Injection molding control system

An injection molding control provides for the programmable control of ram velocity as a function of the position of the ram through closed-loop feed-back of the measured actual velocity. Closed-loop feed-back of the actual mold cavity pressure overrides the velocity program in an analog fashion to stop the ram when a preset cavity pressure has been attained associated with a desired charge size. Programmable control of the ram screw speed and/or back pressure during injection as a function of ram position or time is used to impart a predetermined temperature profile to the charge along the length thereof while it is in the barrel prior to injection. This enables controlled variation in density of the molded article throughout its volume to achieve desired levels in preselected characteristics such as surface wear, gloss, resolution and the like. A closed-loop servo system responsive to hydraulic pressure on the ram, including a flow divider valve which meters flow between the ram pressure chamber and a drain tank, provides accurate and continuous control of injection, hold and back pressure to enhance product quality; smooth pressure transitions between different ram pressure levels utilized in the molding cycle to avoid undesirable effects due to ram overshoot; simultaneous flow and pressure increase during injection when ram velocity falls below programmed level thereby avoiding sluggish response characteristics when restoring ram velocity; and reduction in number of hydraulic components required to effect the injection, hold, and back pressure functions.

 

What is claimed is:

1. In an injection molding machine having a ram screw which advances to inject a plasticized charge of material from an injection barrel into a mold cavity and retracts while rotating to accumulate a plasticized charge, a control system for said ram comprising:

a piston connected to said ram and a cylinder within which said piston moves,

a flow divider valve having an inlet connected to a source of pressure fluid, a first outlet connected to said cylinder, a second outlet connected to a fluid reservoir, and movable means to simultaneously alter the sizes of said outlets in inverse relation such that when one increases the other decreases,

a servomotor responsive to electrical signals input thereto and having an output operatively associated with said movable means for controlling the movement of said movable means in accordance with said signals,

first and second sources of electrical signals associated, respectively, with a deviation between desired and actual ram advance during the injection phase of a molding cycle and a deviation between desired and actual ram retraction during the plasticizing phase of a molding cycle,

means for connecting said first source of electrical signals to said servomotor during injection to control said movable means of said flow divider in a manner to advance the ram at a rate correlated to the fluid flow rate to said piston cylinder from said first valve outlet, and an injection pressure correlated to the size of said second valve outlet, and

means for connecting said second source of electrical signals to said servomotor during plasticization to control said movable means of said flow divider in a manner to permit said ram, when said screw rotates, to retract under a back pressure correlated to the size of said second outlet and at a rate correlated to the fluid flow from said cylinder to said first valve outlet,

whereby said ram screw, during injection and plasticization, is continuously under control of said flow divider valve in response to electrical signals input to said servomotor, to provide the desired ram advance and retraction.

2. The system of claim 1 wherein said first source of signals includes:

a ram velocity programmer which provides electrical signals correlated to desired ram velocity,

a ram velocity transducer having an output correlated to actual ram velocity, and

a comparator responsive to said ram velocity programmer and said ram velocity transducer for developing a ram velocity error signal, said error signal being connectable to said servomotor during injection to control said movable means of said flow divider to advance said ram, at said desired velocity.

3. The system of claim 2 wherein said velocity programmer provides electrical signals correlated to desired ram velocity which vary as a function of ram position, and wherein said means for connecting said velocity error signals to said servomotor is under control of a molding cavity pressure monitoring circuit comprising:

a pressure transducer responsive to the pressure in said cavity, and

a comparator which provides a trigger signal when said cavity pressure reaches a preset limit associated with a predetermined charge, said trigger signal being operative to terminate connection of said velocity error signals to said servomotor, terminating control of said flow divider valve and servomotor by said first source of signals.

4. The system of claim 3 further comprising:

a third source of electrical signals associated with a deviation between desired and actual holding pressure applied to said ram intermediate injection and plasticization, and

means responsive to said trigger signal for connecting said third source of electrical signals to said servomotor to control said movable means of said flow divider in a manner to apply a holding pressure to said ram piston correlated to the amount of fluid diverted to said fluid reservoir via said second valve outlet.

5. The system of claim 1 wherein said flow divider valve includes a valve body having a bore which communicates with said inlet and first and second outlets, a spool which smoothly slides within said bore to gradually alter the outlet areas of said first and second outlets in said inverse relation, and wherein said servomotor output is operatively connected to said spool to gradually alter the areas of said outlets and provide smooth pressure transitions in said cylinder in accordance with the signals input thereto from said first and second signal sources.

6. The system of claim 5 further comprising:

a third source of electrical signals associated with a holding pressure applied to said ram intermediate injection and plasticization, and

means for connecting said third source of electrical signals to said servomotor intermediate injection and plasticization to smoothly shift said spool to increase the area of said second valve outlet, and apply a holding pressure to said ram piston of gradually reduced magnitude than the injection pressure, by gradually diverting an increased fluid flow to said fluid reservoir via said second valve outlet, whereby if said charge in said molding cavity is compressed at the conclusion of injection overshoot of said ram will be avoided upon decompression of said charge.

7. The system of claim 1 further comprising:

a third source of electrical signals associated with a deviation between desired and actual holding pressure applied to said ram intermediate injection and plasticization, and

means for connecting said third source of electrical signals to said servomotor intermediate injection and plasticization to control said movable means of said flow divider in a manner to apply a holding pressure to said ram piston correlated to the amount of fluid diverted to said fluid reservoir via said second valve outlet.

8. The system of claim 1 wherein said first source of signals includes:

a source of electrical signals correlated to a predetermined desired substantially constant ram velocity,

a ram velocity transducer having an output correlated to actual ram velocity, and

a comparator responsive to said constant ram velocity signal source and said ram velocity transducer for developing a ram velocity error signal, said error signal being connectable to said servomotor during injection to control said movable means of said flow divider to advance said ram at said substantially constant velocity.

9. In an injection molding machine having a ram screw which advances at a programmed velocity under an injection pressure to inject a plasticized charge of material into a mold cavity, and which simultaneously rotates and retracts under a predetermined back pressure to plasticize a charge, a control system comprising:

a piston connected to said ram screw and a cylinder in which said piston moves,

a hydraulic pressure source of variable output pressure connected to said cylinder to pressurize said piston and establish said injection and back pressures on said ram,

a first pressure transducer providing an output correlated to the pressure in said mold cavity,

a second pressure transducer providing an output correlated to the pressure in said cylinder, and

program means controlling said pressure source during injection and plasticization, said program means being responsive to said first pressure transducer output during injection to terminate advance of said ram at said programmed velocity under said ram injection pressure when said cavity pressure reaches a predetermined level, said program means being further responsive to said second pressure transducer output during plasticization for regulating the pressure from said variable pressure source at a level to establish said predetermined back pressure on said ram during plasticization, whereby said variable hydraulic pressure source for establishing said injection and back pressures during injection and plasticization is under joint control of both said cavity pressure transducer and said cylinder pressure transducer.

