A lot goes into choosing the "right stuff" in an injection molding parts-removal robot. There are a lot of questions to ask yourself and your prospective hardware supplier. The first question is whether you really need a robot at all, and the second is what you want to do with it.
DRIVES: GO SERVO?
The impressive speed and positioning capabilities of servo drives captured the interest of several molders interviewed, although the steep price still limits servos to more complex operations. According to Bob Hightower, engineering v.p. of Styrex, a custom molder in High Point, N.C., servos are worth the money "if you're doing a lot of external functions from the press," such as hot stamping, degating, or packaging. "But if you're using the servo for nothing more than pulling the part from the mold and putting it on a conveyor, you're spending money that you don't need to."
Art Webster, maintenance supervisor of robotics at Mitchell Plastics, an automotive custom molder in Kitchener, Ont., says a combination of servos and pneumatics on a robot often makes sense from the viewpoint of simplicity. Mitchell limits its use of servos to the robots' traverse axis in order to provide the required positioning accuracy for gate cutters and drop positions. Webster adds that servo drives have eased job changeovers because of easier programming of position changes for physical stops.
While servos undeniably have their place, one robot supplier points out that pneumatic drives should not be dismissed from more sophisticated operations. Often, pneumatic drives are a viable and economical alternative if used with slightly more complex end-of-arm tooling. For example, a heavy undercut on a part may be removed with a particular gripper on the tool, eliminating an extra movement.
GETTING THE RIGHT GRIP
End-of-arm tooling is viewed as critical by molders, many of whom depend on the ability of suppliers to come up with the right designs. Others prefer to fabricate their own tooling. One supplier is Precise Plastic Products Inc., a custom injection molder in East McKeesport, Pa. Director of manufacturing Tom Opielowski has found that "most end-of-arm tools supplied with the robot are very flimsy. They don't hold up for long, and are not adjustable to the extent that we want."
When fabricating its own tooling, the company tries to stick with vacuum whenever possible, says Larry Powell, manager of maintenance, production, and automation. "Vacuum doesn't leave marks on the part, where a gripper could. Picking parts up with vacuum is simpler and verification is easier. And if you rip a suction cup, you can just throw it away." Another advantage of vacuum gripping is light weight, says Powell, especially with multicavity molds. "Any robot may be able to pick up a part, but when you go to flip the wrist, that's when you can run into a problem with extra weight. We have several jobs where we are picking 24 cavities by vacuum only."
Pro Corp. in Florence, Mass., a custom molder of highly polished parts such as polycarbonate windows for copiers, uses silicone cups on its vacuum tooling to protect the part finish. Pro Corp. uses a combination of suction cups and grippers to add stability when removing large parts, says plant manager Bernie Golub, because the grabber can grip the inside of the part while suction cups on the outer face can be used to steady it.
Quick disconnects that ease tooling changes from job to job also rate high on molders' shopping lists. According to Golub, "Some robots are a real bear and some make it easy." Pro Corp. uses tooling designed by the robot manufacturer, in which a combination of suction cups and grabbers can be supplied in many different configurations, so the whole end-of-arm tool does not have to be taken off. Removing the tool is also easy, involving two vacuum lines and two bolts, and new tooling is reattached in the same fashion.
A STURDY FRAME
Basic strength of robot construction is important, according to Gary Hengeveld, production manager of Cal-Mold Inc., a molder in City of Industry, Calif. A frame that's too light "will cause a lot of vibration and mis-grabs, where the robot comes down out of position." Hengeveld also rates accurate guiding of robot movements high on his list. "If a robot isn't well bushed or aligned, or if it isn't engineered properly, it will leave debris and cause heavy rubbing."
Larry Powell of Precise Plastic doubts that there's a robot out there that doesn't wiggle a little. "Even though the robot stops on the money with the prox switch, if it's not lined up in the mold correctly, it may be ten thousandths off." To help steady one 24-cavity job, Precise fabricated special tapered guides on the robot tool to help guide it to the part.
With space at a premium, compactness is an important issue with many molders. "You'll find some robots are bigger than you'll ever need and can't fit on the floor. Some robots are too large for setting up on the press. Also, the controller portion needs to be pretty compact," says Gary Hengeveld of Cal-Mold. For the same reason, Hightower of Styrex favors electric motors over pneumatic ones: "You can put the axial handling unit in a little smaller space, and you don't have a long cylinder sticking out of one side or the other."
