Good & Plenty Mar 1, 2001 12:00 PM
, Pamela J. Davey
JobZone
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An effective way to trigger a replenishment is when pick slot
inventory falls below a preset minimum.
What is the hardest item to pick in your warehouse? That huge,
heavy crate in the far corner? The cartons on the very top level of
the pallet rack? Those tiny beads you wish you could get your
vendor to pack in bags of a hundred? No, it's any item that isn't
in its proper pick location when the worker goes there to select
it. This kind of lag is especially frustrating when you have
sufficient inventory in your distribution center, but it hasn't yet
been moved to its active pick location for order fulfillment.
Replenishments aren't a problem just for manually controlled
distribution centers. Even operations with a warehouse management
system (WMS) can sacrifice productivity if the system is managed
poorly or set up improperly. You must slot items into the right
type and size of active location, set minimum/maximum levels
correctly, and establish optimal replenishment rules to make the
best use of your resources.
Many warehouses are organized into separate active and reserve
areas. The active area keeps the frequently picked SKUs accessible
and arranged compactly, whereas the reserve area provides dense
storage of backup merchandise. Performing replenishments of the
active area on a timely basis and with the right quantities is a
key factor in maintaining effective operations.
Reserve areas often contain pallet racks and drive-in racks
serviced by lift trucks for storage of pallet loads of merchandise.
They may also contain decked racks for storing case quantities of
goods, accessed by using a man-up order picker truck.
Active areas typically use flow racks and shelving to house the
items used to fill customer orders. Since merchandise is stored on
several shelf levels, many facings can be accessed with minimal
walking. Mezzanines may be used to double- or triple-stack the flow
racks and shelving to conserve floor space, while still keeping
locations accessible without lift trucks.
Carton flow racks are suitable for storing fast- and
medium-moving items in the active areas. Full cartons of product
are placed in the back (charge side) of each rack for
replenishment. Individual pieces, inner packs, and even full
cartons are picked from the front of the rack. Since the pickers
and replenishers operate in separate aisles, they do not interfere
with each other, and the slots can be refilled on the same shift
when orders are selected.
Shelving usually houses slower-moving items, and merchandise is
picked and replenished from the same aisles. This results in
interference between pickers and replenishers if both functions
occur during the same shift, unless the pickers themselves refill
the pick slots in their area. Material handlers collect
replenishment cartons from the reserve rack and place them in
staging areas at the end of aisles. The merchandise is typically
placed on the active shelf locations in a second step.
Using carton flow racks on mezzanines makes replenishment more
complicated, since the back of the upper-level rack is typically
not accessible from the ground. Try these techniques to refill the
upper levels:
If the charge side of the flow rack is set outside the mezzanine
platform, a man-up order picking truck can replenish the flow racks
directly. The material handler builds a pallet of replenishment
cartons from the reserve rack, then removes individual cartons and
places them in the back of the flow rack, a one-step process.
If the mezzanine platform extends beyond the charge side of the
flow rack, the pallet of replenishments can be dropped on the
mezzanine and cartons can be delivered to the flow lanes with carts
or a conveyor. In this case, replenishing the carton flow rack will
be a two-stage operation.
Pallet racks can be used behind the flow rack for temporary
staging. The material handler places the pallets in the
“ready rack” using a lift truck, and then the
replenisher moves it to the proper flow rack lane, in a second
stage.
Come and get it
Determining when an active pick location needs to be replenished
and when to perform the replenishment tasks is crucial, and there
are several options to choose from. Most warehouses will select a
primary replenishment strategy plus alternative ones for especially
high- and low-demand periods.
The simplest way to trigger a replenishment is for the picker to
tell his supervisor that the location is empty. Although simple,
this emergency replenishment method is reactive and inefficient.
The material handler who does it is retrieving only a few items at
a time, so he is rushing to the pick face to replenish the stock.
Repeating this for each out-of-stock situation results in many
less-than-full trips between the reserve and active areas. In
addition, any pending orders will need to be held until the pick
slot is replenished.
A better time to trigger a replenishment is when the amount of
inventory in the pick slot falls below a preset minimum. The
material handler will then bring enough product to fill the slot to
its maximum level, so this strategy is called min/max
replenishment. The chance that orders will need to be held pending
the arrival of the merchandise is significantly reduced. This
proactive strategy allows the material handler to group
replenishments together so he is retrieving more cartons each time
he travels to the reserve area.
Min/max replenishments can be generated at different times of
the day, depending on the sophistication of the warehouse system. A
WMS will typically group orders into waves and process several
waves during the picking shift(s). The system can be set to
generate replenishment tasks when creating the waves (before
picking) or processing them (during picking).
Each of these trigger times has advantages and disadvantages. If
tasks are created before picking, the material handler delivers
material before the wave is completed, and may bring more material
than will fit in the pick slot. However, it is unlikely that you
will run out of product. If tasks are generated during picking, you
run the risk of running out of product during times of high demand,
but you won't overfill the slots.
