Injection Molding Machines

There
are different types and capacities of IMMs to meet different product
and cost-production requirements. The types are prin­cipally horizontal
single clamping units with reciprocating and two-stage plasticators.
They range in injection capacity (shot size) from less than an ounce to
at least 400 oz (usually from 4 to 100 oz) and in clamp tonnage up to
at least 10,000 tons (usual from 50 to 600 tons). Other factors when
specifying an IMM include clamp stroke, clamping speed, maximum
daylight, clear­ances between tie rods, plasticating capacity,
injection pressure, injection speed, and so on, as reviewed in this
chapter.

designer should also review Chap. 15 regard­ing micro injection molding.
The type and size of IMM to be used are de­pendent on the molded product dimensions and volume, which determine the processing requirements and the shot size (Chap. 4), as well as the required pressure and material behavior (Chap. 6). Examples of product di­mensions that directly influence the size of the machine required include all part dimen­sions; the number of parts to be molded in a single cycle; the mold runner system needed to produce required number of parts; the mold width, length, stack height (if stacking is

used), and opening distance; and the ejector rod spacing. This information will determine the preliminary requirements for the IMM.


Reciprocating (Single-Stage) Screw Machines
Reciprocating, or single-stage, IMMs are a conventional type where plastic is melted us­ing a combination of conductive heat from heater bands surrounding the barrel and fric-tional heating created by a rotating screw in­side the barrel

moves back to allow melted plastic to accu­mulate ahead of it, then moves forward, in­jecting all the melt into the mold in a single stage. The accumulation of melt at the screw tip forces the screw towards the rear of the machine until enough melt is collected for a shot. The back pressure required on the screw during this plasticating action is low, and when the shot size is produced, the screw stops rotating.
With the mold halves closed, the nonrotat-ing screw acts as a plunger and rams the melt into the cavity or cavities, using controlled in­jection pressure and rate of travel. After in­jection of the melt is complete, the screw ro­tates to prepare the next shot. The advantages of the reciprocating screw IMM over the two-stage IMM include the following: (1) re­duced residence time, (2) self-cleaning screw action, and (3) responsive injection control. These advantages are key to processing heat-sensitive plastics.
chine. In A, the shot (melted plastic) is in front of the retracted screw, which is being used as a ram to force the shot into the mold cavity, B. After the shot has completely filled the cavity and the plastic melt in the mold gate(s) is sufficiently solidified (frozen) so melt will not travel back into the plasticator, the screw starts rotating and retracts to pre­pare the next shot, C. An optional soak peri­od, or idle time, prior to the shot being forced into the mold cavity, may be included as part of the processing cycle. One complete cycle of the IMM operation is shown in Fig. 1-13.
In the single-stage IMM, melt is fed into a shot chamber (in front of the screw). This motion generates controllable low back pres­sure [usually 50 to 300 psi (0.34 to 2.07 MPa)] that causes the screw to retract at a pressure-controlled rate. A preset device (such as a screw position transducer) is activated when the shot size is attained, to stop the rotation of the screw. If the IMM does not have sufficient shot capacity, the screw is instead allowed to continue rotating, permitting additional to enter the cavity prior to shot. However, for plastics with certain melt characteristics, melt flow problems can develop in this case. (Chap. 7).
At a preset time the screw acts as a ram to push the melt into the mold. Depending on the plastic's melt flow characteristics, the injection pressure at the nozzle is between 2,000 and 30,000 psi (14 and 200 MPa). The required pressure is determined by the plas­tic being processed and the melt pressure re­quired in the cavity or cavities, taking into account pressure drops as the melt travels through the mold. While the shot is injected into the mold, an adequate clamping pressure must be used to keep the mold from opening (flashing) during and after the filling of the cavity.
Molds are designed to meet different re­quirements. They include hot runners or cold runners (for TPs or TSs) with different lengths of runners, gates, etc. (reviewed in Chap. 4).
Two-Stage Machines
Another very popular injection molding method uses a two-stage arrangement of screws. Such a machine is also called a pre-plasticizing IMM. The two-stage IMM uses a fixed plasticating screw (first stage) to feed the required melted plastic through a valve mechanism into a chamber, or accumulator (second stage). This screw does not require

