Batch buffers and matrix sortation: What are they and how do they work in pouch sorter technology

Pouch sorter technology has many advantages in being able to accommodate the significantly different demands of B2B fulfilment and B2C fulfilment in today’s fashion warehouse. Some of the defining features of the pouch sorter system are its batch buffer and matrix sortation which are able to bring together an order and even sequence it during sortation. All of this can be done without the system performance being dependent on order structure.

By Harald Hanaweg

To find out more about the pouch sorter system, read our white paper How Pouch Sorter Technology Can Solve Your Omnichannel Challenges here.

In this article we take a deeper look at these features to highlight how they work and can benefit fashion fulfilment centres. We also explain some of the misunderstandings around batch building and discuss the traps warehouse operators can fall into when determining their picking waves.

Batch building in picking and sortation: what’s the difference?

But let’s first discuss the batching concept because there are batches in the picking process and batches in the sorting process and they are quite different concepts when it comes to pouch sorter technology and this can create confusion.

1. The picking batch

Starting with the concept of the picking batch first.

How conventional systems worked

In earlier days, there was no such thing as a picking batch (or picking in batches). There was just the manual approach where workers picked between four to six orders into separate totes inside their cart while walking through the warehouse, collecting what was needed for an order. With rows of shelves and many items in an order, the worker walked almost every aisle of the warehouse to collect all the items in his order.

While this practice created a lot of walking for the workers, it was nevertheless manageable at a time when warehouse operators were shipping large orders to specific shops. In reality, their workers moved just a few metres from one item to the next.

The problem: e-commerce

But with the deluge of much smaller orders driven by e-commerce, picking became incredibly inefficient. Workers were now faced with moving through an entire warehouse to collect just one or two items to fulfil an order. This of course prompted warehouses to adopt a significantly different way of handling their picking.

Along comes batch picking

The easiest way of handling these small orders was through “batch creation” or “batching at picking”. The practice allows pickers to pick multiple items from multiple orders at the same time, so items from a single order can be split between a number of pickers. Each picker works in a specific area only, without the need to trace back and forth through the aisles to pick items later. The items from a single order are then consolidated during the sortation.

What’s new here is the way orders are picked in batches, and what happens before the sortation.

Batch picking can be applied to any kind of sorting technology, whether it’s manual put walls, cross-belt loop sorters or pouch system sortation. However, for put walls or loop sorters the batch size – or number of orders handled – is limited to the batch capacity of the sortation system while for pouch sorting the only limit is the number of items the buffer can hold.

2. The sorting batch

By comparison, the sorting batch is a feature inherent to pouch sorter technology and is completely different from the picking batch (for the reasons mentioned above).

That being said, we should emphasise here that the batch picking influences picking efficiency and how big the batch buffer needs to be designed (we’ll address this further below).

The batch buffer

In the pouch sorter system, the batch buffer (also known as a ‘dynamic batch buffer’) is named as such because it creates the sorting batch.

The items retrieved from picking are inducted to the sortation system where they are held in a batch buffer until all items have been gathered, one item per pouch. The circular batchlines typically hold two picking batches, each consisting of a mixture of orders. The system confirms that all items in the batch are present in the buffer where they stay until they are needed, before the items are then sent through the algorithm-based matrix sortation.

By placing the orders into the dynamic batch buffer first, they are ‘buffered up front’, contrary to, for example, a manual put wall-based batch building or a loop sortation process where orders are only built after the items are discharged into compartments inside the destination chutes. With the dynamic batch buffering process inherent to pouch technology, orders within a picking batch can also be prioritised, for example, according to cut-off times. And the space for the buffer is up in the air and not in compartments on the floor where it takes up space.

The matrix sortation

The pouch system’s matrix sortation – a feature not found in other sortation systems – then determines the sorting batch. The sortation uses the Radix algorithm, a sorting algorithm whose parameters are set by the pouch system supplier. For example, BEUMER Group’s algorithm sorts six lanes of items three times. In other words, 6x6x6 or a total of 216 items (or 216 pouches, as each item is placed in its own pouch) are sorted in one rotation batch.

There are three steps in the matrix sortation, which basically means that items come in no particular order and come out in perfect order.

