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 Many people find that the hardest decisions to make often occur when choices have narrowed to two, with each aspect bearing strong pros and cons. Golf or baseball. French fries or mashed. Servos or steppers.
Electric or hydraulic drive.
A number of industries have been studying the changes in electric motor technology over the last decade, with the intent to replace extant hydraulic systems. Systems under examination include actuators on aircraft; vehicle braking and steering systems; and industrial presses and drives, such as those used in injection molding machines for plastics production. There are also hybrid combinations of electric and hydraulic power for certain applications.
 Pros and cons
Hydraulic power has been around for decades, in the sense of a closed system transferring a liquid at a specific rate and pressure to perform work. Such a setup powered early elevators from floor to floor. However, there are losses and inefficiencies inherent in large-scale, pressure hydraulic systems, as are used in industry. Filtration must be considered, and the probability (not possibility) that a leak may occur in the system, adding to the mess that is normal for hydraulics. More critically, no fluid presently in common use for these systems is environmentally safe. With the need to change out fluid as part of a maintenance program, hydraulic systems are contributing to the ongoing hazardous waste disposal problem, particularly when the machine requires 1000 liters or more.
Over the last 10 years, electric motors have been making inroads in many applications previously powered with hydraulics. Better efficiencies and improved controllers have led to changes in processes — for example, in the injection molding of plastics, electrics offer superior repeatability, as well as subtle control of the shot, in turn opening up opportunities for new product results and materials with new performance and appearance. Electric systems also outperform hydraulics insofar as backpressure control is concerned. Conventional hydraulic machines only control oil on one side of the injection piston, while the other side of the system is sucking oil from a sump.
However, electric motors are not without their limits. Injection molding machines range in ratings of hundreds of tons of force, while running at speeds acceptable for production demands. In high-speed applications, a 200-ton machine requires a motor and drive on the injection unit to be so large that the power demand exceeds profitability (In general purpose application speeds, 400- to 500-ton machines experience the same problems). The mechanical components for these scenarios would be under tremendous stress, which would diminish their reliability. Additionally, the accelerations and decelerations involved between boost and hold pressure can be on the order of 15 to 20 milliseconds. In that time, the large inertia on the servomotor needs to go from zero to around 1800 rpm (or vice versa), consuming most of the machine’s power demands. In deceleration, the energy of the motor is put across banks of resistors as waste heat. This design now involves additional cabinets and cooling arrangements. In some circumstances, the amount of energy wasted means that the machine is operating at only 20% efficiency, with 80% going to heat production.
A hybrid alternative
Various attempts have been made to create an injection molding machine that uses combinations of electric and hydraulic power, with one performing the injection and the other handling the task of clamping the mold. At this year’s National Plastics Exposition Show, Moog Inc.’s Industrial Controls Division, East Aurora, NY, unveiled a sealed, closed-loop module for driving the injection screw.
The PowerShot Injection System, a self-contained, compact electro-hydraulic injection unit, will be scalable for screw diameters from 50 to 170 mm in diameter. Drawing from the Moog’s stock of components for hydraulic and electric injection molding machines, the system will use a 3-way SE3 Servocartridge valve, an RKP radial piston pump, and a FAS Series servomotor controlled by a DS2000 digital drive. The servomotor and pump are turned on only as needed during the recovery portion of the injection cycle to control the rotation of the screw and backpressure, conserving energy. The pump controls the outgoing oil from the piston, with the ability to adjust backpressure to the magnitude of 0.3 bar of
setpoint.
 With this sealed system, there is less chance of contaminants getting into the hydraulic fluid, extending the life of the components. Both the pump and the servocartridge valve use direct drive pilot stage valves, which have greater tolerance to contamination because of high spool driving forces. The system also uses a synthetic ISO-46 grade of non-toxic, biodegradable oil. The sump is also much smaller than standard hydraulic systems — while a typical machine’s capacity may be 1000 liters of fluid, a 150-170 mm PowerShot will require only about 300 liters. Not only does this modification further reduce impact on the environment, it also reduces the size of the machine.
With the system being accumulator-based, the module has the flexibility to attain high speeds. The unitized packaging also dispenses with separate components that, in a hydraulic system, would be joined with leak-prone tubing, hoses and fittings.