reciprocating action (as in a single-stage IMM), since it only conveys melts by means of some type of diverter mechanism (valve) into a holding (injection accumulator) cylin­der (Figs. 2-5 to 2-7). When a sufficient quan­tity of melt has been transferred, the diverter valve again shifts to create a flow path over a prescribed time cycle from the accumulator cylinder into the mold. The second stage (ram injection stage) provides the pressure needed for the desired rate of injection of the melt (shot) into the mold cavity or cavities. Af­ter injection is completed, the diverter valve shifts to direct the melt flow from the first stage into the second-stage holding cylinder, and this operating cycle repeats. During all this action the first-stage extruder is continu­ously rotating; in practice this does not cause problems even when the melt flow is slightly restricted by being cut off from the second stage (1,518, 525).
Thus the diverter or shuttle valve has three positions. One position is the closed mode, during which time the extruder is only prepar­ing the melt. The next position directs the melt from the extruder into the accumula­tor (second stage). The third position directs a shot of melt from the accumulator into the mold cavity.
Injection Hydraulic Accumulator
The injection hydraulic accumulator is a device for increasing the speed of the melt injected into the mold in a conventional IMM (Fig. 2-8). It is a cylindrical pressure vessel that is precharged (filled) with an inexpen­sive inactive gas (usually nitrogen) to a pre­determined pressure level. Hydraulic fluid is pumped into the accumulator opposite the contained gas, with an internal floating pis­ton serving as a gas-oil separator. When the IMM signals to inject, the fluid in the accu­mulator is directed by controlled valving into the injection cylinder.
During all this action, which occurs within seconds, the extruder (first stage) conti­nues to operate, producing melt. When it is not being directed into the accumulator, the melt remains in the barrel, possibly building up slight pressure for a short time. The ex­truder is designed so that the screw can move back some what, allowing melt to accumulate in the front of its barrel without any major buildup of pressure. Designs are used such that controls can be set to prevent damage to the melt.
Compared to the reciprocating screw IMM, the advantages of this technique in­clude: (1) consistent melt quality; (2) ram action in the accumulator, providing high in­jection pressure very fast; (3) very accurate shot size control; (4) product clarity; and (5) easy molding of very thin-walled parts. Disad­vantages include higher equipment cost and possible increased maintenance.


