A plastic molding plant had been running for less than a year. Suddenly parts began testing positive for an electrical charge. What caused it? Mysteries of Science
Transcript
A plastic molding and decorating plant had been up and running for less than a year.
To mold plastic parts, plastic resin would be melted in big molding machines and injected into metal molds. Once cool, the mold would be opened and the parts removed.
Pulling the warm plastic parts away from a mold generates static electricity, and the plastic parts take on a static electrical charge.
The charge was being removed by an ionizer that looked like a small air spray gun that an operator would hold in one hand. The plastic piece was removed from the mold by the operator who then sprayed ionized air onto the part.
Everything was working fine when suddenly the parts began testing positive for an electrical charge. A new operator was working the line when the charge showed up. However, she was spraying the parts with the ionizer and the equipment checked out fine.
What was causing these part to suddenly have a high electrical charge?
Here's the rest of the story.
It sure seemed like the charge was moving to the ionizer gun, but not traveling down the ionizer ground wire. The wire was connected to the steel sheeting on the wooden platform. The engineer pointed the meter at the metal on the platform. The meter read off the scale. The entire platform was charged!
Thinking the problem through, they realized that the ground wire was conducting the charge to the sheet metal floor on the platform. And then where? Nowhere!
The wood insulated the platform from the conductive flooring, and there was no ground path. It was an easy fix; just place a ground connection from the metal platform to a real ground.
Once an operator was on the platform running parts in quick succession, the charge would build up until the entire platform was charged. Each new operator would "reset" the platform by climbing on.
While this looked like an operator-dependent mystery, it was actually a time-dependent problem.
