keep it moving
To understand the role sortation systems play in the DC, it might help to picture the tangle of railcars and tracks in a busy rail switchyard. Though the average switchyard may look like an exercise in chaos, it's anything but. As the trains roll in, track switches swiftly marshal the incoming railcars and route them to the proper spurs or sidetracks, clearing the way for new arrivals. Much like the switches that keep cars moving at busy rail intersections, sortation systems direct the traffic flow in today's conveyorized DCs. Whatever their type—pop-up, tilt tray, cross-belt, bomb bay—it's the sorters that keep the operation humming, rounding up the Star Wars action figures, flannel shirts, tubes of lipstick and whatever else comes hurtling down the conveyor belts and directing them to hundreds of possible destinations.
In their role as traffic cops for the DC, sortation systems direct products from one conveyor line to another. They can be as simple as a single divert that sends cartons to a processing station or as complex as a system of hundreds of destination chutes arrayed around a high-speed tilt tray or crossbelt track. (See the accompanying box for a description of the most common types of sortation systems and their applications.)
Of course, nowhere is it written that a DC can only use one type of sortation system; many facilities have successfully integrated multiple types of sorters into their operations. One such operation is the Blair Co., a retailer that markets apparel and home dÈcor items. Blair uses a variety of sorting systems in its 875,000-square-foot distribution center complex in Warren, Pa. (the DC actually consists of two adjacent buildings connected by a conveyor bridge). The company ships 50,000 units each day using two crossbelt sorters, a sliding shoe sorter and several pop-up diverters, all from Siemens Logistics and Assembly Systems.
Most items are picked in batches from storage locations and picking areas in the first building. The pop-up sorters located among the conveyor lines are used primarily to divert items to stations where tasks like product personalization and monogramming are carried out.
Once all of the value-added processes are completed, items are conveyed to the second building, where they pass through the sliding shoe sorter, which performs multiple tasks. Some items may pass through this sorter more than once during their journey through the building, being diverted to a different processing area each time. For example, the sorter may first send items picked for multi-line orders to one of the crossbelt sorters for order consolidation. The sliding shoe sorter also diverts single line orders to a bagging area. Once bagged, these are sent back through the sorter again, this time to be diverted to packing stations. Some cartons also pass through the same sorter an additional time to be diverted to sealing machines.
The crossbelt sorters serve as the heart of the distribution system at Blair's DC. The first crossbelt sorter breaks down bulk-picked items into individual orders for packing. This unit replaced a tilt-tray system that was previously installed at the facility. Since crossbelts use a moving belt to divert items, this sorter occupies less space than the old tilt-tray unit, yet still contains a whopping 500 destination chutes.
"This crossbelt sorter sorts on three-foot center lines," explains Tim Harlan, director of operations for fulfillment. "The tilt tray could not sort on that short interval. We were able to reduce the footprint and save money." Harlan adds that the sorter is capable of processing 182,000 items over two shifts each day.
Products for about 10 orders (each order averages just over two units) are sorted into each of the sorter's chutes. A worker situated at the bottom of the chute manually removes items from the chute and packs them into shipping cartons. Many of these cartons will pass through the sliding shoe sorter again for sealing.
The second crossbelt sorter serves shipping needs. Once packed and sealed, all cartons enter this crossbelt shipping sorter at five automatic induction points. This shipping unit sorts items to about 45 destinations, depending on weight and how they will ship. Most products are shipped via the U.S. Postal Service. About 21 of the sorter's destination chutes are assigned to bulk mail centers. These items are gathered and then placed on a truck for delivery to postal sorting facilities in distant cities. (By shipping directly to the bulk mail centers, Blair saves greatly on postal costs.) Other diverts are used for very small items (items weighing under one pound) that are gathered into Gaylord boxes for later processing by the Postal Service, and a few additional lanes are reserved for orders handled by UPS.
"It is unusual to use a crossbelt for shipping," Harlan admits. "We could have gone with a tilt-tray system there," he says, but because the company was already operating a crossbelt in the pack area, it opted to install another cross-belt so that it could consolidate its sorter parts inventory.
Others lean toward tilt trays
The Blair Co. may have chosen crossbelts, but it's equally common for companies that place a premium on high speeds to opt for the tilt tray model. One such company is eToys Direct. This direct-to-consumer toy retailer has two large Beumer tilt-tray systems at its distribution center in Blairs, Va. The facility also has a sliding shoe sorter from FKI Logistex that is used during high-volume periods to send products to staging lanes where they are held for later processing.
The first tilt-tray sorter at eToys Direct consists of 495 trays and is used for assigning batch-picked pieces to individual orders. As totes of batched items approach the sorter, items are removed at 12 induction stations and manually placed onto sorter trays. Scanners read the tray numbers and determine which of the 400 chutes arrayed on each side of the track should receive each item. As the tray approaches the proper chute, the tray tilts and the product falls into the chute. Up to seven orders are accumulated into each chute. A worker then manually divides the gathered items into individual order totes, which then are conveyed to a value-added station (like a gift wrapping center) if needed before being sent on to packing stations. Orders average 3.6 items.