Reciprocating vs. Two-Stage Machines
Both types of machines are operated hy-draulically, electrically, or both. The recip­rocating screw design, which has many ad­vantages in a hydraulic power environment, to date has limited the use of of all-electric machines in that it requires large and costly electromechanical drives for shot weights ex­ceeding 80 oz.
An example of a recently developed elec­trical machine for large shot sizes is shown in
This Milacron machine has patented features for a two-stage design that allow first-in-first-out melt handling, quick and easy color change, and precision mini-shot control down to 2 to 3% of barrel capacity. It also provides melt quality, compounding, and venting advantages unique to freestanding extruders (3). Simultaneously it satisfies the need for high throughput and pressure in an all-electric IMM. It eliminates all the guess­work about sizing an injection unit. Intro­duced in 150-, U0-, and 80-oz (4250-, 3100-, and 2300-g) capacities, they are capable of economical large-shot molding at pressures up to 30,000 psi (210 MPa, 2000 bar) (325).
Simple physics shows the advantages of a two-stage electric unit. It takes less power to generate injection pressure with a small-diameter screw while lengthening the stroke to get the required melt volume. There are certain limits on how much these parameters can be varied with a reciprocating unit, be-cause the plasticizing and injection functions are interdependent; in contrast, the two-stage design completely separates these functions. Hydraulic IMMs share a drive for the screw motor and injection unit, using the extrusion screw as an injection plunger to lower ma­chine cost. This capability, in reciprocating screw units, has significantly expanded their use, in preference to two-stage units, since the 1960s. However, as injection volumes in­crease, the diameter of the screw has to be increased correspondingly, because there is an inherent limit on how far one can move the screw to obtain additional volume. These larger diameter screws are no problem to push with a hydraulic system, but are cost-prohibitive with the original electromechan­ical drive designs.
Other tradeoffs with the reciprocating de­sign include that increasing the screw diam­eter to add volume limits the precision at the small end of the shot range. Because the stroke gets so short, it is difficult to have preci­sion melt control. The reciprocating injection unit is usually oversized for the actual mold­ing requirement because the effective diame­ter for plasticizing decreases with increasing screw stroke. As an example, it has become standard to size molding operations to 30 to 70% of a reciprocating injection unit's capac­ity. This sizing keeps the IMM in the best operating range for larger shots—typically 300 oz (8500 g), corresponding to a 150-oz (4250-g) two-stage unit process.
The two-stage unit design is a fundamen­tal departure from the past reciprocating unit design. It frees the design of the injection function from dependence on the plasticiz­ing function, because it uses an indepen­dent shooting chamber. This permits use of a smaller-diameter injection barrel and longer injection stroke for a given volume. The result is to make it easier to generate high injection rates, pressures, and volumes with smaller, precise, and proven electromechanical drives. The two-stage injection unit can shoot its full volume, unlike reciprocating units, which are usually sized twice as large.
The need for affordable high-pressure, large-shot injection with an electric drive led Milacron to look at new approaches rather than simply scaling up the size of a ball-screw, rack-and-pinion, or other linear actuator to accommodate the limits of a reciprocating screw. The two-stage unit evolved as a practi­cal, effective way to dramatically extend the performance range of their electric IMMs, while meeting cost targets.
Drawing on its expertise in extrusion equipment, Milacron used a variant of its single-screw extruder to melt plastic and me­ter it into the injection (second-stage) bar­rel through a port in front. With extrusion

as a separate function, plasticizing rates are sized to exact requirements. Injection control is much more precise than with the nonreturn valve in line with the injection screw plunger, which has to seat before control of the shot occurs. Its longer stroke of a smaller-diameter injection piston is what enables, as an exam­ple, the 150-oz (4250-g) two-stage IMM to do shots down to 4 oz (110 g). This shot is far smaller than would be possible with a 300-oz (8500-g) reciprocating screw. The industry's generally accepted practice is to avoid shoot­ing less than 10 vol% of shot capacity for a re­ciprocating unit, since in such cases the screw stroke becomes so short that it is difficult to control.
The separate extruder allows molders to perform tasks that would be more diffi­cult or impossible with a reciprocating unit, such as compounding glass fiber inline, chan­ging the screw L/D, putting additives in the melt phase, and venting. The melt from the extruder is also more consistent and higher in quality, because each pellet (etc.) passes down the entire length of the screw, in con­trast with a reciprocating screw, where some of the screw feed end may be behind the hopper.
While the two-stage IMM has its advan­tages, it created challenges to the machine designers. Most important was its handling of the melt, which made color change difficult. Also, heat-sensitive plastics could stick to the plunger tip. The electrically driven ball screw behind the injection piston (Milacron; patent pending for tip design) allows a new way to handle melt as it enters the shot chamber, overcoming these challenges. A screw-type tip is used on the injection piston. A one-way clutch rotates the tip while building a shot and retracting the piston, pushing melt forward over the tip (Fig. 2-10). Its first-stage extruder does not move melt directly into the front of the shot chamber at the piston tip; instead, the melt travels through the screw thread to maximize the mixing and forward flow. De­pending on the shot size, this tip gives first-in-first-out, middle-in-last-out, or last-in-middle-out handling. Even when the piston tip is backed past the melt entry port, the ro­tation of the tip continues to wipe the plunger tip against the melt pool. With each shot, the tip starts by pushing new melt forward over the front again, maintaining its cleaning action.

