One Piece Flow Vs Continuous Flow

What is One-Piece Flow?

One-Piece Flow is a lean manufacturing concept that is aimed to achieve a discrete flow of work contrary to 'batch' production approach. In simple terms, it means one-by-one processing of a product. A single unit of product flows between different processes. It follows the old adage - 'One step at a time'. As there is just one unit flowing - there is no wait time in between processes. One-piece flow is also known as Continuous-Flow Processing or Single-Flow Processing.

It is closely associated with the Just-in-time system popularized by Toyota. It requires continuous examination and improvement of the production line that eventually leads to lower costs, reduction of waste, and error-free output.

Let's look at an Example of One-Piece Flow

Example: We start with a 5-step process where each step takes 1 minute to process 1 piece. We have 10 pieces to be moved across the production line. Now, let us compare the results of the process run with 3 different production approaches. (Please refer to the above video for more details)

Approach	1st Piece Output	5th Piece Output	10th Piece Output
One Piece Flow	5 mins	9 mins	14 mins
50% Batch Flow	21 mins	25 mins	30 mins
100% Batch Flow	41 mins	45 mins	50 Mins

As we saw, one-piece flow is much faster. On watching the video, we also observe that total Work In Progress (WIP) in the continuous-flow process is reduced and is limited to 1 unit. However, each step still has to do the same amount of work i.e. 10 mins to process 10 pieces. The difference in time comes from the parallel processing of work.

Reducing batch size is the desirable objective of discrete manufacturing. So why do we need the batch processing at all?

There are a finite amount of resources available in any process; while One-Piece Flow has a huge setup cost & manpower requirement. Additionally, there is a trade-off between Changeover time (time taken by by manpower/equipment to shift from Process A to Process B) and overall time saved.

In this blog, we will go through the criteria and implementation guidelines of one-piece flow in detail. Let us start with a pros & cons comparison.

Pros & Cons of One-Piece Flow

Pros

1. Speed: Work is completed faster with discrete processing as seen in the above example. It can save significant processing time and increase the throughput.
2. Reduced WIP: Also seen in the above example, the total amount of work accumulated at any step is reduced to 1 unit.
3. Cost Savings: There is an implicit cost-saving included with the one-piece flow because of shorter processing time and less waste.
4. Detection of Critical Errors: With one-piece flow, we may identify crucial mistakes at any step quickly because of the faster production of 1st output. For eg. packaging of an e-commerce order at an Amazon warehouse where packing materiel is of incorrect dimensions. Identifying this mistake and fixing it with the one-piece flow will be much quicker than bulk processing.
5. Scale & Automation: Break down a process into smaller steps and add resources at each step. Scaling this production unit should be simple & cost-effective. Automation of a few steps might further reduce processing time.

Cons

1. High Setup Cost: Continuous Flow Process requires a higher setup cost. It needs more workstations, area, resources & equipment, etc. whereas the batch process can be started with lesser space and machinery.
2. Low Variability in Process: One-Piece Flow will only work in processes with a low degree of variation & complexity.
3. Control Mechanism: Sophisticated control systems are required to monitor one-piece systems as opposed to the batch production system.
4. Manpower: One-piece might need a higher workforce if automation is not present.

Conditions to be met for One-Piece Flow

It is not recommended to use One-Piece flow for every process. It's the Toyota vs Rolls Royce logic. Let's look at the conditions that should meet for us to use One Piece flow:

1. Large Scale - One-piece flow has high setup cost so it makes more sense to build a large production capacity with continuous flow. Please refer to the batch process for lower volume or proof-of-concept.
2. Consistent Processes - There should be a low variation of final output at each step of the process.
3. No Downtime - Because all the steps are always involved, there can't be any downtime of any component of the process.
4. Production Planning - Requires expertise in the planning of floor layout, breakdown of cells, control systems, high-tech equipment & automation.

How to Implement One-Piece Flow

The Continuous flow requires a strong assessment in the setup phase. Let's break the process down into 6 phases and investigate the key questions to be asked:

1. Process Analysis: Should I be using a single flow for this process? Does my process meet the above-mentioned requirements?
2. Create a cellular flow layout: How should I breakdown the entire process into smaller cells so that the overall work is equally divided? Will there be any additional "changeover" time?
3. Estimate Cycle Time & Takt Time: Is my changeover time greater than 1 Takt time? How should I associate the Takt time for every step? Is there any imbalance of time between the cells that can create a bottleneck? (Takt Time is the total production assembly time to meet the customer demand)
4. Layout Plan: How do I reduce the motion between multiple processes? Should I use a U-shape layout or an S-shaped layout? Is there any avoidable accumulation of the Work in Progress?
5. Create Standardization: Is the work between each cell balanced? Can I reduce the work content? How much manpower do I need for each step?
6. Continuous Improvement: Is there any scope of improvement? Can we theoretically do better with any changes in the layout?

Use Cases Beyond Manufacturing

There are several uses cases of one-piece flow outside manufacturing & production. Work is processed every day in various shapes and forms. Some common business functions where it can be implemented are Marketing, Product Management, Customer Support, Client Onboarding, Client Request Management, etc.

Ticketing systems are another example of where OPF can be implemented. Be it sales or customer support, the tickets can be processed in batches or one by one. If the team size is smaller, we should process tickets in batches from one department to another. One person might be involved with several tasks and it is generally easier for someone to operate multiple tickets at once and pass it on to other teams.

On the other hand, if there are large, well-established teams equipped with collaboration software, one by one processing will give far better Turn-Around-Time and output quality.

This approach could be applied in many aspects of business & day-to-day life.

The choice between OPF vs BF is more subjective than it looks. An interesting example could be answering to one's emails. This can be done in the batch process or one-by-one.

One-by-one (as it comes) can make the activity/task more efficient. On the other hand, batch processing will make the email responder more productive by reducing the high "changeover cost" (distraction cost) of looking at emails. However, this might make the email responder a bottleneck in the bigger task.

Generally, it is suggested to keep the bigger context in mind while implementing a one-piece flow. It is always good to be equipped with knowledge of the advantages & disadvantages of this system. We hope that this article will expand your knowledge about one-piece flow.

References:

1. https://en.wikipedia.org/wiki/Continuous-flow_manufacturing
2. https://en.wikipedia.org/wiki/Batch_production

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Source: https://getbreakout.com/blog/what-is-one-piece-flow/

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