Batch Picking and Sortation
The Design of High Volume Case Pick Operations
by James M. Apple, Jr.
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Batch Picking and Sortation
The Design of High Volume Case Pick Operations
James M. Apple, Jr.
ABSTRACT
Detailed design of conveyor systems involves analyses that reach far beyond simply the selection of the most
appropriate system concept. This paper discusses issues and trade-offs in systems for batch picking and sortation
of full cases.
INTRODUCTION
As a consultant and system designer, I have always been interested in choosing the right functional technology,
with the proper cost/benefit relationship. More recently, as I moved about in the world of the system supplier, I
rediscovered that choosing the "right" technology still leaves a great deal of system design work to do. Of course,
as in most technical areas, there are rules-of-thumb and accepted practice to serve as guidelines. Ultimately, there
is the panacea of simulation to "ensure" system viability.
However, rules-of-thumb do not break new ground. And, today's imperative is often aimed at "breakthrough"
improvements. Simulation cannot invent trade-offs in system design. It can only help to evaluate those issues that
we remember to test as the model is built. The following text outlines some of the issues I have encountered
regularly in the design of case picking systems. I hope that by reading through these, the reader will be more
educated and will understand what questions are important to ask, and if the reader is a designer, I hope these
comments will help complement what you already know but in a way that improves your designs.
Over the past few years I have become increasingly interested in case picking systems that employ batch picking
and sortation. They are particularly interesting for several reasons:
1. Order picking: Already documented as the dominant cost element in warehousing, has been found to be
mostly "not" picking. Pickers don't spend much of their time at the pick face. Most of their time is spent
traveling around, waiting to get at a pick face, discovering that what they come to pick isn't there, off
loading the picked orders, organizing their work, etc. Consequently, there is a great incentive to reduce
the non-picking portion of the job.
2. Evolving logistics systems: Based on rapidly shifting customer demands, are creating more orders of
smaller size to satisfy the same business volume as before.
3. Centralizing inventories: To improve availability at lower total inventory levels, by capitalizing on creative
transportation alternatives provides interesting economies of scale for mechanization.
4. Now that distribution is everybody's "strategic weapon," the focus is back on reducing costs
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ORDERPICKING ENVIRONMENT
Order picking generally creates a change in the unit of handling for individual products. For example:
Receiving/Storage unit
Truck Load
Pallet Load
Cases
Picking/Shipping unit
Pallet Load
Case
Pieces
Orders are often comprised of products in each handling unit, depending on order size and product popularity.
Usually, each handling unit type comes from different sub-system-i.e. full cases and individual pieces of the same
product are not selected from the same stock location.
Each of these handling unit conversions represents an interesting, and a real part of what must be considered in
warehouse design. To illustrate some of the areas where we can use some help in evaluating trade-offs, I would
like to focus on the pallet-to-case conversion, which is otherwise known as "the case picking process."
Almost all large system environments include a range of products with widely varying popularity and an equally
broad range of customer order sizes. To satisfy these requirements most systems are of hybrid design, comprised
of different levels of technology for portions of those ranges and which ultimately must bring products together at
the same time at the shipping dock. Figure 1 illustrates a typical distribution of product popularity in the grocery
business. Notes on the figure provide
rough descriptions of product/order
activity. Many, many other businesses
exhibit similar problems.
Figure 1.
Distribution of Product Activity as a %
of Total SKUs
CHOOSING AN ORDER PICKING PROCEDURE
Traditionally a picking list for a customer order is given to a picker who travels the pick path, adding cases to a
pallet or roll cage as he/she goes. Large orders may require interrupting the process to drop off full pallets as they
are completed. Alternatively a large order may be segmented and assigned to several pickers, thereby reducing the
elapsed picking time.
Order picking has several advantages, such as:
1. Simple paper pick lists may be used.
2. One person can be made responsible for an entire order.
3. Products may be easily grouped into families along the pick path to ensure that policies, such as placing
the heaviest products on the bottom of an outbound pallet, may be easily observed
4. Activity based location assignment can significantly reduce the pick path for an order.
5. Since pickers are each picking one order and operating independently, it is easy to assign work, keep them
busy and measure results.
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However, as product offerings increase (SKU proliferation), the warehouse begins to fill with more and more
material. Consequently, the pick path gets longer, and as orders or order quantities become smaller, the path must
be traveled more frequently. Congestion and delays can occur near fast moving products in the larger volume
facilities.
