|Evaluating Energy Consumption of Molding Machines: What Have We Learned in 40 Years?|
|by Mark Elsass, Cincinnati Milacron|
|Energy Savings Summer 2010|
Injection molding machines have evolved over the last 40 years from energy hogs,
applying brute force to get the job done, to efficient energy machines elegantly
synchronizing multiple electric servo motors to accomplish the same tasks.
In the 1960s, machines were equipped with individual pumps to accomplish individual tasks. This provided great flexibility and capability but also wasted energy when these pumps were not needed. During the energy crisis of the late 70s and early 80s, machine manufacturers shifted from hydraulic circuits that had pumps dedicated to each machine axis to sharing pumps between axes. This better utilization produced a more efficient but less capable machine. A molder could no longer recharge the shot while the clamp was opening and closing.
In the mid 1980s, all-electric molding machines were introduced. A servo motor powered each of the main axes. This provided not only tremendous capability and flexibility but also, greater efficiency by only consuming power when needed. Capability was demonstrated in the speed and force control that the electric motor provided to an axis. At the same time, hydraulic machines also were evolving in terms of control via servo and proportional control valves. The advent of “PQ” pumps that controlled pressure and flow at the pump provided improved efficiency by eliminating the pressure drop of the servo or proportional control valve. Additionally, machine manufacturers began to add electric screw drives to re-gain some of the capability sacrificed when the “shared” pump system was adopted. Electric screw drives efficiently plasticize the plastic materials and when compared to a machine with “independent” pumps, provide approximately a 25 to 30 percent savings in energy costs.
Electric molding machines dominate the smaller machine market up to 400 tons. As
the machines get larger the load-carrying mechanics get substantially more
expensive, making the larger electrics more of a premium-priced machine.
Hydraulics are very effective at transferring larger forces and amounts of
energy. This is why most 1000 ton and larger machines still build tonnage with a
hydraulic ram and very high volumetric injection rates (cc/sec) are still
accomplished with hydraulic accumulators. This has led manufacturers to explore
Comparing energy balance graphs between an older style hydraulic machine and an all-electric machine reveals the destination of wasted excess energy – down the drain and into thin air. The old machines pumped 25 percent of the consumed energy down the drain via the hydraulic cooling heat exchanger and 50 percent into the air as heat was radiated off of the hot tanks and pipes into the atmosphere (Figure 3).
The evolution in how power is transferred on an injection molding machine has
been driven because of the desire to reduce the energy costs of molding a
plastic part. Measuring and comparing the energy consumption of injection
molding machines has become a favorite pastime of machinery builders and
molders. When doing this it is very important to make apples to apples
comparisons, if possible, and/or understand the factors contributing to the
The above indicates how the material plays a role and it also shows that the
greater the lbs/hr of material processed the larger the kwh/h number will be.
But at higher lbs/hr the machine gets more efficient. The energy overhead just
to get the machine “idling” without processing one lb is spread out over a
larger number and becomes less of a factor. In addition, kwh/lb decreases as the
lbs/hr goes up, which is very typical of molding machines. As the maximum
capability is approached, the dramatic decrease starts to flatten out.
Processing the same lbs/hr on a faster cycle will shift the curve up. This is
because of the additional clamp and injection functions per hour with the faster
cycle. A 12-second cycle has three times as many clamp and injection cycles per
hour for the same lbs/hr as the 36-second cycle.