What are defects?

One of the seven Lean wastes is defects. Defects meaning products, semi-finished products, raw materials, but also services that are not in line with customer expectations.

Where do defects come from in our companies? What are the consequences of their formation? What measures should be taken to eliminate defects?

To begin with, let’s recall the OEE indicator, one of the basic metrics in Lean.

The formula for the OEE ratio is:

OEE = Availability [%] x Performance [%] x Quality [%]

In this article we will not be concerned with availability and performance, but only with quality. It is clear from the above formula that even if we have a great deal of available resources and are very efficient, we still need to have very good quality to achieve the planned result.

As it happens, quality does not go hand in hand with productivity. Therefore, our first piece of advice is, develop a technological process that first ensures quality and then – productivity.

The STOP CALL WAIT principle

A common cause of excessive defects in production is the indifference of workers or their failure to react properly to nonconforming products. If production workers don’t have top-down permission to stop production when they see a nonconformity, they keep producing in the hope that some of the products in that batch will be good after all. Unfortunately, this interpretation of the concept of productivity is wrong. If we produce 100 pieces per hour and for some reason, for example, every 5th piece is defective, this means that in one hour we have produced 20 defective pieces, and in 8 hours of work – a total of 160 defective pieces. To produce these 160 defective pieces, it took 96 minutes. In addition, we wasted material. Isn’t it better to stop production and restore the parameters? In most cases, this would have been possible in much less than 96 minutes.

This means that it is worth giving employees the authority to stop production lines. Such production stops should elicit quick responses from the entire team that is trying to solve the problem. One of the ten principles of Kaizen is that “problems create opportunities.” Of course, this does not mean that it is good to have problems. This principle is about knowing where the weaknesses are and what should be a priority for improvement.

When giving employees the authority to solve problems, don’t forget that this should not be the authority of the “fire department,” that is, to put out fires. Employees will often act on an ad hoc basis, but they should also analyze problems and eliminate root causes.

The later in the process, the worse

We already know that stopping a production line that produces defects pays off. The product gains more and more value after each operation, because there is more and more labor done after each operation and more components and materials are added through manufacturing or assembly. That’s why when a product goes through the entire process and becomes defective at the final operations, then the waste is greatest.

Process variability and stability

We deal with variability everywhere, but especially in the manufacturing process. Variability has a huge impact on the number of defects.

The creator of statistical process control, Walter Shewart, divided the causes of variation into random and special causes. Random causes are characterized by the fact that there are many of them, they are random and usually have little effect on the stability of the process. Special causes, on the other hand, make the production process unstable and require a response. Therefore, it is necessary to strive for process stability. But what does this mean?

Example

Let’s explain this with an example. If we have a process for cutting some product 100 mm long to a tolerance of ±1 mm, and out of 50 pieces measured, the shortest is 98.2 mm and the longest is 101.5 mm, this means that our process is unstable and a large percentage of pieces are out of tolerance, and this means defects.

However, if out of 50 pieces, the shortest piece will be 99.9 mm, and the longest piece will be 100.1, this means that all pieces are in tolerance. In other words, the probability of defects in the first case is very high, and in the second case – very low.

So what should we do? The stability of our processes should be studied. If a process is unstable, find the root cause and through corrective actions bring stability to the process as shown in the following illustration.

When our processes are not stable, then there is a high risk of sending defective pieces to the customer. To prevent this, a 100% selection process is carried out. It often happens that selection is carried out at the express request of the customer, who has already received defective batches of material. All this generates very high costs, especially when selection takes place at the customer’s site and is carried out by special external companies.

Often, however, our processes are already equipped with devices that inspect our products at 100%. On the one hand, this is a very good safeguard against complaints, but on the other hand, it can cause us to ignore the problem. When a process is unstable, we assume that our detection system will detect all defective pieces. Such a situation can cause corrective action to be implemented moderately and not with the highest priority. Unfortunately, 100% inspection devices can also be unreliable. If they are not properly supervised and verified, it may happen that the customer nevertheless sends a complaint. Then you need to analyze the cause of the defect and why it was not detected despite 100% control.

Unidentified problems

Another cause of defects in Lean is the lack of identification of products and semi-finished products. How does the lack of identification affect the defect? It often happens that we use other or similar semi-finished products in the production process by mistake or due to lack of identification. Since this is not in accordance with the design, it causes quality problems. Lack of identification can result from negligence. Negligence, in turn, can result from another Lean loss, namely excess inventory.

Defects are a waste

In summary, producing defects is a waste of all the materials used in production and all the work put in by all employees. The number of defects is the number we have to subtract from our productivity and bottom line.

Any employee who throws a defect into a red box should be aware of this waste and should be empowered to take corrective action. Corrective actions should aim to stabilize processes. In turn, stable processes are a guarantee of our customers’ satisfaction.

If we can’t guarantee the stability of the process, then let’s guarantee the stability of the equipment (not the selection companies) for 100% control.

Finally, let’s add to this the quality of our work, which will translate into product quality as well as eliminate defects – one of the seven Lean wastes.