Know Your Process

In an injection molding factory, most parts will have a mold and cavity ID molded on the back of the part. In this way, when there is a quality problem we can quickly know where the parts came from. Not many processes are set up with such convenience.

I was able to work with a high tech company that had a high reject rate, 2.7% coming from a long and complex process. There were several workstations for each process step so the number of possible paths that each product could have taken was in the hundreds. This is a problem in many high volume manufacturing companies. Not many can run one-at-a-time serial production.

One of the process steps was a heating cycle. Thousands of parts were soaked at an elevated temperature for 24 hours. When they were removed from the ovens, the parts were handed to several people for more work before making their way to a final test station. It was here at final test where the 2.7% failed.

In this assignment I had each product labelled (Sharpie marker) with the shelf number from the oven (from 1 to 12). We were very surprised to see that nearly all failures were labelled 1, 2 or 3. The bottom three shelves in the oven were responsible for almost all the failures! The temperature in the oven dropped of sharply near the bottom and these parts were not getting processed adequately.

There were so many opinions on a possible root cause and the real root cause wasn't on the list! Collecting data may be tedious but there's no other way to know your process.

An Expensive Opinion

Over a year ago, I was involved in a project to solve a weld problem. This company performed thousands of spot welds every day and some (approx one in 300) just didn't take. The failure wasn't obvious unless you tugged at the weld and even that wasn't a reliable test. But a poor weld could be serious if it failed in the field so this had to be solved.

A department manager was convinced that a current monitor would take care of things. In his opinion, the weld fails when the current drops below some critical number. So when the current is too low, the operator discards that last welded part. My argument was that there are many things that could cause a weld to not take; part cleanliness, surface geometry, applied force, squeeze time and more factors that are part of the welding process. The project required much more effort and some data. But he was adamant and he told us that this was what they do in the automotive industry. So we backed off and accepted this as a problem nearly solved.

A year later, the process has a reject rate of one in 300.

Collecting evidence to learn about a root cause takes so little time. Solving a problem based on an opinion can set you back months.

Clue Generating Tools

A client had a high reject rate on one step in the manufacturing process. This is a complex item with several parts and at this stage, near the end, rejects were costing just over $15,000 every month. The defect was causing a leak at a particular location on the part and many efforts over many months were not successful. 

I was invited to assist in this problem solving project and in my line of work, I never accept things as facts without evidence. I've seen too much waste. We rented a high speed camera to record what was happening at the point in the process where the defect was created. To everyone's surprise, we saw the leak start in a different location. It was centrifugal force that moved the fluid to the spot that everyone had thought was the leak. 

The high speed camera showed us where the problem started.  Tools that enhance human perception can often provide clues.