WHY AUTOMATE?Since the Free-Trade debate, the movement of processing capacity to lower-wage countries such as Mexico has been touted as a leading route to lower overheads. Perhaps surprisingly, lower ...
Since the Free-Trade debate, the movement of processing capacity to lower-wage countries such as Mexico has been touted as a leading route to lower overheads. Perhaps surprisingly, lower wages have not blunted the drive toward process automation. David Preusse, general sales manager of Wittmann Robot & Automation Systems (in Canada, ShadowMax Plastics Automation Systems, Uxbridge, Ont.) explains: “Even with NAFTA in place, insufficient supplies of skilled labor have limited the movement of molding to Mexico. With plastic automation’s superior benefits of consistent cycle, reduced handling, increased machine utilization, decreased scrap and mold damage, cavity traceability, etc., molders globally have been forced to automate to remain competitive, including molders in Mexico.”
Preusse notes the move toward total system packages including safety guarding, conveyors, installation, and training.
If you’re a processor thinking of buying a robot, don’t. “Buy an integrated system to build a manufacturing cell”, says Husky Injection Molding Systems’ sales manager for robots, Joe Calomino. “If you compromise one piece of equipment on price, that’s as strong as your system will be. The weakest link will dictate the strength of the entire system. You’re buying a robot, but more importantly, you’re buying a component of a cell.”
Calomino also notes that retrofitting equipment to an existing machine’s envelope can often limit the robot’s speed. Husky manufactures robots for a wide range of the firm’s injection molding equipment. An example is the TE-3 (130), a servo model designed for use with 65 to 180- ton Husky S-Series machines. Both molding machine and robot are controlled from a single station.
For all their advantages, Canadian molders are often trapped in a “Catch-22” situation regarding robotics. With full-time robotics engineering personel too expensive for such a specific task, processors often fail to take advantage of their equipment’s full potential. Rock Lee, vice president of Markham, Ontario-based ASG Automation Services Group (189) (Rep: Plastics Machinery Inc., Newmarket, Ont.) explains: “We’ve been in business for twelve years, and I haven’t seen a mold shop that has the capability to use robots effectively.” Lee attributes much of the knowledge gap to the very nature of Canadian molding: with the easy jobs going offshore, Canadian molders have their hands full coping with ever more complex value-added processes. There just isn’t time to learn a new technology. To realize the best possible return on investment, however, molders must either make the time, or deal with a vendor with solid after-sales support.
CHOOSING THE RIGHT DRIVE
Drives are the muscle of plastics automation, and buyers have multiple choices.
The cam-action mechanical drives of early sprue-picker technology has been replaced in modern robotics by pneumatic, servo-electric, and hybrid systems,
with the choice dependent on cost, speed, and accuracy/repeatability. Pneumatics have traditionally represented the entry level, but the year over year reduction in the cost of servo drives and their controllers is beginning to crowd pneumatics market share. Why servo? Kurt Fenske, vice-president, sales for Engel Canada, is unequivocal about the advantages of servo drives: “Speed and accuracy. Those are the two main factors. It’s as simple as that. The drives have become less costly, and in the long run, that’s all you’re going to see.” Engel markets ERC Series servo robots (131) in multiple configurations, including a high-speed line. Additional automation can be easily integrated with the 32-Bit RISC-based systems. Engel’s entry level ERSP-21 unit (132) is servo-pneumatic, and is also microprocessor-controlled.
Which drive system is “best” depends on the needs of the application. Sprue picking is the most basic function, with simple air-driven systems often supplying the most cost effective solution. Hybrids combine pneumatics with AC frequency or servo systems for applications requiring greater precision. AC frequency and servo designs allow more sophisticated process management, and allow the processor to include second operation functions such as trimming and decorating into the cell. Some suppliers offer multiple drive options within a single machine line. An example is Conair
(in Canada, Stephen Sales Group, Markham, Ont.) which offers it’s Sepro 3010 Series beam robots (134) in all-pneumatic, pneumatic/AC frequency, and AC frequency servo combinations. Sepro 3010 robots are sized for presses from 50 to 150 tons.
Unfortunately, the term “servo” is often used inaccurately, leading to some confusion about drives. AC frequency drives use the sinusoidal wave nature of alternating current to contol motion accurately, while true servos use encoders to position by a coded number of “steps”. While it is generally acknowledged that servo systems offer greater accuracy, overall positioning accuracy and precision is dependent on cumulative errors throughout the system. Flexing of the axial beams or gantries, for example, can easily exceed drive positioning accuracies which can be on the order of two or three thousandths of an inch. The same is true of end-of-arm tooling, with both effects increasing with faster speeds and higher inertial masses. Wittmann (133) has addressed inertia with carbon fibre technologies, and most manufacturers make extensive use of aluminum alloys for the same reason. Vibration is another factor. The point for the molder is that drive accuracy is not necessarily the same as machine accuracy. The “correct” drive is the type built into a machine with sufficient precision to avoid the need for frequent re-alignment.
Choosing a system with sensible controls goes a long way toward the goal of ultimate positioning accuracy. Ken Hoffman, vice president and general manager of Mould-tek Inc., Scarborough, Ont., says it in plain language: “A controller should be plain and simple to understand, with easy to read functions. You want to enter (parameters) without going to college. You should also have a good storage memory for molds.” Mould-tek’s Kawaguchi robots (135) achieve this with large, pendant-mounted LCD displays with touch screens, and memory cards for up to 50 molds.