10. The control system of claim 9 further comprising:

a transducer associated with said ram screw for providing a signal correlated to actual ram screw velocity,

a servovalve and a pump incorporated in said variable hydraulic pressure source, said servovalve having an inlet connected to said pump, a first outlet connected to said cylinder and a second outlet connected to a fluid reservoir, said servovalve being responsive to electrical signals for altering the sizes of said first and second outlets in inverse relation,

a first source of programmed electrical signals correlated to a programmed ram velocity during injection,

a second source of programmed electrical signals correlated to a programmed back pressure during plasticization,

said first and second programmed electrical signal sources being incorporated into said program means,

a first comparator incorporated into said program means responsive to said first signal source and ram screw velocity transducer for providing a velocity error signal to said servovalve to control the sizes of said valve outlets and the fluid flow to said cylinder via said first valve outlet and the pressure thereof for maintaining the ram screw velocity at the programmed ram screw velocity,

said comparator being ineffective to control ram velocity when said cavity pressure reaches said predetermined level as sensed by said first pressure transducer,

a second comparator incorporated into said program means and responsive to said second signal source and said second pressure transducer for providing a back pressure error signal to said servovalve to control the sizes of said first outlet and the pressure in said cylinder in accordance with the programmed back pressure.

11. The control system of claim 10 wherein said program means further comprises:

a third source of electrical signals correlated to a predetermined holding pressure to be applied to said ram between injection and plasticization, and

a third comparator responsive to said third signal source and said second pressure transducer for providing a hold pressure error signal to said servovalve to control the sizes of said first and second outlets and hence the hold pressure in said cylinder in accordance with said predetermined holding pressure applied to said ram between injection and plasticization, said third comparator being effective to initiate control of said servovalve when said cavity pressure reaches said predetermined level as sensed by said first pressure transducer.

12. In an injection machine having a ram screw which during injection advances under an injection pressure to inject a plasticized charge into a molding cavity and during plasticization simultaneously rotates and retracts against a back pressure to plasticize a charge, a control system comprising:

means to generate said injection pressure and apply it to said ram screw during injection to advance said ram screw and inject said charge,

means to generate said back pressure and apply it to said ram screw during plasticization,

means to rotate said ram screw during plasticization, and

control means controlling said back pressure generating means for varying the level of said back pressure during plasticization whereby the plasticized charge is given a predetermined temperature profile along its length in accordance with the manner in which said back pressure is varied during plasticization,

said means to generate said injection and back pressures including:

a single servovalve under control of electrical signals, said servovalve having an inlet connected to a source of pressurized fluid, a first outlet connected to said ram screw and a second outlet connected to a fluid reservoir, said outlets being inversely variable in size dependent on the magnitude of the electrical signal input to said servovalve during plasticization,

a programmed source of electrical back pressure signals correlated to the desired programmed back pressure,

a pressure transducer responsive to the actual injection and back pressures on said ram screw during injection and plasticization, respectively,

a first comparator responsive to said pressure transducer during plasticization and said programmed back pressure signal source for generating a back pressure error signal, said error signal being input to said servovalve during plasticization for controlling said ram screw back pressure,

a source of electrical signals correlated to said injection pressure, and

a second comparator responsive to said pressure transducer during injection and said injection pressure signal source for generating an injection pressure error signal, said injection pressure error signal being input to said servovalve during injection,

whereby the injection pressure and back pressure applied to said ram during injection and plasticization is achieved with a single servovalve under sequential control of injection pressure and back pressure error signals developed by comparing injection and back pressure signals correlated to desired injection and back pressure levels against ram screw pressure transducer signals correlated to actual injection and back pressure levels.

13. In an injection machine having a ram screw which during injection advances under an injection pressure to inject a plasticized change into a molding cavity and during plasticization simultaneously rotates and retracts against a back pressure to plasticize a charge, a control system comprising:

means to generate said injection pressure and apply it to said ram screw during injection to advance said ram screw and inject said charge,

means to generate said back pressure and apply it to said ram screw during plasticization,

means to rotate said ram screw during plasticization, and

control means controlling said ram screw rotating means for varying the speed of said ram screw during plasticization whereby the plasticized charge is given a predetermined temperature profile along its length in accordance with the manner in which said ram screw speed is varied during plasticization,

said means to generate said injection and back pressures including:

a single servovalve under control of electrical signals,

a source of electrical back pressure signals correlated to the desired back pressure,

a source of electrical injection signals correlated to the desired injection pressure,

a pressure transducer responsive to the actual injection and back pressures on said ram screw during injection and plasticization, respectively,

comparison means responsive to said transducer signals and said signal source to generate injection and back pressure error signals correlated to the difference between said desired injection and back pressures and the actual injection and back pressures, said injection and back pressure error signals being sequentially input to said servovalve during injection and plasticization, respectively, to maintain said injection and back pressures on said ram screw at the desired levels,

whereby the injection pressure and back pressure applied to said ram during injection and plasticization is achieved with a single servovalve under sequential control of injection pressure and back pressure error signals developed by comparing injection and back pressure signals correlated to desired injection and back pressure levels against ram screw pressure transducer signals correlated to actual injection and back pressure levels.

BACKGROUND OF THE INVENTION

In the art of injection molding, machines are employed which cyclically supply plasticized material to a mold. These machines are usually of the reciprocating screw type in which the material to be molded is plasticized through the application of heat and the mechanical working of the material by the rotation of a screw within a plasticizing chamber. As the plasticized material is accumulated within the chamber, the screw, which also serves as a ram, retracts away from an injection orifice communicating with a mold cavity. When enough plasticized material has been accumulated, the ram advances toward the orifice in an injection stroke to inject the plasticized material into the mold.

One of the general problems in molding articles is that of insuring that the mold is filled properly with material. Because frequently the molds are quite intricate and irregular in shape, the material tends to flow through the mold in an erratic manner, first flowing into one region then another, and sometimes prematurely solidifying, blocking the flow to certain portions of the mold. This affects the surface finish of the objects and causes non-uniform density and irregular shrinkage of the objects. The rate at which the material flows through the passages of the molds will cause a change in the temperature and thus the viscosity of the material as it flows. By controlling this rate, the filling of the mold can be controlled to some degree. It has been found that injection of material into the molds at a precise programmed rate will greatly enhance the quality and uniformity of the molded products.

