Injection Molding: A Technology in Transition to Electrical Power

A technological sea change has been un­derway in injection molding machinery. With little fanfare, there was one company exhibit­ing EMT during the 1985 NPE show. Just 12 years later, there were at least twelve. The Japanese were racing to make the transi-

tion to EMT. At least seven Japanese compa­nies started pursuing it, with Fanuc having a dedicated factory to build electric machines. Purchasing patterns in the United States and Europe indicate a significant increase in mar­ket acceptance, too.
Hydraulic machinery will always be strong Before we go further, it is important to note that hydraulic injection molding machines will continue to be strong contenders in the market. There will always be regional mar­ket preferences for these machines because of differing labor costs, work-force skill lev­els, and industrial infrastructures.
These machines also enjoy a cost advan­tage, so builders will continue to invest de­velopment money to provide better value. Fi­nally, hydraulic machines will, at least for the foreseeable future, probably have a secure market in high-tonnage applications (primar­ily over 1,500 U.S. tons), because of the cost premium for high-power servomotors.
EMT offers many environmental, energy-reduction, and performance benefits, which largely drove development of the technol­ogy. Three designs from Japan, the United States, and Germany show how different ma­chine builders have developed products that delivered these benefits to customers in con­formance with specific regional market re­quirements. This is the same pattern of design proliferation that has occurred with hydraulic machinery and will continue as long as there are specific customer needs driving deve­lopment.
Priority preferences The Japanese were first with an electric machine, because envi­ronmental issues are a high priority in that densely populated island nation. Compact size, low noise, and elimination of oil as an en­vironmental and fire hazard led the Japanese to create the first commercially viable EMT. In a market dominated by precision mid- to low-tonnage machines and relatively small shot requirements, early electric drive tech­nologies could most easily be adapted to in­jection molding in the Japanese market.
In the European market, speed and precision are high priorities, along with environmental benefits. The first electric ma­chine developed in Germany reflects these priorities, with subsecond dry cycle times. This machine configuration is ideal for quick, precision molding of thin-walled parts, such as CD jewel boxes and medical disposables.
The U.S. market also wanted the benefits of electrical machinery, but within a market con­text of custom-molding requirements with mid- to high-tonnage machines and larger shot sizes. The first U.S.-built machine was therefore designed to appeal to the core mar­ket of toggle-machine users, with features, ca­pabilities, and controls analogous to popular hydraulically powered toggle machines.
In addition to all the environmental and energy factors driving electric-machine de­velopment, an overriding consideration in all these markets is the broad imperative for reduced-labor and unattended produc­tion. A key enabler for this is high process repeatability—removal from the process of variables that require operators to monitor and adjust the machine (Fig. 2-22). Hydraulic oil and all the hardware needed to manage it exposes machines to variations.
Repeatability improvement is why electric drives completely supplanted hydraulics in machine tools in the 1970s and in robots in the 1980s. And it is repeatability improvement that will pull EMT into the mainstream of in-

jection molding. EMT did not arrive on that scene until the 1980s, since power require­ments for IMMs were significantly higher than those for machine tools and robots. There was not much development in servo­motors above 50 hp until the 1980s, and at the time the few that were available were not in a package suitable for IMM applications.
The earliest applications of electric motors in molding were on extruders, which oper­ate continuously, in contrast with the inter­mittent nature of injection molding (3). This experience, combined with developments in drives and controls, served as a foundation. Today, the demand for servomotors of all sizes is attracting development funding and driving down the cost of manufacturing.
The transition to electric injection mold­ing will proceed rapidly, and most machines below 1,500 U.S. tons in developed coun­tries will probably be all-electric in 20 years, though the basic configuration for clamp and injection functions will change relatively little.
Repeatability potential is inherently higher for EMT The repeatability potential for EMT is inherently higher than that of hy­draulic power, for fundamental engineering reasons. Hydraulic drives are typically dis­tributed systems, employing a compressible