Other Machine Types
Other types include machines with plas-ticators in other positions than those de­scribed, multiple clamping units, clamping for different mold motions (shuttle, rotary, Ferris wheel, etc.), and ram-plunger plastica-tors [Fig. l-19(a), (b)J with one or more rams instead of screw systems (so-called screwless machines). Another type of machine, with a rotary platen system, is shown in Fig. 2-11. Figure 2-12 shows Husky's 660-T machine with 96 cavities, using a three-position water-cooled takeoff plate mold; it molds 96 FET preforms per fast cycle, which are later injection-blown into carbonated-beverage bottles (Chap. 15). Husky's multiinjection rotary-platen machine using two plasticat­ing (injection) units is shown in Fig. 2-13. Multiclamp IMMs can be used with a single injection unit as shown in Fig. 2-14. There are also IMMs with three or more plasticators.
Machine Operating Systems
There are basically three different types of IMM operating systems: those with hy­draulic, electrical, and hybrid drives. The hy­brid system is a combination of hydraulic and electrical. At present the hybrid system provides a technically effective and economi­cally reasonable compromise. At the current pace of electrical-drive development, more economical and efficient electrical drives will make them much more acceptable.
The following three sections provide infor­mation on the three types. In some cases tech­niques described for one type are also appli­cable to other types.
Hydraulic Operations
In an IMM with an all-oil hydraulic sys­tem, oil pressure provides the power to turn the screw to plasticate the plastic, inject the melt into the mold cavity or cavities, close the mold clamp, hold the clamp tonnage, release the clamp, and eject the molded part(s) (Fig. 1-7). A number of hydraulic components are required to provide this power, includ­ing motors, pumps, directional valves, fittings,
tubings, and oil reservoirs or tanks. A single central power source is used for supplying the main and secondary functions of such IMMs (7). Control pumps and hydraulic accumula­tors are used to drive pumps.
The cycle of a hydraulic IMM may be sum­marized as follows;
1. Oil is sent into the clamp ram, closing the mold. Pressure builds up to develop enough force to keep the mold closed during the in­jection of melt into the mold cavities.
2. Previously temperature-controlled pla-sticized material in front of the reciprocating screw or two-stage ram is forced into the mold cavity or cavities by the hydraulic injection cylmder(s).
3. Controlled pressure is maintained on the plastic melt to mold one or more parts

free of sink marks, flow marks, welds, frozen stresses, and other defects. During this part of the cycle the temperature in the mold is con­trolled to eliminate of defects and assure de­sired part performance (dimensional require­ments and stability, surface finish, etc.).
4. At the end of this part of the molding cycle, the reciprocating screw starts to turn, plasticizing material for the next shot. For a two-stage plasticator, the first-stage screw is continuously turning, preparing melt to enter the second stage for delivery into the mold. Techniques and/or devices are used during this phase of plasticizing to prevent drooling from the nozzle in reciprocating systems.
5. While this plasticizing action is occur­ring, the thermoplastic melt is cooling in the mold and solidifies to a point where it can be successfully and safety ejected. For
thermoplastics this cooling is accomplish­ed by circulating a cooling medium, usually water, through drilled holes or channels prop­erly located around the cavity. Thermoset plastics require heat, usually via electric cal-rods, in the mold to complete their solidifica­tion (chemical cross-linking).
6. The next step in the cycle is sending oil under controlled pressure to the return port(s) of the clamping ram, separating the mold halves.
7. As the moving platen returns to its open position, knockout or some type of ejection system (usually mechanical) is activated, re­moving the molded part(s) from the mold.
Examples of these hydraulic IMMs are shown in Figs. 2-15 and 2-16.
An example of advanced hydraulic tech­nology is shown in Fig. 2-17. Two of these HPM 5,000-ton, 400-oz-injection-unit, two-platen, hydromechanical NEXT WAVE™ series machines were installed (1999-2000) in