The beauty of the pouch system bringing 216 items in a perfect sequence is that the 216 items can belong to any number of orders. This makes the process completely independent from order structure. It simply doesn’t matter if the 216 items belong to 50, 70 or 100 different orders. To the system, the task is always the same; bring 216 items into a desired sequence. This makes the system so perfectly suitable for use in the e-commerce market that is typically characterized by many orders with relatively small order sizes. The matrix sortation with a pouch sorter system is the only sorting solution that is not dependent on order structure.

How picking relates to the batch buffer

While we said that a picking batch is different from a sorting batch, the two are interrelated in that the size of the batch buffer will depend on the size of the picking batch.

So, if the picking batch is bigger, the dynamic batch buffer will also need to be bigger in order to buffer more pouches. But the warehouse manager doesn’t need to take care of the sorting batch at all, the system will still sort 216 items in a batch. The only thing the manager has to think about is the picking batch.

Here are some important considerations.

Constraints within the picking batch working against each other

Fashion fulfilment managers can often fall into the trap of thinking the more time they spend on accumulating a lot of orders before picking, the more efficient their operations. But in practice, what this means is that their orders can’t be shipped speedily.

This is because the picking batch has two constraints: volume and speed. Unfortunately, these two interests are in conflict with each other and work against each other.

The conflict can be illustrated in a number of scenarios:

  1. The manager chooses a pick wave of 30 minutes: This means the manager needs 30 minutes to accumulate all the orders in the order pool (that is, the time needed to create the picking batch), 30 minutes for picking, 30 minutes to induct them and 30 minutes to pack them. In total, therefore, it will take the warehouse two hours, from receipt of the order until when the items are shipped.
  2. The manager chooses a pick wave of one hour: This means the manager needs one hour to accumulate the orders in the picking batch, one hour to pick, one hour to induct and one hour to pack. In total therefore, it will take the warehouse three hours from receipt of the order to shipment of the order. But an overlap is possible, whereby the warehouse can start inducting the first items to the pouch sorter before the picking has been completed. And there is another potential for an overlap when packing the orders. In reality, though, there is very little difference between a one-hour pick wave and a 30-minute pick wave.
  3. The manager chooses a pick wave of two hours: In total, this will be seven hours from receipt to shipment of the order.
  4. The manager chooses a pick wave of four hours: This is great for a manager in terms of picking efficiency, but it’s unlikely to meet same-day or next-day delivery.
  5. The warehouse manager wants to pick one, large batch in one day: In reality, this means that the manager needs to factor in one day for waiting time and another day for fulfilment time. In total, therefore, it will take the warehouse two days to ship the items.

The key takeaway here is that the bigger the batch size, the more efficient the picking is going to be. Conversely, the smaller the batch size and the smaller the window of time, the shorter the time will be to get an order out the door.

So, a warehouse looking for small batch waves must be prepared for less efficiency in their picking. Our experience shows that the optimum picking wave is between 30 minutes and an hour for typical e-commerce fulfilment.  But the pouch sorter system will always operate the same.

‘Waveless’ picking: Is there such a thing?

In the USA – where the picking batch is called a ‘picking wave’ or pick wave – some warehouse operators are seeking ‘waveless picking’, while still aiming for optimal efficiency. Waveless picking is a way of optimising the picking even further, with workers starting to pick their orders before operators have accumulated all the orders and adding orders to the picking list later, as they come in.

The pouch system supports waveless picking, and while this approach might decrease the picking efficiency, the pouch will still flatten the workload on the packing process.


The concept of batch building in picking is very different from the concept of batch building, inherent in the sorting capabilities of pouch sorter technology. While the former evolved as a practice to drive greater picking efficiency in the face of new e-commerce demands, the latter is a way pouch sorter technology assembles and stores items until they are needed, through its dynamic batch buffer. It is a way in which the system also sorts items completely independent of order structures and even sequences them to meet B2B store requirements through its matrix sortation.

That said, the size of the picking batch will impact on the dynamic batch buffer size of the pouch sorter system and warehouse operators ought to be aware of this in designing their systems. There is, however, always the possibility to start with a small buffer and add more buffer circles later if needed.

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