Electric Technology Simplifies Powertraln fluid and a complex network of hoses, tubes, and valves to allow one or two pumps to drive all machine axes. By contrast, an electric ma­chine has a motor for each axis. An all-electric power train may consist of as little as a belt, two pulleys, and a ball screw. With a sepa­rate motor for each axis, all-electric machines have the inherent ability to drive and coordi­nate all axes of motion simultaneously, which can greatly reduce cycle times.
Whatever its configuration, the electrome­chanical power train is rigid, solid-on-solid. Hydraulic power transmission is dependent on a compressible fluid. Any conditions that affect the fluid or its flow properties affect po­sitioning of the machine. These conditions in­clude viscosity variation, compressibility, oil degradation, and thermal effects, as well as sticking valves and expanding hoses.
Many advantages of EMT affect the bottom line While energy costs remain relatively low worldwide, it is important to remember they are probably as low as they will ever be. By its very nature, hydraulic power wastes energy converting electricity to mechanical motion, and this alone is a strong advantage for EMT. An axis drive draws no power when there is no motion along the axis (Fig. 2-23), significantly reducing energy use. EMT cuts power use by 50 to 90%.
Head-to-head testing of 550-ton electric and hydraulic machines producing a 33-oz

shot for an HDPE bucket on a 20-sec cy­cle showed annual energy savings of about $25,000, at 6 £kWh and 6500-h/year opera­tion. As Fig. 2-24 shows, kilowatt-hour sav­ings with the electric IMM increase dramati­cally as the melt requirement increases.
In addition to its shortcomings as a power transmission medium, hydraulic oil can be an environmental problem. Some injection molding plants have been compared to oil-patch drilling sites. There are molding plants in the United States that cannot afford to move, nor can they be sold, because the ground below is contaminated with hydraulic oil. EMT eliminates oil from the workplace, along with spills, fugitive oil mist, hazardous-waste disposal, oil-related employee falls, fire hazard, and inventory and storage costs.
There is already a move underway to elim­inate fugitive oil mist in machine shops, and it is possible that the vapors from hy­draulic power reservoirs will come under the same scrutiny. The mere presence of hy­draulic oil in a plant can increase insurance premiums.
Hydraulic machines are maintenance-intensive (Chap. 11). The chief failure mode of hydraulic machines is valving—eliminated with EMT. Also eliminated are nuisances such as leaking hoses and sticking valves. There are always downtime and maintenance labor costs to remedy these stoppages, none of which occur on an electric machine.

The noise is lower for electric machines— less than 70 dB, not much more than an office copier. This lowers stress on employees, and can allow molding machinery to be located in nontraditional manufacturing areas. Product development teams and engineers can work side by side with EMT, if needed.
The labor costs are lower for EMT because setup times are shorter and there is less art to the molding process. The machines are so consistent that setup data can be transferred from one machine to another, and few or no adjustments will be needed to get acceptable parts right away. Shorter time to first good part, coupled with better process consistency over time, makes EMT better suited for JIT molding to feed an assembly operation.
Costs for EMT new plant construction are lower because of reduced electrical service connection hardware and size of bus bars. Because electric machines throw off 65 to 75% less heat than hydraulic machines, air-conditioning loads are greatly reduced.
Real cost advantage in EMT is improved process capabilities While there are many peripheral advantages to EMT, the real drivers behind it are the same economic and competitive issues that put electric drives on machine tools and robots. They are (1) tighter

part quality (more good parts per shift) and (2) high repeatability without constant oper­ator attention. Their benefits derive from a simple concept that is widely understood and utilized in the metalworking industry, but is in the very early stages of acceptance in the injection molding industry. That concept is process capability.
Process capabilities Process capabilities can be defined and measured. A process ca­pability study determines whether a manu­facturing operation is capable of producing parts within a specified tolerance or range of limits. Such a study is performed be­fore making parts with machine tools, and is widely used to benchmark and grade the performance of machines in a shop. With this knowledge in hand, users can relegate certain machines to roughing work, and reserve more-accurate machines for higher-paying, more-demanding work.
Hunkar test The Hunkar class standards are the closest thing plastic processors have to a counterpart of the formalized, ASME-defined test regimen for machine tools. The purpose of all such tests is to attempt to establish machine capability before making parts, rather than inferring it from statistical
process control (SPC) studies on parts al­ready made. One graphic result of a process capability study is a bell-shaped curve, which many processors with SPC backgrounds use. As Fig. 2-25 shows, the tighter window of the electric machine's process capability allows operations to be moved confidently toward the lower specification limit where savings ac­crue in material, scrap, energy, and so on. In simple terms, the curve for an electric ma­chine is much steeper, allowing upper and lower control limits to be moved in tighter. The curve for a hydraulic machine is flatter, dictated by the variables of hydraulic power (Chap. 13).
This higher process capability appears to be inherent in electric machine design. Even general-application electric machines have

