Now that all-electric and hybrid-electric machines are really beginning to catch on, machine builders are exploring a variety of approaches to get the most out of servo-drive technology. That means molders will have to work harder to keep up with the new technologies and growing variety of machine designs. This article explores some of the latest innovations and previews others on the horizon.
All-electric or hybrid machines are now available from almost all Japanese makers and a growing range of U.S., European, and Taiwanese suppliers. Newer developments include sizes up to 1550 tons, vertical clamps, two-stage and three-stage injection, two-color and injection-compression capabilities, and special models for compact discs, LIM, and micro-molding. The proliferation of electric units has lowered the price premium for these machines to as little as 15-20%.
All of these new machines are said to save energy and improve accuracy, repeatability, cleanliness, and quietness. But all servo-driven machines today are no more alike than conventional hydraulic models. Some new designs are faster, more precise, or more durable than other versions. For example, some machines sport "direct" servo drives instead of belt pulleys. Suppliers are also debating the potential benefits and limitations of linear servo motors in place of standard rotary types. More new developments on the horizon may improve or replace current DC brushless, AC vector-drive, or AC servo motors.
What's more, electric drive on the clamp end is no longer limited to toggles. Electric motors are being combined with low-oil hydromechanical designs to spawn new types of hybrid clamps. Suppliers say these can provide the best of both worlds: the precise and efficient operation of an electric motor with the proven pressure generation of hydraulics. There's even one new system on the market that reportedly provides the "direct" clamping action of hydraulics using all-electric drive.
Since the first all-electric machines were commercialized by Nissei and Fanuc of Japan around 1983-84, the dominant electric drive mechanism has involved transfer of power from a servo motor to a ball screw via a belt. Since then, alternatives such as roller screws and rack-and-pinion mechanisms have been introduced by Sumitomo and Ferromatik in Germany (now Ferromatik-Milacron).
Ferromatik-Milacron moved away from ball screws when its Powerline series (220-935 tons) succeeded the Elektra line in 1998. Belt-driven roller screws are used for injection and ejection. "This is the most space-efficient device, and it transmits more force per diameter than a ball screw," says Barr Klaus, technical director of the Elektron Technologies Div. of Ferromatik-Milacron. The roller screw uses a special threaded nut that meshes with the screw. Either the nut can be stationary and the screw travels through it, or vice versa.
Powerline models also use a rack and pinion instead of a ball screw to drive the clamp. This mechanism improves acceleration/deceleration response by reducing inertia, say Klaus. As a result, clamp dry-cycle time on a 330-ton Powerline machine is 2.4 sec, vs. 2.9 sec for a similar-sized Elektra model that used ball screws and the same-size motor. In addition, Klaus notes, "The rack is very quiet and is self-lubricating, which is a benefit to the user."
In the last couple of years, new forms of beltless "direct-drive" electric designs have emerged. In these designs, the ball or roller screw is an extension of the AC motor spindle. Proponents of beltless drives claim faster response to the lightning-quick acceleration and deceleration rates of newer electric machines. They also speculate that such stresses could cause wear or stretching of belts, resulting in potential loss of precision and generation of dust particles that could contaminate clean-room operations. On the other hand, suppliers such as Ferromatik-Milacron reply that many years' experience with belt drives has not produced evidence of any such problems.
The first of the new beltless direct drives appeared on Sumitomo's SE-S series of electric machines. Models up to 198 tons use ball screws for clamping, injection, and ejection. They translate rotary into linear motion through a rotating nut. (SE-S models larger than 198 tons still use belt-and-pulley drives.) According Jerry Boggs, executive v.p. of Sumitomo Plastics-Machinery in the U.S., the rotating nut results in less inertia, so quick starts and stops are easier and more precise. Injection speeds are up to 300 mm/sec and response times for acceleration/deceleration are 30 millisec.
Although Sumitomo manufactures its own proprietary beltless drive, other machine builders have access to beltless electric drives offered off-the-shelf by Nisso Electric-Machinery Co. Inc. of Japan. The first user is Nissei, which introduced direct drive on its new ED200/TM (22 tons) and ED3000 (170 tons) all-electric models at the International Plastics Fair (IPF '99) in Tokyo last fall. These machines have a novel electric clamp not based on a toggle. Beltless "center drive" is also offered on Nissei toggle-clamp electric presses such as the new high-speed Elject ES200 (22 tons) and ES6000 (308 tons). It is also used on the new N40DE (44 tons) for DVD molding and DCE60 (66 tons) for multi-material molding. According to Atsushi Koide, director and general manager of R&D at Nissei in Japan, the center drive is simpler, quieter, and cleaner than belt drives, generating no dust particles. He also says the direct drive has half the size and weight of a belt drive and three times faster response.
Another brand-new direct-drive mechanism comes from Niigata. At IPF '99, the company introduced the super-high-speed MD 30SR-III (33 tons). Two ball screws drive the injection ram through a nut filled with precision ball bearings, says Mark Zulas, U.S. technical manager. This unit reportedly achieves injection speeds up to 600 mm/sec and response time of 0.04 sec from zero to maximum motor speed. Such acceleration would stretch a drive belt, according to Niigata spokesmen in Japan.