GM's Saturn plant (Spring Hill, TN) to mold interior parts and body panels for the divi­sion's new midsize LS sedans. To date the many existing IMMs at this 4-million-sq ft plant have been large toggle-clamp machines. The new IMMs include (1) parabolic platen design, leaving mold mounting surfaces flat and free of distortion, (2) retractable tiebars,
(3) platen movement using very little oil,
(4) GE Fanuc process control (similar to that used throughout the plant), (5) improved en­ergy efficiency, and (6) 20 to 30% reduc­tion in floor space compared to their existing machines.


Reservoirs
The reservoir (or tank) provides hydraulic oil to the system for use in powering the various IMM actions. The reservoir must be sized to ensure that an adequate supply of oil is available to the system and also
allow sufficient capacity for the system to return oil. In the past most oil reservoirs had to be rather large to meet the require­ments of the various operations, particularly moving the platens. More recently, through simplifying these motions, very little oil is needed, so that oil leaks are practically eliminated and much less maintenance work is needed.
Oil lines are located to meet oil delivery and return requirements. As an example, suc­tion lines are placed near the bottom of the reservoir to ensure ample oil supply. Re­turn lines from the system discharge beneath

the oii level to avoid spraying into the air and foaming. Some type of antisiphon device should be used to stop the back flow of oil through the return lines in case of line break­age or the removal of a hydraulic component for service. A standard guideline for sizing a reservoir is that it be three times the pump output in one minute, but all system require­ments should be carefully considered before the final reservoir size is determined.

ponents are used together in control systems for injection molding machines.
For linear movements in injection molding machines, hydraulic cylinders are generally employed. These cylinders are controlled by either analog or digital valves.
The mold height adjustment in toggle-clamp machines is a linear motion involving an electric or hydraulic motor that drives a chain or bull gear to turn the tie-bar nuts of the clamping unit. Screw recovery is a rota­tional motion for which a hydraulic motor is usually employed.
A number of competing valve concepts come into consideration foT the control ele­ment for all of these drives: on the one hand, proportional valves or servovalves as contin­uously acting valves, and on the other, digital hydraulic components for pressure or volume adjustment.


Hydraulic Controls
The objective of open and closed-loop con­trols on injection molding machines is to ob­tain the most reproducible molding process possible. Because of their yes-no logic, digital components are considerably less sensitive to external disturbances than analog ones. How­ever, closed-loop position control is accom­plished much more readily at present using analog technology (1,7), though digital tech­niques are increasingly used. In principle, any process variable can be presented in digital or analog form. The difference is shown in Fig. 2-18. In practice, digital and analog com-
Proportional Valves
Proportional valves represent the link between open- and closed-loop control tech­nology. They provide continuously variable

adjustment of pressures and speeds and are employed primarily for open-loop control functions. By means of an amplifier, the con­trol signal (for example, 0 to 10 V) is con­verted into a proportional current and fed to a solenoid. The solenoid, in turn, generates a proportional displacement or force in the valve. The force or displacement is converted by the valve into a pressure or volume flow.
Servovalves
Servovalves are employed for closed-loop control. For the linear axes, pressure, speed and position can be controlled; for the screw drive, the screw speed. The flow characteristic of a servovalve exhibits two operating ranges, to which different tasks can be assigned. In operation range A, pressure or position is controlled; in operating range B, linear or ro­tational speed.
For applications in operating range A, a valve with zero or negative overlap must be selected. Positive overlap cannot be used, since signals within the range of overlap are not transmitted and those outside the range can become garbled. When the closed-loop control of clamp functions is involved, the achievability of position control is the de­cisive criterion as to whether servovalves should be employed. Such control permits the positioning of a mold or ejector with a variation of only a few tenths of a millimeter. This is particularly desirable when inserts are placed in the mold or parts must be removed with precision.
Compared to digital hydraulic compo­nents, continuously acting valves exhibit a few weaknesses that must be taken into con­sideration when selecting the valve concept:
• The valves are driven by an analog sig­nal. This requires more extensive shielding than if the valves were driven by a digital signal.
• Oil filtration is more critical than for digital hydraulics.
• The valve characteristic around the zero point is subject to a certain variability and changes as the result of wear during con­tinuous operation.