Cartons of packed orders next head to the second tilt-tray unit, which handles shipping tasks. This sorter has 525 trays and feeds 18 shipping lanes. It has a rated capability of performing 8,760 tilts per hour and provides accuracies above 99.8 percent.
So far, at least, it appears that the tilt trays have not only met, but exceeded expectations. "On our best day last year, we shipped 48,000 cartons," reports Kenneth Scruggs, eToys Direct's facilities manager. "And [we] could have handled even more."
all sorts of options
Sorters come in a variety of styles, from simple low-cost systems that handle a few hundred diverts per hour to complex and blazing fast systems that sort up to 10,000 units per hour. Here's a rundown of some of the most popular sorters on the market:
POP-UP SORTERS, which are designed to divert moving cartons or totes to a different conveyor line, are the sorters most commonly found in today's DCs. Systems vary, but basically they consist of wheels embedded below the conveyor's roller surface at the point where two or more lines meet. When a carton needs to be redirected to another line, the embedded wheels pop up from the conveying surface to change the direction of the box to the desired conveyor. Some pop-up wheels are also designed to rotate slightly left or right as the carton encounters them. These steerable wheels allow for higher conveying speeds and can accommodate diverts to either side. Pop-up sorters handle only a few hundred diverts an hour and are designed for low-volume sorting applications. They can be placed anywhere in the DC where conveyor lines are found.
PUSH DIVERTERS consist of an arm or pusher panel placed next to a conveyor that swings or pushes out when a carton approaches. The carton is then redirected to a new conveyor or into a sorting bin. These simple units are low cost and easy to maintain. Like pop-ups, they are used in low-volume applications where only a few hundred sorts per hour are needed. However, they can handle higher amounts when many push diverters are placed side by side, such as in baggage handling operations. It's important to keep in mind that with push diverters, the swing arms come into physical contact with the product, so they should be employed only in those applications where solid packaging is used. Push diverters are not recommended for use with fragile goods.
VERTICAL SORTERS, which are not as commonly used as pop-up sorters or push diverters, are designed for applications where space is at a premium. These small-footprint sorters take advantage of vertical space and consist of layers of belts stacked one on top of another. When there is a need to divert an oncoming product, a conveyor belt tilts to meet the upper or lower belt stacked above or below it. These units offer speeds of up to 2,000 sorts per hour and are well suited for irregularly shaped items, such as golf clubs, garment boxes or multi-sided cartons. They are available in multiple widths for added flexibility.
SLIDING SHOE SORTERS are the most common type of high-speed sorter used in today's distribution facilities. They are capable of delivering up to 10,000 sorts per hour depending on length and speed, which can vary greatly. The basic design consists of a conveying platform that connects to a main in-feed conveyor line. As cartons reach their sort destination, small blocks, known as shoes, slide across the conveying surface to gently push the cartons down another conveyor spur or chute. Sliding shoe sorters are most commonly located in shipping areas for sorting to dock lanes. They can also be used to sort to packing stations, to picking areas and from receiving to processing areas. Many facilities use the same sorter for multiple duties, feeding different process areas each time the carton is sorted. To work effectively, the items sorted have to be suitable for conveying in cartons and totes.
BOMB BAY SORTERS represent an infrequently used, yet effective high-speed sorting option for operations handling non-fragile items. Primarily used in apparel and other soft-goods industries, they consist of a conveying surface that opens at the bottom, like the bomb bay door on an airplane, when products reach their sort destinations. The items drop into a carton, tote or accumulating chute below. They typically represent a lower-cost option than other high-capacity systems, with speeds of up to about 7,000 sorts per hour.
TILT-TRAY SORTERS are among the fastest on the planet, able to achieve rates of 8,000 sorts per hour and above. They consist of a circular conveying path with small trays affixed to the top of the path. Items are placed on the trays either automatically or manually at induction stations. The items move around the circular track until they reach their sort destinations, at which point each tray tilts and the item slides off into sorting chutes. These systems are ideal for catalog and e-commerce applications where batch picking is performed before sorting to a large number of customers. They are also used in packing and parcel applications. In order to work properly with these sorters, items must be small enough to fit on the tray and able to ride without falling off. In other words, apparel is a good fit, bowling balls are not. Though capable of achieving high speeds, tilt tray sorters are complex and expensive systems that require more maintenance than other sorting systems.
CROSSBELT SORTERS are siblings of tilt trays, using the same basic design of a circular track. The main difference is the conveying surface. Trays are not used. Instead, items sit atop small conveyor belts placed perpendicular to the path. When an item reaches its sorting chute, the belts power on to force the product to its destination. This "positive" discharge is a key advantage of crossbelt sorters, giving them the edge over tilt trays in operations that move items that don't easily slide off of a tilt tray. Crossbelt sorters offer sort rates similar to those offered by tilt trays—in the range of 8,000 sorts per hour and above. Purchase and maintenance costs are similar to those associated with tilt trays, which means the decision of which system to use often comes down to the specific application and personal preference.
