Low-tech/no-tech parts-handling solutions.
Some injection molders still handle their parts the old-fashioned way--human hands do all the picking, separating, moving and packing chores. The hands-on approach, though, makes less and less sense as labor costs spiral upwards and quality concerns extend well beyond the actual molding process. These two factors, plus growing worker-safety concerns and the need to maximize productivity, point more and more molders in the direction of downstream automation of one type or another.
The most obvious impact of downstream improvements can be found in labor savings. All the equipment manufacturers we talked to, however, remind us that automation doesn't necessarily mean an end to the patter of human feet around the plant. It could, instead, simply facilitate an increase in production capacity without additional hiring.
Mechanical parts "catchers," conveyors and separators may not really fall on the cutting edge of new technology, but these relatively prosaic items can offer other advantages that go beyond labor savings. In keeping with the heightened demand for post-mold quality management, equipment makers have added so-called diversion options to aid in quality sampling and off-spec part rejection. And newer downstream equipment can also provide accurate parts counting capabilities and help prevent part mix-ups or damage. Some equipment, by fostering faster cycles and reducing waste, can bolster a molder's bottom line.
Perhaps equally important in our litigious society, automation has the potential for reducing workplace injuries. With the simple addition of an underpress conveyor, for instance, "You can get the operators out of the machine environment," says Anthony Andraitis, president of EMI Corp. in Jackson Center, Ohio. At LaRos Equipment Co. Inc. in Portage, Mich., general manager Timm VanNess has also found increasing interest in "ergonomic considerations" on the part of his customers. He notes that LaRos recently supplied one conveyor system built purely to protect workers from injuries sustained while bending over to handle parts.
Parts-management systems run the gamut from high-tech to low-tech to no-tech. For those companies without the resources, need or ambition to shoot for a space-age, "lights-out" plant, some inexpensive and flexible downstream devices can still help reduce labor costs and ensure post-mold quality while improving safety and productivity at the same time.
There's certainly no shortage of companies addressing the parts-handling side of injection molding. In fact, several dozen companies--each with its own machine construction philosophy--supply nearly every item we examine here. We invited all of them, by mail, to enlighten us on the trends in post-mold handling. We ended up interviewing a dozen or so representative suppliers who told us about some interesting changes in this often-overlooked area. (For a complete list of suppliers, see PLASTICS TECHNOLOGY'S annual Manufacturing Handbook & Buyers' Guide.)
PARTS REMOVAL WITHOUT ROBOTS
Threats to quality don't always begin in the mold but can emerge further downstream instead. Mishandling, for example, can damage even the best parts or result in equally wasteful mix-ups when good parts end up in the wrong places. At least two companies offer devices to help eliminate bungled handling by automatically capturing parts the moment they exit the mold, creating a closed system that doesn't rely on robots.
"Guideskirt" products from Molding Automation Concepts Inc. (MAC) of Woodstock, Ill., direct parts by gravity from the mold to a box, under-press conveyor, or other auxiliary equipment. Made from waterproof fabric, the bellows-shaped devices attach directly to the bottom of the mold, opening with it to catch the falling parts. MAC president Frank Eltvedt says the device primarily prevents part contamination and damage, adding that it stops parts from "flying out of the mold like popcorn" and landing on the floor.
Available in both standard and custom sizes, the patented Guideskirt has several variations. The standard tapered model serves to funnel smaller parts downstream, while a straight design handles larger parts. A dual-exit model keeps multiple parts separate, while a right-angle model provides clearance from the framework of a small machine when the mold is recessed inside.
To limit the effects a free fall might have on easily damaged parts, MAC adds detachable chutes of fabric or metal at the bottom of some Guideskirts to soften the ride. Molders can run especially fragile items through a "delayed-drop" model. Its bottom end closes to catch parts, bringing them to a dead stop before releasing them downstream.
For improving quality control, an optional Guideskirt system connects to the press controller. During start-up, or anytime the controller senses an off-spec shot, the Guideskirt can divert bad parts through an air-operated "deflection chute." Molders can also use this diversion option to collect parts reliably for quality control sampling at more regular intervals than some human operators can manage.