Digital Hydraulic Control
As an alternative to servovalves, hydraulic logic elements can be driven by digital elec­trical signals. In this case, the hydraulics per­form the analog conversion. Separate control manifolds are required for pressure and vol­ume flow control. The number of logic ele­ments (bit number) determines the resolu­tion. With a 7-bit pressure manifold, 128 steps can be achieved, including the value zero. For a system pressure of 160 bar, the resolution is then 1.25 bar. This resolution is sufficient for all requirements in injection molding.
Digital pressure controls operate without hysteresis and perform reproducibly over a long period of time. The effects of seating appear during the first few actuations of the valves under load, but diminish quickly. For this reason, a recalibration is conducted after the injection molding machine has completed its test run. This procedure assures good long-term reproducibility of the set pressures.
Along with their advantages, digital hy­draulic control elements also exhibit limita­tions. The electronic digital-to-analog con­verters used to drive continuously acting valves offer as a rule higher resolution than digital hydraulics. With a ramped output, steps are no longer visible. This high resolu­tion, however, applies only to measurement of the process variable and output of the ma­nipulated variable. The accuracy of the pro­cess variable being controlled is always less than that of the measurement.
Digital hydraulic systems are built with a maximum resolution of 8 bits. Their strength is open-loop control of process variables. Pressures and speeds are reproduced with high accuracy. When switching the binary stages of digital hydraulic control elements, however, small pressure spikes and pressure drops with a magnitude equal to that of the in­cremental resolution occur. With the present state of the art, the pressure spikes resulting from actuation of the directional valves can­not be reduced to the same extent that the resolution can be increased.
For closed-loop control, the dynamic re­sponse of the final control element is just as important as the resolution. The restrictions

resulting from the design principle employed for discrete output of the manipulated vari­able mean that digital valves should not be employed as the final control element for closed-loop control of the process variables pressure and speed. Their use should be kept to open-loop control, where the advantages of this design predominate.
For similar reasons, digital hydraulic con­trol elements are not suitable for closed-loop position control. The positioning of the mold and ejector in digital hydraulic machines does not achieve the accuracy of closed-loop posi­tion control. As in the case with digital tem­perature control, the digital measurement of position is becoming more common in high-quality injection molding machines.
There is still no standard approach for open- or closed-loop control of hydraulic functions. Nevertheless, it can be seen today that there will eventually be two attractive versions of digital systems:
• Open-loop machine with digital hydraulics. This version will find its greatest use in machines with sequential functions. It can meet stringent requirements with regard to reliability, while offering simple operation and needing minimal maintenance.
• Digital closed-loop machine. This version will find its greatest use in machines with simultaneous functions and automated equipment that requires closed-loop posi­tion control for high reliability and conve­nience of operation.
Both concepts represent good technical so­lutions for their areas of application.

Hydraulic Fluids and Influence of Heat
A hydraulic fluid is a liquid or mixture of liquids designed to transfer pressure (and thus power) from one point to another in a system on the basis of Pascal's law: pressure on a confined liquid is transmitted equally in all directions throughout the liquid.
The pressure due to excessive heat in the operation of machine-tool hydraulic systems, such as that of an IMM, can degrade the op­eration of the entire system. Heat affects five
major areas of machine hydraulics, which in turn affect the cost and/or performance of the molded plastic product(s): (1) hydraulic-fluid life, (2) energy loss, (3) erratic operation of components, (4) formation and removal of sludge and varnish, and (5) operating condi­tions that cause overheating, which in turn causes leakage of check valves, relief valves, and so on.


