Distribution and Materials Handling Focus: Our Weekly Feature Article on Topics Related to Distribution Management and Material Handling Strategies and Technologies  
 
 
  - February 22, 2008 -  

A Sortation Systems Dictionary

 
 

Half the Battle is Understanding Key Terms; from Pick Modules to Induction

 
 


SCDigest Editorial Staff

Half the battle for companies and individuals new to sortation systems is getting comfortable with all the terminology used by vendors and consultants. Below are a number of key terms that it pays to know going into a sortation project. These terms are taken from an article on from The Supply Chain Digest Letter on Sortation Systems for Distribution. Visit our Sortation System Resource Page.

Shoe Sorters: These sorters, sometimes also referred to as slat sorters, use a series of “horizontal shoes” that slide across the conveyor surface to push product (cases/totes) down a divert lane. Bi-directional shoes allow items to be offloaded to both sides of a sorter, which can improve space efficiency while providing greater design flexibility. Capable of very high speed sortation, but also used in more medium-speed/throughput environments, this is probably the most common form of sortation technology today. Maximum rates now in excess of 300 cartons/per minute.

Pop-Up Wheel Sorters: Rows of powered wheels “pop up” above the main conveyor line, generally at an angle, and make contact with the bottom surface of the product to be sorted. The wheels recede after diverting the product. Once the primary technology used in carton sortation, Pop-Up Wheel sorters are slower than Shoe sorters and as a result have lost market share, but can still provide an effective and economical alternative for smaller and medium throughput requirements. Maximum rates can be as high as 130 cases per minutes.

Tilt-Tray Sorters: Designed for high-speed sortation with a very high quantity and density of divert locations, a tilt-tray consists of continuously moving train of independent “trays” that tilt when directed by the control system, causing the product to slide down a chute. Tilt-trays are often used at an individual item level, diverting a series of picks into shipping cartons for example in retail or ecommerce, especially for shoes and apparel. Tilt-trays can handle as many as 10,000 or more pieces per hour, and are generally best suited for small- to medium-size items and high-throughput operations where many sorting locations are required. Picked product is brought to the tilt tray sorter either manually or by conveyor. Enables batch picking of split case picks. Sometimes used for sorting cartons.

Cross-Belt Sorters: In a sense, a form of a tray sorter is where the tray is replaced by a short belt conveyor section with its direction of travel orientated perpendicular to the line of travel of the sorter. When directed, the belt motor is energized, which propels the product to a divert lane. Cross-belt sorters can achieve higher rates than tilt-trays due to having powered conveyor belts on every carrier in the system. Cross-belt sorters are used in many of the same markets as tilt-tray sorters, but can handle a wider variety of items.

Pick Modules: Dedicated areas, specially constructed areas for full case or split case picking served by a takeaway system such as a belt conveyor. Pick modules are often multi-level rack structures using pallet or case flow storage (see below). Generally in a pick module, there is storage on either side of the takeaway conveyor, and pickers on both sides can place cartons on it for conveyance to the sorter.

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Induction: The process of identifying the product to be sorted, getting the product moving at sorter speed, and ensuring there is the appropriate gap between boxes for the specific sorter application. The sorter needs to be fed by the induction system at the rate of the intended sorter performance (e.g., 125 per minute). Usually involves a bar code scanner and a series of belts that move at different speeds to create carton gaps – a process made more challenging by today’s very high speed sorters that can operate with very small gaps between boxes.

Merge: As the name implies, a merge point is basically an area where product from one part of the total conveyor system is inserted into the main conveyor line leading to the sorter itself. Increasingly sophisticated controls and technology are being used to accomplish merge processes at ever-higher speeds today. A typical example would be a merge coming out of a given pick module. Merges come in several types, generally named according to the way the physical design looks (sawtooth merge, herringbone merge, etc.), and each is best suited to different applications.

Print and Apply: Sub-systems that apply a label in-line on the conveyor system prior to the merge point. Sortation systems generally have each carton uniquely identified, usually by a bar code (in the future, perhaps RFID). If cartons are not pre-labeled with a unique bar code identifier, then companies have two choices: pickers can apply labels to cases during the picking process, which degrades productivity, or it can deploy an in-line print and apply system. Generally, a scanner reads a generic case bar code, the WCS looks up order and other information for that SKU, a label is then printed and applied at high speed (approximately 60 per minute) with a unique serial number for that case. That label often serves as the shipping label, such as the UCC-128 label used by consumer goods companies shipping to retail.

Pallet Flow Rack: A storage type often use in full case picking modules that presents full pallets of product from which cases are picked and placed on take-away belt conveyors. Pallet flow rack can be single or double deep (or sometimes more); fresh pallets are replenished from the back; empty pallets are removed from the front.

Case Flow Rack: A storage type often used in split case pick modules that presents a single open case to order pickers. When the case is emptied, it is removed, and gravity forces the next case to the picking position. Replenishment of case flow racking is from the back.

Accumulation Conveyor: A special type of conveyor designed to stop, hold and eventually release products on a conveyor system. So-called “zero pressure” accumulation conveyors do this without creating pressure between boxes being accumulated. This type of conveyor, used before merge points and sometimes other areas of a system, is very expensive on a per-foot basis.

Slug: A group of boxes that moves as a set on the conveyor system. The most common application may be merging a slug from a pick module onto the main conveyor. The control system will determine the size of the slug, which may be all the boxes awaiting a merge, or just a portion of them, depending on conveyor conditions at the time.

Wave: A grouping of customer orders for release for order picking. Typically, pick waves are designed to last between 45 minutes and two hours. Any number of criteria may be used to group orders, such as outbound carrier(s), customer(s), priority, value-added services, and other criteria. A wave must be synchronized with sortation system activity.

Utilization: The actual throughput achieved versus the theoretical maximum. If the system is designed to sort at a rate of 100 cases per minute, and over a given period of time the average number of cases sorted is 60 per minute, the utilization of the system is 60%.

 
     
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