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Shop floor 4.0: Adapting Production Batch Sizes to Flow

By Bernard Milian
production batch sizes

The 4.0 shop floor is about being agile, responding to real customer demand, and making priorities visible and decisions intuitive for all.

In this series of articles, we look back at the key principles of a demand-driven operating model for the Industry 4.0.

The Ideal of “One-Piece Flow”

Lean has promoted an ideal of reducing batch sizes down to one unit. If you can effectively reduce your lot sizes and efficiently produce different items one after the other, you can more easily adapt to a constantly changing demand. What may seem like a utopia at times works very well in some areas, such as highly differentiated products. It has become the norm in some sectors. No one is surprised anymore that every vehicle on a car assembly line is different, with specific colors, options, and variants. Additive manufacturing enables that also on custom parts.

However, this target has probably acted as a deterrent to the adoption of pull flow tactics. For example, any producer of injection-molded plastic parts knows that disassembling and reassembling a mold to make just one part does not make sense. When they hear about one-piece flow, they turn their back and say: “It doesn’t work for us.” Working to reduce changeover times is OK, but the primary motivation for this is often to lose less capacity in changeover times, rather than making more changes, which is perceived as unproductive.

Your transportation team or purchasing team will have the same thought: unless it’s a giant wind turbine blade or airplane wings, we’re not going to source and ship by the unit — let’s do full containers! In many cases, the further upstream you go in your value chains, the larger the lot sizes will become…

Having led multiple Lean and Demand Driven transformations, my recommendation, if you want to get your teams on board, is to start by pragmatically acknowledging that yes, of course, certain groupings, certain lot sizes are required. But right away engage in the conversation about: how do we determine the right batch size?

Calculating the Right Production Batch Size

To determine the right lot sizes, start by forgetting about Wilson’s formula.

Take a couple of steps back to get a bird’s eye view of your system and its flows.

Describe your grouping rules and your batch sizes along this flow and ask yourselves as a team some questions to challenge their adequacy. There are factual analysis elements to consider, but above all collective intelligence to mobilize!

Step 1: Manage constraints

On a production flow, the first of our recommendations is to start by identifying the major constraints. What are the transformation steps that set the tempo, that determine the flow? For each of these constraints, what is the capacity required to meet the market? Based on this, what percentage of the opening time of these resources can we devote to changeovers while ensuring sufficient throughput?

It is on these constraints that you must concentrate your improvement efforts: SMED, TPM, OEE make sense on these work centers! For example, if you aim at 10% of opening time dedicated to setups, the challenge will be to achieve a maximum of changes in this allocated time. The proportion of time to be devoted to changes will be the key to determining the batch sizes.

The constraints are the resources that you must optimize, schedule at finite capacity, and ensure the most productive sequences that result in the least amount of wasted time and material. Intuiflow can help you with this…

Step 2: Determine and formalize frequencies

In a production environment, determine the products to be manufactured daily, weekly, fortnightly, monthly, quarterly…

If you are replenishing inventory, set this interval between campaigns as the number of days to size your green zones. You will size your inventory accordingly and ensure that your pull flow respects your campaign frequencies.

Step 3: Translate batch sizes into lead times

If your batch size is 10,000 and you process 1,000 per hour, the next batch will wait at least 10 hours before being processed. When you describe your operating model — determining your constraints, the batch sizes, the number of variants, the time buffers — you induce the lead time for a new order to flow through your system.

Step 4: Plan in a grouped manner

Using grouped planning logic, such as planning wheels, can help you avoid having to impose lot sizes on each SKU. It takes advantage of product families to stabilize the flow while accelerating it and facilitates the organization of efficient recurring cycles.

Step 5: Leave a degree of freedom

Often the rule in the workshop is to follow a strict sequence, and not to give the choice of doing this or that. However, there are circumstances where it is quite appropriate to give the workshop the latitude to group orders intelligently. This allows for optimization of resources without slowing down the flow if it is governed by clear rules. The best example is the use of time buffers.

A time buffer is the equivalent of a queue placed under control at key stages of the process. The available jobs are visible in red/yellow/green time buckets. Normally we would take the red ones in sequence, then the yellow ones, then the green ones. However, if it’s clever to combine a red order with a green one and it won’t put other reds or yellows behind, no worries — the operator or supervisor is empowered to make the call.

We will come back to time buffers later in this series of articles.

Adapting Production Batches to the Flow

In summary, batch sizes, grouping rules and sequences are at the heart of the design of the operating model and are crucial to ensure a flow paced to actual demand. This is what should guide you in sizing them and making them evolve over time. By modeling this control in a digital twin of your workshop, Intuiflow will allow you to simulate its operation and validate the demand response model.

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