Designing WIP Systems (CONT’D FROM PAGE 64)
Transfer cars must be able to keep up with material flow requirements. This can be accomplished by operating at higher travel speeds, increasing car capacity to move more material in each trip, or a combination of both. As the number of conveyors in each bank increases, resulting in increased travel distance, this becomes increasingly sig- nificant. Increasing travel speed is a fairly straightforward solution but may be insufficient. Capacity is determined by the number of conveyors on the car, and their width and length. Are there at least two parallel conveyors that can handle two side by side stacks? Is each comprised of paired narrower conveyors that can accommodate either a single wide stack, or two narrower side by side stacks? How many stacks will fit on each length of conveyor? Here too, control algorithms play a part in cart utilization. Material should be released to the cars in car-load quantities while maximizing capacity utili- zation, and cars should travel when loaded with as much
be pulled. Manhandling material on an inflexible system is a really bad idea and can result in injury. If material is shuf- fled using manual controls, be sure to correct the locations in the system. Plant infrastructure plays a part here as well. Aside from power and compressed air, secure network connections for remote troubleshooting are necessary. In the past, conveyors were typically roller conveyors with three inch centers, but flat belt conveyors are sup- planting them because they don’t damage the bottom sheets in a stack, and don’t cause sheet creep. Further, they use variable frequency drives with gradual accelera- tion and deceleration which makes for more stable stacks, so material may be piled higher without tipping over. This increases storage capacity, reduces number of stacks that need to be handled, and reduces prefeeder cycles.
material as possible. The control algorithm should optimize the parking position of the cars, when not awaiting a call to transport material, as this will shorten the response time when the system calls for material. For the corrugator, that would be near its dis- charge, but converting cars should be po- sitioned based on the production schedule. Car design is also of concern. Cars typ- ically use urethane tires, and a steel guide that is embedded in the floor. The tires should run on steel beams or plates as, over time, operating directly on concrete will eventually degrade the surface. Power can be supplied by overhead rails or fes- toons, in-floor power bars, or inductively from wires buried in the floor. Control sig- nals can be supplied in the same manner, or over a local or dedicated WIFI network. What configuration of conveyors is best? That depends upon plant layout and mix. Clearly, if you have a jumbo press you will need wider conveyors. If you have a mix of sizes, perhaps from mini to 50-inches, or to a 125-inch die cutter, you can use a mix of conveyor widths tailored to your plant’s dis- tribution of sheet widths, but it’s simpler and more flexible to use uniform width convey- ors. As on the transfer cars, each line should ideally be comprised of narrow paired con- veyors that - under computer control – can be operated in paired mode for wider mate- rial, or independently for narrower material. Based on your mix and scheduling, and how close to just-in-time you plan to sched- ule, determine the MSF of product you wish to store in your WIP system. Your plant con- figuration will constrain how many lines can
CONTINUED ON PAGE 68
April 4, 2022
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