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The Plexicorp logo overlooking the factory floor.

Plexicorp was a 20th century manufacturing plant located near San Francisco, Earth - operated under the management of Dr. Nichols. Plexicorp was well known for its work with polymers and its output of plexiglass.

The floor of the Plexicorp plant

In 1986, they received an unscheduled visit by "Professor Scott", from Edinburgh, by appointment to study methods of manufacturing by Plexicorp.

Plexicorp, however, was unaware of the appointment, but nevertheless Dr. Nichols offered to take "Professor Scott", and his assistant McCoy, around the plant personally.

At the conclusion of the tour, "Professor Scott" offered Dr. Nichols the formula for a material called "transparent aluminum". Nichols was given the formula in exchange for a number of sheets of plexiglass. (Star Trek IV: The Voyage Home)

The scenes for Plexicorp were filmed on location at a company called Reynolds & Taylor, Inc. (now Reynolds Polymer Technology, Inc.) in Santa Ana, California. (Star Trek Encyclopedia (4th ed., vol. 2, p. 171))
The plant's location is in dispute. It is either located in San Francisco (Star Trek Encyclopedia (4th ed., vol. 2, p. 171)) or in Burlingame (revised shooting script for the movie. [1])

Employees of Plexicorp