Introduction
Unit sortation is the backbone of any batch picking operation. The concept of unit sortation has been widely used for order picking in retail distribution for decades, and more recently in eCommerce fulfillment. Despite its long-standing presence in the industry, there continues to be innovation in unit sortation technology that enables enhanced speed, accuracy, and capacity in warehouse operations. Furthermore, the wide range of unit sortation solutions, some being entirely manual and others fully automated, means unit sortation has a place at all scales of operation.
What is Unit Sortation?
Put simply, unit sortation is the process of sorting an individual item (unit) to a desired destination. In practice, the items being sorted are typically batch picked and the sort destinations usually represent a single order. Unit sortation groups the exact items required for an order, preparing them to be quickly packed, even though they were not picked together.
Unit sortation differs from other warehouse automation methods in that unit sorting solutions accommodate a wide range of product types. Everything from a bottle of shampoo to a shoe box or even a knit hat in a polybag may need to be handled within a single system. The items being sorted, as well as the required sorting rate, will influence the unit sortation solution that is right for your operation.
Benefits of Unit Sortation
The benefits of unit sortation go hand-in-hand with the benefits of batch picking. In fact, in order to enable batch picking, some level of unit sortation will be required.
Basic Warehouse Operations Processes
1. Increased Efficiency & Accuracy
While unit sortation does add an additional step between picking and packing, it is justified by the increased efficiency realized through batch picking. Depending on the type of unit sortation solution implemented, this additional step can also serve as a valuable verification of order contents, increasing order accuracy.
2. Multi-Purpose Use
Some unit sortation solutions can even be configured to handle other processes beyond their primary use case. Common examples include parcel sorting and returns processing. The multi-purpose use of a unit sorting system can strengthen the business case for investment in such equipment.
Types of Unit Sortation
With a wide range of unit sortation applications comes several types of unit sorters. In this section, we will examine different unit sortation solutions and when it makes most sense to use them.
Unit Sortation Landscape
1. Put Walls
Manual Put Wall
Among the most basic unit sorting solutions, put walls offer a simple, yet effective, way to sort batch picked product to the order level. A manual put wall consists of open shelving that is subdivided to create multiple cubbies or slots. A single order is assigned to each slot, and as items are delivered to a put wall, an associate will scan the item and be directed to put it in a slot with order demand for that item. When all items for a single order are placed in the slot, an order packer, typically stationed on the opposite side of the wall, will retrieve the items and pack the order. The now open slot will be assigned a new order, and the cycle continues.
In the context of retail distribution, a put wall slot may represent a store replenishment order, and a shipping carton may be pre-set in the slot. The product typically flows to the put wall area in full-case volumes, and associates are directed to pick the necessary units out of the cases and put them in slots based on each store order. This is called put-to-store.
The manual put wall is the lowest rate unit sortation system and is labor intensive. The larger the put wall, the greater the batch picking opportunity, and the higher efficiency gained. However, the sortation rate is also a function of the length of the put wall, therefore, as more sortation destinations are added, the sorting rate decreases.
A put-to-light system can be installed on a put wall to improve putting performance. A put-to-light system uses lights and buttons on each put wall slot to direct associates to their next location by illuminating the associated light and extinguishing it when the put is confirmed.
Example of a Put to Light Enabled Manual Put Wall
Automated Put Wall
A recent advancement in unit sortation is the development of automated put walls. These impressive systems require only a single associate to induct individual items into the system, which then uses mini-shuttle technology to sort the item to a destination bin or chute.
The automated put wall offers higher sortation rates with less labor compared to the manual put wall, however current systems are limited by the size of product they can handle. This level of technology also comes at a high cost, and may be difficult to justify outside of idealized used cases.
Example of an Automated Put Wall - OPEX Sure Sort
2. Line Sorters
Line Sorters are space efficient sorters that use a belt, slat, or similar design and return under or within the sorter chassis. A traditional example of a line sorter is a shoe sorter; however, these are often not used for unit sortation, but rather parcel sortation. Line sorters that are applicable for unit sortation include:
Cross Belt
The trays of the sorter are themselves conveyor belts that run perpendicular to the direction of the sorter. When the tray reaches the divert destination, the belt conveys the product off the tray. This design can divert products to either side of the sorter and is gentler on products than other solutions. Multiple trays may be used together for larger items, such as parcels.
Example of a Linear Cross Belt Sorter - Beumer Group
Sweeper or Pusher Bar
Overhead bars or sweepers move perpendicular to the line sorter. When the product reaches the divert destination, it is pushed off by the sweeper arm. This design can divert products to either side of the sorter and is best for small and lightweight items. The rate of this sorter is however lower than other line or loop sorters.