Another option is to fill the pick slots on an off-shift basis,
a strategy called lean time replenishment. The WMS generates
replenishment tasks for active locations that are below the minimum
quantity, then batches them together to minimize trips between the
reserve and active areas. Using lean time replenishment greatly
reduces the need to perform replenishments during picking shifts
because the slots are filled before picking begins. Some systems
also permit “topping off” all the slots in an area
whether they are below minimum or not.
Manually generated replenishment works if the warehouse knows
that an especially heavy demand is coming. An operator walks
through the active area and scans each location that needs
replenishment, using a radio frequency (RF) terminal. The system
then generates the replenishment tasks for those locations.
Take it to the max
The next challenge is to set up the SKUs and locations so that
replenishment strategies will work efficiently. Establish the
minimum and maximum levels for each item/pick slot combination. The
minimum quantity, if used, sets the timing for triggering the
replenishments. If it is set too high, slots will be replenished
with small quantities, which will result in too many tasks. If the
quantity is too low, you may run out of product before completing
the replenishment.
The maximum quantity is the capacity of the location. The WMS
uses this to tell the material handler how many cartons are
required to refill the slot. If the maximum is too low, he will
bring too few cartons. If the maximum is too high, the material
handler may bring more cartons than will fit into the pick slot, so
the excess will have to be staged and put away when room becomes
available. Merchandise may have to sit in the aisles or under the
flow rack before being handled a second time.
Items should be placed in pick slots that are sized to hold
about a week's supply of each item. Some products may need multiple
facings to meet peak demand. If the minimum quantity is less than a
one-day supply, the material handlers must be sure to replenish
items promptly to avoid running out of product in the active
area.
In replenishing, the material handler gathers a group of cartons
from the reserve area to take to the active area. He will usually
be directed to build a pallet of cartons that are all going to the
same active area. For each destination active zone, the WMS should
guide him through the reserve area in a best path sequence to
minimize his travel time.
Alternatively, larger warehouses may have a conveyor system to
sort and deliver the cartons to the active areas. In this case, the
material handler does not need to build pallets by destination
zone, so he can work more efficiently.
When putting the cartons away in their active locations, it is
usually more productive for the material handler to select the
cartons to process, rather than be directed by the system. Using an
RF terminal, he scans the carton label and the WMS tells him where
to put the product. Some computer systems also let you use a
“best path” methodology in the active area, where the
system tells the worker what location to fill and which cartons to
use. You may want to use this option if your active zones are
large. However, because the material handler must find specific
cartons, it is important to build smaller replenishment pallets to
reduce the amount of time he spends searching.
To ensure picking accuracy, replenishments must be placed in the
proper active slot. Make sure that pick slots are clearly labeled,
in both the front and the back for flow racks.
When RF terminals are used to fill pick slots, the replenisher
should scan each carton or pallet into the active location to
verify that it is put away properly. He should scan the carton
label rather than the UPC or SKU barcode to verify that it is the
correct carton. If UPC or SKU barcodes are scanned, it is too easy
to scan the same label several times, then potentially put a wrong
carton in the pick slot.
If the slots are filled without use of RF terminals, the label
at each pick location should show the item number and its
description in addition to the location number so that the
replenisher can verify visually that he is putting the cartons and
pallets away properly.
Easy does it
Even using the best techniques, sometimes the number of
replenishment tasks may exceed what you can accomplish. Here are
two other strategies to consider to reduce replenishment needs. You
must evaluate your requirements, the layout of your facility, and
system support to determine whether they are possible in your
distribution center.
Using the “hot pick” method can reduce the
replenishment burden if most of your customer orders are downloaded
to the WMS the night before they need to be picked. In the morning,
the system's first task is to check the total requirements of each
item for the whole day's orders and compare that to the available
amount in the active location. If more product is needed for the
day than exists in the active location, the WMS generates
instructions to pull full cases from reserve and move them to a
temporary active picking location. This hot pick location will then
be used to fill the orders until it is empty. Then, any remaining
demand for that SKU will be pulled from the active pick
location.
The hot pick method is most efficient when you need to perform
multiple replenishments for the whole day's requirements. It is
particularly useful in active shelving areas where the pick slot
may not be large enough to accommodate a peak day's demand.
You can also send material directly to the active pick location
when it is received, if there is space available in the active
location. This strategy, called immediate needs, cannot be used
when FIFO (first-in, first-out) or lot control is required. It
reduces material handling effort by replacing the putaway to
reserve storage and then replenishing active steps with a single
putaway instruction. Some warehouse systems support automatically
directed receipts to the active location, if they can be
accommodated there.
By selecting and implementing the appropriate strategies, you
should be able to refill your active slots efficiently. If you
monitor outstanding tasks regularly, you can ensure that
replenishments are being performed as directed. You can implement
alternative strategies when special needs, such as very high or
very slow demand, arise.
The result? The distribution center equivalent of the
satisfaction you feel when you can reach the slice of pie you want
in the midnight kitchen.
Pamela J. Davey is a consultant with Sedlak Management
Consultants, specialists in information systems selection and
fulfillment center planning. Contact her by phone at (330) 908-2286
or by e-mail at pdavey@jasedlak.com.