Optimal motion control also aids repeatability, with curved, gentle action yielding the best results. Rectilinear motion, although the easiest to program, requires the sudden reversal of sometimes considerable inertial masses of tooling and part, contributing to vibration and loss of alignment. In most applications requiring high speeds, curvilinear motion is also more efficient, especially when the arm must reach around obstacles such as tie bars or cores.
ROBOT DESIGN BY APPLICATION
While beam or linear gantry designs predominate in plastics robotics, articulated models are growing in popularity where cells include second or third operations. Pedestal mounting allows greater press-side reach, although at the cost of floor space and additional guarding considerations. Large tonnage machines with slower cycle times are commonly teamed with articulated units, where more time is available for the robot to perform post-mold tasks.
M-6iT and M-16iT robots, from Fanuc Robotics, Mississauga, Ont. for example, (136) feature articulated “joints” for load/unload and post-processing motions and a linear axis for repositioning, allowing them to make flexible, human-like moves. A six-axis arm design accommodates most operations, from part rotation to intricate part placement and assembly, without expensive options or fixturing. The payload for the two new robots ranges from 6 kg to 16 kg (13 lbs. to 35 lbs.), and each is designed to operate with most plastic injection molding machines with clamping forces from 350 tons to 1100 tons.
Process-specific models are another way to optimize part extraction and handling, with most suppliers either offering special purpose units or models which are readily reconfigurable. Yushin robots (in Canada, En-Plas Inc., Scarborough, Ont.) are an example, and can be ordered
with compounding arms or side entry for restricted ceiling heights and clean-room applications (138). Yushin also offers high-speed machines for compact disc take-out. Motoman Inc. (West Carrollton, OH) is another supplier of process-specific equipment with its PackWorld series (139). PackWorld systems combine a high-speed six-axis robot, controller, software, gripper package and powered conveyors with available add-on modules for specific packaging configurations. The system features reconfigurable grippers to accommodate a wide range of consumer products.
ROBOT “FINGERS” NEED DEXTERITY, VERSATILITY
End-of-arm tooling (EOAT) represents the “fingers” of a system, and like a human operator, must combine strength with dexterity. The current trend is away from custom EOAT in favor of a more modular approach. Ken Heyse, president and general manager of Plastics Automation Engineering (Windsor, CT) says: “Standard tool components provide three benefits to molders: Faster and lower cost development of the initial tool for their robot due to less custom engineering and manufacturing; the ability to reuse tool components and adapt them to new parts and projects, and greater ability for molders to make or modify their tools in-house.” PAE supplies standard tool components such as aluminum extrusions, grippers, clamps, pullers, sensors and actuators (141).
Versatility and cost are obvious factors in the EOAT decision process, but molders who develop in-house solutions must consider additional potential problems. Custom tooling may add additional mass to the arm, creating higher inertial forces which at best slow the robot, and which in the worst case can degrade accuracy and accelerate drive wear. Another factor relates to the operating envelope of the robot. Some tooling will require extra reach, especially where multiple grippers are added and where air or vacuum lines may interfere with platens or tie bars. The extra reach must also include sprue or part clearance since in many cases the part or sprue is considerably larger than the gripper. Similarly, robots operating at the limits of stroke or reach require special attention when retrofitting modified EOAT.
GOING DOWN THE LINE
Getting the part out of the mold is the first step, but many downstream operations require moving parts over considerable distances. Conveyors remain an option for low-cost continuous delivery, with or without dedicated robotics. SAS Automation, (Xenia, OH), a manufacturer of end-of-arm-tooling and materials handling auxiliaries has developed a line of conveyors dedicated to plastics processing (142). The flexible belt conveyors height adjust with a single crank and feature adjustable hinges for incline and nose-over applications.
Conveyor systems receive little attention compared to more glamorous forms of process automation, but the short lead times and frequent mold changes common to most custom molders often necessitate frequent changes to the conveyor set-up. Robotics store different set-ups in memory, but at the press, conveyors are most often reset manually. Hydraulics and jack-adjustable systems speed this process. Since overall machine downtime only ends with the first delivered part, cost savings through undersized conveyors can degrade overall cell efficiency regardless of the press or robotics.
Sterling Systems’ Finished Parts Transfer Systems (143, in Canada, New Tech Machinery, Burlington, Ont.) are a novel method for moving moldings downstream. Parts are either blown or “vacuumed” through tubing from a pick-up point to an “air brake” where parts are slowed down for delivery without damage. Positive-pressure systems deliver parts at up to 300 feet, while negative-pressure installations may span up to 1000 feet, both with a single blower. Tubing features a soft wall design to avoid part damage, and is available in diameters up to twelve inches. The system’s air brake adds another component to the system, according to Sterling Systems marketing manager Darrell Childress: “It also serves as a dust collection system. You’re dealing with a vacuum, so we modified the air brake to actually pull dust out of the system.” If a manufacturing cell is only as strong as its weakest link, parts handling systems must be as strong as any other component.
Plastics automation is mature and reliable enough to enhance productivity in most processing operations, but like any other piece of high-value capital equipment, a positive return on investment means matching the right equipment to the task. CPL