When I talk to customers about zone picking systems, we often come to the question of whether they should fill their flow racks in the zone picking system with the products‘ original cartons or with standardized plastic totes. There are good arguments for both, but I have the feeling that the arguments for using totes tend to be underappreciated.
Let’s unpack this.
Some Context First: Zone Picking Systems
If you are reading this, you probably know what zone picking systems are. If not: do not worry, I got you covered. Follow this link and read up on it. Here’s the short version: zone picking systems are a crossbreed between person-to-goods and goods-to-person picking systems. Pickers move to storage locations – but only a little. The picking area is divided into different zones, and each picker is responsible for one zone, and only moves within that zone. Order load carriers move from zone to zone, but only to the zones where there are SKUs needed for the particular order.
Zone picking systems can be simple or quite complicated. No one wants them to be complicated, but if you want them to outshine goods-to-person picking systems (which is absolutely within the realm of what has been done before and can be done again; ping me if you want to know more), you have to design them right. And that can be complicated.
In zone picking systems, the products you want to pick are often stored in flow racks. Flow racks are great: they enforce FIFO, they allow you to separate picking from replenishment, and they are affordable. So far, so good.
Some food retailers, especially e-grocers, use zone picking systems for order fulfillment. For instance, German grocery retailer Rewe is using this concept in its recently opened online fulfillment center in Frankfurt and has announced they’re rolling out this concept to more locations, and I’m glad they do because it is a fantastic concept. Zone picking eats goods-to-person picking for breakfast when implemented correctly – and at a fraction of the cost.
But one question that always comes up is, „Should we use original cartons or plastic totes?“. In this article, I will explore this question and discuss the arguments for either option.
The Case for Using Original Cartons
The arguments for filling flow racks with original cartons (cases) are simple and clear, and it is what companies do by default unless they have good reasons not to do so (but see below for good reasons). There are two principal arguments for leaving products in their original carton for picking:
First, you do not have to repack every item. Repacking items into totes is a non-value adding process and takes time. It looks like (!) a productivity sink. Each item gets touched eve before it gets picked.
Second, original cartons are usually narrower than standard totes. While totes typically measure 600mm x 400mm or a little more, original cartons are often 300mm x 400mm or less. Because they are narrower, more different SKUs fit on the flow racks, increasing picking density, and allowing more different SKUs to be picked overall in the zone picking system.
The second argument is the more important one. SKU density is a critical parameter for zone picking systems. The first argument, the argument that productivity is lost, is controversial, as we will see in a moment.
The Case for Standardized Totes: The Not-So-Obvious Option
The arguments for using standardized totes are not entirely obvious.
You have to repack all products into totes, which costs time and therefore money, and reduces productivity – at first glance. But that’s not necessarily true if we look at the picking and replenishment process more holistically.
Lower SKU Density – but Higher Item Density and Inventory Reach
I mentioned above that the totes are usually larger than the original packaging of SKUs, which reduces SKU density on flow racks. Because this is so, it is also true that more items can fit in each bin, thereby increasing the inventory reach of that SKU on the flow rack and decreasing replenishment frequency. I contend that for every original carton that you do not need to replenish, you can repack a whole bunch of cartons into totes before your productivity is negatively affected. It’s not unlikely that for some SKUs, especially those that move faster, this is already a productivity gain.
If the zone picking system is complemented by a goods-to-person system (or vice versa, however you want to put it), SKU density does not matter as much because the long tail is covered by the goods-to-person system. And since the goods-to-person system requires a decanting process anyway, it makes even more sense to use the same process for replenishment totes going to the zone picking system.
Less Troubleshooting in the Flow Rack
It’s not a big problem, but it does happen that the cartons do not flow as they should and sometimes get stuck. Of course, there are a number of things you can do to alleviate this problem, such as adjusting the slope of the flow bed, the width of the channel, and the resistance of the rollers. Each carton is different in terms of bottom material, weight, footprint, height, shape, fragility… while the only difference between totes is weight. Therefore, it is easier to adapt the flow rack to totes than to the variety of different cartons.
No Trash Handling at the Picking Stations – and no Trash Carton Conveyor
If you unpack all cartons in one central location, you do not have to worry about trash disposal at the picking stations. It’s not that trash disposal is particularly difficult, but it takes time and interrupts the picking flow. Trash disposal takes time no matter where you do it, but it’s very likely to take less time if someone is unpacking cartons and disposing of trash at a fast pace and not switching between picking and trash removal.
And importantly, you can get rid of that darn trash carton conveyor that keeps causing trouble. I have visited several warehouses where the main activity of the (expensive) maintenance staff was troubleshooting the trash carton conveyor. And that thing is not cheap to build either, so you are better off avoiding it altogether or keeping it short and simple in a central location.
Empty totes are simply pulled from the flow rack and placed on the conveyor to join the return flow to the central decanting stations.
Totes Allow Automated Handling
Finally, the use of standardized totes enables automated handling. More precisely, standardized totes enable
- automated transport,
- automated replenishment and
- picking by robots.
And that’s quite something.
Imagine the following process: Totes are packed at central decanting stations. From there, they are picked up by small AGVs that take them to the destination rack where replenishment is needed. The AGVs set the totes down at a transfer station and a lightweight linear robot picks them up and pushes them into their destination lane in the flow rack. On the other side of the flow rack, a picking robot picks orders and removes empty storage totes. Every time the stock in a channel falls below a certain threshold, a replenishment order is automatically triggered and the process starts again.
The process I just described can be done with today’s technology, and the solutions are available on the market.
In conclusion, when replenishing flow racks in zone picking systems, the choice between using original cartons and standardized totes is not clear-cut, especially since some of the arguments in favor of totes are often overlooked. Original cartons have the advantage of not needing to be repacked and allow for higher SKU density. Meanwhile, standardized totes have the advantage of increasing inventory reach, reducing the frequency of replenishment and avoiding troubleshooting on the flow rack. Also, the centralized trash removal is more efficient and saves the otherwise expensive and petulant trash carton conveyor. So, while repacking into totes may seem like a productivity loss at first, the other benefits can make it a productivity gain in the long run.
Questions? Contact me!
- 2023-05-15: Note added on the combination of zone picking with goods-to-person systems