The Day the CPK Report Came Out, the Room Fell Silent for Three Seconds
I still remember years ago, when our factory first started implementing "mixed-model production," it truly turned everything upside down. Once, for a product line I was in charge of, the routine CPK report before shipment came out, shockingly showing a CPK of 1.08. The conference room instantly fell silent for three seconds, filled with an air of dread. My boss's face turned pale, because we all knew that this figure was utterly unacceptable to customers, indicating a high probability that this batch of goods would be rejected. It was later discovered that a parameter hadn't been properly adjusted when this product was co-produced on the same line with another model, and the issue went unnoticed throughout the production run.
Where Did the Problem Lie? The Myth of 'Sharing'
Simply put, mixed-model production means running multiple different product models on the same production line. The benefits are obvious: high equipment utilization, great production flexibility, making bosses smile. However, the challenge lies in the unique "nuances" of each product. You might think that sharing machines and fixtures is perfectly fine, but often, one product passes while another gets stuck. The most common issues are differing tolerance standards, varying process parameters, and even potentially different inspection standards. When you run these "different" items within the same SOP without sufficient segregation and management, it's only a matter of time before problems arise.
Therefore, the key point is that the challenge of mixed-model production is not in "mixing," but in "managing." You must ensure that each product maintains its respective quality standards within the shared environment.
How Is It Actually Done? Tiered Management Is Key
We later learned that to effectively manage quality in mixed-model production, "tiered management" is crucial.
- Parameter Tiering and Automation: For different products, categorize process parameters into "shared parameters" and "dedicated parameters." Dedicated parameters must be automatically switched by the machine to reduce the chance of human setting errors. In the earlier CPK 1.08 example, it was because the temperature parameter was not automatically reset during a product changeover, resulting in the yield for that batch dropping to DPMO 6210. If barcode scanning can automatically load the Recipe, it would eliminate a lot of trouble.
- Fixture and Tool Management: Fixtures, clamps, and even test probes for each product must have clear identification labels and undergo regular calibration. We even later implemented RFID to ensure that the system verifies correct fixture usage during every changeover.
- Differentiated Inspection Standards: You cannot use product A's inspection standards to test product B. Therefore, inspection equipment programs must also be able to switch automatically, or at least require mandatory manual double-confirmation. For instance, we once experienced a situation where, due to an unswitched inspection program, the Electrical Test (E-test) standard for a product was incorrect, nearly resulting in a batch of non-conforming products being shipped as good.
In other words, the "differences" must be forcibly "segregated" by systems or processes, rather than relying on human memory.
The Most Common Pitfalls: Cutting Corners and the 'It Was Fine Last Time' Mentality
The biggest pitfall I've encountered is the "cutting corners" mentality. Sometimes, to meet deadlines, people might think, "Oh, a slight difference in this parameter should be fine," or "This fixture is similar to the last one, let's just make do for now." Such thinking is the root cause of all quality evils!
I recall one instance when, to expedite an urgent order, the production line supervisor directly said, "Ah, these two products are similar in shape; let's just use the old fixture for a few pieces and see." As a result, due to a 0.02mm difference in fixture tolerance, the products experienced uneven stress on the machine, causing several pieces to crack directly and ultimately requiring scrap. The losses incurred then far exceeded the time saved and the cost of the fixture.
Honestly, problems often arise because people think, "It was fine last time when we did it this way," only for it to explode this time. This mentality is absolutely to be avoided in a mixed-model production environment.
One Thing You Can Do Today
Check your production line's SOP to identify which steps during product changeovers rely on "human memory."