process capability that is significantly bet­ter than Hunkar class 1. Table 2-1 provides typical Hunkar test results from a series of 60 shots on a 300-ton U.S.-designed general-purpose Elektra IMM used for molding pipe elbows.
With a more capable process, molders can produce more good parts per day, adding to their profit margin and competitive ad­vantage in the market. This occurs be­cause:
1. It provides quick startup and setup with­out oil preheating.
2. Mold setup parameters can be deter­mined once, then used on reruns with little or no adjustment. EMT reduces the art in mold­ing, just as computer-controlled servoelectric
drives reduced the art in metalworking to a programming exercise.
3. Less scrap is caused by changes over time than with hydraulic machinery. Long-term repeatability reduces operator interven­tion, allowing unattended production.
4. Greater reliability and more productive hours are achieved by using a machine with fewer parts.
Many routes to EMT Machines that have emerged in three different markets demon­strate that the higher process capability is in­herent in EMT, not due to a specific machine design. The different designs reflect the inge­nuity of engineers in transforming the rotary motion of an electric motor to linear motion for injection molding. The differences also re­flect the needs and wants of different target markets for each machine.
The U.S.-designed Elektra (Fig. 2-26) is essentially a proven toggle-machine chassis with dedicated electric drives for each axis. The objective of this design is to mimic the look and feel of a popular general-application toggle machine. It uses the same controls as its hydraulic counterpart, greatly easing the transition of a molder from hydraulic to elec­trical machinery.
Although aimed at the broad-range cus­tom molding market, the Elektra has the

speed and repeatability to make inroads into both high speed applications such as packag­ing and closures, and precision applications such as electrical connectors and medical dis­posables, True to its broad-range design ob­jective, this machine has ample room be­tween the tie-bars, large daylight and long stroke, oversized platens, and three-way parts removal capability. The dry cycle time is very competitive at 2.2 sec, with a 350-mm clamp stroke.
Clamp, ejection, injection, and sled pull-in motions are driven through computer-optimized ball screws developed for the machine-tool industry (Fig. 2-27). Servomo­tors are connected to the ball screws through a heavy-duty timing belt and pulleys. The die height is set by a servo-driven chain-and-sprocket arrangement. The plasticator is driven directly through a timing belt.
The design objective for the German Fer-romatik electric injection molding machine (Fig. 2-28) was high speed, and it meets that objective with subsecond dry-cycle times. It is ideal for thin-wall parts and packaging ap­plications, And true to its purpose, the man­ufacturer reports that roughly 50% of these machines are purchased for speed, and 25% are purchased for clean applications, such as food and electronics.
Key to this design is the use of mechani­cal transmission devices to amplify torque or
speed, and to generate specific force-velocity profiles for injection and ejection. The clamp is driven through a high-speed double rack and pinion, with an upstream two-stage spur-gear set.
As shown in Fig. 2-29, the crank arms are used for injection and ejection. The left view shows a full stroke of the extremely fast crank-driven injection. The right view shows its crank-driven ejection system.

A crank arm is able to deliver large force or high speed, depending on its position. The injection crank arm, powered through a mul­tistage gear drive, is able to double the ve­locity on the screw at the beginning of the move, and double the pack pressure at the end of injection. The crank arm performs only about one-third of a revolution. The die height on this machine is adjusted automat­ically through a servo-driven ring gear. The
plasticizing screw is driven through a two-speed gearbox.
The Japanese electrical design (Fig. 2-30) is also a market-driven machine, created for compact size, low noise, and precision. Screw geometry and shot size, relative to platen area and clamp tonnage, have been optimized for molding precision parts with engineered materials.
The injection unit provides a wide range of adjustment for low-speed, high-pressure or high-speed, high-pressure injection. In­jection rates of 9.32 cu in./sec (14.9 x 10~5 cu m/sec) and pressures up to 35,000 psi (240 MPa) are standard on machines in the 100-ton range.

