Tips

One way to increase production and throughput and decrease waste is to utilize web accumulators at the unwind and/or rewind in your converting process. Accumulators allow you to store large amounts of web.

At the unwind, accumulators operate by storing the necessary amount of web to allow the machine to continue running while the machine operator makes the splice to a new, full master roll. The accumulator must be designed to store enough material for a splice to be made in a pre-determined amount of time. In other words, if it takes the machine operator one minute to make a splice, the accumulator must store one minute (plus some for a safety factor) worth of web.

Accumulators at the rewind location are similar, except they operate running empty. When a cut-over to an empty core is required, the accumulator fills with web allowing enough time for the cut-over to be made on the fly.

One example of control logic for an accumulator for use in a rewind application is:

1. The accumulator is placed after an existing independent tension zone, most likely after a driven nip roll system.
2. Your web is threaded thru the accumulator.
3. The accumulator has an exiting nip roll assembly, prior to a closed loop tension control (like a load cell), then out of the closed loop tension controller to the rewind station. The accumulator exiting nip assembly has its own drive.
4. Your machine is started. The accumulator position (sensed with the linear transducer) controls the speed of the accumulator exiting nip roll assembly. The accumulator normal running position is minimal (almost empty but off the mechanical limit) web storage. The accumulator is operating like a dancer roll assembly and its position is maintained by controlling the speed of the exiting nip assembly speed. If web speed changes for any reason, the accumulator will move (empty or fill) and its position will tell the exiting nip roll to speed up or slow down, again maintaining the almost empty roller assembly position as constant.
5. Your existing closed loop tension control system controls the web tension in the tension zone between the accumulator exiting nip and the rewind roll.
6. When you want to stop your rewind to make a cut-over, a machine operator activates a stop button. The stop button stops the accumulator exiting nip and the rewind system.
7. Your machine is still producing a web and the web continues to feed into the accumulator. However, because the accumulator exiting nip roll is stopped, no web exits the accumulator. The accumulator motion "stores" the web with roller assemblies that separate as the web continues to feed into the accumulator. Web tension is maintained on the web with a rodless air cylinder that "loads" the accumulator roller assemblies.
8. The cut-over is made.
9. The machine operator then activates the start button. Now, the accumulator is full of material and needs to empty out to the near empty position (see number 4, above). The linear transducer accumulator position sensor sees the position of the accumulator roller assemblies (full position) and tells the nip roll drive, "hey, you have to run faster than line speed to get me to the near empty position". Typically, this may just be 10% faster than line speed. So, the accumulator exiting nip runs slightly faster than line speed to empty the accumulator, because the accumulator exiting nip roll is literally pulling material out faster than the web processing line is feeding it in.
10. Your existing load cell roller and control will sense (thru web tension) the rate the accumulator exiting nip roller is feeding material and will adjust the rewind drive to accommodate the extra 10% faster web speed.
11. When the accumulator rollers and position sensor reach the near empty point at which they are supposed maintain (waiting for the next cut over), the position sensor controls the speed of the exiting nip roll speed, like a dancer would, to maintain near empty position.
12. The process starts again at number 4, above.

It is important to consider web speed, accumulator storage and time needed to make a cut-over. Make sure the accumulator is designed with enough storage to accommodate the maximum amount of time for a cut-over.

Use of web accumulators can allow you to run the machine continuously, eliminating stops for splices at the unwind and cut-overs at the rewind. Your production will increase; waste due to machine stops will be eliminated. When you consider lost production time during the course of a normal work day for machine stops, you can easily calculate return on investment when utilizing web accumulators.

Compound motion accumulators operate with two carriages (upper and lower) of idler rolls that move simultaneous up and down to accumulate web. This design has many advantages over other types of accumulator design. Compound motion accumulators are self threading for speedy web-up, they provide superior tension control and reduce the effects of gravity on the system.