Slotting is the process that determines how stock is distributed in a warehouse. The pick face slotting strategy defined within your WMS will determine where stock goes when you put it into the pick face and it has to compliment the picking strategies that are going to take it out again. Slotting logic will be applied whenever you move stock into the picking face as part of your receipt process or when you replenish the picking face from storage locations.

The advantages of ‘dynamic’ pick locations

A very simple warehouse operation can associate each SKU with a fixed location in the picking face. This makes it very straightforward to ensure each SKU is in the right place and to adjust this for particular SKUs as required. However this approach becomes more unworkable as the number and turnover of SKUs increases. Using fixed picking locations requires each SKU to be assigned enough space to hold the maximum quantity you expect to hold in the picking face. This usually results in very low stock densities, wasting space and increasing the distance your pickers have to walk.

If the picking face needs to hold a wide range of different SKUs the mechanism that decides where stock is held should be dynamic. It should be possible to hold a SKU in multiple locations, to track the available space that remains in each location and to be able to configure your WMS environment to automatically make the right decisions when it determines the target location for each putaway. This will increase stock density, increase pick efficiency and allow you to refine your slotting strategies automatically without having to continually tweak the pick locations associated with each SKU.

Your slotting logic has to be tailored to suit you

The slotting logic in each warehouse will be different. The logic has to balance the efficient use of the space available with the need to reduce pick times, the optimal solution might need to accommodate some or all of the following constraints:

• Ensuring that each SKU is held in the appropriate location type (e.g. loose SKUs in bins, large SKUs on deep shelves, heavy SKUs at ground level, light SKUs on top shelves, frozen SKUs in the freezer etc.).

• Ensuring that the most frequently picked SKUs are in high value pick locations (e.g. at chest height, in the aisles nearest the packing stations, at the entrance of closed aisles etc.). Conversely, that slow moving SKUs are held in low value pick locations (e.g. top or bottom shelves, the end of closed aisles or furthest away from the packing stations etc.).

• Ensuring that the most frequently picked SKUs are spread around, if your popular items are present in every picking aisle you can dramatically increase the proportion of multiline orders that can be picked within a single aisle.

• Ensuring that two batches (product lots) of the same SKU are never mixed in the same pick location.

• Ensuring that SKUs that are frequently picked in combination are kept in adjacent locations.

Accessible ground-floor space suitable for picking faces is usually in short supply, one of the most effective and obvious ways to use it efficiently is to allow pick locations to hold multiple SKUs. If all your products are barcoded this should not increase picking errors but it will slow down the picking process significantly. When there is more that one SKU in a pick location pickers have to stop and choose the required item before scanning the one they need. In an ideal world each pick location would only hold one SKU but where this is not possible the slotting logic can help by making things as easy as possible for the pickers.

One of the easiest ways of doing this is to divide your product catalogue into SKU categories that look significantly different. The slotting logic can then ensure that a pick location never holds two SKUs from the same category and the handheld device driving the pick process can show a picture of the item required. The slotting logic should also be able to ensure that SKUs are spread evenly within the pick locations, so that you don’t get into a position where user are prompted to put a SKU away in a location that already holds three or four SKUs while there are still locations with space available that are only holding one.

An ongoing process of refinement

Finding the slotting logic that works best in your environment is an ongoing process. In order to be able to refine it over time your WMS needs to be able to capture the key metrics that reflect the aspects of your stock distribution that are most important to you. These are likely to include:

• % of empty pick locations
• % of pick face empty by volume
• % of pick locations in use that contain a single SKU
• % of pick locations in use that contain two SKUs
• % of pick locations in use that contain three or more SKUs

These figures will need to be tracked over time and analysed in combination with the metrics that measure your picking efficiency, which is the subject of the blogpost here.