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The Graying White Plastic

The Graying White Plastic
A plastic injection molding plant was suddenly seeing white plastic parts come out grey. What was causing the discoloration?
Mysteries of Science


A large plastic injection molding plant was suddenly seeing their white plastic parts come out gray.

The raw material arrived as plastic pellets that were transferred to molding machines where they were melted, injected into metal molds, cooled, and ejected as plastic parts.

The plant first looked at the raw materials to determine the problem, but the white resin checked out fine. There did not seem to be any correlation between the molding press used and the gray parts.

They did notice that if the problem occurred in one press, it would also occur in others that same day, or a few days later. Also the coloration would change in intensity, diminishing completely as the run continued.

What was causing the white plastic discoloration?

Here's the rest of the story.

A technician was observing the resin deliver process. A trailer truck arrived and parked at the side of the loading area. The actual resin transfer required a large flexible hose connected from the truck to the silo. The truck pumped the plastic pellets using a separate pump on the truck.

When the truck first pulled in, its exhaust stack was close to the end of vacuum hose that was supported with a spring-loaded arrangement. It's a common practice to leave diesel engines running while loading and unloading. So the vacuum hose was pulling in the diesel exhaust while the truck engine chugged along.

By the time the driver climbed up on his rig to suck the resin up into the silo, quite a lot of carbon soot had entered the storage silo. The source of gray contaminant has been diesel truck exhaust particulate.


I enjoy reading the mysteries of science. It's like reading a mystery story. Sherlock Holmes would be impressed.
Richard Grime, Analytical Technology, Inc.

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