Molders also value the ability of robot manufacturers to customize designs when needed. One pet peeve of Precise Plastic's Powell is what he sees as an unwillingness of some suppliers to extend their robots' traverse reach. "They don't give any leeway on it. One robot may be fine for you, in terms of the weight it can handle, until you realize that you can't go any farther in traverse unless you go up to a larger size robot. That's overkill, just to get more traverse." In one instance, Precise Plastic paid $800 to extend the traverse of one of its robots by 2 in. so as to permit moving it from a standard 200-ton press to another 200-tonner with an extended platen.
One relatively expensive option that's favored by some molders is the ability of the robot to place parts below the centerline of the press. "There are only about a half-dozen robot suppliers who do that," according to Pro Corp.'s Golub. He says that option can be important on some larger presses in which the centerline of the mold is much higher than the working table of the operator, and may eliminate the need for conveyor systems to bring the parts within the operators' reach.
WHAT COUNTS IN CONTROLS
Ease of use is a top priority among molders when it comes to robot controls. "If you look at a panel with 50,000 switches on it, you're scared," says Powell of Precise Plastic. Pro Corp.'s Bernie Golub adds that programmability eases changeover from one job to another. "Along with that, I look at what kind of memory storage it has, if its memory can store 10 jobs or 100 jobs, or whether it has a memory bank at all." John Rake, corporate director of operations of Superior Plastics, a custom molder in Rochester, Mich., notes that controls programmed in simple ladder logic eases the job of reprogramming the robot's movements.
One useful control feature is troubleshooting capability, according to Precise Plastic's Opielowski. "Troubleshooting is nice because when running a robot at high speeds, it doesn't take much for a switch to move a few thousandths and not make contact. When you have a machine down, determining the source of the problem quickly is very important."
One safety-related issue, according to Powell, is that functions in the manual mode should be in the same sequence as in the automatic mode. "We have one robot that can do functions in the manual mode which I don't think should be there." For example, a manual override on the kick stroke permits the robot to kick out above the mold and can damage end-of-arm tooling if it reaches into the mold with the stroke extended.
Ruggedness and accessibility of components were rated high among many molders who perform their own maintenance. "A robot will run as much as the press does," says Opielowski. "It must be built well. We look at how the robot is put together--what type of bearings it has, how it interfaces with the press, and the kind of stops it has so it doesn't beat itself to death." A few molders look for name-brand pneumatic components and control components. Also important is the placement of components. In pneumatic robots, Powell looks for stacked cylinders, finding that arrangement the most accessible.
Hightower of Styrex is looking forward to the day when his robots will be self-lubricating, "so we won't have to grease them at night or wipe off the extra grease. We can't do it while they're running, and they collect a lot of dust before you can clean them up. If you had everything lubrication-free, you'd be in Fat City."
Webster of Mitchell Plastics finds that "a full set of electrical prints" is vital so he can do his own troubleshooting. Often, he works with his robot vendor over the phone to pinpoint a problem, and he puts a premium on having such technical backup readily available.
Virtually all molders contacted viewed support from the supplier as a critical factor in choosing a robot. One particular sore point is spare-parts availability. Comments Pro Corp.'s Golub, "They |robot suppliers~ don't seem to stock parts like they used to. I just ordered two parts and have to wait eight weeks. My machine will be down because I don't have another part for it. Molders can't afford to stock every part on a machine."
Many molders welcome a supplier's expertise in providing ideas for plant automation, sizing of the robot, or the right end-of-arm tooling. "What we really need is a supplier who can integrate a whole system, rather than just supply a robot picking up parts," Cal-Mold's Hengeveld explains. He also regards training from the supplier as important to developing a molder's in-house capability for programming and maintenance. Adds Golub, "I want a person to come in here and show people how a robot works and how to keep it working. And if something happens to a robot, how quickly will they come out to see me?"
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|Title Annotation:||Straight Talk on Buying|
|Author:||De Gaspari, John|
|Date:||Oct 1, 1992|
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