The challenge in larger facilities is to develop a system that maintains most of the advantages of smaller, order
picking environments, while addressing the problem of increasing levels of "not" picking.
Batch Picking
One alternative is to batch the picking requirement for several orders and then sort the cartons that are picked to
individual orders as the picker goes along. This is known as multi-order picking or cluster picking. In its simplest
form, a picker may take two pallets at a time along the pick path and simultaneously pick two orders. This
approach can be extended by pulling a "train" of several pallets or roll cages. In this way most of the benefits of
simple order picking are maintained. The technique is limited by the length of the train to about 4 or 5 orders. As
the train size grows, aisle widths and congestion become bigger factors.
Wave Picking
The next variation is radical. An interesting and even more powerful concept is to accumulate the requirements for
a larger number of orders (50-100); pick them as a batch and convey the cartons to a high speed sortation machine
and then divert them to chutes/spurs dedicated to each order. At the end of the spur they are loaded onto a pallet
or roll cage by a second operator. This is known as "wave picking". The general principle is to handle each case
efficiently two times, rather than inefficiently once. Clearly, the rate for each of the two handing steps, batch
picking and container loading, must be considerably more than twice as fast as the single step in traditional serial
order picking to make the concept worthwhile.
The table in Figure 2 shows that labor savings sufficient to justify the cost of mechanization only occurs when the
rate for picking individual orders is expected to be slow and the total case picking requirement is high. This occurs
when the same product is required in small quantities for many orders at the same time. This is a common
occurrence in centralized distribution of high volume consumer products to a chain of retail stores. Groceries are a
typical example. Shoes, Hardware, and PC Software are other examples.
Figure 2.
While batch picking is more efficient, it only starts to pay off at higher volumes
Because very popular products may be ordered several cases at a time for each store/order, they generally can be
picked at higher rates in the traditional order picking fashion. The midrange and slower moving products are more
time consuming to pick and represent the best opportunities for wave picking and sortation. Once a major
equipment investment can be justified for those products, it sometimes makes sense to use the same method for
the fast movers as well, if there is sufficient capacity.
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CHOOSING MATERIAL HANDLING EQUIPMENT
The picking of cases of product in wave picking environments may be accomplished in a number of ways. Several
common ones are listed below. A few are illustrated and described below.
Pick-to-pallet on pallet jack
This is the simplest and most common way to accumulate cartons
for a batch of customer orders. The investment in equipment is low,
however, it is difficult to pick from any location except at floor level,
so the pick path may be quite long. Aisles must be wide enough to
accommodate pickers passing one another and most probably, lift
trucks for storage and replenishment moves. Cases must be
rehandled to induct them onto the sorter.
Pick 2-4 batches to pallet or roll cage train
This method is very similar to picking to a pallet jack, but permits
several batches to be combined to reduce the number of times that
a picker must travel the pick path. To make the picking most
efficient, a special pick list aggregating the requirements for the
batches and then indicating the allocation by batch must be
prepared.
Order Picking Truck
This method still requires
a separate handling step to induct the picked cases onto the sorter, but
the pick path can be much shorter because the orderpicker can access
several levels in the pallet rack. It is especially useful for very slow
moving products. With a narrow aisle, the picker can reach products on
both sides of the aisle, but it is difficult(if not impossible) for more than
one picker to share the aisle or to perform replenishment operations
during the time of picking.
So far, all of the picking technologies described have required that the
wave picker perform two operations: first pick the item and then induct
the item
onto a conveyor to go to the case sortation system.
In
order to improve the throughput and efficiency of
pickers, the following case pick technologies can be
employed:
Pick-to-belt
Cases are placed by the picker directly on a belt
conveyor from pallets along the pick face. Pickers
walk from one pick to another to complete the
requirements for a batch in their zone, and then
repeat the process for each batch. A zone may be defined as large or small as necessary to permit the picking of all
cases within the allocated batch time. Belts are typically placed on mezzanines over one another to improve space
utilization.
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Pick car
This is a special device that combines the features of an
orderpicking truck that can access multiple levels of pallet rack
and pick-to-belt in which picked cases are conveyed directly to
the sorter. Because only one pickcar may be placed in an aisle,
the case flow rate from that aisle is limited by the capacity of
one picker.
Pallet-to-picker
When the number of cases of a single product required
for a batch will consume a full pallet, the pallet may be
brought directly from reserve storage to a special
depalletizing workstation where cases may be efficiently
transferred to a conveyor infeed to the sorter. In some
systems, this is done even when the batch quantity is
less than a full pallet. However, the unused portion of
the pallet must be returned to storage or to a staging
area until it is needed again. The ergonomics of the
depalletizing station can be improved with the use of
lifts, turntables and even mechanical lift assistance as
shown in the picture to the left..
