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Get most of the energy savings of an all-electric injection press for less money by choosing a hybrid of hydraulic pumps and variable-speed electric drives. You'll also run faster and quieter than with a conventional press.

Greater energy efficiency is one reason that electric servo drives have received so much attention lately as alternatives to hydraulic power for injection presses. Another factor is the desire to minimize the presence of oil in cleanroom operations. Quieter electric drives can also enhance the work environment. And electric drives permit screw recovery during the clamp holding phase, which can reduce cycle times. More precise operation of plastication and other functions may also result with electric drives.

These advantages are available from all-electric machines - but they don't come cheap. Fortunately, most of these benefits can be obtained at much lower cost with an electric servo drive on the screw and/or on the hydraulic pumps. The resulting combination of hydraulic and electric power is what we call a "hybrid" machine. It brings additional benefits of lower maintenance costs and more familiar overall machine design and operation, as compared with an all-electric machine.

How much can you save?

In an increasingly competitive environment, molders are driven to reduce their cost per part by every means available. There are many operational factors to consider in pursuit of this goal, but energy consumption is one element that is relatively quick and easy to address.

How much opportunity is there to save money by reducing energy costs? Industry estimates on energy usage as a percentage of overall production costs vary, depending on geographic location, local utility costs, efficiency of a molder's operation, type of molding performed, and so forth. But we can safely say that about 2% to 5% of the total cost of an injection molding operation is energy cost. A recent assessment of the operating costs for one of our customers molding a crate on a 650-ton machine appears in Fig. 1.

Once you decide that paying attention to energy costs is worthwhile, you need to know what is the main consumer of energy in your plant. Figure 2 shows how energy is used in a typical molding plant. About 60% of the energy cost can be assigned to your injection molding machines. So their operation obviously presents the greatest opportunity for energy savings.

The next question is where to look to reduce energy consumption by the injection machine. Figure 3 gives a clue: Almost 55% of energy used by the machine is for screw recovery. That's the most logical place to start in the search for energy savings.

To reduce energy in plastication, machine builders replace the hydraulic screw motor with a variable-speed electric motor, such as an AC servo, and a gear box or some other reduction mechanism (belt or chain drive). This is where the most significant savings are quickly obtained. What's more, it is a simple application of servo-motor technology, since these electric drives have a rotary motion, which is translated straightforwardly into the rotary motion of the screw.

The primary advantage of having a separate electric screw drive is not energy savings but the ability to overlap plasticating and clamp or ejection functions in parallel operation. This ability is available as an option on some hydraulic machines. It is sometimes referred to as "continuous screw rotate." But on a hydraulic machine, this option comes at the price of more horsepower, manifolds, and valves, making it costly, more maintenance-intensive, and an energy hog. Electric screw drive is a more efficient, less costly, and simpler way to achieve continuous screw rotate.

But this advantage is useful only when the plasticating phase represents a large proportion of the cycle time - longer than the cooling or cure time. In this circumstance, up to 70% of the total energy use of the machine is consumed in plasticating. For these applications, two benefits are obtained - lower energy consumption and shorter cycles. But if cooling is what determines the overall cycle time, then overlapping cycles will be of little benefit. The screw is simply forced to wait until the cooling phase is complete.

Servo teams with hydraulics

There are many different ways to apply electric variable-speed drives on injection machines. One is to use them on individual components like the screw drive. At the other extreme is the so-called "all-electric" machine, which typically combines a number of variable-speed motors with ball screws or other devices to convert rotary motion into linear motion of the clamp, injection unit, and ejectors. As many as eight motors may be required to provide all the necessary functions on a machine with purely electric power.

Hybrids combine AC servo motors with hydraulic pumps. The Van Dom HTE Series hybrids have both an AC servo screw drive and a separate variable-speed electric motor to power a hydraulic fixed-volume pump. This system retains a hydraulic system to handle injection, clamp movement and locking, and ejection functions. This solution retains the capabilities of hydraulics to achieve precise pressure control throughout the cycle, particularly during the transition phases such as the transfer from injection boost to cutoff to the hold-pressure phase.

The hybrid eliminates the use of high-cost ball screws and avoids the challenges that all-electric machines face in the complex translation of rotary motion (of the electric drive) to linear motion (of clamp, ejector, and injection unit), especially during pressure-transition phases of the cycle. Hybrids tend to be lower in cost than all-electric machines. While their energy-saving performance is comparable to that of an all-electric machine, they may not be quite as energy efficient, nor will they be quite as quiet. However, long-term maintenance costs for hybrids are lower and they represent a more "comfortable" technology, in terms of spare parts, training, and maintenance.

