In order to get a better understanding of today’s available machine
technologies, it is important to use a common language for the different
drive technologies for modern injection molding machines. Varied uses of
servo motors is one of the complicating factors. This article will discuss and
explain 15 of the most common terms and concepts involving machine drive
technology, regarding cost, performance, maintenance, and employee
training.
1. Full-electric IMM
Following the standards of the Plastics Industry Association (formerly SPI),
a full-electric injection molding machine is one that has at least its three
main axes (clamp movement, injection, and metering/plasticating) driven by
servo motors. The remaining three axes (nozzle touch, ejection, and moldheight adjustment) can still be driven by hydraulics. The most efficient
machines are capable of
using a kinetic-energy recovery system (KERS). This patented process makes
use of the servo motors as generators and can collect all deceleration energy
from each axis and convert it back into electric energy. The converted energy
is then fed back into the machine, usually for heating or control functions.
2. Hybrid IMM
Machines that have one or more of the three main axes driven by hydraulics
but do have at least one electric-driven axis are called hybrid machines. One
common example of this is a hydraulic machine with servo-driven metering.
The metering is the biggest energy draw (besides barrel heating) on any IMM.
Therefore, it is a smart decision to at least use a servo-electric drive for the
metering. Hybrid machines are not machines with a servo-driven pump,
because all movements on the latter are still driven by hydraulics.
3. Servo (driven) hydraulic IMM
A servo-driven hydraulic (or servo-hydraulic) machine is a full- hydraulic
IMM with a servo motor to drive the hydraulic pump. These machines often
have a fixed-displacement-pump, but could also have variable-displacementpumps (often DFEE or DFEC) with a swash-plate.
4. Standard hydraulic IMM with variable-displacement pump
These machines usually have a constant-speed motor and a variabledisplacement pump with a swash plate that can be adjusted by the control to
regulate the oil-volume flow of the pump adjusting to the current demand of
the hydraulic system. If the machine does not move at all, the pumps will
still idle and waste energy.
5. Energy efficiency
The European plastics industry has developed a system to classify energy
efficiency of IMMs. The Euromap 60.1 standard categorizes machines in
certain categories and has developed certain standards to compare the
energy efficiency of similar machines from different suppliers or executions
(full-electric, hybrid, and so on).
Servo-driven pump (or servo-pump) machines unsurprisingly show much
better energy efficiency than machines with a constant-speed motor and a
variable-displacement pump. So what is a servo pump and what does it do?
The servo motor on the servo-driven pump machine only operates if the
hydraulics demand oil flow. During the time when no axis is operating, there
is no pump movement, and therefore no energy consumption by the pump.
Of course, the amount of energy savings depends highly on the process point
of each specific application. A fast-cycling pack- aging application will have a
lower percentage energy savings than an application with a long cooling time
when the constant-speed motor would be idling for seconds or minutes.
Full-electric machines, where the electric energy is directly converted into
kinetic energy, have the best energy efficiency, since there is no
transformation from electric energy to hydraulic pressure and then to kinetic
energy.
A very rough classification would be:
• Hydraulic machine with constant speed motor: 100% energy consumption;
• Hybrid machine with only metering fully electric: 75% to 80% energy
consumption;
• Servo-driven pump machines: 65% to 70% energy consumption;
• Full-electric machines: 45% to 60% energy consumption.
6. Repeatability of electric vs. hydraulic drives
On full-electric machines, the repeatability of a servo-driven axis is at least
10 times higher than hydraulic-driven axis. An active servo-driven axis has
almost no delays, while delays with hydraulics are inevitable. Acceleration,
and specifically deceleration, are also more accurate with a servo drive than
on hydraulic-driven machines. Full-electric machines also allow for instant
production stability, as there is no necessary oil warm-up, and start-up is
faster after production has timed out. This is an advantage for full-electric
machines, not for servo-driven pump machines.
7. Noise level
Full-electric and servo-driven pump machines usually have a noise level that
is far lower than a standard hydraulic machine. Modern full-electric and
servo-driven pump machines should have a noise level around 68 dbA.
