P₂ (STEP 3). To the contrary, in case the detected value Z is equal to or smaller than the deceleration starting position Z₁ (Z ≦ Z₁), the slide 6 is controlled to decelerate in accordance with a deceleration pattern depending upon an exponential function and given by an expression where e : base of natural logarithm t : time after the decelerating starting point t₁ In this instance, if it is possible to control the amount of movement of the slide 6 by means of the velocity controlling pumps P₁ and P₂, then the movement of the slide 6 is controlled by the velocity controlling means P₁ and P₂. On the contrary, if the amount of movement of the slide 6 is too small, the remote operation relief valves 41 and 42 will be operated to effect the control of a flow rate to the main cylinder 4 (in this instance, the flow rate of the velocity controlling pumps P₁ and P₂ is zero). The time constant α may have a default value given by Where the time constant α is used, the transition from the fixed velocity region (STEP 3) to the deceleration region (STEP 4) can proceed smoothly. Fig. 12 shows a part of a controlling electric circuit where all of the hydraulic pumps included in the hydraulic circuit shown in Fig. 3 are variable pumps, and Fig. 13 shows the operation of the controlling circuit shown in Fig. 12. Where the controlling circuit shown in Fig. 12 is employed, a change-over valve 57 and a flow controller 58 are interposed in parallel in the hydraulic circuit. Thus, in case the controlling flow rate of the main cylinder 4 can be controlled by the variable pumps, the control is effected by means of the variable pumps, but on the contrary in case the controlling flow rate is too low, the change-over valve 57 is turned on and the flow rate is controlled by means of the flow controller 58 so as to effect such exponential function control as described above. Where such an exponential function is used for the deceleration control, the slide 6 can be decelerated smoothly during the pressurizing downward movement thereof (STEP 4), and, consequently, a uniform material flow can be attained. Accordingly, the pressure control at a next step can be effected under an optimum condition, and a molded article of a high quality can be produced. Further, if the deceleration position Z₁ and/or the time constant α are changed suitably, optimum molding conditions can be set for every kind of resin material. The pressure control may also include an exponential function control at STEP 6 and STEP 8. In particular, referring to Fig. 14, the internal pressure P of the cavity 13 is maintained for a predetermined fixed period of time T₀ after starting of the pressure control (STEP 5), and, after then, the internal pressure P is changed in accordance with an exponential function where P : internal pressure of the pressurizing cylinder 4 or 7 P₀ : initial internal pressure P₁ : internal pressure upon completion of the compression molding β : time constant t : time For such exponential function control as described above, an electric signal corresponding to the equation (3) above is transmitted from the controlling means 28 to the relief valve (electromagnetic proportional pressure controlling valve) 48 to control the hydraulic pressures of the pressurizing cylinders 4 and 7 to a preset value. It is to be noted that the coefficients P₀, P₁ and β in the equation (3) above depend on the resin material used. The reason why the exponential function of the equation (3) is used in the pressure control is that a variation of the internal pressure of the cavity 13 illustrated in Fig. 15 conforms to an exponential function. Where the pressure control is effected in accordance with an exponential function, a pressurizing force can be applied to the resin material, which undergoes a change of the condition of expansion, shrinkage and cure upon compression molding, in accordance with a current condition of the resin material. Accordingly, molded articles of a high quality can be produced.