MOLD TEST #1
Cover mold on 300-ton toggle with 30-sec cycle

                             Hydraulic      Hybrid      All-Electric

Actual Energy
Use, kwh/hr                         24          14             12

Production Hr                     7500        7500           7500

Total Energy

Use, kwh                       180,000     105,000         90,000

Annual Energy
Cost (@4[cents]/kwh)             $7200       $4200          $3600

Annual Savings                       -       $3000          $3600
                                             (42%)          (50%)

Why AC servo is better

Variable-speed pump drive, also known as variable-frequency drive, replaces the traditional fixed-speed AC pump motor with a newer AC servo drive for the pump. This feature ramps the speed of the drive motor up or down, depending on the demand required by the molding cycle. During long cooling times, when screw recovery has been completed and the machine is sitting in a locked-up waiting state, the motor rpm can be ramped down to a low energy-consumption level, since energy demand is at a minimum.

Since the machine runs at lower rpm, it runs cooler, reducing the amount of cooling water required to flow through the heat exchanger - a collateral source of energy savings. Also, the rpm reduction brings the sound level down dramatically when compared with conventional motors running at full rpm. Our noise tests on a 300-ton HTE model registered noise reduction from 3.5 to 10 dbA, bringing it to the mid to low 70s.

Since the motor speed can be ramped up and down with a servo motor, the fixed-displacement hydraulic pumps in a hybrid machine can be utilized as effectively as variable-volume pumps to deliver only the oil volume required. Fixed-displacement pumps are quieter, less costly, and less maintenance intensive than variable-volume pumps.

Full-hydraulic systems, even with variable-volume pumps, use as much as 25% of their full-load power even when idling. But electric drives produce power on demand only, expending virtually no power during the idle phase of a cycle. The effect of servo drives, which use only as much power as is needed to meet the instantaneous load, is shown in Figs. 4 & 5. The sudden increase in power consumption by the standard machine occurs where the second fixed-speed pump kicks in. The standard machine loses efficiency at this point because most of the flow of the second pump is not contributing useful work in this load range.

MOLD TEST #2
Brush-handle mold on 300-ton toggle with 1-min cycle

                             Hydraulic      Hybrid      All-Electric

Actual Energy
Use, kwh/hr                        19          12             8.5

Production Hr                    7500        7500            7500

Total Energy
Use, kwh                      142,500      90,000          63,750

Annual Energy
Cost (@4[cents]/kwh)            $5700       $3600           $2550

Annual Savings                      -       $2100           $3150
                                            (37%)           (55%)

Fundamentally, electric servo motors convert electric power to mechanical force more efficiently than do hydraulic motors. A typical electric drive system converts 84% of input electrical energy to mechanical power at full speed, compared with 74% for a hydraulic motor/pump system. This is a 10% advantage in energy efficiency at full torque and full speed.

In addition, electric servo motors use incoming AC line power at a relatively high power factor. Electric servos achieve an input power factor of 90% compared with 80-85% for AC pump drive motors on hydraulic machines. Further, the power factor of the electric machine remains high at reduced load, while the traditional AC pump motor's power factor declines as load is reduced.

There is at least one more opportunity for savings: In many geographic areas, electric utilities impose peak-load penalties in the billing process. The penalty can be as much as 50% of the total power bill. These penalties can be reduced because electric servo-driven machines do not require the high starting power of constant-speed AC motors.

Documenting the savings

How much energy can be saved with variable-speed drives depends on what kind of a cycle you are running. For example, a 300-ton machine running a 9-sec cycle in a packaging application is putting virtually all its energy to work. Very little energy is wasted on idle time and the opportunity for savings is slight. But with a 35-sec cycle on a 200-ton machine, more energy is being wasted and the potential for savings is greater. The variable-speed pump-drive option is a good choice when you are mainly running parts that require long cooling times.

AVERAGE INSTANTANEOUS POWER, kw
300-Ton Toggles

                             Hydraulic      Hybrid      All-Electric

Clamp Close                      30           23             19
Lock-Up                          22           17             14
Injection                        86           25             14
Hold                             13           11              9
Rotate                           23           19             17
Idle                              8            5              4
Clamp Open                       35           23             20
Ejection                         27           11             10

Results of actual tests with different molds comparing 300-ton models of a standard hydraulically powered toggle machine, a hybrid toggle, and an all-electric toggle are shown in the accompanying tables. Average overall energy usage in three mold trials was 20.8 kwh/hr for the standard Van Dorn Demag HT Series and 8.8 kwh/hr for the hybrid HTE Series - almost a 58% reduction.

A hybrid machine can be built with either an electric screw drive, a variable-speed drive on the hydraulic pumps, or both. A variable-speed pump drive can add $10,000-38,000 to the price of a machine in the 120-900 ton range. An electric screw drive can add $24,000-61,000 for machines of this size. For some molders, an electric screw drive alone may be the most cost-effective solution.

To determine the payback available to you with electric drives, use the formula below.

To Calculate Payback in Years

1. Subtract the price of a conventional machine from the price of a hybrid machine.

2. Subtract the estimated power consumption (kwh) per cycle for a hybrid machine from the estimated kwh/cycle for a conventional machine.

3. Multiply the result of step 2 times the estimated cycles per year.

4. Multiply the result of step 3 times the cost of electricity ($/kwh).

5. Divide the result of step 1 by the result of step 4.