Other Guideskirt options include a cushion insert for breakable parts and a rubber-coated fabric for parts hotter than 300 F. Prices are below $100 for most models.
Two parts-guiding systems from Husky Injection Molding Systems Ltd. of Bolton, Ontario, differ considerably from the less expensive MAC Guideskirts. Husky's patented Guiderails and Swing Chutes are custom-designed to work with one specific part. And both systems actively control the part's exit from the mold. Bruce Coxhead, Husky's manager of robotics and product-handling equipment, explains that both systems are actually "part of the mold itself," so the decision to use them must come at the mold-design stage.
Mounted on the mold, Guiderails consist of two slotted tracks that come to rest adjacent to the mold cores during ejection and capture parts by their edges when the ejectors pull back. Parts then slide down the tracks, moving away from the mold and on to other downstream equipment. Applications so far have included 3.5-in. microfloppy-disk shells, audio cassettes, VHS cassette parts, and petri dishes. Husky recommends the system for relatively flat parts that have edges for the rails to direct.
Husky's swing chutes have arms that swing 90|degrees~ to face each core as the mold opens. These arms then pick the parts, "grabbing" them with vacuum cups. Husky uses cams to ensure accurate positioning of the arms in front of the cores.
What they sacrifice in flexibility, Swing Chutes and Guiderails make up in quality and productivity advantages, Husky says. Both systems protect fragile parts from free-fall and contamination by "foreign" parts or other objects. Husky claims molds with Swing Chutes can cycle up to 10% faster than conventional molds by eliminating the delay in clamp-closing times while parts drop free of the mold. Guiderails likewise offer productivity gains, with Husky claiming parts-removal times of 0.4 sec. Coxhead says that this speed element gives the systems an advantage over robots. "When molding cycles get extremely fast, most standard robots' in/out time becomes too great," he explains. Last, both the Guiderail and Swing Chute orient parts as they leave the mold, simplifying secondary operations such as assembly or decorating.
CONVEYORS GET MORE RESPECT
Traditional belt conveyors may not inspire much excitement. "They're not the most noble of machines," acknowledges Rudy Colombi, president of Crizaf Inc., a new branch of an Italian conveyor manufacturer in Sterling Heights, Mich. (see PT, May '91, p. 179; Aug. '91, p. 89). Still, these unglamorous devices form an important link in the parts-handling chain. And the emergence of parts-handling robots has enhanced the role of conveyors. "A robot will move a part 6 ft from the press, but then what?" asks EMI's Andraitis. EMI's answer is an indexing conveyor with an electrical interface to the robot control so the conveyor indexes forward only when the robot has finished placing the part. LaRos also builds special conveyors with robot interfaces.
Today, conveyors often do more than just move parts from point A to point B. For example, several conveyor manufacturers target their equipment's contribution toward maintaining quality once parts have left the press. The Smart-veyor option from LaRos ties its conveyors into the statistical process-control (SPC) monitoring function of molding machine controls. Based on a signal from the press controller, the conveyor can temporarily reverse direction to either divert bad shots or to provide quality-control samples at regular intervals. The connection can be made via a PLC signal or even a simple contact closure. EMI, MAC, Crizaf, and others have similar approaches to integrating their conveyors with molding process control.
Other conveyors that transcend the simple carryall function have become available from several companies. LaRos, for one, sells a Water-veyor that cools parts by ferrying them first through an integral water trough and then past a blow dryer.
MAC offers conveyors with enclosed belts for air-cooling, heating, and eliminating static charge with de-ionized air. "You can condition a part right on the conveyor belt," Eltvedt says, adding that the cooling can permit faster cycle times and minimize part distortion. Parts enter the metal enclosure through an infeed hopper or Guideskirt. A variable-speed blower atop the enclosure, meanwhile, creates an air flow around the parts as they ride along on a mesh belt. Air temperature can be controlled with treated water from either a chiller or the mold-temperature controller.
