Automation for Whiter, Brighter Teeth

Engineeredinjection molding" is a new motto Brewster Plastics Inc.has added since IMM last visited (March 1998 IMM,pp. 18-22). It joins another slogan at this full-service custommolder/moldmaker that avid readers might recall: "We moldmore than plastics, we mold relationships." If you don'tbelieve the company means what it says in both cases, just considerthe advanced injection molding cell it recently engineered andbuilt on its own dime to produce a new customer's tricky product­ a disposable prophy angle dentists use to clean and polish
a patient's teeth.

Twist2it (Flushing, NY) approached Brewster with its new ideafor an all-plastic prophy angle that features a patented stop-startaction. Its tip oscillates back and forth in 90° segmentsrather than rotating 360° like conventional devices, improvingperformance. Brewster looked at the idea and took it from there.

"We suggested that its body be polypropylene and thatits internal parts be acetal," says Brett R. Wallace, vpof operations. "We also designed an internal cam action thateliminates its ever needing any lubrication." The elastomerictip is molded elsewhere. Brewster molds the body and its innerworkings-a shaft, a locking ring, and the cam (Figure 1). Brewsteralso engineered an economical means of putting everything togetherin a highly automated two-press cell.

A competitor's best idea was a molding and assembly systemthat would have required as many as eight operators. Brewster'scell does it with three. Resin goes in and individually packagedfinished assemblies come out the other end every 1.5 seconds.Not a single part has been returned since it started running inearly 2000. Brewster's new customer has already asked it to takea long-term look at eventually adding tip molding and assemblyto the cell.

Auto-polished Productivity

The cell operates with minimal manual intervention. An operatorplaces an empty, cavity-numbered parts handling/assembly jig ontoa conveyor that transports it down to press 1's clamp end. Atthe fully automated parts-preparation station there, the emptyjig is elevated near where device bodies are removed from a 20-cavitymold by a press-fit EOAT on press 1's robot (Figure 2A).

The bodies, degated in the mold, are placed by the robot intothe jig, which then slides into position next to press 2 whereit awaits the cams. Meanwhile, the robot deposits sprues and runnersinto a direct recycling system on the other side of press 1 (Figure2B).

An inverted U-shaped EOAT on press 2's servo robot comes downover the top of the center section of a 60-cavity stack mold.Each side of the inverted U advances to each mold face to removesimultaneously knocked-out parts from both sides. The robot grabsparts by gripping subrunners, and then collapses and rises, carryingthe parts into position at the prep station's assembler/spruecutter. Shafts and locking rings are on top; cams are on the bottom(Figure 2C).

The robot's EOAT, executing the same motions used to removethe parts, inserts the hot runnered cams into the device bodieson the waiting jig while putting the locking rings and the shaftsinto place. Locking rings and shafts are still on their runnersfor easier handling.

While the assembler/sprue cutter slides over to drop spruesthrough a chute into a box, the jig elevator descends to the conveyorbeside press 2, which transfers the filled jigs back to the operatorstation (Figure 3).

Painless Extractions

The first operator transfers the jigs to a hold-down fixtureand then into an air-powered station that separates the lockingrings from the shafts, forces them together, and pushes them intoplace past a molded-in locking indent inside the device bodies.Runners are dropped through cutouts in a homemade slide that depositsthem in a direct-recycling granulator nearby.

The second operator manually removes parts from the holdingfixture using an ergonomically correct single-stroke knockoutdevice. The empty holding fixture is then placed on another slideunderneath the runner slide. This one transports the empty holdingfixture back to the first operator.

Assemblies are placed in a vibratory bowl-fed system that pneumaticallyattaches tips, one part at a time, and then photo-optically checksthe finished parts as they are delivered to the automated packagingstation. Meanwhile, the second operator places the empty partshandling/assembly jig back on the press 1 conveyor, and the cyclerepeats.

This cell is one example of how automation needs have givennew meaning to the term "reverse engineering" at BrewsterPlastics. Ken Newton, vp of engineering, designed the molds. But,as Wallace explains, "We had to know how we were going toput the parts together before the molds were designed," hesays. "If we took the easy way out and built easy molds,we might not have been able to automate the process."