To achieve this, it has been attempted in some prior art systems to program the pressure exerted on the ram in order to achieve a more desirable flow pattern into the mold cavity. Some of these prior art devices have attempted to program this injection rate on a time basis. This has, however, not been entirely satisfactory, particularly in view of the fact that the flow rate into the mold is dependent on several variable factors, such as the viscosity of the material which is being injected and on various pressure fluctuations within the hydraulic ram driving system. Other systems have attempted to program this injection rate by mechanically synchronizing the position of the valve which supplies fluid to the ram directly to the position of the ram by mechanically actuating cam followers and switches. This again has not entirely overcome the injection rate problem in that intervening variables affect the relationship between the injection rate and the valve position.

It is one principal objective of the present invention to overcome the problems of the prior art and to more precisely control the flow rate of material into the mold in a precise programmed fashion.

Accordingly, the present invention as set forth in the parent application and the improvement set forth herein provides a means for programming the injection rate of material into the mold through the programming of the velocity of the ram. The present invention incorporates a velocity program module which operates to control the ram velocity as a function of the position of the ram in direct response to a closed loop feedback signal representative of the actual ram velocity.

The advantage of this particular aspect of the present invention is to overcome certain uncontrollable variables which affect the velocity of the ram such as the material viscosity and hydraulic system variables. Further advantages of this aspect of the invention are the ability to reduce "jetting", "blush" and warping of the product. For example, when mold filling starts too rapidly, material shoots into the empty mold and solidifies. This phenomenon is known as jetting and can be eliminated by the invention. Also, it has been found that surface stress is generally dependent on flow surface velocity, which is in turn dependent on a combination of the flow surface area of the material and the material flow rate. Irregular stresses can cause product warping. In accordance with the present invention, these irregular stresses and resultant warping are eliminated by precisely controlling ram velocity.

Another problem encountered by those systems of the prior art which have attempted to program the flow rate of material into the mold has been that the program has been inalterably tied in to the exact ram position. However, when the density and viscosity of the material vary, it is desirable to expand or contract the program or to alter the end points of the program in relation to the position of the ram so that the program need not be materially altered nor the mechanical linkages be moved on the machine.

It is another object of the present invention to provide a velocity program means which will provide velocity programming as a function of ram position but which will allow the program to be alterably associated with the actual position of the ram through simple adjustments within the programming module.

Accordingly, the present invention as set forth in this and the parent application provides a means which will associate a predetermined velocity control signal with a specific relative domain of the ram stroke and which will furthermore automatically divide the entire operable ram stroke into a plurality of distinct regions. The specific embodiment of the present invention will automatically divide a portion of the ram stroke between two arbitrary selectable end points and to associate the function directly to these regions. As these end points are moved for any reason, the present invention further provides that the program be automatically revised to redivide the new ram stroke domain into the same fixed number of regions and to associate the programmed velocity function with these corresponding regions. Since it may be necessary to revise the end points of this domain to accommodate for varying viscosity or density of the material to be molded, thereby lengthening or shortening the ram stroke, by the provision of the present invention, each portion of the ram velocity program will be directly related to the actual quantity of material fed as the stroke is varied to accommodate material density rather than in previous systems where the program was directly tied to fixed positions of the ram.

Another problem encountered in the prior art has been the difficulty in maintaining a predetermined flow rate into the mold while simultaneously insuring that the fill pressure of the mold cavity does not exceed certain critical values. If a critical value is exceeded, it is found that material will extrude from the junctures of the mold, resulting in what is referred to as a "flash". This not only reduces the actual material within the mold, but results in many cases in imperfect finish of the molded object and undesirable defect in the part.

It is a further objective of the present invention to provide means for insuring that the critical pressure of the mold is not exceeded when a precise injection rate program is employed.

Accordingly, the present invention provides means for monitoring the pressure within the mold cavity and utilizing this pressure to override the velocity program at the end of the injection stroke to limit the amount of material compressed in the mold to some desired value. Furthermore, the present invention provides for a closed loop feedback of the pressure signal from the mold cavity and the comparison of the signals of predetermined value and utilizing the result of this comparison in an analog fashion to override the velocity program. The overriding of the pressure in an analog fashion provides for more precise control of the servo valve which is supplying fluid to the ram, to regulate the deceleration of the ram while preventing loss of control and overshoot of the valve spool element, which may introduce an unpredictable effect on the final pressure within the mold.

Another major area wherein problems arise in the injection molding of objects is the difficulty in insuring that the objects produced fall within high dimensional and weight tolerances. It is important that objects be made in successive molding cycles of the machine in a highly predictable and repeatable manner.

One of the more critical problems in attaining precise repeatable articles of high dimensional and weight tolerances has been the phenomenon involving shrinkage of the molded article upon cooling. This shinkage is generally inversely related to the pressure and compressed density of the material within the mold at the time the mold is filled. In order to overcome these problems, some prior art attempts have been made to regulate the pressure within the mold at the time that it is filled. Another common practice in the art is to provide a cushion of material at the orifice of the extrusion device upon which a steady holding pressure is exerted so that material is forced into the mold to accommodate for the shrinkage of the material within the mold. However, as the mold cools, it becomes increasingly difficult to control the pressure within the mold cavity by the exertion of pressure by the ram against the cushion. One reason that this practice has not been wholly successful is that, as the viscosity of the material changes, the density of the material varies and thus the cushion size varies from cycle to cycle. Thus, the effect of the holding pressure operating through the cushion has differing effects from cycle to cycle upon the material within the mold cavity, and thus the density or weight and ultimate shrunk dimension of the molded products varies from cycle to cycle.

The factors which result in changes in viscosity and its effects on the molded material are discussed in detail in the copending application of the inventor of the subject matter of this application, filed Sept. 15, 1971, and entitled "Extruder Control System", now U.S. Pat. No. 3,759,648.

It is another important objective of the present invention to provide means for controlling the shrinkage of the objects molded from cycle to cycle in a precise and repeatable manner, and furthermore to control in a precise manner the quantity of material extruded to the mold in each cycle of operation. More particularly, it is an objective of the present invention to overcome the cycle to cycle effects of variable changes, such as the viscosity of the molded material.

Accordingly, the present invention, as set forth in this and the parent application, provides means of maintaining the cushion developed at the end of each injection stroke constant from cycle to cycle and furthermore provides additional means for insuring that the mold cavity fill pressure is also maintained constant from cycle to cycle. In addition, this constant pressure and constant cushion coexist at the same time in each cycle so that a precise pressure and volume relationship exists. This insures that a precise quantity of material, which is dependent on the combination of pressure and volume factors, is the same in each molding cycle. By maintaining the constant pressure in this manner, and by maintaining the constant cushion length through which a holding pressure applied by the ram is exerted, the cooling and consequent shrinkage characteristic of each product will be maintained in very close tolerances from cycle to cycle.