This machine has a very compact foot­print. It reflects the builder's focus and ori­gins in manufacture of positioning drives for machine-tool and robot applications. The die height (Fig. 2-31) is set with a motor-driven precision ring gear set.
An other available feature is the servo-driven power ejection through ball screws (Fig. 2-32). Clamp and ejector positioning are repeatable to ±0.0005 in.
Expect new designs, more choices It is worth noting here that the terms high­speed, broad-range, etc., for these machines are not definitive and rigid, any more than they are for hydraulic machines. Market
classifications do not have walls around them. They simply represent notions that many molders and machine makers can recognize, Molders can, and do, adapt to using machines in applications that may not be ideal for the job. This is also true for hydraulic machinery. Molders want choices, and there will be many choices in electric machines, as there are in hydraulics. The same type of design
proliferation, specialization and overlap seen in hydraulic machines will occur in electric machines to match the perceived needs and wants of customers. This is a plus for the industry.
An example of new designs entering the market is the all-electric Powerline 330 (Fig. 2-33) with advances in performance, size, and simplicity. This is a step forward
in value and performance for the general-application machine, with the use of digi­tal hrushless servo drives with direct-drive clamp, open-architecture PC-based control, and low-inertia motors with air-cooled drives.

Digital servos allow performance improve­ments through software changes, rather than hardware changes as with analog systems. The digital servos are also more resistant to electronic noise in the plant, and change over time such as occurs when analog circuitry ages. This improves process consis­tency and control for the general-application machine, and increases productivity and cost-effectiveness.
Coupled with the new generation of open-architecture, PC-based controls, digital ser­vos complete an important link in informa­tion and control systems, from the machine level to the enterprise level. Digital control and communications, particularly the PC-based variety, are the common denominators for established channels of data communica­tion on the plant floor and throughout the en­terprise. Analog control systems are isolated from this information infrastructure, while digital control facilitates everything from satellite-communicated production schedul­ing to SPC at the machine.
Trends, predictions There is a cost pre­mium for EMT at the present time, but that will change. When comparing prices with those of hydraulic machines, specifications and capabilities should be balanced. By the time you enhance the hydraulic circuits and controls on a hydraulic machine to approach the performance of a general-application electric, the cost difference narrows signifi­cantly; and hydraulic technology remains, by its nature, less precise. For example, in some molding applications it is advantageous to have simultaneous operations occur in the clamp and injection ends of the machine. This capability is inherent in the electric machine with its independent axis drives, but it signifi­cantly affects the cost of a hydraulic machine.
In the area of servos, cost will come down and capability will improve as critical mass develops in market demand and manufac­turing. The changes will be analogous to the rapid evolution of everything digital. The uni­versality of digital devices will allow general-purpose hardware solutions, with specialized software taking the place of custom circuitry, again reducing cost. Designers of electrical machinery are looking at all the rapidly evolv­ing alternatives in servos to find the highest performance for the lowest cost.
Within 10 years, expect to see 70 to 75% of all 800-ton-and-below injection molding machines in developed countries to be elec­tric. EMT will enable developed countries to remain competitive—and retain jobs— through higher quality and productivity, even while labor costs and environmental regula­tions add to overall costs. Hydraulic oil, be­cause it increases molding costs in so many ways, will be seen as a business hazard as well as an environmental hazard. Any increase in electricity prices will also drive demand for EMT.
Hydraulic machines will still be in favor in undeveloped countries with low-cost, low-skilled labor and low quality requirements.
There will also be changes for mold buil­ders. They will begin producing more ser-voelectric systems to actuate core pulls and other functions. Mold builders will not want to be seen as the sole cause for bringing hydraulic-oil contamination to a clean pro­duction floor.
The environment of the molding plant will change considerably in a few years, as will the standard of quality that we take for granted in the process. In just over a decade, we have gone from the first electric machine at NPE to having at least 12 electric-machine manufac­turers in the market. This, alone, is a leading indicator of the market's appetite for cleaner, more precise, more energy efficient molding.

Hybrid Operations
Many different combinations of hydraulic and electrical machine operating systems are used that provide advantages such as fast moving of platens, reduced size of hy­draulic cylinders, and reduced operating cost. These hybrid operating systems have pro­liferated to meet the molders' different re­quirements. Popular examples that have been used for many decades are the electric screw drive system designs in hydraulic operating IMMs.
All-hydraulic drive components not only offer a good price-performance ratio; they also have numerous technical advantages. It is therefore beneficial to develop combina­tions of hydraulic and electrical systems so as to have the advantages of both. Depending


