by Mark Elsass, Cincinnati Milacron
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 hybrid machines.
Hybrid machines are models that utilize the most effective methods to achieve the needed results. This leads to machines with combinations of hydraulic- and electric-driven axes – hydraulic clamps with precise electric injection units and electric clamps with hydraulic accumulator-powered injection units.
The energy that a machine consumes is a function of the drive mechanism choice and the maximum capability to process plastic. A machine designed with the ability to recover the shot while the clamp opens and closes increases the plastic output of the machine (lbs/hr). This machine can process greater quantities of plastic because the available recharge time increases from just the cure time to the cure time and the clamp traversing time. Greater lbs/hr translates into more actual kW hours but a lower kWh/lb of plastic processed number. The machine becomes more efficient. In general, it requires more horsepower to have this parallel recharge ability. If this function is not utilized, the additional hp is wasted as the extra dedicated pumps idle. To prevent this waste from taking place, an electric screw drive can be added for the parallel capability. The electric drive uses power only when it runs. The typical savings when using an electric screw drive versus a hydraulic-powered parallel circuit can be seen in Figure 1.
Pump type and control of that pump also contribute heavily to how efficient a molding machine is in processing the plastic. Figure 2 shows a graph comparing three 300 ton presses with different hydraulic circuits versus an all-electric machine. The variable volume pump circuit was an improvement over fixed pumps because it pumped flow only when needed. The P/Q pumps were an improvement over the variable volume because they could control pressure and flow at the pump and eliminate the energy-consuming control/servo valve. Finally, todays servo motor/fixed pump improves the P/Q system because it pumps flow when it is needed, removes the control valve, and stops completely during idle times.
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 differences.
There are two main elements in the molding process that dominate the energy usage given one particular machine design. For example, when comparing two all-electric machines the material and the throughput (lbs/hr) will dominate the results. Because each material requires different levels of energy input to melt the plastic, different materials will show vastly different results on the same machine. Compare the test results obtained from an all-electric 550 ton machine processing 407 lbs/hr of flexible PVC and a 750 all-electric machine consumed processing 275 lbs of HDPE. The 550 machine consumed 21.4 kWh/h while the 750 consumed 43.5 kWh/h. This significant variance in energy consumption can be explained when the materials specific enthalpy are compared. To raise flexible PVC to its process melt temperature requires approximately .048 hp.hr/lb. To raise HDPE to its process melt temperature requires approximately .13 hp.hr/lb. This specific enthalpy ratio compares closely to the measured results (Figure 4).
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.
In overview, all-electric machines are the most efficient machines on the market today. The servo motor fixed pump design may be the most cost effective design and P/Q pumps typically provide the best full hydraulic energy performance. If two machines are consuming significantly different amounts of energy for the same performance, there has to be a reasonable explanation. Compare the basic design and hydraulics. For example, take two machines doing the same work with the same hydraulic circuit design: one machine equipped with 100 hp and the second with 50 hp. The 50 hp machine will draw less energy because it has to be closer to its maximum capability than the 100 hp, so it will be more efficient.