8. Speed and/or acceleration
Comparison of servo versus hydraulic speed and/or acceleration is matter of
lively debate in the industry. Full-electric drives have great acceleration—
much better than a pump ramp-up in a hydraulic system, which suffers due
to compressibility of the oil and the hydraulic hoses and lines that expand.
The counterpoint cited by some suppliers is that a perfectly designed
hydraulic system with accumulators and strategically placed valves can be
even more spontaneous and agile than the servo-driven machines.
9. Parallel movements
Full-electric IMMs generally use a separate servo drive for each axis. This
makes those machines very flexible since it allows all axes to be used with
parallel movements, which makes sense when it is necessary to meter during
mold opening and ejection, inject during mold closing, eject during mold
opening, and so on. Servo-driven pump machines and standard hydraulic
machines do not necessarily have that advantage. Most suppliers offer a
second or even a third pump and hydraulic accumulators as an option.
10. Hydraulic Design
A standard hydraulic machine creates a lot of wasted heat through shearing
and friction of the oil during idling, which is mainly related to the design and
work principle of a variable-displacement pump and the constant-speed
motor. The servo-driven pump machine does create a smaller amount of heat
in the hydraulic system. In fact, some manufacturers run a special oilheating sequence to keep the oil at operating temperature during production,
depending on the process point, the machine’s operating environment, and
the process sequence of the specific product/process.
11. Machine cost
A full-electric machine is always more expensive than a hydraulic machine
due to the necessary number of electric drives and motors and gears
(ballscrew or gearbox or rack and pinion) for the former. However, in many
cases it is easy to calculate an ROI using energy savings and improved
utilization (faster cycle due to parallel movements and greater uptime due to
less maintenance), as well as energy rebates that some states and energy
companies offer. Servo- driven pump machines can (with some suppliers)
still be slightly more expensive than standard hydraulic machines, but since
servo-driven pump technology is being pushed hard by the industry as a
whole, there is more and more competition even among component
suppliers, which allows the difference in price (where it still exists) to be
narrowed down to a minimum.
12. Service of pumps, servo motors
In cases where a hydraulic pump must be changed, fixed-displacement
pumps that need no calibration are faster and easier to change than variabledisplacement pumps. Fixed-displacement pumps also are usually much less
expensive than variable-displacement pumps. Even though a servo motor
may be more expensive than a constant-speed motor, it is more likely over
time that a molder will need to exchange or maintain the pump.
Of course, full-electric machines either never, or only rarely, need to deal
with pump maintenance at all. The well-known Rotex coupling (spider-gear)
does not exist on servo-driven pump machines and full-electric machines,
and therefore does not need to be exchanged. This is some- thing that the
maintenance crew in every molding shop knows all too well.
In general, modern machines offer much better analysis features than their
predecessors. Servo drives can usually be analyzed with ease through
embedded oscilloscope functions, which some suppliers offer as a standard
addition to their modern machine controls. This, together with the
possibility to access the machine through web service, creates a powerful tool
to service and troubleshoot machines remotely. Constant-speed pump
motors usually do not offer that possibility. Even alarm messages on the
control of a servo-driven machine are more precise since the monitoring of
servo drives is so much more detailed.
13. Oil quality & maintenance
On servo-driven pump machines, the oil does not get stressed very much and
the oil quality therefore degrades slower than on a standard hydraulic
machine. Of course, this always depends on the process and how much the
oil is moved and sheared; but as a general rule of thumb, the hydraulic
machine puts more strain on the oil. Full-electric machines might not have
to deal with oil at all.
14. Complexity of machines
Hydraulic machines in general, whether they are servo-driven or not, have
electronics, mechanical parts, and hydraulics. Full-electric machines might
only have electronics and mechanical parts, which reduces the complexity. A
good electrician can work on the machine without needing to understand
hydraulics.
15. Training of maintenance staff
One point that is easily forgotten is the training of the molder’s own
maintenance staff. For standard hydraulic machines, it is recommended that
the hydraulics be calibrated once a year. Better training of staff can lead to
better calibration and spotting of problems. No pump-related calibration is
needed on electric machines, so the necessary training of the staff is less
complex.
These 15 critical points of interest and comparison about partially and fully
servo-driven and hydraulic machines are important for every molder to
know. With these points in mind, suppliers can help work with molders to
find the best machine for their desired process.