A Minnesota medical moulder needed every critical characteristic on a connector above Cpk 1.67 to win a long-term supply agreement. Centring two features in steel and balancing the hot runner lifted the lowest Cpk from 0.97 to 1.69, won the agreement and cut inspection cost by two thirds.
Key numbers
A medical moulder in Minnesota was close to winning a long-term supply agreement for a medical connector. One condition stood in the way: every critical characteristic needed a demonstrated Cpk of at least 1.67. Three of the five did not reach even 1.33. Without the capability evidence, the agreement would go to a competitor.
The situation
The connector had five critical characteristics: the luer taper, bore diameter, overall length, flange thickness and thread pitch. The parts were functional, but the capability data showed that several characteristics either sat off-centre in their tolerance or varied too much between cavities. Heavy inspection was being used to protect the customer, at a cost of around $64,000 a year.
What the data showed
The analysis, following the approach in understanding Cp and Cpk, separated the problems. The luer taper (Cpk 0.97) and bore diameter (1.18) had good Cp values but were off-centre: their spread was acceptable, but sat too close to one tolerance limit. Flange thickness (1.29) had excess variation, which analysis by cavity traced to imbalance in the hot runner. Overall length and thread pitch were already capable.
Different problems needed different fixes. Adjusting the process to move one feature would have moved others too.
What changed
- Steel adjusted to centre the taper and bore, so the process could run in the middle of its window rather than being pushed towards one limit.
- Hot runner tips balanced to reduce cavity-to-cavity variation, the same principle that resolved an eight-cavity imbalance years earlier.
- The process window confirmed with a designed experiment, so the final settings were robust to normal variation.
- Stability confirmed with control charts before final capability was calculated.
Results
All five characteristics exceeded Cpk 1.67, with the lowest at 1.69 and the taper at 1.71. The supply agreement was signed. With demonstrated capability, inspection was reduced, and inspection cost fell from about $64,000 to $22,000 a year.
Lessons for your operation
- Separate centring problems from variation problems. They need different solutions.
- Centre features by adjusting steel, not by running the process at the edge of its window.
- Analyse capability by cavity on multi-cavity tools. Imbalance hides in combined data.
- Confirm stability before calculating capability.
- Demonstrated capability is a commercial asset as well as a quality measure.
