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Controls push speed limit

U.S. Sugar fills two 5-lb bags/sec with better than ±0.1% accuracy thanks to a state-of-the-art sugar-filling line powered by the latest in servo drive, PLC and networking technology.
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In designing high-speed packaging lines engineers eventually reach a point where they must sacrifice accuracy for speed. U.S. Sugar with its two high-speed sugar bagging lines found a way to keep both speed and accuracy.

Each of the two lines forms fills and seals two-layer paper sacks on equipment from SIG Pack (Raleigh NC). One line fills at a top speed of two 5-lb bags per second.

Further accuracy is extremely high better than ± 2 g on a 5-lb (2-g) bag according to Dave Pelham chief of bulk shipping warehousing and packaging. That works out to be a variance of less than a tenth of one percent. U.S. Sugar attributes much of the extremely high filling speed and accuracy to the state-of-the-art controls from Siemens (Alpharetta GA).

Packaging World recently visited the year-old packaging lines near the Florida Everglades where U.S. Sugar’s plant is located. Long a processor of raw cane sugar U.S. Sugar had decided to enter the sugar refining and packaging business instead of selling its raw sugar to other firms. That’s what prompted construction of the Clewiston FL plant and the packaging lines contained within which were put into production in March 1999.

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On the day of PW’s visit U.S. Sugar was packing 5-lb bags of sugar for Pillsbury on the line dedicated to that size which was running at 100 bags/min. The other line packages 2- 4- or 5-lb sizes all in multiwall paper sacks.

Triple volumetric filling

Each line starts out by forming double-wall paper sacks from two separate rolls of paper one for the printed outer ply and the other for the unprinted inner ply. The paper is cut wrapped around a mandrel and sealed with liquid adhesive. The empty bags are then stuffed in a pocket-style conveyor that brings them to the filling section.

Filling is done via the bulk-and-dribble method. First bags are filled with the rough amount by three in-line volumetric fillers that fill three bags simultaneously. Next filled bags are conveyed over an integral checkweigher that checks for gross weight accuracy feeding back over- or under-weigh information to the volumetric fillers for automatic on-the-fly adjustment.

The bags continue beneath the high-accuracy top-off filling station. Bags are conveyed over another integral checkweigh station this one with a reject mechanism.

After filling bags are closed at a bag closer/sealer station. Finally bags are conveyed through a Ramsey Icore metal detector (Minneapolis MN).

Bags then enter a SIG Pack bailer that collates them into eight-bag bundles which are overwrapped with kraft paper.

Finished bundles are ink-jet coded via a Marsh® large-character ink-jet printer from Marconi Data Systems (Belleville IL) before being conveyed into another room for automatic palletizing and stretch wrapping.

Controls make it possible

There are several Siemens components that are used on the packaging line:

• Simatic S7-400 programmable logic controller (PLC).

• Simatic TP27 human-machine interface (a color touchscreen panel).

• 17 servo drives and motors on each machine 13 of which are electronically synchronized to a master drive.

• a Profibus network that connects all of the above items along with sensors valves and other components.

Servo drives and motors enable high speeds and accuracy because they are controlled via precise digital electronic signals instead of physical cams or gears which cannot transmit as accurately and efficiently at high speeds and are also subject to wear.

The Profibus network connects all of the above components. To shield network communications from electromagnetic noise SIG Pack chose a fiber-optic link for the Profibus connection between the components on the actual machine and the PLC housed in a separate electrical cabinet.

SIG Pack confirms that the Siemens “totally integrated automation” concept lives up to its billing reporting that the different components resulted in streamlined configuring programming data management and communications for U.S. Sugar’s machines.

Efficiency edging up

Pelham points out a few examples of the benefits from U.S. Sugar’s perspective of tight integration of the servo controls PLC and HMI: “You can adjust the amount of glue on the fly or even change position of the [front label graphics] left or right on the fly.”

On the machine that handles 2- 4- and 5-lb bags changeover is also relatively simple says Pelham though it’s not instantaneous due to some mechanical adjustments that need to be made. Changeover from 2- to 4-lb bags takes six to eight hours; from 4- to 5-lb takes about two hours. Nevertheless changeover would be even longer were it not for several parameters that can be changed electronically.

“If you have to change bag length” says Pelham “you just punch in a number.” U.S. Sugar is pleased with its investment says Pelham. “We’ve been spoiled by it. We haven’t had older systems to compare it to” he says. Occasional demonstrations of the equipment to other firms from the sugar industry have confirmed to Pelham that U.S. Sugar chose the right technology. “They come in and look at what we’ve got and say ‘Wow this is nice.’”

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