Example of a Linear Sweeper Sorter - EuroSort
Product can be inducted onto line sorters manually, semi-automatically, or fully automatically. Cross belt line sorters can reach sort capacity above 10,000 items per hour but generally do not offer as high of capacities as loop sorters. Additionally, line sorters are typically restricted in length by their drive mechanism, which limits the number of sort destinations possible.
3. Loop Sorters
Loop sorters are defined by their loop-shaped layout. Rather than returning the sorting tray under or within the chassis, as a line sorter would, loop sorters circulate. This provides a few distinct advantages:
- Items can be inducted onto the sorter anywhere along the sorter.
- Divert destinations can be placed both on the interior and exterior of the loop, maximizing footprint.
As with line sorters, loop sorters can also have manual, semi-automated, or fully automated product induction. Loop sorters can support the same cross belt design seen with line sorters, but also offers several other styles, including:
Tilt Tray
An articulating tray that can “dump” an item into a divert to either side of the sorter. Multiple trays may be used together for larger items, such as parcels.
Tilt Tray Mechanism - Falcon Autotech
Push Tray
A fixed tray with a that uses a bar to push items out of the tray onto a divert. Most designs are only capable of diverting to a single side. Double trays, which employ two independent bars on a single tray further increase sorter capacity.
Example of a Push Tray Sorter - Eurosort
Bombay or Flat
A fixed tray with an opening “trap door” bottom. The sorter opens the doors when over the sort destination, allowing the item to fall completely flat. Frequently used in apparel applications, items can be dropped directly into shipping cartons below the sorter.
Example of a Bombay/Flat Sorter - Bowe Intralogistics
When coupled with automated induction, loop sorters are among the most efficient and effective unit sorting solutions available. Capable of reaching sort capacities in excess of 20,000 items per hour, they also are not limited in overall size, making them ideal for operations requiring a high quantity of destination locations.
4. Pouch Sorters
Pouch sorters, or bag sorters, use cloth pouches to convey products on overhead rails. These pouches can be manual or semi-automatically loaded, with each pouch accepting a single unit, pouch sorters use a series of buffers and matrices to organize pouches by order and deliver them to a packing station.
These systems can be scaled to handle over 20,000 items per hour; however, they are limited by the size of the item that can fit inside the pouch.
Two key features that distinguish the pouch sorter from other unit sorters are its ability to deliver units to pack stations in a specific sequence, facilitating store-friendly shipments, and its usefulness in returns processing. Returned products can be inducted into the pouch sorter and temporarily stored in its buffer until needed.
Example of a pouch sorter - Dematic
5. Autonomous Mobile Robot (AMR) Sorters
Autonomous Mobile Robot (AMR) unit sorters take the tray-based mechanisms found in loop sorters, and mount them atop small, wheeled robots. The system is highly flexible, as it can handle a wide range of products and sort items across long distances.
AMR solutions can also be scaled up quickly through the addition of more robots, or even scaled back through the removal of robots during periods of lower volume. The growing availability of robots as a service (RaaS) makes scaling a system even more feasible.
AMR sorters are highly space efficient, especially when able to take advantage of vertical space. Multiple levels of drive platforms for the robots can be stacked, creating more capacity in the same footprint. Some layouts are even portable and reconfigurable.
Example of an AMR unit sorter - Tompkins Robotics
Unit Sortation Challenges
While unit sortation can improve order accuracy through additional handling and item verification, certain high speed unit sortation solutions can be prone to mis-diverts, sending items to the wrong destination. Additional handling may also increase the likelihood of damage to products.
The efficiency of any unit sortation system is reliant on the ability to feed the sorter with product. Without consistent product flow, the sorting process can become unbalanced, which can lead to wasted sorting time and long transitions between waves.
As the critical link between picking and packing, a unit sorting system may be identified as a single point of failure in many operations. Fortunately, unit sorters are often built with robustness and stability in mind. Nonetheless, operators should prioritize care and maintenance on these systems.
Key takeaway: Sort destinations in any unit sortation system are highly valuable. The quicker an order can be completed, the sooner another order can take its place, and the more efficiently the system can run.
Unit Sortation: Scale and Complexity
When scaling up from a discrete picking operation to a batch picking operation, it can be difficult to upgrade existing infrastructure to meet new demands. The introduction of a unit sortation system will not only require new equipment, but also a warehouse execution system (WES) to control it. And, once installed, not all systems can be easily modified or expanded. Therefore, it is critical to understand what unit sorter type is best for your operation, your product mix, and will accommodate your current and future needs.
Not sure how to integrate unit sortation into your operation? Maveneer has extensive experience designing optimized warehouse operations and installing complex material handling systems for our clients. Contact us today to get started.
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