More particularly, the present invention provides a means for measuring the cushion length in each cycle of operation and for feeding this information in closed-loop feed-back manner to a control circuit which affects the cushion length during the next succeeding cycle of operation in a manner which will tend to maintain this cushion dimension constant from cycle to cycle. More particularly, the present invention provides means for measuring the cushion length at the precise instant that the cavity pressure has attained a predetermined value and for comparing this measured cushion dimension with a predetermined dimension. Furthermore, the present invention provides for utilizing the information derived in comparison of the actual and standard cushion dimensions to vary the shot size, or the retracted position of the ram at the beginning of the injection stroke, in the next injection cycle in a manner which will tend to correct for differences between the measured actual cushion dimension and the predetermined desired cushion dimension. Furthermore, the present invention provides means for setting a predetermined correction factor to a ram stroke so that, upon each comparison of the cushion dimension, the shot sizes vary by a predetermined fixed amount.

The automatic shot size correction capability which the present invention provides yields a particular advantage in allowing the injection molding machine to compensate for slowly varying changes in material density and viscosity and also provides means to automatically correct for any improper setting of the shot size by the operator and to allow for only a rough initial setting which will be automatically followed by the adapting of the machine to the optimum shot size for the given product being molded.

Furthermore, the effect of the present invention is to provide precise control, not only of pressure, but volume and temperature at the time of mold filling. By this provision, it is possible with the present invention to precisely control part size by adjustment of cavity pressure. This has not been provided before by any system of the prior art since, because of other uncontrollable variables, no prior art system has provided the precise relationship between part size and cavity size.

Furthermore, an additional objective of the present invention is to provide means which are economical and efficient to adapt an injection molding machine to complete computerized control.

Accordingly, the present invention provides a programming module which may serve as an interface between a conventional injection molding machine and a computer. To achieve this, the present programming module of the present invention undertakes to furnish and control those aspects of an injection machine operation which are peculiar to the injection molding process and the particular machine being used and, in addition, the particular molded object which is being used and formed. In this manner, sophisticated computer master process controls may be used without the necessity of programming these computers to the particular characteristics and properties of different injection molding machines.

Another problem encountered in injection molding is the phenomenon known as blush. This occurs when the cavity pressure is released too rapidly while the material is still molten. In many cases it is desirable to fill a cavity to a relatively high cavity pressure and then to relieve to a somewhat reduced holding pressure which is sustained until the material solidifies. In lowering the pressure to the holding pressure, the ram will normally retract some finite dimension. If the pressure is dropped too rapidly, the ram will tend to overshoot in this retracted position.

It is one of the objectives of the present invention, and particularly of the improvement disclosed herein, to alleviate this problem by controllably decreasing from the cavity pressure to the holding pressure along a ramp of limited slope.

Accordingly, the improvement of the present invention provides means for switching the ram pressure from the relatively high preset cavity pressure at the end of injection to the relatively low holding pressure in a manner which provides a smooth and gradual ram pressure decrease during the transition between injection and hold. In the preferred embodiment, this is accomplished by means of servomotor-controlled flow divider valve of the shiftable spool type which, in a controllable manner dependent on spool position, divides the flow from a pump between the ram pressure chamber and a drain tank. Since spool position, which is accurately controlled by a servomotor, establishes the operating pressure, and any positional shift thereof to alter pressure is reasonably smooth, change in pressure from one level to another during transition from injection to hold occurs in a gradual manner, avoiding problems associated with ram overshoot occasioned by a sudden pressure drop.

A further advantage of using a flow divider between the ram pressure cylinder and the drain tank, particularly in a system where ram velocity is programmed, is that the response of the system to a sudden resistance to ram motion, and hence decrease in ram velocity below the programmed level, is significantly improved over prior systems. More particularly, in this invention when a decrease in ram velocity below the programmed level is sensed and the flow divider spool shifted, to increase fluid flow to the ram, there is a simultaneous and complementary increase in blockage of the flow path to the drain tank. As a consequence, the pressure of the hydraulic fluid increases concurrently with the increase in flow rate to the ram, assuring that the increased flow to the ram will occur at a pressure sufficient to overcome the increased resistance to ram motion which initially caused the velocity drop, thereby restoring the ram velocity to the programmed level without undue sluggishness typical of velocity control schemes heretofore used.

A futher object of the present invention is to provide means which more precisely regulate the ram fluid pressure throughout the entire molding cycle. Particularly, the present invention involves using the flow divider valve, which is connected to the ram cylinder, in a closed-loop servo system during all portions of the cycle, i.e., during injection, hold and plasticize. This single flow divider valve replaces the multiplicity of valves previously required. The hydraulic equipment, as disclosed in the parent application, normally included separate valves for setting the holding and plasticizing pressure. By direct servo-control of the flow divider valve and provision of electronic means for generating the proper servo signals during all phases of the molding cycle, the improvement of the present invention requires only a single valve to control the various portions of the machine cycle. In addition, it provides a great conservation of power in that unused fluid from the high pressure pump which drives the system is bypassed by the flow divider at low pressure back to the fluid reservoir, thus expending a minimum amount of energy.

A further object of the present invention is to provide a means for switching from cavity pressure control to fluid pressure control. In the first instance, cavity pressure is utilized as a feed-back control to terminate the injection stroke portion of the cycle, while fluid pressure control is used during the holding and plasticizing portions of the cycle. Separate transducers are provided in the molding cavity and in the fluid lines to generate these respective feed-back signals for the servo-control. This enables cavity pressure and ram pressure during hold and plasticization, critical parameters in an injection molding process, to be directly monitored by the mold cavity and ram pressure cylinder transducers, respectively, in turn permitting control of charge size during injection in direct response to actual cavity pressure, and control of the ram during hold and plasticization in direct response to actual ram cylinder pressure.

A further object of the present invention is the provision of means for controlling the back pressure and/or screw speed in a programmed manner as a function of ram position or time during plasticization as the ram is being retracted. This aspect is provided through the use of a separate patch panel which takes over control of the operation during the plasticizing stroke. By programming screw speed and/or back pressure during plasticization, a predetermined temperature profile can be imparted to the charge along the length thereof while in the barrel prior to injection. This enables controlled variation in density of the molded article throughout its volume to achieve desired variable levels in preselected characteristics such as surface wear, gloss, resolution and the like.