These sources note that 600 mm/sec represents a doubling of the injection speed formerly obtainable with electric drive. Previously, standard electric machines typically offered 150-200 mm/sec, and "high-speed" versions went up to 300 mm/sec.
Niigata is not alone in reaching this new speed benchmark for all-electric machines. Toyo recently introduced its ST Series in 55- and 110-ton models. Top injection speed of the standard version is 300 mm/sec; the "high-speed" model offers 600 mm/sec; and a "high-pressure" version (42,630 psi) can reach 400 mm/sec. Incidentally, the ST series also offers a new toggle mechanism, more rigid die plates, and a new frame design that cuts vibration by 90%. At IPF '99, Toyo demonstrated the ST-100's capabilities by molding PP speaker cones only 0.25 mm thick.
Nissei and at least one other electric-machine builder are investigating another kind of electric motor--the linear servo. Linear servos utilize a stationary coil and a movable magnet to convert electromagnetic forces directly into linear motion. The result is to eliminate the large motors and gear reducers of current servo drives, cutting size and weight. There is also no need for roller or ball screws or rack-and-pinion mechanisms. Linear servos are said to be extremely efficient but also more expensive than standard servos.
Linear servos have been used by a few robot makers in the U.S., Japan, and Europe. They also provide oil-free core pulls on new Electrocore molds from Ufe Inc. Nissei's Koide says linear servos offer superior positioning accuracy but are currently suited only to small loads (Nissei is testing a 2-ton machine) and may also be more sensitive to dust contamination.
During the past year, Sodick in Japan has sold over 100 CNC-EDM systems built with linear servos in place of ball screws. Sources there say linear servos are much faster than other types. Spokesmen for Star Seiki, a Japanese robot maker that is experimenting with linear servos, say they are equivalent in speed and accuracy to other servos. On the contrary, a competing Japanese robot maker says linear servos are slower.
Until recently, "hybrid" machines meant putting an electric screw drive on an otherwise hydraulic-powered machine. This option is available from a wide range of machine suppliers. Lately, the term hybrid has taken on a broader range of interpretations.
At the K'98 show in Germany, Demag Ergotech introduced Elexis hybrid machines that use a new "hydrostatic drive" to generate high injection pressures using just a smidgen of oil. Available on models from 60 to 350 tons, the hydrostatic drive uses multiple hydraulic piston pumps that are driven by variable-speed AC servo motors instead of the usual fixed-speed DC motor. This "distributed" system uses separate motors and pumps to drive different machine functions. Each motor and pump work only as hard as is required to meet the instantaneous demand of the process. The motor and pump stop during idle portions of the cycle, rather than wastefully pumping oil over a relief valve back to the tank. This hybrid system saves energy and just sips oil, says Rick Shaffer, v.p. and general manager of the Demag Ergotech Div. of Van Dorn Demag Corp. "A 200-ton conventional hydraulic machine may need 100 gallons of oil, but our unit needs only 1.5 liters," he says. He also notes that the response time of the injection acceleration and deceleration profile is 0.5 millisec, and positioning is accurate to 0.0004 in.
Toshiba has taken a somewhat similar approach with the 500-ton IS450GSW II hybrid machine it showed at IPF '99. This hydraulic-clamp press has an AC servo screw motor and a variable-frequency synchronous electric motor to drive the hydraulic pump. This wide-platen machine has new Actrol III controls tailored for automotive low-pressure molding. U.S. sales manager Fred Ferris says 610- and 720-ton versions are on the way. "The larger-tonnage hybrids are currently more cost-effective than fully electric," he adds.
Nissei introduced the new model FN 1000 HD hybrid (90 tons) at IPF '99. It has an electric injection mechanism and an AC servo motor to drive a dedicated hydraulic pump for the clamp. It reportedly uses only 30% as much oil and 50% as much energy as a standard machine.
Hybrid clamp mechanisms are another recent innovation that combine energy savings with the continuously controllable pressure generation of hydraulics. At least three suppliers have introduced "direct clamping" alternatives to toggles. Sodick of Japan offers new Tuparl EH machines from 22 to 242 tons. They have a hydromechanical clamp in which a servo-driven ball screw opens and closes the clamp and small hydraulic cylinders apply clamping pressure. After the mold closes, a mechanical locking mechanism engages the ram driven by the ball screw. Electric motors provide faster acceleration and deceleration, as well as greater position accuracy, say Sodick sources. Direct hydraulic clamping reportedly provides finer pressure control, more uniform pressure distribution, delicate mold protection, and greater die-height range. These two-stage machines also have AC motors on the screw and ejector.
A similar hydromechanical hybrid clamp is used on the new Nadem 800-DM (88-ton) and 1200-DM (132-ton) machines from Meiki. They have six-stage clamp-pressure profiling, said to be useful for injection-compression molding of optical lenses. Plasticating and injection are all-electric.
