By Jan H. Schut
Injection molding machine makers at the upcoming Kunststoff Show in Dusseldorf, Germany, in October are touting energy efficient presses that will save 30%, 50%, even 70% of energy use. But is anything really new? Energy efficient all-electric and hybrid injection molding machines have been available for nearly 30 years, and energy saving devices like servo pumps and barrel insulation have been retrofitted onto injection molding machines for almost as long.
A cynical explanation for the K Show fuss is that after years of slower business conditions, processors are newly aware of the importance of energy saving. There is also a perceived opportunity for all-electric presses in Europe, where all-electric still accounts for only 15%-20% of injection molding machine sales (see chart), while hydraulic and hybrid account for 80%-85%, based on a 2011 study for Euromap, Frankfurt-Main, Germany (www.euromap.org). In Japan roughly 80% of injection molding machine sales are all-electric vs. 20% hydraulic or hybrid, and in the U.S. sales are roughly 50/50, industry sources say.
How do original equipment manufacturers (OEMs) calculate their energy savings percentages? Typically by comparing energy use for a new efficient machine function with old-fashioned hydraulics for the same function. An injection molding machine has seven main functions that use energy – plastification, barrel heating, platen movement, injection, clamping pressure, ejection, and barrel retraction. All seven can be made more energy efficient than conventional old-fashioned hydraulics, where a standard induction electric motor runs at constant RPM powering 1-2 hydraulic pumps, which runs even when idling.
Energy efficiency, however, isn’t just a machine function. Efficiency, which is typically measured in kilowatt-hours per kilogram (kWh/kg), is different for each molded part and depends on part weight, cycle time, machine size, and machine efficiency. The SKZ Plastic Institute in Wurzburg, Germany (www.skz.de), which researches energy saving potentials for OEMs and performs energy audits, published a study in 2011 that gives a rough idea of how much energy each machine function uses (see pie chart).
The SKZ tested a 100-ton mostly electric press with hydraulic injection, molding an 83-gram ABS storage tray in 16.9 second cycles and found that plastification consumed by far the most energy at 48%. Next came barrel heating at 17%, followed by clamp force at 12%, and injection force at 11%. Linear movements to open and close molds and retract the barrel for cooling used only minor amounts of energy – 4% each. So plastification and barrel heating are the big opportunities to save energy.
Heat is the only energy user that doesn’t involve movement, so the fixes are different. Insulating the barrel saves “an average of about 25% of heat energy,” says Bernhard Lettner, product manager for large two-platten presses at Engel Austria GmbH in St. Valentin, Austria (www.engelglobal.com). Replacing hydraulic heat with radiant I.R. heating adds cost, but saves even more. “Based on a complete retrofit of a small machine, we expect I.R. ceramics to save 45-50% of heat energy—up to twice as much as insulating blankets,” says Dean Francis, president of Windsor Kunststofftechnologie GmbH in Hanau, Germany (www.windsor-gmbh.com), which retrofits hydraulic presses for energy saving and sells all-electric presses from JSW Plastics Machinery Inc. in Japan (www.jswpmi.com). Cast in insulating ceramic, I.R. elements from Rex Materials Inc., Fowlerville, MI (www.rexmaterials.com), can convert all zones or only the feed zone.
OPTIMIZING ELECTRIC MOTOR EFFICIENCY
For both OEMs and retrofits the basic energy saving devices on injection molding machines are variable frequency drives, also known as inverters, and servo drives. Standard AC motors can be retrofitted with variable frequency drives to control motor speed, so the motor turns at exactly the RPM needed to pump the correct oil pressure for the process.
Direct electric power with a variable frequency drive and AC motor will be 50% more energy efficient than old-fashioned hydraulics and 20%-30% more energy efficient than modern servo-hydraulics for any machine function because hydraulics lose 20%-30% in two conversions. First, electric power is converted to hydraulic power; then hydraulic power is converted to mechanical power. Since plastification uses the most energy, converting plastification to electric power saves the most. It also reduces the size of remaining hydraulic pumps needed and the amount of hydraulic oil in a hybrid machine.
Servo motors controlled by servo drives do the same thing as an inverter/AC motor, but are more precise. The kW size of the variable frequency drive and servo drive has to match motor size. Large servo drives and servo motors over 50 kW to 100 kW are expensive because of the high price of copper and rare earth magnets. So a servo drive/servo motor is likely to replace positional motors for energy saving on functions like clamp opening and closing or part removal.
A direct-drive synchronous torque motor instead of an inverter/AC motor with gearbox saves energy lost in the gearbox (1.5%-2% of energy for that function). Oswald Elektromotoren GmbH in Miltenberg, Germany (www.oswald.de) offers torque motors for plastification and clamping up to 70,000 Nm. Etel S.A. in Motiers, Switzerland (www.etel.ch) makes standard torque motors up to 31,000 Nm and larger custom motors.