Pumps
The hydraulic pump provides hydraulic flow and pressure to the system. It receives oil from the reservoir at low pressure and increases the pressure to that required by the system. Several different types are used. The most common are fixed- and variable-displacement pumps. Different designs are available, the most common being vane, pis­ton, and gear types.
Variable-volume and variable-pressure compensating pumps are being used more frequently in an attempt to conserve energy. These pumps are capable of varying output to meet a particular flow requirement, or dis­pensing only enough flow to develop a partic­ular pressure requirement. There is no single pump type that is perfect for every class and size of IMM.
Figures 2-19 and 2-20 show fixed and variable pumps. Fixed pumps can be single units or staged in multiple-pump configura­tions for powering large-clamp-tonage ma­chines. Big machines theoretically could use multiple variable-volume pumps, but such systems would be rather expensive. Fixed-volume balanced vane pumps are quite popu­lar and generally operate at 2,000 to 3,000 psi (13.8 to 20.7 MPa) with 90% volumetric efficiencies.
In vane pumps, a slotted rotor is splined to the driveshaft and turns inside the cam ring. Vanes are located in the rotor vane slots and follow the inner surface of the cam ring as the rotor turns. Centrifugal force and out­let pressure under the vanes hold them out against the cam ring, and they are enclosed by inlet and outlet support plates. The vary­ing, continuous pressure under the vane area

reduces wear and usually assures high pump efficiency.
Vane-type fixed-volume pumps are not the only types. For bigger-tonnage machines (above 800 tons), use is made of multiple groupings of fixed-displacement internal gear pumps. They can be matched and sized to a variable-volume-type range of outputs. Also, they are rugged and forgiving, so that they are often used in heavy-duty industrial equip­ment such as earth-moving machinery.
Oil output based on machine-cycle status requirements is the key feature of variable-volume pumps, making them very popular. The cylinder block is turned by the drive shaft. Pistons fitted to bores in the cylinder are connected through piston shoes and a re­tracting ring so that the shoes bear against an angled swashplate. As the block turns, the piston shoes follow the swashplate, causing the pistons to reciprocate. The displacement is determined by the size and number of pis­tons and piston stroke length, as well as the swashplate angle.
The swashplate is installed in a movable yoke for variable displacement. Pivoting the yoke changes the swashplate angle to in­crease or decrease the piston stroke. The yoke can be positioned manually, with a servo control or a pressure-compensation con­trol, or by other means. There are variable-displacement pumps that provide at least 96% volumetric efficiency. Most can operate above 3,000 psi (20.7 MPa). There are also radial-piston variable-volume pumps for self-contained presses.
Generally, fixed-volume pumps are easier to maintain, and variable-volume pumps pro­vide more energy efficiency. However, there are pumps of each type that can match the other's benefits.


Directional Valves
Directional valves are used to direct the hydraulic oil from the pump to where it is needed. Spool, check, and cartridge valves are commonly used for this control.
The spool-type directional valve is com­monly used on IMMs. Spool valves can be
either two- or three-position. In a two-position valve, a solenoid is energized for one position, and normally a spring will return the spool to the second position when the solenoid is deenergized. The three-position valve is obtained by adding a second solenoid.
Small valves can be directly operated by the solenoid; on larger valves, solenoid-operated pilot valves direct pilot flow to the main spool for shifting.
A check valve is a single valve that allows flow in only one direction.
An extension of the check valve that is be­ginning to find greater use is the cartridge valve. It is essentially a check valve that is powered open normally by a small spool di­rectional valve. Cartridge valves are grouped to provide the same directional flow capa­bility as spool valves. Figure 2-21 shows a schematic of a cartridge valve; note that the sleeve and its internal parts are mounted within the manifold.

servovalves. These valves can be used to con­trol flow and pressure. The main difference in performance between the two is speed of response, the servo being much faster.
Proportional valves substantially simplify a machine's hydraulics, as they circumvent the need for separate flow and pressure reg­ulators. Machine-cycle pressures and speeds can be conveniently set directly on the ma­chine's control panel by decimal preselectors, allowing as many as 99 different values to be entered.