These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the drawings illustrating a preferred embodiment of the injection molding control system of the present invention, and modifications thereof, in a reciprocating screw type injection molding machine.

FIG. 1 is a diagrammatic illustration of injection molding apparatus utilizing a control system which incorporates the principles of this invention;

FIG. 2 is a schematic circuit, in block diagram format, of a control circuit incorporating the principles of this invention for controlling the injection molding apparatus depicted in FIG. 1;

FIG. 3 is a schematic circuit, in block diagram format, of a modification useful with the control circuit of FIG. 2 which includes provision for programming screw speed during plasticization; and

FIG. 4 is a schematic circuit, in block diagram format, of a further modification useful with the control circuit of FIG. 2 which includes provision for programming screw back pressure during plasticization.

Conventional injection molding apparatus modified to incorporate the controls of this invention is shown in FIG. 1. With reference to this figure, the injection molding apparatus, which may in a preferred form be of the reciprocating screw type, includes an extrusion or injection apparatus 11, mold assembly 12, and an electrically controlled hydraulic circuit 13. The injection apparatus 11 includes an injection cylinder or housing 14 which is generally elongated in shape and provided with an interior cylindrical plasticizing or melt chamber or barrel 15. Axially disposed within the melt chamber 15 is a screw or ram 16 which is both rotatable about its longitudinal axis, as well as axially translatable within the barrel or plasticizing chamber 15.

The upstream end of the melt chamber 15, which is the left end thereof as viewed in FIG. 1, communicates with an input hopper 21 via a passage 21A connected to the lower end of the hopper. Molding material, typically in the form of a pelletized thermoplastic composition, is loaded into the open upper end of the hopper 21 where it is gravity-fed by passage 21A into the rearward or upstream end of the melt chamber 15 atop the rear end of the screw 16. The downstream end of the melt chamber or barrel 15, which is located rightwardly as viewed in FIG. 1, terminates in an injection nozzle 22 which at its downstream end communicates with a mold cavity 24 via an orifice 23. The mold cavity 24 of the mold assembly 12 is established or defined by a pair of cooperating mold elements 25 and 26 which are relatively movable toward and away from each other by a mold opening actuator (not shown) to allow molding of an object in cavity 24 and the subsequent removal thereof. The mold opening actuator and associated control accessories such as actuator-controlling timers, limit switches for sensing whether the mold is open or closed, etc., can be constructed in accordance with well-known techniques and form no part of this invention. For example, an illustrative timer-controlled mold-opening actuator and mold position sensing limit switch arrangement is disclosed in copending U.S. patent application Ser. No. 371,390, filed June 19, 1973, entitled "Injection Molding Control", assigned to the assignee of this application. The entire disclosure of the above-identified co-pending application is specifically incorporated herein by reference. Suitable conduits 12B are preferably provided in the mold elements 25 and 26 to facilitate the circulation of coolant to permit rapid and controlled cooling of the molded article.

The screw or ram 16 is selectively bidirectionally axially movable within the barrel 15 by a hydraulic piston 27A fixedly secured to the ram. The piston 27A is slidably movable in a hydraulic cylinder 27B. Cylinder 27B is divided into two variablsize chambers 27C and 27D by the movable piston head 27A. Rotation of the screw or ram 16 is obtained by a motor 28, preferably of the hydraulic type, which has a rotatable output shaft indicated by dotted line 28A drivingly connected to the piston 27A. A source of pressurized fluid, such as a pump P, is connected to the motor 28 via a solenoid-type electro-hydraulic ON/OFF valve 57 controlled by signals on input line 56. Preferably, a constant flow valve 55 having adjustably variable flow rates determined by the adjustably variable setting of a flow rate potentiometer 54 connected thereto by line 53 is connected between solenoid valve 57 and pump P to provide constant screw speed operation when valve 57 is open.

In normal operation, the injection molding apparatus repeatedly cycles through a predetermined molding sequence, with a molded article being produced in the mold cavity 24 during each cycle. The molding sequence of each cycle may be considered to start upon ejection of a molded article from the cavity 24. Specifically, following a suitable cooling period initiated after plasticized material has been injected by the ram 16 into the mold cavity 24 through orifice 23, the mold actuator (not shown) is operated to separate the mold elements 25 and 26 and eject the molded article, now solidified, from cavity 24. The mold elements 25 and 26 remain open for a predetermined period of time, whereupon the mold actuator returns the molding elements to their closed position shown in FIG. 1.

Following closure of the mold cavity 24 an "inject" signal on line 40 is provided to a servo-amplifier 41 by a control circuit to be described, which in a manner also to be described causes a suitable electrical control signal to be input on servo-amplifier output line 42 to an electro-hydraulic servovalve 43 causing hydraulic pressure of controlled magnitude to be input to chamber 27C via hydraulic line 44. This urges the ram 16, which has previously accumulated a predetermined charge of plasticized material in the melt chamber 15 downstream of the ram tip 29, rightwardly as viewed in FIG. 1. Rightward movement of the ram 16, particularly the tip 29, causes the accumulated charge of plasticized material for the next mold cycle to be injected into the cavity 24 in a controlled fashion via the nozzle 22 and ultimately the orifice 23.

Depending upon the nature of the inject signal on line 40 to the servo-amplifier 41, the injection pressure applied to the ram 16 during the injection phase may be maintained at a constant value or varied as a function of time or as a function of ram position. In a preferred form of this invention, the inject signal on amplifier input line 40 is such that the ram 16 moves toward the cavity 24 with a velocity which varies with ram position in accordance with a predetermined program.

Rightward injection motion of the ram 16 under the action of the servovalve 43, which in turn is controlled by the signal on inject line 40 to the servo-amplifier 41, continues until the pressure in the mold cavity 24 sensed by a pressure transducer 48 communicating with the cavity reaches a pre-set cavity pressure, whereupon a trigger signal is developed, terminating the injection phase. At this point, and in response to the trigger signal, a "hold" signal is provided to the servo-amplifier on line 45, by circuit means to be described later, causing the servovalve 43 to apply a holding pressure to the chamber 27C via line 44 of a magnitude substantially less than the injection pressure which previously existed during the injection phase when molding material was injected into the cavity. The holding pressure established by the signal to servo-amplifier 41 on line 45 is maintained for the duration of a predetermined holding interval, for example, 10-12 seconds, established by a hold timer to be described.

