Electronics Assembly Knowledge, Vision & Wisdom
What Caused the Mysterious Spots?
What Caused the Mysterious Spots?
One afternoon big round blank spots in a roll of metalized film were detected. Were the defects in the film-coating process?
Mysteries of Science

Dr. Gilleo
Mysteries of Science by Dr. Ken Gilleo
Dr. Gilleo is a chemist, inventor and general problem solver. Ken has been tracking industrial forensics and collecting case histories for decades. These cases are taken from the vast world of industry and commercial enterprise.

Check out Dr. Gilleo's eBook, 100 Mysteries Solved by Science. We hope you enjoy these case histories. You need not be an engineer or scientist to understand the problems and appreciate the solutions.
Metalized films have a variety of uses including electronics shielding. The process begins when a jumbo roll of plastic film is loaded into a vacuum metalizing chamber. In the chamber aluminum wire is heated to vaporize the metal and coat the film to produce a bright silvery finish.

The film is visually inspected while being rolled up an operator watches the film race past a light positioned underneath. A good metal coating blocks the light, but even tiny holes show up.

One afternoon big round blank spots in the center of the roll were detected. The defects almost certainly had to be in the film coating process.

A chemist noticed that the blank spot areas felt greasy. A spectroscopic analysis of the grease spots determined that they were vegetable oil, not an industrial lubricant. 

There was no way that oil could come from the inside of the chamber to make such a pattern; it must be a downstream problem. 

What was the cause?

Here's the rest of the story.

Our industrial detective went to the film treating room. Here, the film is heated to cause it to preshrink and to drive out moisture. The oven looked fine; nice and clean, with no dripping oil or grease. It was time to quiz the oven operator, Otto. "Otto, when did you start working on this film line?" His response was "On Monday".

This was very day the mystery started. Now it's noontime, but the roll of film will continue to run right through lunch just as Otto had planned. Otto opened the oven access door and pulled out a long, thick sausage. Otto then placed the dripping sausage onto a plate and walked to his desk to eat his lunch. The "Mystery Circles" case was solved.

Otto would hang homemade sausage inside the oven at 11:30 AM every day. The sausage would heat up and slowly drip grease on the moving film below. When Otto removed the sausage, the drips suddenly stopped.
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