For sensitive applications, MAC also sells an enclosed conveyor equipped with HEPA filters to serve as a "portable clean room" designed to eliminate particles as small as 0.3 microns. Another enclosure option, the company's fabric conveyor cover, connects to a Guideskirt and protects parts by creating an oil-and water-free environment along the length of the conveyor.
Foregoing the belts and rollers of traditional conveyors, air-conveying systems use a blower-generated stream of air to move parts between two points via a tubing pathway. Though often associated with scrap removal and rough handling in the past, air conveyors can transport finished products as well. Sterling Systems Inc., Forest, Va., has several features on its equipment designed to protect parts during air conveying, says marketing manager Greg Snowman. The company's Venturi System provides adjustable air velocity and blows finished parts through cellulose acetate butyrate tubing, a softer material than the more commonly used PVC. A nylon relief sock slows parts as they leave the system to limit the potential for damage. So far, the system has been used with pen parts, battery tops, bottle caps, and medical components.
LaRos's VanNess notes that air conveyors generally cost less per foot than mechanical belt conveyors over long distances and can fit in tighter spaces. He concedes, however, that LaRos doesn't recommend even its own air conveyors "whenever cosmetic considerations are important."
BOXING CAN BE PAINLESS
High-quality box filling, whereby the correct number of parts winds up in the right container after molding, is sometimes hard to achieve by hand. Alternatively, a number of automatic systems perform this normally labor-intensive task while controlling count and limiting the potential for parts mix-ups. At the same time, many newer systems have provisions for rejecting bad parts. From a labor-cost perspective, EMI's Andraitis estimates that a typical box-filling system can allow a single worker to handle the production from eight to 10 presses.
In the absence of a robot, a common approach to box filling pairs two conveyors: one carries parts from under the press, while the second holds both the empty and full boxes. EMI, LaRos, Crizaf, MAC and many more offer such systems with cycle-count or weigh-scale controls to determine when to switch filling from one full box to the next empty one.
EMI designed its newest box filler to address space limitations by stacking two conveyors in a common frame connected by an elevator section. Called the Over/Under system, it sits next to the press and can hold twice as many boxes as a linear system with the same "footprint." Robots or an under-press conveyor can fill the boxes on the top or bottom conveyors or on the elevator section that connects the two levels.
MAC also has a vertically stacked system. The horizontal "Mac Stack" operates on cycle-count, weigh-scale, or visual signals. Both systems can lift filled boxes off the floor for an added ergonomic benefit--no workers bending over to pick up heavy boxes.
Rather than move boxes under the parts, MAC's arm-fill conveyor takes an alternate path. Boxes sit under chutes off the conveyor, and a programmable deflector arm travels the length of the conveyor to send a predetermined count into each box. The arm can also disengage to allow QC samples, rejects or overflow to proceed off the end of the conveyor.
Another box filler from MAC eliminates the need for multiple conveyors. While bringing parts from under the press, the Robotic Conveyor can shift between two containers and dump a preset count or weight in each. The unit, when hooked up to a machine controller's SPC monitor, could also be used to divert samples or rejects.
For parts that must go on totes before heading to auxiliary operations, MAC's portable tote magazine holds a stack of up to 30 empty containers of either plastic or wire-reinforced fiberglass. The unit feeds parts from a stainless-steel hopper and can be controlled with either an automatic counter or weigh-scale. EMI's tote stacker holds up to 20 wire-reinforced totes and also has count-or weight-based filling systems.
Conair Martin of Agawam, Mass., designs its tote filler specifically to work with robots by electronically integrating the two devices. The Autostocker positions empty totes horizontally and vertically for filling by a robot. It then restacks the full totes on an accumulation conveyor running parallel to the press. Aside from uniform parts handling, the system requires minimal labor. According to Conair v.p. Ed Skiba, an Autostocker has enough capacity to require attendance only once per shift in many cases.
