Zone Picking: Why and Why not

First let me say that zone picking isn't a complete order picking methodology. It's really just one of several characteristics of an order-picking methodology. In other words, there are many variations on zone picking. If someone tells me they are doing zone picking, it tells me something about what they are doing but I would need a lot more information to understand how they actually pick orders. I wanted to clarify that because when I start talking about pros and cons of zone picking, the pros and cons don't necessarily apply to every variation of zone picking.

The basic concept of zone picking is simple. In a non-zoned warehouse, an order picker grabs an order and picks all items on the order, travelling through the entire warehouse (if necessary) to complete the order. In a zoned warehouse, you separate the pick area of the warehouse into two or more physical areas (zones). Order pickers are typically assigned to one zone and only pick items from that physical area. Therefore, an order that has picks in multiple zones will be picked by multiple order pickers.

Why zone pick?

There are several reasons for considering zone picking. Here are the main ones.

Reducing travel distances.

If you have a very large warehouse with a large number of SKUs, and orders that tend to have multiple line items (multiple picks), you could potentially have an order picker travel a quarter mile or more to pick an order. Now if that order has 100 line items on it, that isn't terrible because the travel time between picks will be rather low, but if that order had 4 line items, that's a long way to travel for 4 picks. Zone picking in an operation like this would likely incorporate automated conveyors to move the order to each zone where a pick station would be closer to each physical pick, thus reducing the distance a person needs to travel.

Eliminating traffic congestion.

This is actually kind of the opposite to the above reason, but oddly enough could potentially apply to the same operation. If you have a larger number of picks to process each day, and therefore a large number of order pickers working in the warehouse, you can end up with too many pickers in the same place at the same time. This is especially true of fast-pick areas where you group your fastest moving items. Setting up fast-pick areas is almost always a good idea, but traffic congestion is a potential problem when you have multiple pickers. Zone picking allows you to limit the number of people that will be in the same place at the same time. It doesn't necessarily eliminate all traffic issues unless you only have one picker per zone, but it can prevent having 5 workers potentially needing to pick out of the same spot at the same time, or constantly needing to pass each other in the aisles.

Using single-person equipment, methods, and technologies.

Equipment and technologies like pick-to-light, carousels, AS/RS, and others are typically designed to be operated by one worker at a time. Unless you can have all your inventory in one set of carousels, and one worker can pick all your orders from these carousels, you're almost certainly going to need to use some type of zone picking.

Reducing order-cycle time.

This only applies to pick-and-consolidate zone picking (pick-and-pass actually results in longer order-cycle times). Reducing order cycle time is especially true with larger orders where it may take a single order picker several hours to otherwise pick the entire order.

Pick-and-pass versus pick-and-consolidate zone picking.

The first big decision related to zone picking is deciding how you are going to execute it at the order level.

Pick-and-pass.

The most common way zone picking is executed is through what is typically referred to as "pick-and-pass". With pick-and-pass, an order starts at the first zone, the items in that zone are typically picked into a tote or carton, then the tote or carton is physically passed on to the next zone where another picker will pick the next items on the order.

How does the order get routed to the next zone?

There are a variety of ways of doing this—from completely manual to fully automated.

The most basic method has the order passing through each zone, and the person working in that zone needs to look at each order to see if there are picks for that zone. If most orders have picks in most zones, this isn’t terrible. However, in my experience most zone-picking operations don’t have most orders having picks in most zones. This then results in a lot of wasted time when order pickers need to look at orders only to see they don’t have any picks in their zone. I’m not saying this method should never be used, just make sure it makes sense for your operation.

The next manual method has the order picker that just picked a zone identifying the next zone the order needs to go to and routing it directly to that zone. This would typically be via conveyor (would have multiple conveyors, each going directly to a specific zone), or carts (would have a cart for each zone). This can be workable if you don’t have a lot of zones.

A typical automated pick-and-pass system would use automated conveyor with diverters that automatically route the order to the next zone. You would typically have totes with barcodes on them. You assign an order to a tote by scanning the tote bar code. The conveyor system will then “know” each zone the tote needs to go to, and route it accordingly. This isn’t the only way this is accomplished, but it’s a fairly common method. This does require integrating the conveyor system control software with your warehouse system.

Problems with pick-and-pass zone picking.

Additional handling of orders. Each time an order is “touched” by another worker, that touch takes time. I’m not talking about the time it takes to pick the items—that you’ll have to do anyway, I’m talking about the time it takes for a worker to start a new order. This may include picking a tote off of a conveyor and putting it on a cart, scanning a tote to assign the order to a picker using portable computers to pick orders, or whatever else occurs in the transition from picking one order to picking another order. This may only be a few seconds, but those seconds add up quickly in a high-volume operation.
Order cycle times.

Order cycle time is the time it takes an order from being put out to be picked, to being picked completely (or picked and packed). This is not the time (in labor) you have actually picking that specific order. You may only have 5 minutes of labor picking the order, but it may have taken 2 hours to get picked. Each time an order is passed on to another zone, it gets in line to be picked in that zone. There will be orders in front of it that were actually dispatched later than it. This is simply the result of some orders only going to one or two zones, while others go to 4 or 5 zones. The 2-hour cycle time I mentioned is not unusual at all in pick-and-pass operations. In fact, the cycle time could be quite a bit longer for some orders. You may have an average cycle time that sounds reasonable, but some orders could take significantly longer to get through the system.