The trigger signal developed at the end of the injection phase when the cavity pressure reaches a pre-set limit, in addition to terminating the injection pressure and initiating the hold pressure, also operates to cause the position of the ram tip 29 to be sampled and compared with a predetermined desired position known as the "cushion". To facilitate monitoring the location of the ram tip 29 at the conclusion of the injection phase, a ram position transducer 49 is provided which is mechanically connected to the ram as indicated by dotted line 49A and provides on its output line 50 an electrical analog signal correlated to the position of the ram within the barrel. If the position of the ram at the end of the injection phase when the mold cavity pressure has reached the present value represent a ram position further from the mold cavity 24 than the predetermined cushion position, a correction signal is developed to decrease the amount by which the ram retracts during the ensuing plasticization phase, to thereby restore the cushion to the desired level at the conclusion of the next injection phase. If the sampled ram position at the conclusion of an injection is such that the ram is closer to the molding cavity 24 than the desired preset cushion, indicating the ram did not retract a sufficient distance during the preceding plasticization phase, a correction signal is provided which increases the amount by which the ram retracts during the next plasticizing operation, to thereby restore the actual cushion position to the desired preset level.

When the hold timer times out, the hold pressure supplied as a consequence of the hold signal input on line 45 to the servo-amplifier 41 terminates, and a "plasticize" or "back pressure" electrical signal is applied on line 51 to the servo-amplifier 41 by circuit means to be described. The back pressure signal causes the servovalve 43 to apply a controlled back pressure to the ram 16 via hydraulic line 44. A signal is also input on line 56 to the solenoid-controlled ON/OFF hydraulic valve 57, causing hydraulic motor 28 to rotate the ram screw 16 in a controlled manner, preferably at a constant speed established by flow rate potentiometer 54 and constant flow valve 55. The ram 16 is rotated in a direction which causes the screw threads thereof to feed material toward the orifice 23 and to accumulate material downstream of the tip 29 of the ram 16. This action, together with heat which is applied to the wall of the cylinder 14 by means not shown, causes the material to plasticize within the barrel or chamber 15.

Depending upon the nature of the electrical signal input to the servo-amplifier 41 on back pressure line 51, the back pressure applied to the ram 16 during the plasticize phase may either be maintained at a constant level, or, for reasons to be apparent hereafter, varied as a function of a ram position or as a function of time. Similarly, depending upon the nature of the control signal input to the constant flow valve 57, the rotational speed imparted to the ram 16 by the hydraulic motor 28 may be maintained at a constant level during the plasticizing phase using potentiometer 54, or may be varied as a function of ram position or as a function of time if the signal input to the constant flow valve 55 on line 53 is derived from a programmed signal source (not shown in FIG. 1) rather than potentiometer 54.

The rotation of screw 16 and accumulation of charge forward of ram tip 29 builds up pressure in the chamber 15 forward of the ram tip which eventually overcomes the back pressure exerted on the ram piston 27A by the servovalve 43 under control of the back pressure signal on line 51, causing the ram to retract away from the nozzle 22 until it reaches a predetermined retracted position at the upstream end of the chamber 15. As noted previously, the predetermined ram retraction position is corrected at the end of each injection phase as a consequence of a comparison of a preset desired ram cushion position and the actual ram cushion position at the end of that injection phase. Upon reaching the corrected ram retraction position, which position is in part determined by the output of the position transducer 49 on line 50, the back pressure provided by the servovalve 43 under control of the plasticized signal input to the servo-amplifier 41 on line 51, terminates, as does the rotation of the screw by hydraulic motor 28. Plasticization and accumulation of the desired charge forward of the ram tip is now complete.

At this point in the cycle, ram pull-back pressure is applied to the hydraulic cylinder chamber 27D via hydraulic line 61 from a ram pull-back circuit 60 which is under the control of an electrical signal input thereto on line 62. The pull-back pressure applied to piston 27A functions to retract the ram 16 a fixed amount which is designed to decompress the plasticized material located between the ram tip 29 and the orifice 23. Such decompression eliminates the need for providing a valve at the orifice 23 since the plasticized material accumulated between the orifice and the ram tip 29, once decompressed by retraction of the ram under the action of pull-back circuit 60, will not flow into the cavity 24 via orifice 23. Such flow is prevented from occurring while the charge is accumulating and prior to termination of screw rotation and retraction under the action of pull-back circuit 60, by reason of the fact that the material in the orifice 23 from the preceding injection cycle has solidified almost immediately following injection. With the material in the orifice 23 solidified shortly after completion of the injection of the charge into the cavity 24, the cavity 24 is effectively sealed with respect to the melt chamber 15 including the nozzle 22.

Coincident with application of the pull-back signal to line 62 of the pull-back circuit 60, a signal is input to the servo-amplifier 41 on line 59 to control the servovalve 43 in a manner which connects chamber 27C to a drain tank to permit fluid in chamber 27C to escape when the pull-back pressure is applied to chamber 27D.

When the ram 16 has reached the "pull-back" position following termination of screw rotation, the apparatus goes into a standby mode until a predetermined cooling time interval has been completed during which time the injected material in the mold 24 is cooling and the molded article comletes its solidification. Upon expiration of the cooling time interval, the mold elements 25 and 26 are separated and the molded article ejected from the cavity 24. The mold members 25 and 26 remain open for a predetermined time whereupon the mold elements move together to close cavity 24. At this time, a start signal is produced and the previously described molding cycle repeated to produce another molded part.

The pull-back pressure circuit 60, which retracts the ram 16 to decompress the charge accumulated forward of the ram tip 29 following plasticization, can be constructed in accordance with known hydraulic design principles. In the preferred form, the pull-back hydraulic circuit 16 includes a pump 60P having its input line connected to a fluid reservoir 60R and its output line connected to an electrically controlled valve 60S via a pressure relief valve 60PR set at the desired pull-back pressure. In operation, when a pull-back command signal is input to the pull-back circuit 60 on line 62, fluid pressurized to the setting of the pull-back pressure relief valve 60PR is input to the cylinder 27D via line 61. Obviously, other hydraulic circuits can be provided to provide the desired pull-back pressure, the description provided being only for the purpose of illustration.

Considering the servovalve 43 of this invention in more detail, it includes a servomotor 43A and a flow divider valve 43B. The servomotor 34A may be of any of the well-known and commercially available types which in response to an electrical signal input thereto on line 42 produces pivotal movement of an arm 46, known as a flapper, in either a leftward direction or a rightward direction depending upon the polarity of the input signal on line 42. The magnitude of the leftward or rightward pivotal movement of the flapper 46 is determined by the magnitude of the input signal on line 42.