Claims: A method of controlling a hydraulic press which comprises a pair of metal mold halves (11, 12) movable by a hydraulic cylinder (4) to compression mold a work (56) within a cavity (13) defined between said metal mold halves (11, 12), said method comprising the steps of controlling the velocity of movement of said metal mold half (11) by said hydraulic cylinder (4) until the pressurization of said work (56) is started, and, after starting of said pressurization, controlling the pressurizing force of said hydraulic cylinder (4), wherein an internal pressure of said cavity (13) is successively detected during the velocity control, and a changing over from said velocity control to said pressure control is effected in dependency on said internal pressure, characterized by the steps of detecting said internal pressure at a predetermined stroke position of said hydraulic cylinder (4) during said velocity control, and comparing the detected internal pressure with a preset value for said internal pressure at said predetermined stroke position to obtain a difference therebetween, wherein the control is changed over from velocity control to pressure control at a point of time when said hydraulic cylinder (4) has been moved by a stroke amount corresponding to said pressure difference. A method according to claim 1, characterized in that said changing over from velocity control to pressure control is effected at a point of time when said hydraulic cylinder (4) is moved to a position of a predetermined stroke value Z₀, and that said internal pressure P of said cavity (13) is detected upon such changing over and compared with a predetermined internal pressure P₀ to obtain a stroke modifying amount δZ for modifying said internal pressure P of said cavity (13) to said predetermined internal pressure P₀ in accordance with the expression δZ = α (P₀ - P) in which α is a coefficient, wherein a stroke value of said hydraulic cylinder (4) at which said changing over from velocity control to pressure control is to be effected in a subsequent molding cycle, is modified to Z₀ + δZ, and wherein such modification is repeated for each molding cycle. A method according to claim 1 or 2, characterized in that said pressure control is effected at a plurality of stages in accordance with a change of the current condition of said work (56). A method according to claim 3, characterized in that said multi-stage control is effected in accordance with a preset time. A method according to claim 3, characterized in that said multi-stage control is effected in response to a pressure of said work (56) within said cavity (13) detected by a pressure detecting means (14). A method according to one of the claims 3 to 5, characterized in that said multi-stage control is effected by controlling a preset pressure of a pressure controlling valve (48) provided in a hydraulic circuit (25) of said hydraulic cylinder (4) to control the internal hydraulic pressure of said hydraulic cylinder (4) to said preset pressure. A method according to claim 6, characterized in that said pressure controlling valve (48) operates in response to an electric signal. A method according to one of the preceding claims, characterized in that said work (56) consists of a resin material, wherein, in a deceleration control of said velocity control before said changing over from said velocity control to said pressure control, the velocity V of movement of said hydraulic cylinder (4) is controlled in accordance with the expression V = (Z₁ - Z₂) 1/α e -1/α t in which Z₁ is the stroke position of said hydraulic cylinder (4) at a point of time when said deceleration control is started, Z₂ is another stroke position of said hydraulic cylinder (4) upon said changing over from said velocity control to said pressure control, α is a time constant, e is the base of natural logarithm, and t is the time after starting of said deceleration control. A method according to one of the preceding claims, characterized in that said work (56) consists of a resin material, wherein said pressure control proceeds such that the pressure P or said hydraulic cylinder (4) is maintained at a fixed value P₀ for a fixed period of time T₀ after starting of said pressure control, and, thereafter, the pressure P is controlled in accordance with the expression P = P₁ + (P₀ - P₁) e -1/β t in which P₁ is the pressure of said hydraulic cylinder (4) upon completion of the compression molding, β is a time constant, e is the base of natural logarithm, and t is the time after lapse of said fixed period of time T₀. A method according to one of the preceding claims, characterized in that said work (56) is a thermosetting resin which undergoes a variation in condition of expansion, shrinkage and cure during compression molding.

Inventors: FUKUDA YOSHIHIRO, IMANISHI ETUJIRO, KAJIYAMA KAZUYUKI, KONDO HIROAKI, KURUMAJI MASANOBU, NAKAGAWA NORIHARU, SANO TUTOMU, TAKEUCHI NAOKI, UEDA HIROSHI, FUKUDA, YOSHIHIRO, IMANISHI, ETUJIRO, KAJIYAMA, KAZUYUKI, KONDO, HIROAKI, KURUMAJI, MASANOBU, NAKAGAWA, NORIHARU, SANO, TUTOMU, TAKEUCHI, NAOKI, UEDA, HIROSHI
Applicants: KOBE STEEL LTD, KABUSHIKI KAISHA KOBE SEIKO SHO ALSO KNOWN AS KOBE STEEL LTD.
Application year: 1988
Main classification: B29C43
IPC codes: B30B15