Zone balancing. Zone balancing involves maintaining a steady flow of orders through each zone. While the concept of zone balancing is simple, actually achieving a steady flow through all zones can be very challenging. The more zones you have, the more challenging it gets. Out-of-balance zones can result in backups/bottlenecks. This can result in starving downstream zones of work while physically backing up conveyor systems gong into the bottleneck.
Conveyor cost and space requirements. If you plan on using conveyor to facilitate pick-and-pass there are costs associated with this. A fully automated system can get a little pricey. In addition, conveyor systems take up space and can complicate physical flows and traffic in the warehouse.

Pick-and-consolidate zone picking.

Pick-and-consolidate zone picking, also called simultaneous zone picking, or wave picking, involves picking the zones for each order independently of the other zones. Therefore, if an order has picks from 5 zones, 5 pickers could theoretically be picking items for that order at the same time. I say “theoretically” because even though it is technically possible, it’s actually unlikely that 5 people would be picking the same order at the same time. However, this does eliminate the very long order cycles associated with pick-and-pass.

To achieve pick-and-consolidate, you need a consolidate step. If you are picking into totes (very common), that order that had picks in 5 zones will come around in 5 totes, and those totes will not be together.

Manually consolidating orders? I don’t see this done often, but it is doable in some operations. This is more feasible with a relatively small number of orders. You can use a multi-step manual sort onto gravity conveyor lanes or maybe some manual type of Put-wall. You can partially automate the put-wall concept by using put-to-light to gain greater efficiencies without going to a fully automated system.

Automatically consolidating orders. This has historically been the more likely methodology for consolidating orders. It typically involves an automated sortation conveyor that will divert all totes for an order into a lane once all zones are picked. There are many variations of sortation conveyors, but a common application would have a recirculation loop that allows totes to essentially keep going in a big loop until all zones for an order have been picked. It will then have multiple lanes for accumulating finished orders. Depending on volume, that may be all it needs to do. But is many applications, the lanes set up to accumulate the order are only temporary until all totes for the order are there, then it goes back one moving conveyor to be routed to a packing station.

Problems with pick-and-consolidate zone picking.

While pick-and-consolidate can alleviate the long order cycle times, it does share some problems with pick-and-pass as well as introduces some new issues.

Additional handling of orders. While the “touch” is different than pick-and-pass, you still have multiple workers having to handle the same order. In addition, you are dealing with more totes (covered next).

Additional totes. An order going to 5 zones in a pick-and-pass operation may be picked complete into one tote, but in pick-and-consolidate picking that order will be in 5 totes. This means either someone is combining the items into one tote at some point, or 5 totes are going to a pack station where the packer will need to know there are 5 totes for the one order that is being packed. While this may not sound like a big problem, it can be.

Zone balancing. Zone balancing is still an issue here, though the impact is different. If a zone gets behind, it doesn’t necessarily cause an immediate impact on other zones. What it will do is delay orders that have been picked in other zones from being able to be consolidated and forwarded to pack stations. This can potentially result in filling up that recirculation loop, and that’s not a good thing (covered next).

Sortation conveyor capacity issues. When you start getting into automated material handling systems, they tend to have hard capacities. A sortation conveyor can only sort so many totes at a time, and a recirculation loop can only recirculate so many totes at a time. While your system may have the capacity to handle your typical daily volume, it may not be able to handle your hour-by-hour or minute-by-minute volume if you can’t keep everything wonderfully balanced. Or it may have trouble keeping up if you have days that are significantly busier.

Sortation conveyor cost and space requirements. As previously mentioned, a fully automated system can get a little pricey. In addition, conveyor systems take up space and can complicate physical flows and traffic in the warehouse.

Mitigating problems associated with zone picking.

Be conservative with zones, use them only where they benefit you enough. I can’t overstate the importance of this. It’s easier to balance 3 zones than it is to balance 5 zones. Fewer zones will result in fewer touches. Fewer zones will result in fewer totes in a pick-and-consolidate operation. Obviously there are valid reasons for zones or you wouldn’t be doing zone picking, so I’m not saying less is always better. Just make sure you are evaluating the benefit you get from each added zone to make sure it is worth the potential problems. As mentioned earlier, some zones need to be one-person zones due to equipment or technology used, but other zones may be perfectly fine with 2 or 3 people working in them. One-person zones are challenging to keep balanced and will have a fixed capacity, so before committing to technology/methodology that forces one-person zones, make sure it provides enough benefit.

Other picking methodologies.

I just want to clarify once again that there are many ways to pick orders. Some have nothing to do with zone picking, while others are variations of zone picking. The intent of this article is not to cover all Order Picking methods, it is just to cover the basics of zone picking. Some of the problems solved by zone picking can also be solved by other methods. For example, multi-order picking can help to reduce travel distances, so it may be an alternative to zone picking in some operations. To complicate things further, multi-order picking can be done within zone picking, so they are not mutually exclusive methods.

Robots and zone picking.

There has been quite a bit of experimentation with robots in the warehouse in recent years. A lot of this has been in the "parts to picker" area, but there are providers looking at using robots for zone picking. Not that the robots are doing the picking, but they are being used to route the orders through the zones, so pickers in the zones can pick to the robots. From my perspective, the big advantage is it gets rid of all that conveyor. Early adoption is expensive, but eventually some of these technologies will become more mature, and prices should drop.

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Dave Piasecki, is owner/operator of Inventory Operations Consulting LLC, a consulting firm providing services related to inventory management, material handling, and warehouse operations. He has over 25 years experience in operations management and can be reached through his website (https://www.inventoryops.com), where he maintains additional relevant information.