The flow divider valve 43B, which may also be known commercially available types, includes a valve body 70 having an elongated bore 71 in which a spool 72 is slidably positioned. Spool 72 has a center section 73, a left-end section 74 and a right-end section 75 which have equal diameters and which are dimensioned to snugly fit for sliding motion in the bore 71. Spool members 74 and 75 are spaced from the central spool section 73 by reduced diameter rigid interconnecting elements 76 and 76A. Depending upon the position of the spool 72 within the bore 71, ports 77 and 78 which are connected to the cylinder 27C via line 44 and to a drain tank 79 via a line 80, respectively, are fully open, fully closed, or partially open to varying degrees, for reasons to become apparent hereafter. The sizes or areas of outlet ports 77 and 78 vary inversely in a complementary fashion as the spool shifts such that the blockage of port 77 increases as the blockage of port 78 decreases, and vice versa.

The flow divider valve 43B also includes a cavity 81 which connects via a passage 87 and a hydraulic line 86 to a source of pressurized fluid such as a pump 85. The cavity 81 communicates with the left-hand portion of the bore 71 via a passage 82 and with the right-hand side of the bore via a passage 83. The left and right-end regions 84 and 84A of the bore 71, which have volumes which vary in a complementary manner depending upon the position of the spool 72 in the bore 71, communicate with a source of pilot pressure 88 via passages 89 and 90, respectively, formed in the valve body 70 and hydraulic lines 91 and 92, respectively, which connect the passages 89 and 90 to the pilot pressure source 88. The pilot pressure source 88 functions under static conditions when the spool 72 is stationary to apply equalized fluid pressure to the chambers 84 and 84A with the result that no net axial force is applied to the spool 72 in either the leftward or rightward direction as a consequence of the pressurized fluid in the chambers 84 and 84A which act in opposite directions on the ends of spool sections 74 and 75. The chambers 84 and 84A communicate via passages 93 and 94, respectively, formed in the valve body 70 and hydraulic lines 95 and 96, respectively, to oppositely directed nozzles 97 and 98. The nozzles 97 and 98 are spaced apart a predetermined amount to define a gap 99 in which is positioned an intermediate section of the servomotor flapper 46, the lower end of which is mechanically connected to the central spool section 73 for feed-back purposes to be described.

In operation, when a zero level signal is input to the servomotor 43A on line 42 from the servo-amplifier 41, no torque is applied to the flapper 46 by motor 43A with the result that the flapper 46 is neither pivoted to the left nor the right, but rather assumes a central position in the gap 99 equidistant from the nozzles 97 and 98. With the flapper 46 centered in the gap 99, the back pressures in the nozzles 97 and 99, which are reflected back to chambers 84 and 84A via hydraulic lines 95 and 96 and passages 93 and 94, are equalized. As a consequence, chambers 84 and 84A which are subjected to equal pressures from the pilot pressure source 88 via lines 91 and 92 and valve passages 89 and 90 have equal pressures therein, applying no net axial force to the spool 72 in either a leftward or rightward direction. With no net axial force on the spool from the pressurized fluid in the chambers 84 and 84A and with no torque applied to the flapper 46, the flapper remains centered in the gap 99 with the spool 72 centered in the bore 71. Under such conditions ports 77 and 78 are each partially open to an equal extent with the result that pressurized fluid in chamber 81 is applied to the chamber 27C via hydraulic line 44 and to the tank 79 via line 80. Since chamber 81, which is supplied with pressurized fluid from the pump 85, is connected to the tank 79 via unblocked port 78 and line 80, the pressure applied to the cylinder chamber 27C via port 77 and hydraulic line 44 is minimal.

During the injection phase in response to an injection signal input to the servo-amplifier 41 on line 40, a control signal is input to the servomotor 43A on line 42 which functions to pivot the flapper 46 rightwardly in an amount dependent upon the magnitude of the signal. Rightward movement of the flapper 46 shifts the spool 72 rightwardly closing the port 78 and opening the port 77 in complementary fashion selectively varying amounts dependent on the signal level input to servomotor 43A on line 42 produced by the injection signal input to the servo-amplifier 41 on line 51. As a consequence of the increased blockage by spool section 73 of the port 78, which connects to the drain tank 79 via line 80, the pressure in the chamber 81 and hence in the chamber 27C of the ram actuator builds up. Simultaneously, the fluid flow to the chamber 27C of the ram actuator increases by virtue of port 77 becoming less blocked as spool section 73 moves rightwardly.

The movement of the spool 72 in a rightward direction by servomotor 43A causes the flapper 46 to move in the gap 99 to a point closer to the nozzle 98 and more distant from the nozzle 97, with the result that the back pressure in nozzle 98 increases and the back pressure in the nozzle 97 decreases in complementary fashion. With the back pressure in nozzle 98 increased and the back pressure in nozzle 97 decreased, the pressure in chambers 84A and 84 increases and decreases, respectively, applying a net axial force to the spool 72 in a leftward direction. When the net axial force on the spool 72 occasioned by the pressure differential in chambers 84 and 84A equals the force applied to the spool 72 by the flapper 46 due to the servomotor 43A as a consequence of the signal input thereto on line 42, the spool 72 reaches a condition of force equilibrium whereupon it moves no further in a rightward direction. The new equilibrium position of spool 72 locates the spool rightwardly of its center position in the bore 71 an amount dependent upon the magnitude of the signal on line 42 input to the servomotor 43A from the servo-amplifier 41 produced by the presence of an input signal on inject line 40. Consequently, the fluid flow rate to the chamber 27C of the ram actuator from flow divider 43B and the pressure thereof reach an equilibrium level correlated to the servo-amplifier inject signal level on line 40. During the injection phase, the signal level on line 40 input to the servo-amplifier 41 (which is determinative of the signal level input to the servomotor 43A on line 42, which in turn determines the extent to which the spool 72 moves to the right, and hence the flow rate and injection pressure in the chamber 27C) can be maintained at a constant value or alternatively may be programmed to vary in some predetermined manner dependent upon ram position or dependent upon time. Of course, if the injection signal to servo-amplifier 41 on line 40 is programmed to vary with ram position or time, the servo-valve spool 72 will assume successively different equilibrium positions corresponding to the successively different programmed injection signal levels input on line 40.

In a manner similar to that described above, wherein a signal on line 42 to servo-amplifier 41 produces a predetermined injection pressure and flow in the ram actuator chamber 27C, a signal input to the servo-amplifier 41 on line 45 produces a predetermined holding pressure in ram actuator chamber 27C during the holding phase of the cycle. Insofar as operation of the servo-valve 43 is concerned, the operation thereof during the holding phase is the same as during the injection phase, except that the equilibrium position of the spool 72 and hence the flow rate and pressure to the chamber 27C via line 44 is controlled by the hold signal on line 45 to the servo-amplifier 41, rather than by the injection signal on line 40 input to the servo-amplifier 41.