Electrical Operation
Completely electrical IMMs are availa­ble from machine manufacturers worldwide, including Battenfeld, Engel, Fanuc, JSW, Milacron, Nissei, Sumitomo (543), Toshiba, and UBE. However, at present they are less used than hydraulic and electrohydraulic hy­brid IMMs. The advantages of electrical de­signs are energy efficiency, high power, vari­able controlled power, and brushless motors (331).
Electrical IMMs provide decentralized power generation with individual electrical drives for the main and secondary functions.
Servo and Proportional Valves
With the advent of more advanced micro­processor systems for process control, greater use is being made of proportional valves and
They use servo drives and main-spindle drives that are comparable with the drive technology already used for many years in machine tools (455).
Additional advantages include cleanliness due to the elimination of oil, closed-loop liq­uid cooling, avoidance of the need for exten­sive air conditioning, use of dynamic brak­ing resistors, quick startup and setup, high molding quality, high productivity, repeata­bility without operator attention, and low noise (below 70 dB). The simpler solid-on-solid power train (servomotors, pulleys, belts, and ball screws) eliminates the major causes of molding variations in hydraulic IMMs, with their motors, couplings, pumps, hoses, niters, valves, tubing, heat exchangers, and tanks.
Electric IMM designs offer various engi­neering features. There are high-speed, directly connected rack-and-pinion clamp drive systems. Also in use are 64-bit mi­croprocessor and digital communication be­tween the ac servomotor and controller to create a closed-loop feedback circuit for each of the four axes of motion, namely clamp­ing, injection, screw recovery, and ejection. Repeatability accuracy of ±0.004 in. is attai­nable on both clamping and injection. The four servomotors work independently, so that control can overlap the motion of each axis to shorten the cycle time. As an example, the IMM need not wait for full screw recov­ery prior to opening the mold. The JSW ma­chines are designed to provide for injection-compression molding (coining).
Electric Motors
Practically all basic and auxiliary process­ing equipment uses electric drive motors. To date the dc motors are the most popular. They can be controlled through solid-state circuitry that rectifies the ac supply. Apart from being among the most efficient motors in the speed range of 20 to 100% of maximum, dc mo­tors give a wide range of controllable speeds, better than 30 : 1. A major disadvantage is the tendency of the speed to drift as the mo­tor warms up, though this can be reduced by feedback speed controls (293).

Variable-speed ac drive motors are also used. The main's frequency supply is recti­fied to dc and then converted to a variable-frequency waveform using solid-state switch­ing devices. The resulting nonsinusoidal waveform can cause power loss; use of more silicon-controlled rectifiers (SCRs) can re­duce it. However, this recourse increases the cost of the motor, lessening its advantage over the dc types. Two major advantages of the variable-frequency ac drive motor are its better power factor and lower maintenance.
Adjustable-Speed Drive Motors
A way to cut energy use dramatically is through adjustable-speed drives (ASDs). At the same time they can improve process effi­ciency and minimize machine wear and tear. The energy savings and efficiency stem from ASDs' precise electronic control of motor speed. They provide soft starts, extending the life of the components they drive, such as hydraulic pumps, fans, and seals on rotating shafts.


Servo Drives
A servoelectric drive can be used to pro­vide screw rotation independent of other machine functions, replacing the more con­ventional hydraulic drive with significant operating energy savings. This more expen­sive drive allows screw recovery simultane­ously with other machine functions. Three-phase servo drives can be precisely controlled and easily integrated into the machine con­trol. Their high positioning accuracy and high repeatability have met users' increased de­mands in this respect on the clamping unit and ejector mechanism.
Microtechnology Moldings
To mold micron-scale precision parts with shot weights of only 0.0022 g, all-electrical IMMs are being used (see the section on Micro Injection Molding in Chap. 15).