One of the most significant items on the list of wastes in lean manufacturing is in-process rejection, commonly referred to as defects. This form of waste results in either scrap or rework—both of which disrupt the smooth flow of the manufacturing process and directly erode productivity.
To truly understand the impact of defects, it is worth examining how in-process rejection affects the operations team.
The Operational Impact of In-Process Rejection
- Accountability and Firefighting
Production and quality teams are immediately held accountable when defects occur. They are expected to implement short-term containment actions while simultaneously working on long-term corrective measures. This reactive mode often diverts focus away from value-adding activities. - 100% Inspection as Containment
In most cases, the quality team is forced to implement 100% inspection as an immediate containment measure. Until exit criteria are satisfied, this “firewall” must remain in place. While necessary, it places an additional burden on resources and often has a negative impact on team morale. - Customer Risk and Escalation
Even with 100% sorting, there is no absolute guarantee that defective parts will not reach the customer. When they do, the consequences can be severe—ranging from line stoppages to punitive actions—further demoralising the team and damaging customer confidence.
The Root Cause: Poor Process Capability
At the heart of these problems lies poor process capability. Process capability is commonly measured using the index Cp, which represents the ratio of specification tolerance to the natural tolerance of the process.
Process variation is influenced by factors such as machine condition, tool selection, fixtures, and operating methods. While natural variation can never be completely eliminated, it can—and must—be minimised through standardised conditions and robust process design. Ideally, a Cp value of 2 or higher should be targeted for critical characteristics.
Engineering’s Role in Defect Prevention
In my view, it is the responsibility of the manufacturing engineering team to establish strong Standard Operating Procedures (SOPs) that ensure high levels of Cp and Cpk (more on this later). This foundation must be laid early—during the APQP stage—and formally established as part of the PPAP process.
When process capability is built into the system from the start, defect prevention becomes proactive rather than reactive. The result is stable processes, predictable output, improved morale, and uninterrupted flow—key objectives of any successful lean transformation
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