During the plasticizing phase of a molding cycle, a back pressure is applied to the ram actuating chamber 27C via line 44. However, instead of the ram moving toward the molding chamber 24, as occurs to a large extent in the injection phase when the plasticized charge is injected through orifice 23, and to a very small extent in the holding phase as a consequence of shrinkage of material in the mold, during the plasticizing phase the ram moves rearwardly as the charge accumulates torward of the ram tip 29. To enable the back pressure to be applied to the ram actuating chamber 27C via line 44 and yet permit the ram to move rearwardly, a control signal is input to the servo-amplifier 41 on line 51 of a polarity opposite to that present on line 40 and 45 during the injection and holding phases. The opposite polarity signal input to the plasticized line 51 produces an input to the servomotor 43A on line 42 which moves the flapper 46 leftwardly. Leftward movement of the spool 72 partially closes the passage 77 to throttle the flow of oil from the chamber 27C flowing in line 44 as a consequence of the piston 27A moving rearwardly as the ram is urged rearwardly by the accumulating plasticized material forward of the ram tip 29. The fluid from the chamber 27C flowing through partially blocked throttling passage 77 flows into the tank 79 via passage 82, chamber 81, passage 83, and passage 78 and line 80. The extent to which the spool 72 moves leftwardly, and hence the extent to which the passage 77 is blocked and the resultant degree of throttling and level of back pressure, is dependent upon the magnitude of the plasticize signal input to the servo-amplifier 41 on line 51. The larger the signal on line 51, the greater the distance the spool 72 moves leftwardly, and hence the greater the throttling and back pressure in cavity 27C. The leftward movement of the spool 72 produced by the plasticize signal on line 51 causes the spool 72 to move leftwardly, increasing the blockage of port 77, until a new equilibrium spool position is reached. At equilibrium the leftward force applied to the spool 72 by the flapper 46 equals the net rightward hydraulic force applied to the spool 72 by the pressure differential in chambers 84 and 84A, which pressure differential is occasioned by the relatively increased back pressure in nozzle 97 and the relatively decreased back pressure in nozzle 98 produced when the flapper 46 moved closer to the nozzle 97 and further from the nozzle 98 under the action of the back pressure signal on line 51.

During the pull-back phase, the spool 72 is centered as a consequence of the input of an appropriate signal to the servo-amplifier 41 on line 59. If during the pull-back phase the pump 85 communicates with the flow divider valve chamber 81 via line 86 and port 87 as shown in FIG. 1, the setting of the pull-back pressure relief valve 60PR must be such that the force applied to the piston 27A via chamber 27D and line 61 is sufficient to overcome the force applied to the piston 27A via the chamber 27C and line 44. Alternatively, it may be desirable to place an electrically-controlled hydraulic valve in line 86 which is placed in an OFF position disconnecting the pump 85 from the chamber 81 during the pull-back phase in which event the hydraulic pressure in line 61 developed by the pull-back pressure circuit 60 during the pull-back phase would not have to overcome any forward pressure applied to the ram as a consequence of the pump 85 being connected to the flow divider 81 which has its spool 72 centered during pull-back. Such an ON/OFF valve placed in line 86 would, of course, be placed in the ON condition connecting the pump 85 to the flow divider chamber 81 during the injection and hold phases of a molding cycle when a flow of fluid into the chamber 27C via line 44 is required to substantially advance the ram to accomplish injection and very slightly advance the ram during the holding phase to maintain the cavity 24 full as the molded article shrinks. During the plasticizing phase wherein fluid flows to the flow divider 43B from the chamber 27C via lines 44 as a consequence of the ram moving rearwardly as the charge accumulates forward of the tip 29, the pump 85 can be disconnected from the flow divider chamber 81 via the OFF/ON valve placed in line 86.

At this point it should be noted that the injection, plasticize, and hold signals on lines 40, 51, and 45 are generated by comparing the actual pressure in the chamber 27C output on line 100 from a pressure transducer 101 communicating with the chamber 27C against desired injection, plasticize, and hold pressure signals provided by suitable signal sources, either fixed or programmed to vary as a function of ram position or time, as the case may be, all in a manner to be described.

Utilization of a servovalve 43 provides a number of unobvious advantages when compared to ram control schemes heretofore utilized, such as disclosed in previously referenced copending application Ser. No. 371,390. In such previously used schemes the injection, hold, and plasticize pressures were obtained by providing separate pressure sources, each usually including a pump and a pressure relief valve set to the desired pressure, which were selectively connectable to the ram actuating chamber 27C via selectively operable individual solenoid ON/OFF valves. In such prior schemes if injection pressure is desired, the solenoid valve connected to the injection pressure circuit, which as indicated includes a pump and pressure relief valve set to the desired injection pressures, is placed in an open condition, while the solenoid valves interconnecting the hold pressure and back pressure circuits to the ram chamber 27C are placed in a closed condition. When holding pressure is desired, the previously closed solenoid associated with the holding pressure circuit is opened, and the previously open plasticizing circuit solenoid is closed and the back pressure solenoid which had been closed during injection remains closed. Similarly, when back pressure is desired the previously closed solenoid associated with the back pressure circuit is opened, while the previously opened solenoid associated with the hold pressure circuit is closed and the injection pressure circuit solenoid allowed to remain closed. As is apprent, in these prior schemes a substantial duplication of hydraulic circuit components was necessary to achieve the injection, hold, and back pressure functions. By way of contrast, by using the servovalve 43 in this invention, a single flow divider 43B and servomotor 43A under common control of a single servo-amplifier 41 input with injection, hold, and plasticize signals on lines 40, 45 and 51, can be employed to provide the necessary pressures on the ram during the injection, hold and plasticizing phases. By avoiding the duplication of hydraulic circuitry heretofore necessary to accomplish the injection, hold and plasticizing functions, significant cost reductions result.

In addition to the reduction in component costs occasioned by eliminating duplication of hydraulic circuitry, a number of other advantages have been provided as a consequence of utilizing the servovalve 43. For example, a problem encountered in injection molding during the transition from the injection phase to the holding phase, known as blush, is avoided. During the injection phase the ram has applied to it a very substantial hydraulic pressure with the result that the material in the molding cavity upon conclusion of the injection phase is in a compressed state. Such a condition is desirable to provide high resolution and enhance surface finish of the molded product. However, once the mold cavity has been filled, it is desirable to relieve the pressure to the reduced "holding" pressure which is maintained until the material in the mold cavity solidifies.

In prior schemes, a switch from injection pressure to the lower holding pressure was accomplished by closing the injection pressure circuit solenoid and opening the holding pressure circuit solenoid, in the course of which the