Clamping Systems
The clamping unit is that portion of an IMM in which the mold is mounted on sup­porting platens and usually guided by four tic-bars (though basic concepts described here are applicable to tie-barless systems as well). The clamping area is the largest rated molding area the machine can hold closed un­der full molding pressure. The clamps pro­vide accurately controlled motion and force to close and open the mold. They also hold the mold closed during plastic injection. When the clamp is closed in a horizontal direction with the platen vertical, (by far the most pop­ular arrangement), the system is referred to as a horizontal clamping system. When the clamp is closed in the vertical direction, it is a vertical clamping system.
The stationary (fixed) platen is where half of the mold is fastened. This member usu­ally includes a mold-mounting pattern of bolt holes or T slots; a standard pattern is rec­ommended by SPI. For certain machines, it usually includes provision for a mold with a sprue to be properly aligned with the platen's opening and to be secured to the platen so that the IMM nozzle can be firmly fitted. This platen, with the nozzle leaning against the mold's sprue, docs not move or separate un­der normal operation. The movable platen secures the other half of the mold and moves to close and open (separate) the mold halves.
The term "mold halves'" refers to the two ba­sic parts of a mold; they are usually not equal in size.
The clearance between two platens of a press is called the clamping daylight opening. It provides space for the mold height plus the space needed after the mold opens and the pan has to be removed from the mold cavity. There are a maximum and a minimum day­light opening distance (Figs. 2-34 and 2-35).
The clamping force in a hydraulic IMM is provided by various drive systems. There are three main types of force: hydraulic, tog­gle, and hydromeehanical. Electrical drives and combined electrical-hydraulic drives are also used. These different combinations of hydraulic and electric machine operating sys­tems are used to provide advantages such as fast movement of platens, reduced size of hydraulic cylinders, and reduced operating

costs. Examples of these hybrid operating sys­tems are many.
One common technique is to direct hy­draulic fluid to a booster tube to move the clamp ram forward. Oil fills the main area by flowing from the tank through the prefill valve to the main area. As the ram moves forward, a slight vacuum is developed in the main area, pulling fluid from the tank into the chamber. Once the clamp is closed, the refill valve is closed, trapping the oil in the main cylinder area. High-pressure fluid is put into this area, compressing this volume of oil and thus raising the pressure. A pressure control valve that closely controls the clamp tonnage thereby controls the maximum pressure. The tonnage is the maximum hydraulic pressure times the area it pushes against.
To open the clamp, hydraulic fluid is di­rected to the pull back side of the cylinder while the prefill valve is open, with fluid from the main cylinder being returned to the tank. One of the major advantages of the straight hydraulic clamp is its very precise control of the clamp tonnage.

are all-electric drive systems and hydraulic-electrical hybrid systems. The mechanical mechanisms include toggle and straight ram systems among others. Each of these differ­ent systems has its advantages.
Pressure forces The pressure force, also called the clamping force or locking force, is the force, in tons, that is exerted to hold the two platens or mold halves together when melt under pressure fills the mold cavity.
Pressure measurement Different meth­ods are used for pressure measurement, de­pending on the type of clamping system used. They include: (1) use of a pressure transducer between closed platens, (2) summation of the tie-bar forces, (3) measuring the force in a toggle mechanism, and (4) determining the force from the oil pressure in a hydraulic sys­tem or the electric power used in an electrical system. Measurements in the tie-bars, usually via some type of electrical strain gauge, of­fer the additional advantage of monitoring the forces in the individual bars. Thus, un­even loads or overloading of individual bars caused by unbalanced or worn molds, as well as other problems, can be identified quickly to avoid major problems.
Pre-close clamping Often one closes the mold to some point near the fully closed po­sition before and after final closing. This per­mits bumping, improved parison pinch areas for blow molding, mold safety measures, etc.
Clamping actions IMMs can provide close slowdown clamping action. This means slowing down the moving platen for an ad­justable distance before the mold faces come into contact. There may also be a close low-pressure clamping system to lower the clamp closing force in order to minimize the dan­ger of mold damage caused by molded parts caught between the mold halves. A clamp-opening-stroke interruption is a complete stop of the clamp opening stroke to allow auxiliary operations before completion of the opening stroke.
The maximum distance over which the opening and closing mechanism can move a platen is called the maximum clamping ac­tion. This action can be adjusted to meet mold or molding requirements. The clamping shut height is the minimum distance between ma­chine platens when the clamp is closed.
The clamping ejector, or knockout, is a provision in the clamping unit that actuates (mechanically, pneumatically, hy-draulically, and/or electrically) a mechanism within the mold to eject the molded prod­uct from the mold cavity. A close pre-position ejector mechanism is a provision in the ma­chine control circuit to allow a clamp to open fully and then close to a predetermined posi­tion. It is also used to allow the mold ejector

(knockout) mechanism to retract so inserts can be placed in the mold.


