Mysterious Drop In Production Yield

Mysterious Drop In Production Yield
A factory manufactured membrane switches. After installing exhaust fans the yield dropped. What was causing this decrease in production?
Mysteries of Science


A factory manufactured membrane switches, flat control panels found on microwave ovens and other appliances. Business was booming, and more capacity was needed. Analysis showed a bottle neck was applying the clear over coat giving the desired level of gloss. All of these coatings required several minutes in the oven.

The factory decided to try Rad-Cure, a new process for radiation curing.  Using this method the coating could be applied and cured within seconds using a small UV processor unit.

Everything seemed to be running well during two months of testing and the decision was made to set up a full UV process line. The new process was set up and running well except for one minor problem. The UV coating had a very strong acrylic odor that bothered the workers. A bigger exhaust fan on the roof eliminated the odor.

Two months later during a quality audit the coating yield which should have been higher was now only 90%, although the scrape rate was only 1%. 

What was causing this decrease in production?

Here's the rest of the story.

One operator found some of the membrane switches in the parking lot. She mentioned that every so often she heard a fluttering in the exhaust pipe.

The strong exhaust fan was occasionally sucking a membrane switch up from the conveyor and blowing it through the roof. The fix was to extend the "draw" area to pull exhaust before and after the UV processor to reduce the air velocity. The lesson is to localize the problem zone and carefully observe.


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