Order Picking, Warehouse Layout, Methods, Equipment, Storage Methods

  • Post last modified:1 June 2021
  • Reading time:12 mins read

What is Order Picking?

The order picking or order preparation operation is one of a logistic warehouse process. It consists in taking and collecting articles in a specified quantity before shipment to satisfy customer orders.

It involves taking and collecting articles in a specified quantity before shipment to satisfy customer’s orders. It is a basic warehousing process and has an important influence on supply chain’s productivity. This makes order picking one of the most controlled logistic processes. It is one of the warehouse management system functionalities.


Definition of Order Picking

Order Picking involves the process of clustering and scheduling the customer orders, releasing them to the floor, picking the items from storage locations, and disposal of the picked items.


Methods of Order Picking

  1. Piece-Picking Methods
  2. Basic Order Picking
  3. Zone Picking
  4. Batch Picking
  5. Wave Picking
  6. Case-Picking Methods
  7. Basic Case-Picking Method

Piece-Picking Methods

Piece picking, also known as broken case picking or pick/pack operations, describes systems where individual items are picked. Piece pick operations usually have a large share in the thousands of items, small quantities per pick, and short cycle times.

Mail order catalog companies and repair parts distributors are good examples of piece pick operations.

Basic Order Picking

In the most basic order-picking method, product is stored in fixed locations on static shelving or pallet rack. An order picker picks one order at a time following a route up and down each aisle until the entire order is picked. The order picker will usually use some type of picking cart.

The design of the picking flow should be such that the order picker ends up fairly close to the original starting point. The picking document should have the picks sorted in the same sequence as the picking flow.

Zone Picking

Zone picking is the order picking version of the assembly line. In zone picking, the picking area is broken up into individual pick zones. Order pickers are assigned a specific zone, and only pick items within that zone. Orders are moved from one zone to the next as the picking from the previous zone is completed (also known as “pick-and-pass”).

Usually, conveyor systems are used to move orders from zone to zone. In zone picking it’s important to balance the number of picks from zone to zone to maintain a consistent flow.

Batch Picking

Batch picking / Multi-order picking in batch picking, multiple orders are grouped into small
batches. An order picker will pick all orders within the batch in one pass using a consolidated
pick list. Usually the picker will use a multi-tiered picking cart maintaining a separate tote or carton on the cart for each order.

Batch sizes usually run from 4 to 12 orders per batch depending on the average picks per order in that specific operation. Batch picking is rarely used in case of pick operations primarily because of the physical size of the picks. You are unlikely to have enough room on a pallet to pick multiple orders.

Wave Picking

It is a variation on zone picking and batch picking where rather than orders moving from one zone to the next for picking, all zones are picked at the same time and the items are later sorted and consolidated into individual orders/shipments.

Wave picking is the quickest method (shortest cycle time) for picking multi item orders.However the sorting and consolidation process can be tricky. Operations with high total number of SKUs and moderate to high picks per order may benefit from wave picking. Wave picking may be used to isolate orders by specific carriers, routes, or zones.

Case-Picking Methods

Case picking operations tend to have less diversity in product characteristics than piece picking operations, with fewer SKUs and higher picks per SKU.

Basic Case-Picking Method

This is the most common method for case-picking operations. Rather than product stored on static shelving, case-pick operations will have the product stored in pallet rack or in bulk in floor locations.

The simplest picking method is to use a hand pallet jack (or motorized pallet truck) and pick cases out of bulk floor locations however many operations will find that going to very narrow aisle (VNA) pallet racking and using man-up order selectors or turret trucks will provide high storage density and high pick rates.


Order Picking Equipment

  1. Static Shelving
  2. Carton Flow Rack
  3. Carousels
  4. Automatic Storage and Retrieval Systems (ASRS)
  5. Automatic Picking Machines
  6. Pick-to-Light
  7. Bar-Code Scanners
  8. Voice-Directed Picking

Static Shelving

The most common equipment for storage in piece pick operations, static shelving is designed with depths from 12” to 24”. Product is either placed directly on the shelving or in corrugated, plastic, or steel parts bins. Static shelving is economical and is the best method where there are few picks per SKU or where parts are very small.

Carton Flow Rack

Carton flow rack is similar to static shelving with the exception that rather than shelves, there are small sections of gravity conveyor mounted at a slight angle. Product is stocked from the rear of the flow rack and picking is done from the face.

The product can be stocked in cartons or small totes or bins. As a carton or tote is emptied, it is removed from the rack and another one will roll into place. Carton flow rack is most useful where there is a very high number of picks per SKU.

Carousels

Horizontal carousels are a version of the same equipment used by dry cleaners to store and retrieve clothing. They have racks hanging from them that can be configured to accommodate various size storage bins. Generally an operator will run 2 to 4 carousels at a time avoiding the need for the operator to wait while one unit is turning.

Picking is usually performed in batches with orders downloaded from the host system to the carousel software.

Automatic Storage and Retrieval Systems (ASRS)

An ASRS is a system of rows of rack, each row having a dedicated retrieval unit that moves vertically and horizontally along the rack, picking and putting away loads. ASRS systems are available in mini-load types that store and transfer product on some type of tray or in bins, and unit-load types that transfer and store pallet loads or other large unitized loads.

In addition to the automation features, ASRS units can provide extremely high storage density with capabilities to work in racking up to 100 feet high. Unfortunately, the high costs of ASRS equipment and the length of the retrieval times make it difficult to incorporate into a piece picking operation.

Automatic Picking Machines

Fully automated picking machines (such as A-frames) are still pretty rare and are used only where very high volumes of similar products are picked such as music CDs, or, where high volume in combination with high accuracy requirements exist such as pharmaceutical fulfillment.

