Plastics Are Still in Demand. This Stock Stands to Profit—but Beware the Risks.

My piggy bank is beating the stock market—the pig, not the coins. It’s made from rigid plastic, and I’ll bet you two old milk jugs and a shampoo bottle that it’s HDPE, or high-density polyethylene. The price of that stuff has doubled in a year. I’m leaving my pig where it stands on my shoe rack—let your winners run, momentum investors say.

HDPE is the world’s most common plastic. The business was barely a decade old in 1967, when The Graduate hit screens and Mr. McGuire gave Ben his famous one-word career advice. McGuire...