A third version of this sort of hybrid clamp appears on the new Rheomaster Eon Series (55 to 165 tons) from Shinwa Seiki. It has electric plasticating and injection as well.
While several machine builders are using hybrid approaches to marry energy-saving electric motors with the direct-clamping characteristics of a fully hydraulic machine, Nissei has developed what appears to be the first all-electric "direct-pressure" (i.e., non-toggle) clamp design. Few details have been revealed about this proprietary mechanism other than that it employs two ball screws and uses the electric motor to maintain clamp force after the mold closes. Nissei's Koide says direct clamping yields better pressure control, comparable to a hydraulic-clamp machine. He claims the direct-pressure mechanism is better suited to ultra-precision molding than a toggle. Electric direct clamping was first used on a 7-ton machine in 1998. It is also featured on the new ED200/TM and ED3000 machines discussed above. For now, Nissei does not intend to offer direct clamping on larger machines.
Mitsubishi, which recently came out with its first medium-size all-electric line, is also looking for a direct-clamping alternative to toggles, according to company sources in Japan.
Although Toyo's new all-electric ST machines have toggles, company spokesmen in Japan say clamp tonnage is controlled automatically, comparable to a fully hydraulic machine. This reportedly permits such tricky maneuvers as injection-compression with a toggle clamp. It is accomplished with an extra mold-mounting plate on the moving platen. Load sensors between the two plates provide closed-loop control of tonnage.
Electric machine technology presents opportunities to revamp traditional approaches to process control. This is clearly demonstrated by the new Roboshot i series from Milacron-Fanuc, which will make its U.S. debut at NPE 2000. Its controls have three optional artificial-intelligence (AI) functions that take advantage of the machines' ability to sensitively monitor torque loads on the servo motor. The controls "learn" the load profile of a good shot in order to sense any deviation from that profile and respond instantaneously.
The first of these AI functions is cavity-pressure profile control, which has been available on earlier models. By detecting any slight deviation from the stored pressure trace, and by "looking ahead" to anticipate pressure conditions farther along the curve, the controller can make same-cycle adjustments in velocity to maintain the needed pressure. For example, it can bump up injection speed to compensate for a leaky check valve, worn barrel, or inconsistent shot recovery, according to Robert Kadykowski, v.p. and general manager of Milacron's Specialty Equipment Business.
One brand-new feature is AI metering control, which senses screw-motor torque and adjusts the rpm to maintain consistent recovery rate despite changes in resin feeding.
The other new feature is super-sensitive AI mold protection, which memorizes the load curve of the clamp-closing motion and halts movement the instant there is any deviation.
Numerous electric machines have appeared in these pages in recent months. Here are some more new entries.
Electric debut: At NPE 2000, Mitsubishi will introduce the 390-ton, 40-oz all-electric model 390 ME-40. Mitsubishi also builds 500- and 610-tonners and plans additional models of 715, 935, and 1100 tons.
Two years ago, Mitsubishi came out with a series of high-precision, small electric models with an unusual injection mechanism. These Mitsubishi-Hotsuma presses of 16.5, 33, and 55 tons have an in-line two-stage arrangement called a "plunger in cone." The "screw" is a short cone whose wide back end is said to improve pellet feeding compared with a normal small-diameter screw. The cone feeds directly into an in-line shooting pot, which is emptied by a coaxial plunger inside the cone.
Hybrids include the DASH-60-52LR, a 52-ton, 60-cc LIM machine with hydraulic clamp and servo injection. It also comes in a 33-ton model. Another hybrid has horizontal injection. This HV-100-75P (82 tons, 100 cc) is the company's first two-stage machine. Only the screw is electric driven. Injection speed is up to 2000 mm/sec, 1400 cc/sec.
Another new hybrid for LIM is the RV4-1 from Nissei. This 1.1-ton machine has electric injection and four pneumatic clamps on an electric-driven turntable.
Two colors: Nissei came out with the DCE60, a 66-ton, two-color, all-electric model.
In January, automotive custom molder Kamco Industries, Inc., West Unity, Ohio, took delivery of the largest all-electric machine yet built. This 1100-tonner was manufactured by Ube Industries in cooperation with Niigata, which supplied the servo-drive technology. At NPE 2000, the two firms plan to push the size envelope further with the introduction of a 1550-ton Ube Ultima UN model.
Ube and Niigata also collaborated on the second-largest electric machine ever built, a 950-tonner installed last year at Stanley Electric U.S. Co., Inc., London, Ohio, for molding auto headlamp lenses. The Ube/Niigata partnership has sold more than 10 electric machines of 720 and 950 tons.
Sizes of all-electric machines have been limited by the cost and availability of large servo motors. Suppliers say 55 kw (74 hp) is the largest currently available, but they expect to see 75- and 100-kw (100- and 134-hp) servo motors come out soon. According to Japanese reports, Fanuc Ltd. already has a 100-kw size.