Electric motors lose about 8% of energy converting rotational power to linear power, typically with a conventional ball screw. Knoedler-Getriebe GmbH in Ostfildern, Germany (www.knoedler-getriebe.de), offers an alternative ZS rack and pinion electric motor, controlled by a variable frequency drive, that also converts rotation to linear movement. Knoedler’s rack and pinion motor can replace a hydraulic cylinder for plastification and injection and is offered for up to 450 ton presses.
More savings are possible with “stop/go” movements like clamp open and close by regenerating electricity from braking. A regenerative inverter AC unit returns that energy to the grid or applies it to other movement. Regeneration can recover 10%-15% of energy for clamp movement with higher recovery for bigger molds, notes Reinhard Schiffers, head of machine technology and tests at KraussMaffei Technologies GmbH in Munich, Germany (www.krauss-maffei.com). Regenerative inverter drives are commercially available from Siemens AG in Erlangen, Germany (www.siemens.com/drives) and Bernecker + Rainer Industrie-Elektronik GmbH (B&R) in Egglesberg, Austria (www.br-automation.com) and are applicable both to hydraulic and all-electric machines and can be retrofitted. Some regenerative devices are also proprietary to OEMs, like a new mechanical fly-wheel system, which Engel will bring to K.
All-electric presses also have size limits because of the cost of large motors. The largest all-electric press is 3300 clamp tons vs. around 7000 tons for the largest hydraulic presses. Ube Machinery Corp. in Yamaguchi, Japan (www.ubemachinery.co.jp), and JSW have both built 3300-ton all-electric presses for automotive applications. To reach that tonnage electrically, both are believed to use four large motors in sync. Most all-electric machines are less than 350 tons. Over that size, hybrids with servo hydraulics become more cost effective.
OPTIMIZING HYDRAULIC PUMP EFFICIENCY
Another reason hydraulic injection molding is still popular in Europe is that hydraulics have roughly five times higher energy density than electric motors (kW/cu meter). Many existing molds also use hydraulics for core pulls, so on hybrid machines European OEMs typically use hydraulics for injection and for clamp force done with energy efficient servo-hydraulic pumps. The main saving from putting variable speed drives on hydraulics comes from shutting pumps down during cooling time, which can be long for large parts.
Bosch Rexroth AG in Lohr am Main, Germany (www.boschrexroth.com), combines a variable frequency drive with synchronous torque motor for hydraulics control. Siemens offers Simatics servo hydraulic pumps, launched at the K Show in 2010, with a frequency converter and servo motor for retrofits, reportedly saving up to 70% energy vs. conventional hydraulics.
More efficient drives on hydraulic pumps also allow less expensive gear-type hydraulic pumps to be used instead of variable displacement pumps, though not all OEMs agree. Negri Bossi SpA in Milan, Italy (www.negribossi.com) combines a variable frequency drive with a variable volume hydraulic pump on its Canbio VSE press, introduced in 2011, claiming oil output is more consistent. Engel also combines a servo drive/servo motor with a variable displacement pump on two-platen machines. Software optimizes servo motor setting and pivot angle on the variable displacement pump for greater efficiency, Engel says.
Better controlled hydraulics are quieter and cooler. Siemens in a 2012 publication describes retrofitting a hydraulic machine with servo motors, after which hydraulic oil ran 10 degrees C cooler (35 °C down from 45 °C), and oil cooling was “practically no longer required.” Reducing friction in oil lines, manifolds and fittings also saves energy. KraussMaffei’s GX series at K sports a new hydraulic manifold with two valves instead of one and bigger oil lines, which uses 30% less energy with lower oil temperature. And cooler oil lasts longer.
The area with genuinely new developments at K is software and control packages to monitor energy use. B&R, which supplies energy saving servo pumps for injection molding, is introducing APROL EnMon (energy monitoring) pre-installed in its PC to optimize energy use throughout the molding process. ABB Group in Zurich, Switzerland (www.abb.com) this year introduced energy monitoring capabilities in its new ACS 880 drive motors.
OEMs will show new energy optimizing software at K, too. KraussMaffei will introduce MC6 (motion control) software, which automatically optimizes screw speed and other settings for energy efficiency, then asks the operator to check the part and approve the adjustment with an Eco Button feature. Engel will introduce new “ecobalance” software to reduce peak energy consumption automatically. Instead of heating and injecting at the same time with high peak energy use, the software injects first, and then heats gradually.
Will processors get the same energy savings as OEM tests? The short answer is actual energy savings still depend on part size and cycle time, not just energy efficiency. An all-electric machine will be inefficient if it runs low throughput of small parts. Choices between all-electric and servo-hydraulics won’t be resolved soon. “All systems have usefulness,” notes Werner Knoedler, managing director of Knoedler. “The future will need more than one system.”