WHEN RUNNERS LOOK LIKE PARTS
In general, separating parts from runners and sprues boils down to "discrimination on the basis of size," as Crizaf's Colombi puts it. Common finger-type separators typically attach to or sit at the end of a conveyor. Mounted on a moving belt, the fingers snare the runners but not parts. They can then send the captured runners, sometimes via another conveyor, to an auger grinder for reclaim purposes.
Separation sounds easy enough, but it can get complicated when parts and runners share a similar geometry. Close resemblance of parts and runners nowadays occurs more frequently due to the popularity of multi-runner, multi-cavity designs, according to LaRos' VanNess. These "modified runnerless" mold designs can render sprue pickers and traditional separators useless, he notes. So LaRos approaches the problem with "refinements of the separation zone," including a repertoire of 11 different finger designs, as well as agitation capabilities. LaRos' S-400 separation units can be built right into the conveyor itself and can be enclosed to prevent parts from straying.
Columbi likewise notes that parts and runners don't always diverge enough in size for easy separation. Accordingly, Crizaf's approach doesn't employ finger-based designs at all. Instead, the company's end-of-conveyor models use various transverse roller configurations. These devices let parts fall through the rollers while the runners ride over the top to the end of the unit.
For difficult separation tasks, Crizaf's sprue separator models consist of two or three parallel cylinders arranged in the direction of intended parts-travel rather than transversely. The parts fall between the rotating cylinders but sprues travel along the length of the unit aided by the cylinders' grooved surface. Colombi says these S-Series sprue separators can distinguish sprues from parts that differ by as little as 0.02 in. in size. And he guarantees 100% separation as long as the molder follows Crizaf's recommended equipment configuration.
Crizaf and several other companies also sell rotating drum separators for fine separation. Typically conveyor-fed, these tumbling units have a gapped surface to let parts fall out while the runner continues to the drum's end. Molders can adjust both the rotation speed and exit openings to accommodate a variety of products.
Parts handling grows more complicated when post-mold operations don't end with parts going into a box. EMI's Andraitis notes that operators have to assemble many products before shipping and adds that "most molders still need some sort of visual inspection." Several equipment suppliers recommend temporary storage devices as a way to bring secondary operations like these on-line.
EMI, for instance, sells turntables to provide a centralized place for parts to accumulate during hand assembly or inspection while presses continue to run automatically. Conveyor trunk lines carry parts automatically from under the presses to the turntables, allowing workers to inspect, assemble and pack as press operations continue. Andraitis concedes that this approach may not eliminate human operators, but does distance them from the machines. Crizaf and others make their own versions of turntable accumulators.
Another type of on-line storage can help cut labor costs by reconciling secondary operations with automatic press runs. "In-process storage can eliminate the bulk of parts handling," says Frank Seggio of Plastiplant Services Inc., an engineering firm in Wayside, N.J. Plastiplant custom designs and builds bins to temporarily store molding machine output, allowing faster decorating or assembly machines to run for fewer shifts. The company frequently
joins the bins to butyrate air conveyors, creating a closed system between press and decorating operation to help maintain part quality by eliminating contamination.
Suppliers and molders we interviewed agree that the installation of even one of the devices mentioned here can provide attractive benefits, especially to molders who might not have considered even the barest degree of automation in the past. But more and more molders want comprehensive "manufacturing solutions" for parts handling problems that include assembly and decorating operations, according to John Mallon, president of Yushin America Inc. in Warwick, R.I.
Mallon points out that the more complex manufacturing operations often require more than a piecemeal approach to automation. "Ideally, we like to design turnkey systems," he says. Yushin, which supplies Japanese robotics, and other suppliers even prefer to be brought in all the way back at the part-design stage. The suppliers say they can then better match their automated systems to the downstream problems at hand.
Larger molding operations that we talked to swear by their elaborate, automated parts-handling systems for both productivity and quality benefits. Yet all of these companies noted that a small custom molder who does only short runs or doesn't have modern presses would be a different case. "For these guys," one molder says, "total automation just isn't worth it."
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|Title Annotation:||plastic injection molding automation|
|Date:||Apr 1, 1992|
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