Robotic picking / Depalletizing
Using robots or other mechanical devices to transfer cases
from a pallet to a conveyor is an extension of the palletto-picker method. Some systems use a robot to transfer
one case at a time, while others transfer an entire layer
from the pallet.
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Figure 4.
Choosing mechanization
based on item activity
The selection of the right
material
handling
technology depends on
analysis of total volume,
individual product volume,
frequency of pick of a
product, space availability,
cost and a number of other
factors. A combination of methods may be used, but using more than three may be complicated to plan and
manage. Figure 4 on the following page uses the product activity distribution to illustrate some possible
combinations of technology for varying levels of mechanization.
ADDRESSING THE ISSUE OF LABOR IMBALANCES WHEN WAVE PICKING
When orders are picked by wave, a facility will be organized into zones and operators assigned to work within
those zones. This is done to keep the pick tours relatively short as an operator may only be assigned picks that are
within his zone.
Unfortunately, for each wave/batch of orders to be picked, the biggest challenge is to synchronize the flow from
the various picking zones. Some of the factors to consider in both system design and operation include:
1.
The number of cases from each area for each batch may vary greatly.
2.
The workload in a zone usually requires fractional operators for each batch, so that when "whole" people
are assigned, the zone will have a slight over, or under capacity and as a result, will finish ahead or behind
the other zones.
3.
Maintaining a family grouping of products further complicates the assignment of stock locations and the
control of picking activity.
The implication of these design issues is that when products are picked in batches from multiple pick zones, they
do not all arrive at the sorter in the same time window. Travel distances, varying pick rates and an imbalance in the
number of cases for a batch from each zone will cause some zones to finish early and others to finish late. This can
lead to a lot of operators standing around waiting for the next wave to begin while others in more heavily hit zones
scurry to finish their assignments. This also means that for some period of time towards the end of the wave, the
sorter isn't being fully utilized.
Buy Excess Capacity
In many facilities, this issue is simply, although expensively, addressed by purchasing more-than-sufficient capacity
in all areas. Picking is then controlled to use only as much of that capacity as is required for each batch. This
approach still does not address the loss in sorter utilization resulting from these factors, as illustrated in Figure 5.
(Note: In recent years, a dynamic wave planning logic, commonly referred to as “waveless,” has been used in
some systems to address the start/end of wave losses in capacity and productivity).
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Figure 5.
Sorter utilization loss
at the end of waves
Floating Zones
Alternatively, the control system for picking may shift capacity (people) between, or even during batches. One
method for shifting capacity is illustrated in Figure 6. The pick path may be viewed as continuous. The length of
path assigned to an operator will vary from batch to batch depending on the density of demand along the path,
and even may be based on individual operators performance capability. This concept is known as a floating zone
because each operator will be assigned a different set of continuous locations to pick from during each wave, but
the amount of work that any given operator has at the start of the work is the same. By doing this, the work in the
different zones complete at more or less the same time.
Multiple Forward Pick Locations
Another method of addressing zone imbalances is to provide picking locations for the most popular products in
several zones. The control system can shift the source of these products from batch to batch(i.e. zone to zone) to
help balance the workload. For example, imagine that an Item #123 is available in Zone A and Zone B within a
warehouse. If during the wave planning phase, Zone A's workload looks to be noticeably greater than Zone B's, the
system can shift all or some of the picks for Item #123 to Zone B.
ADDRESSING THE ISSUE OF SORTER UTILIZATION
In wave picking environments, cases will be flowing from each of the picking sub-systems and from several zones
within some of these sub-systems at different rates. The sorter will run at a constant speed and when a low flow
results in underutilization, that capacity is lost forever. So, if it is important to achieve high sorter utilization to
satisfy the total demand,
then the picking and
conveying
system,
together, must maintain a
steady flow of products. To
do that, some method
must be designed to
accumulate and release
cases within the system.
Figure 7
Schematic flow from
picking to sorter
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How Much Accumulation Should You Buy?