Hydraulic Clamps
The hydraulic clamp system uses a hy­draulic cylinder and piston to develop clamp force directly. The two-platen version typi­cally features a drive mechanism that pulls rather than pushes the moving platen (Figs. 2-36 and 2-37). Hydraulic systems include other designs, particularly the use of a se­ries of smaller hydraulic cylinders (Fig. 2-38). Common arrangements include the three-
platen, two-platen, C-clamp, rotating-platen, and tie-barless machines, each providing dif­ferent benefits. As an example, the two-platen usually is much the shortest IMM, re­quires less floor space (by 20 to 40%), and weighs less than a three-platen hydraulic or toggle systems.


Toggle Clamps
Toggle, or mechanical, clamps use the me­chanical advantage of a linkage to develop the force required to hold the mold closed during the plastic melt injection portion of the cycle (Figs. 2-39 to 2-41). Figure 2-41 shows the process (from top to bottom): partial in­jection, degassing, final injection, and ejec­tion after the product is sufficiently solidified. Normally the linkage is designed so that slow­downs are built in. The advantage of a toggle clamp is that less hydraulic fluid is required to open and close the clamp than with a conven­tional hydraulic clamp. A main disadvantage is that the actual clamp tonnage is not pre­cisely known.
A small hydraulic cylinder is used to close the clamp. This cylinder travels at a constant speed with the slowdown for mold close built into the linkage. The mechanical advantage of the linkage is extremely high, so a rela­tively small closing cylinder can develop high tonnage.
A single toggle applies the correct clamp­ing force by amplifying the force exerted on it. The multiplying factor so obtained ranges from 15 to 20 times for the single type, and from 25 to 50 times for the double. Thus, with a mechanical advantage of 20, a 100-ton clamping force can be obtained from a single toggle in which a hydraulic force of 5 tons is applied.
The single toggle was used in the past by a number of machine manufacturers for ma­chines with a clamping force up to 200 tons, and occasionally more. Currently, most are under 70 tons. For the same applied clamp­ing force, the power consumption of a single-toggle is higher than that of a double-toggle machine.
Double-toggle machines are currently the most widely used, particularly for those with a clamping force up to 1,000 tons. The rea­sons for their wide use are to be found in the fact that this system allows higher moving-platen speeds to be attained, shortening the mold clamping and opening times, and conse­quently reducing the total molding cycle time. In addition, power consumption is reduced to about one-half, and the force applied to the moving platen is better balanced than one ap­plied by a single toggle. It acts along two lines that are generally aligned with the mold unit's tie-bars. However, a double toggle is more ex­pensive, as it uses more links and involves a more complex construction of the toggle unit and moving platen.
Hydromechanical Clamps
In a hydromechanical clamp, forces are created partly by a mechanical system, such as a toggle system, and partly by a hydraulic system to increase speed of operations, re­duce operating costs, and provide a means for high-speed close and open (Figs. 2-42 and 2-43). The hydromechanical clamp system from Engel (Fig. 2-43) features two small cylinders to open and close the clamp, and four locking cylinders in the baseplate.
A short-stroke cylinder is used to develop tonnage identical to that for the straight hydraulic design. This concept offers the
advantage of toggle clamps1 high-speed close and open, and the advantage of a straight hydraulic for precise control of clamp ton­nage. The hydromechanical design normally has a high-speed clamp close and open device that is usually a hydraulic cylinder or actua­tor. The closing and opening modes occurs with relatively low force. Once the clamp is closed, a blocking action takes place, allowing a large-diameter hydraulic cylinder to build tonnage similar to that for the straight hy­draulic design popular in the past. When the

clamp is to be opened, the blocking mem­ber is removed, and the clamp opens rapidly. The blocking member is normally a mechan­ical device, and the tonnage is applied by hydraulics.


Hydroelectric Clamps
A system may use a combination of hy­draulic and electrical systems to take advan­tage of their distinct benefits.