Pick-to-Light

Pick-to-Light systems consist of lights and LED displays for each pick location. The system uses software to light the next pick and display the quantity to pick. Pick-to-light systems to have the advantage of not only increasing accuracy but also increasing productivity.

Bar-Code Scanners

Though very useful in increasing accuracy levels, bar-code scanners in a fast-paced piece-pick operation tend to become cumbersome and can significantly reduce your pick rates. With proper training, tracking, and accountability, you can get very high accuracy rates in order picking without scanners. I find they are better suited to case pick, pallet load, put away, and order checking operations.

Voice-Directed Picking

Voice technology has come of age in recent years and is now a very viable solution for piece pick, case pick, or pallet pick operations. Automated conveyor and sorting Systems. Automated conveyor systems and sorting systems will be integral to any large-scale piece pick operation. The variety of equipment and system designs is enormous.


Warehouse Layout

Before explaining storing, order batching and routing methods is convenient to have an overview of the layout warehouse and its principal elements. Warehouses are divided by aisles, and the aisles contain shelves, where the products will be stored.

The goal of warehouse layout is to optimise warehousing operations and achieve high efficiency. In order to achieve it, some elements play a key role. A brief description of these elements is given below:

  1. Depot
  2. Aisle
  3. Cross Aisle

Depot

Normally, pick lists are generated or received electronically at the depot, and then the picker starts to retrieve products. Depending on the situation of the depot, and the facilities used in the warehouse there are different sort of depots:

  • Central Depot: The picker only can start and end at the same point. Commonly, it is located in the front end of the aisles.
  • Decentral Depot: It is the alternative for central depot. It is used when terminals or RF scanners and conveyor operate in a warehouse. Conveyor allows drooping off products at any location of itself; therefore, it facilitates the picking process. In order to maximize the advantages of decentral depot the starting point needs to be larger than in central depot.

Aisle

The aisles of a warehouse are the main spaces where pickers travel in order to retrieve products from the shelves. Some of the variables related to aisles are:

Length: Is the distance between the front end and the rear end.

Distance between Aisles: Is the distance between the centre of one aisle and the centre of the next aisle. Depending on the width, aisles can be sorted as:

  • Narrow Aisles: The picker can retrieve products from both sides of the aisle, without the need to realize any lateral displacement.
  • Wide Aisles: As opposed to narrow aisles, in this kind of aisles, due to a major distance, the picker has to realize lateral displacements in order to pick products from both sides.
  • Numbers of aisles.

The total width of a warehouse is the distance between aisles multiplied by the number of aisles.

Cross Aisle

Is an aisle perpendicular to the aisles used to storage products, the main aisles. It enables aisle changing and facilitates moving around the warehouse. If a warehouse has cross aisles it is divided in blocks by the cross aisles. The variables related to cross aisles are:

  • Width: Is the distance between different blocks.
  • Number of cross aisles.

Storage Methods

Products need to be stored, and there are several storage methods for assigning storage locations to the received items, they are explained below:

  1. Random
  2. Closest Open Location
  3. COI Based (Cube-Per-Order Index)
  4. Volume Based
  5. Class Based (or ABC storage)
  6. Intelligent Storage Methods
  7. The cloud
  8. Product Group Affinity

Random

Items are randomly assigned to an available location. On the one hand, random storage increases the average travel time compared to other storage methods. On the other hand, it reduces aisle congestion and increases the uniform utilization of the warehouse.

Closest Open Location

This is probably the simplest storage method. Incoming items are allocated to the closest empty location. Some studies show that in a long run random and closes-open-location methods converge.

This method is mainly used when order pickers have to decide locations by themselves. The main problem of this method is that in a long term items are scattered over the warehouse.

COI Based (Cube-Per-Order Index)

This method defines COI of an item as the ratio of the required storage space to the order frequency of the item. Items are stored by increasing COI ratio and locations on increasing distance from the depot.

Volume Based

it assigns items to storage locations based on their expected order or picking volume. The most accessed items are located near to the depot area. The main advantage, compared to random storage, is the reduction in travel time. However, uniform warehouse utilization and aisles congestion increase.

Class Based (or ABC storage)

It is based on the division of items and storage locations in the same number of classes, in order to assign the items to one location. Class-based storage fuses randomized and volume-based methods.\

The difference between this method and the volume-based is that this one assigns items to storage location following a group basis; however, volume-based follows an individual basis. And regard to randomized, it provides a saving on travel distance.

In order to divide items into classes Pareto’s method is used. The items are subdivided into three categories, based on the nature as well as the size.

Intelligent Storage Methods

Some of the strategies described above are suboptimal from the point of view of space utilization. In several markets consumer’s demand is cyclical, this leads to an optimal space utilization during peak seasons but to an inefficient during the majority of the year. Another problem of these strategies is that they do not take into account consumer purchase.

Take into account consumer purchase can lead to a reduction of travel distance, for this reason some alternative storage methods were developed. Despite of these methods can increase savings, they have received little attention.

The cloud

This strategy distributes items randomly to several different warehouses zones, which are called clouds. The objective of this strategy is to create clouds that at any point in time contain the majority of all items necessary to fulfil a customer order. Thanks to disperse the items in different clouds the travel distance is minimized, therefore savings increase.

The benefits of this strategy outweigh the additional costs that can result of storing the items in several zones. Rubenstein (2006) predicted a 10-15% cost saving applying this storage method for Amazon’s Fulfillment Center.

Product Group Affinity

Product group affinity direct items to virtual warehouse zones based on product group, it is very similar to class-based storage. This strategy is based on the hypothesis that customers tend to order items of the same category, therefore thanks to this the distance travelled between picks will decrease and productivity will increase.

Another feature of this method is that items are stored randomly in the virtual warehouse zones, therefore it fuses the best of direct and random methods.


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