The merging of flows and the buffering of peaks and valleys in the instantaneous rates is a critical part of any wave
picking system design. Figure 7 illustrates a combination of picking modes, all feeding into the same sorter. There
are a variety of different ways and areas in which accumulation could take place. Of course, the ideal solution
would be to provide conveyor lanes to accumulate all the cases from all picking areas for a single wave, and then
to release it to the sorter at a constant rate. This is often a practical solution for the sortation of small items which
can be accumulated with 10-20 pieces in a tote box. But, the conveyor length to hold a complete batch of full cases
is usually prohibitively expensive. So, a trade-off of accumulation, control of picking and sorter capacity must be
evaluated.
Where Should Accumulation Take Place?
It would seem to make the most sense to put the accumulation as close as possible to the sorter so that it can be
shared by all picking zones. However, in a large system, the distances from the various picking zones to the sorter
could require that conveyors for transport also be used for accumulation. This solution may not be cheap;
accumulation conveyor is much more expensive than traditional motorized conveyor.
For the very high volume products that may be needed in full pallet quantities at the sorter or for the very slow
moving products that are batch picked to a pallet or roll cage to be inducted into the sorter, accumulation is easier.
These products may be picked or retrieved from storage in advance of the batch for which they are needed and
kept in pallet form until the time comes for their induction. This of course is not an option for pick-to-belt or pick
car situations.
CHOOSING THE RIGHT SORTER
There are a number of sorter types to choose from for case sortation. Selecting the one that is most appropriate
for a specific system depends on the sorting rate required and the handling characteristics of the products passing
through. Regardless of which type of
sorter is selected, many of the other
design issues are similar.
Figure 8 indicates an in-line sorter with
a recirculation loop.
Figure 8
In-line sorter schematic
Within this simple configuration, a
number of important trade-offs exist.
They relate to:
 Number of Spurs
 Spur Length
 Recirculation
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Number of Spurs
In general, a spur is assigned to each customer order in the wave, however, there are some reasons to create
more, or even fewer than this number.
 Large orders, which may have very high flow rates at times during the sortation of the wave, may be
assigned two or more spurs to provide some temporary accumulation or to permit more than one
operator to palletize cases for that order at the same time.
 Additional spurs may be used to provide a separation of product families for each order. This may be
traded-off against more sophisticated control of the picking process.
 Additional spurs may be provided to permit the overlapping of waves. Cartons arriving early for orders in
the in next batch may be assigned to a spur not utilized in the previous batch.
In some cases, fewer spurs may be provided if, for example, two small orders are sent to the same spur and
manually separated during the palletizing process.
Spur Length
If there were always a loading operator available when a case arrived in a spur, the spur would need no
accumulation capacity. But, because there are usually 5 to 10 spurs/operator, it makes sense for the operator to
move from spur to spur when several cases have accumulated, so that the walking time is reduced.
When a spur is full because the loader has not been able to service it, newly arriving cases cannot be diverted and
must recirculate.
So, trade-offs exist here between spur length, response time of servicing, number of spurs assigned to an order
and utilization of sorter capacity for recirculation.
Recirculation
Cases which cannot be diverted on their first pass across the sorter may do one of three things. They may:
 Exit the sorter in a "dump" lane and be, manually moved to the end of proper spur.
 Recirculate directly back to the sorter induct point.
 Divert into a separate recirculation / accumulation lane.
The first alternative is an easy solution if a company elects not to do the analysis to choose between alternatives
#2 and #3; however, it should not be overused. On the other hand, it should not be overlooked as a solution for
exceptional occurrences, when a mechanical solution might be quite expensive.
Alternative #2 is acceptable as long as there is sufficient sorter capacity to handle recirculation and accept new
incoming cases. If there is insufficient capacity to do so, priority may be given to the new cases, but when the
recirculation line fills, the system will slow down. Or, priority may be given to recirculation, slowing the rate of
accepting new cases. This could, in turn, slow down the picking process. This is not desirable, but might be the best
way to automatically adjust the system, so long as it it does not occur so often as to adversely affect picking
productivity. The additional lane in alternative #3 may be used to hold cases that have arrived early and then
release those cartons when spurs are available for them.
SUMMARY
In summary, when constructing higher volume case pick facilities, there are number of questions to be answered in
the design process.
 What picking methods are appropriate?
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



How should products be assigned?
o to picking method
o to zones within each method
How should picking be controlled?
Where should accumulation be provided?
At the sorter,
o How fast should the sorter be?
o How should recirculation be managed?
o How many spurs are required?
o How should orders be assigned to spurs'?
Of course, there are many more detailed design issues to address, but those mentioned here begin to illustrate the
complexity of system design involved in making the "right" concept work effectively.
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