Hybrid in operation
In the PowerShot, the oil is discharged from a piston accumulator through an active cartridge to the servocartridge during injection. The injection cylinder rod side is always attached to the accumulator, which allows for very precise injection control. During this time, the main servomotor and RKP Pump is turned off to conserve energy.
With the injection cylinder rod side attached to the accumulator, the 3-Way servocartridge only needs to allow oil out of the injection cylinder during decompression until the desired plasticizing screw position setpoint is reached. The servocartridge is used for closed-loop control of injection velocity, pressure, and position.
During the recovery portion of the process, the servocartridge is de-energized. The main servomotor and pump are turned on to control the outgoing oil from the injection cylinder. The pump is controlled by a servo-proportional valve to maintain closed-loop backpressure control.
All of the leakage oil from the pump case drain and valves is passed through a small heat exchanger, and then collected in an oil recovery vessel. When the oil reaches a certain level, an auxiliary pump/motor assembly turns on to return the oil to the piston accumulator. The pump/motor assembly turns off when the oil reaches a low-level indicator.
As a safety feature, during a shutdown of the unit or in an emergency stop condition, the active cartridge closes to isolate the accumulator from the injection cylinder. The main motor and pump are turned off, and the servocartridge de-energizes. Solenoid valves in the system de-energize, allowing the piston accumulator to discharge into the oil recovery vessel, along with any other pressurized oil in the system.
Controlling the hybrid
 Driving the PowerShot is a multi-axis servo controller (Moog Servo Controller, or MSC) with integrated PLC functionality. The unit performs closed-loop control of the injection ram position, injection speed, holding pressure, screw speed, decompression (suckback) and backpressure. The individual parts of the injection cycle can be customized, allowing the user to stay in control of the process and come up with unique solutions. In its recommended configuration, the MSC communicates with the existing machine controller using digital I/O signals and/or serial communication. A more traditional control structure with the existing machine controller supplying analog command signals for the injection process to the MSC can also be accommodated, if required. The machine controller uses digital I/O to signal the MSC to perform the individual task of the injection cycle. Digital outputs on the machine controller connect to digital inputs on the MSC to signal injection start, plasticizing start, etc. Digital outputs on the MSC connect to digital inputs on the machine controller to signal a task is completed. As a starting point the injection process is configured as follows:
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Speed control is configured as a 10-step speed vs. position profile.
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Holding pressure control is configured as a 10-step pressure vs. time profile.
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V/P switch between speed and pressure control is selectable between ram position, injection pressure, cavity pressure and time.
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Plasticizing control is configured as a 5-step backpressure vs. position, and a 5-step screw rpm vs. position profiles.
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Decompression (suckback) is available before and after plasticizing as two sets of speed vs. stroke parameter sets.
 The operator can adjust the MSC by first visualizing all these process parameters on the existing machine controller. The parameters and current cycle data can then be downloaded from the existing machine controller to the MSC using a serial communication line. This configuration allows for the display of current injection speed and pressure, and parameters such as actual values at the V/P switch point, in turn permitting tight integration between the existing machine controller and the PowerShot Injection System, as no additional display/HMI is needed.
On the drawing board
Under development at Moog is the second element in hybrid design, the PowerShift Actuator System. Similar to the PowerShot, the device will use a Moog MaxForce Electro-Mechanical Actuator, connected to a hydraulic intensifier, to provide precision control of the clamp opening and closing, tonnage build and mold break. Useable in both injection and blow molding machines of medium and large tonnage, the system is expected to produce high power with energy efficiency. It will also surpass an all-electric system with the ability to transfer from high-speed/low-force to low-speed/high-force while using less installed power.
The benefits of the design will duplicate its sibling — smaller sump, no hoses or other leak points, fast speed and high repeatability, energy efficiency and a unitized package. The modular design of both the PowerShot and PowerShift lends the units one additional aspect — potentially, they can be retrofit into existing machines for companies that wish to make a hybrid choice.
Thanks to David Geiger, Moog Industrial System Engineering Manager — Hybrid Solutions, for assistance in developing this article.
For more information:
Moog,
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